My Fisher 400 restoration and modification project

[klamsoss]

I picked up this Model 400 locally last summer with the intention of restoring it for use in our living room. I love my other Fisher gear, and my fiancee likes the glow from these big Fisher receivers. I won't say no if she endorses any audio purchase! Other projects have kept me busy, but I have been feeling increasingly guilty that I haven't done anything with this jewel.

The chassis is in really good shape and will clean up well once I establish the electronics in safe working condition. It isn't rusty, doesn't stink or have sticky stuff all over it and most of the lettering is in great shape. I've posted a few "before" pictures to show the condition.

Electrically:
It has a good tube compliment (mostly original and a few American replacement tubes), OPTs test good (I get 264 and 273 ohms between pins 9 on each pair of output tube sockets, and then between the two red leads coming out of the OPTs and pins 9 on V14 though V17 I measure 127, 137, 133, and 140 ohms respectively. I believe these two sets of measurements are how one checks that OPTs are in good shape).
The small value high voltage caps in the power supply were physically broken, the power switch/volume pot is burned, but the volume works (photos of these are in the next post). I don't know how to test the main power transformer but for some reason thought that if I don't get open or continuity across the AC plug itself, that's an indication that the main transformer is probably not dead. I can't find where I read that though, so I can't rely on it. I hear the power transformers on these rarely blow, though, but the busted 0.01 caps and bad power switch worry me. The power switch is a common issue on these, though, I understand

My intention is to get it running properly first, then decide how far I want to take the modifications. I have some experience with very similar circuits in my FM-100-B and X-100-B which I have modified to some extent. Some, but not all, of the modifications and reliability improvements for those two units will transfer neatly.
I knew about the power switch when I got it, and considered just adding an inline switch and mounting it hidden on the side of the chassis, just behind the face plate. I may still do that but I like the properly working switch on my X-100-B, so I've inquired with Mark Oppat about a replacement for this model 400 and will use that if I can get one for a reasonable price.
I've ordered the hayseed hamfest can cap kit to cover the large value caps up top, and the "soda can" dual section 1000uF cap underneath. I did NOT get the "can" for underneath because I may end up wanting that real estate for EFB(tm), but that question is for a later post. I have stuffed cans in the past and it's not that hard, but it's messy and time consuming. The extra cost of the hayseed kit is worth the saved time and mess to me, at the moment. The kit should be here this coming week.
Other parts on order include the large wattage resistors that @larryderouin often recommends replacing with uprated pieces, and a few other parts I think I'll be needing. Most of the rest should be covered by parts I have on hand at my repair bench, though there are a few exceptions/questions, which I'll discuss in my next post.

I've also downloaded the "late model" (SN >48001) service manual since my SN is 72499 AM. For the record, I have the -AX OPTs and 6 IF/RF limiter tubes. I believe this is the correct manual for me, but I know there are at least a few examples of model 400s out there which appear not to exactly match any of the available service manuals. I have reviewed the thread on the various OPT and manual issues, but I can't follow them in their entirety. Hopefully folks will weigh in as the differences become relevant.

I'm modeling much of my work off of several of the very helpful threads by resident expert @dcgillespie Dave Gillespie. This includes the "Reference 400" thread (https://audiokarma.org/forums/index.php?threads/a-reference-400-restoration-modification.870612/ ), the thread on Fisher FM improvements ( https://audiokarma.org/forums/index...er-fm-stereo-mpx-tuners-and-receivers.708033/ ), the thread on improving the 400 (https://audiokarma.org/forums/index.php?threads/improving-the-fisher-400.511867/ ), and a few others. I'm incredibly grateful for the wisdom shared by all in these threads, especially Dave and Larry. I am aware of the threads. I have studied them. I will return to them as I make progress. However, I don't yet have the knowledge to fully understand them and confidently implement each of the updates/improvements described therein. I'll return to/reference them as my work progresses so that my thread can hopefully serve as a bit of a guide for others who, like me, aren't quite beginners but are only maybe sophomores (an occasionally sophomoric ) in terms of knowledge about these units, and want to learn more.

Okay... on to the work!!
Even though I haven't yet tested the main power transformer in a way that I feel I can rely on, I've gone ahead and started with a few things that I believe are consistent regardless of what "version" 400 I have:
Triple cleaning all the contacts, pots and switches with appropriate deoxit formulas: D5 for switches and contacts, F5 for the treble, bass, and balance pots.
I've replaced the physically broken 0.1 uF power supply bypass caps (C63 and C64 in my service manual) with appropriately rated film pieces.
I've installed a CL-90 ICL on the incoming AC leg which goes to the rectifier, NOT the leg that goes to the fuse (which is how I did it in my X-100-B). I think it doesn't really matter which leg of the power cord it goes on. It just needs to be in there.
The 100uF 25V electrolytic which is crammed over in that corner (C42 in my service manual) has also been replaced with a new Nichicon 50V rated piece. It is interesting here that the positive side of this capacitor goes to ground, rather than the negative. That's something that I double and triple checked.
The 1uF electrolytic capacitor in the tuner section (C210 in my service manual) has been replaced with an appropriately rated film which I have not yet tied in fully (more on that in my next post).

My plan is to replace all of the electrolytics and other the main power supply components (diodes, rectifier) before powering up to establish general functionality. From there, I'll perform updates and modifications.

This seems like enough for one post, but I'll be coming back. In my next post (later today) I'll describe some of the issues I want to understand better, questions I have related to appropriate parts, and outline some of the goals I have already developed for this resto-mod. Meantime, if anyone can advise on how to verify that my main power transformer is good, I'd appreciate it!

 

[klamsoss]

Unfortunately, I didn't take "before" pictures of everything "under the hood" but I haven't done TOO much, yet, so it still looks reasonably close to "before", except for a few coupling caps and the busted 0.01 uF power supply bypass caps. I still don't know how to check the power transformer with a non-powered test, so would welcome guidance on that.

Once I have this functioning, I need to revisit the tuner thread here https://audiokarma.org/forums/index.php?threads/improving-the-fisher-400.511867/ to refresh my understanding of the tuner and MPX in these Fisher units. I think most of the modifications should transfer well, except my FM-100-B has a stereo indicator beacon which is absent in the 400, which employs a tuning eye tube. I haven't yet learned about that, but I will. Let's not get ahead of ourselves with this post, though... The extent of the tuner section work I have done is replaced the 0.0047 uF coupling cap on V101 in the MPX section and the 1uF electrolytic(pictured). The coupling cap had a visible cold solder and would be replaced eventually, anyway, so I went ahead and did it. I will replace the 8uF electrolytic before trying to power up, too.

Other things I have done so far are replace the ceramic interstage coupling, and 0.022 uF phono stage caps, and then also the 400V 0.047uF main coupling caps with PIO pieces, as I did in my X-100-B. One of the phono section capacitors looked physically damaged, and when I pulled them out, they were both out of spec. I lifted legs of the 400V main coupling caps and measured them to be leaky, but realize now that my test procedure was flawed. I guess there is no harm in having replaced them for right now, but I might end up needing to change the main coupling cap values depending on EFB/grid resistor modifications... more on that later, though.

I also replaced the mylar 0.1 uF C32 (in my service manual) with a film type. But something fishy is going on with that little terminal strip. It doesn't match the schematic (to my understanding). The thumbnail image shows the strip in question. The terminal strip has four lugs(one unused at the bottom). Second lug from the top is the ground lug. The purple wire connected to lug 1 (top) goes to the function switch.
Lug 1 connections:
purple wire to function switch,
470K resistor to ground (lug 2).
0.1 uf cap to lug 3

Lug 3 connections:
1800 pF ceramic disc to ground (lug 2)
0.1 uF cap to lug 1
47k resistor to tuner section

I've also posted pictures of a schematic sketch comparing the circuit as it physically exists in the receiver, and as it is depicted in the service manual, and a shot of this part of the service manual.
I really do not get what's going on here. My work here is of the "remove and replace" variety. I don't actually understand what this terminal strip and it's parts are doing in the circuit other than that that it sits between the function selector and the tuner section. So I guess we are at another question..
3. What the heck is up with this?

Some questions about parts that I have on hand and their suitability for use in this receiver:
1. I have UF4508 on hand left over from another project (probably the X-100-B?). Would those be a suitable replacement for the power supply diodes, rather than 4504 diodes recommended by Larry elsewhere?
2. I have a spare GLB06 single phase bridge rectifier (600V, 3A) left over from when I did my FM-100-B and am wondering if that is a suitable replacement for the rectifier in this 400. If not, what would be the suitable replacement part?

Eventually I will address EFB(tm), phase inverter, IBBA/IBAM, grid resistor and coupling cap values, tuner updates, equalizing inputs, RIAA filter adjustment, pre-out/main-in jacks, and maybe adding a soft serve ice cream machine. A lot of these items are tied together, and I will need to resolve some questions before making those modifications, but that's for another day.

Some things I'll tackle once I get a baseline include:
Install the RIAA circuit modification outlined by Dave in his thread on the topic (https://audiokarma.org/forums/index.php?threads/the-fisher-phono-preamp-and-riaa.600690/ ). It was a pretty easy little mod on my X-100-B and I was really pleased with the results.
I'm also going to do 10 ohm cathode current sensing resistors to the appropriate pins on each output tube socket.

For now I'll leave things where they are and continue to read more about EFB and (potentially) related issues while I await input from you good people of AudioKarma.

 

[thornev]

If you're having issues with the circuitry as it is now, I would solve those issues before adding anything new. Adding something new will just add more variables to sort through and you'll still have the original issues to solve.

 

[klamsoss]

@thornev , I'm with you. It's always important to establish an operation baseline, once a unit is safe to power up. Otherwise you're probably going to chase your own tail.
With the exception of the handful of coupling caps I replaced, everything thus far has been geared towards replacing 60 year old electrolytics and getting the power supply situated so that it is safe to turn on and begin establishing that baseline.
Before going into EFB(tm), tuner mods, or even the RIAA update, I'll definitely get it going and verify functionality of each section of the receiver.
At this point, I'm trying to get a few clarifications on things I need to complete before that first start up, and then gathering information for future study. Hence my questions about the suitability of my diodes and rectifier (which I kinda consider necessary to replace for safe startup), as well as the question about testing the main power transformer before trying to power it on (which I don't plan to do until I get the hayseed caps installed).
Input beyond that is requested for guiding my study of what to do after I have a basically functioning Fisher model 400.

 

[Kingbleu]

I have a Fisher 400 I just pulled out of the pile. I have had this for a couple years and always wanted to restore it. I will be following your thread and possibly mimicking your advances. I'm very interested in the modifications you have mentioned. My unit is working and I am listening to it at the moment. I hooked up some cheap Klipsch computer speakers and it sounds great, nice tuby bass. If you need any base line specs or pictures I can help you out. Mine is serial number 61938. I didn't know there were so many versions.

 

[kappclark]

I am planning to get the Hayseed kit for my 400 - please share your experience and pics in replacement process when the time comes...

 

[thornev]

Bill - It's a simple process but must be done with detailed attention. What I did was take pictures of all the cap can wiring that will be replaced making sure the wiring is clear. That way you have a pictorial reference from which to operate. I also made my own drawing of the wiring which helped me understand what the rewiring project is. When the kit arrives, replace the cap cans one at a time being very careful to unsolder each wire and component (be aware that some components are heat-sensitive so you don't burn them out by holding the soldering iron to the solder too long or to remove solder). You likely will have to "unturn" the can tabs that hold the can in place. You can cut them if need be. Just make sure you remove any solder that would prevent you from putting the new tabs in their slots. Twist the tabs carefully and slowly to secure the new cans. Also be careful not to destroy the phenolic gasket that separates any external metal on the cans from the chassis. That gasket is there for a reason - the can is probably can-positive and cannot make contact with chassis ground. Then rewire that can using your drawing and picture as a guide. Very important that you rewire exactly the same as with the old can. Remove as much old solder as you can. It's OK if you cannot remove solder - you can cut the old component's leads leaving a 1/4 inch or so with which you can solder the new wiring.

That's pretty much it. Also don't forget that capacitors, especially the can capacitors, can hold a charge long after you power down the unit. You have to ensure those cans have no charge in them by grounding the terminals together or some other process that is described on many internet web sites. I'm sure others will chime in who may have specific 400 knowledge. Good luck and be careful !

 

[bhamham]

Removing cans can be challenging.
Tools I use in order of importance: Desoldering gun (Hakko), high-wattage soldering iron (I use one for stain glass), solder wick (get a good one like Chem-wick), manual desoldering sucker like Edsol? I'm not sure I have the name right, but it's a big hand operated solder sucker and works great. Paste flux like Nokorode is good to use on solder wick as it helps draw the solder.

 

[klamsoss]

I don't know if my 400 is standard, but there are luckily only 2 solder tabs on the can caps that are actually soldered to the chassis. I haven't tried to melt that chassis solder yet, but I'm confident I can make it happen using both my soldering iron and desoldering gun. My backup plan is to use a high speed Dremel tool to carefully cut the solder away from the chassis if I can't melt it. I've done that once before on a rebuild.

When I was getting started and collecting tools, I couldn't stomach the cost of the Hakko desoldering gun and instead picked up a Stahl tools DSDS from Parts Express (which happens to be up the road from me). I've been really pleased with how well it works, and recommend it for folks who are looking for a cheaper alternative to the Hakko. To be honest, though, I use the desoldering pump more than the gun with this kind of point-to-point wired stuff. The gun really excels on PCBs.

Anyway, still looking for input on testing the main power transformer, using UF4508 diodes rather than the recommended UF4504, and the GLB06 single phase bridge rectifier (600V, 3A). Plus question 3 in my 2nd post in this thread... still don't understand what's up with that setup.

 

[larryderouin]

Question 1 & 2. You're good to go with those parts (diodes and rectumfrier.)
Question 3. Electrically it should be the same. If you built a receiver exactly as the schematic shows, it would be over 2.5'wide and almost 2.0' deep. Schematics show the circuit in an ideal way, that's easy for diagnostics and layout. Actual units will look different, but be electrically the same.

 

[kappclark]

Thorn - Thank you for the very nice detailed pep talk ... Yes about draining caps -- safety first ! I understand the 400 has bleeder resistors...but .. trust but verify and measure voltages... trust me, I will drain caps... Have started new employment, and find my time even more precious ...so have put this off.. I read somewhere about having to drill out rivets ... the guys at hayseed said no way - any truth to this ? do I need to buy a dremel ?

Yes - pictures pictures etc ... watched blueglow's youtube about replacing can caps -- same idea of sketching out how everything is connected, take pictures etc ... measure twice, cut once .. and of course if the job goes well, I will share experience .....and if not well, I will still share the experience..

 

[thornev]

Bill - Re drilling out rivets - Are your cap cans secured to the chassis with rivets? On my 500-C they aren't. My cans have tabs that one inserts into chassis slots and then one twists the tabs slowly, gently but firmly no more than 90 degrees (and less than 90 is good as long as the cans are securely attached). Now, if your cans are secured via rivets, one option is to cut the cans at the chassis. People have done it that way. I forget how one attaches the new can (a search of these threads may uncover more info in regards to this option). But I'd say drilling out rivets is last resort. Let's see what others say.

 

[grindfix]

The 100uF 25V electrolytic which is crammed over in that corner (C42 in my service manual) has also been replaced with a new Nichicon 50V rated piece. It is interesting here that the positive side of this capacitor goes to ground, rather than the negative. That's something that I double and triple checked.

That is a bias supply capacitor. Positive to ground because it supplies negative voltage to bias output stage tubes

 

[larryderouin]

2 of the caps on the 400 are held in with tabs. If I'm replacing with Hayseeds, I use 2 irons to heat up the solder on the tabs, suck up as much as you can. At this point you have 2 options. Twist tabs so parallel with holes, and loosen by wiggling in a circle while keeping the heat on the soldered tabs or grind them off with a dremel.

The 3rd will be on a phenolic wafer that will be more often than not brittle to any movement. Grind the rivets off with a dremel and install the new one with 1/2" long 4x40 machine screws, lockwashers and nuts on the bottom side. Orient the cap on the wafer 1st, twist the tabs and install the assy. It DOES NOT get grounded to the chassis. It's part of the voltage doubler and will connect it's negative side to the adjacent can cap. Twist and ReSolder tabs to the chassis (use 2 irons on the tab/chassis point, one on each side of the tab). Wire it up and double check all wiring connections . Pictures and drawings help.

Larry

 

[RS Steve]

The soldered tabs are the biggest challenge for me in replacing the cans. I have a very powerful soldering gun and use de soldering wick with it having good results. Once I get a fair amount of solder removed, I use plyers to twist back and forth until the two tabs snap off. I have tried removing all the solder, it takes what seems forever. Once the soldered tabs are broke off, and the other tabs perfectly straightened, I wiggle the can free. Sometimes I need to hit the soldered tab areas with the solder gun again while wiggling. This process has really decreased the amount of time involved. The HH can caps are my favorite option for replacing the cans, not a fan of trying to hide stuff underside.

 

[klamsoss]

I put in another couple hours on the 400 tonight now that the hayseed hamfest capacitor kit arrived. The kit includes all 3 top side capacitors, plus two nichicon 1000uF caps of appropriate rating to replace the dual section cardboard can underneath. The cans all went in without drama. I wish I had used hayseed for some past projects. It's much easier than restuffing, and to me, it saves enough time and headache to be more than worth it.

I have had trouble in the past with removing cans, but either I'm getting better or these were easier than my past attempts... turning the iron up to 800 degrees melted enough solder around the tab/chassis interface to be able to break the tabs off. I was able to line up most of the non-soldered tabs with their holes and only had to break a couple off to remove all 3 cans. I was able to remove the top-side cardboard can without damage to the resin insulator.

While doing the cans, I replaced the pair of 330ohm 3w dropping resistors coming off the "front" can with 5w resistors. The old 3w guys were strung up between a solder lug strip and the lugs on the can itself. I skipped the chassis mounted lug strip and mounted them just between the can lugs. My thought with that is that under hood space is limited, and the area those resistors occupied might be useful down the road. Just keeping my options open... before and after in the first two pics.

The two rear cans (one is the cardboard covered single cap and the other is the 40/40/40/20) went in as close to the original setup as possible.
The cardboard unit was no problem. Shown in picture 3 (before/mid way) and picture 4 (after).
On the 40/40/40/20, I damaged one of the 2 resistors on the 20uF lug and the other was outside spec (not by much, but out, nonetheless). Photo 5 shows the bottom of the can before I got started. The large blue replacement 2w resistor seen in photo 6 is waaayyy overkill. I used it because I was worried the heat necessary to melt the chassis solder could potentially damage the resistor. As it turned out, the solder wasn't a problem and a standard 1/2 watt resistor would surely have been fine (and been easier to install because of its smaller size).

The two 1000uF caps in the corner under the transformer (in place of the huge cardboard covered dual section cap), and the replacement rectifier were a little tricky. I was trying to install the new rectifier in the same place and physical orientation that the original one was in, but it would have been better/neater to have done the two 1000uF caps, the 100uF cap, the two 1/2 watt resistors, and the rectifier all on a perf board. In fact, I will probably end up doing that when I install an EFB circuit (and put it all on the same board...which would mean I didn't need to try to save the space up front... ), but let's do some baseline functionality testing first.
As a result of my approach to keeping the rectifier in the original position and orientation, the area isn't as neat as it could be and the pictures don't do a great job of telling the story.
Pics 7, and 8 show the big cardboard dual section capacitor still in place as I did the rectifier. Then 9 and 10 show after I installed the new caps.

Now that the power supply is completed, and the tubes have all been tested, I think the next thing is to put it on the DBT. My plan for tomorrow is:
1. DBT at 80VAC with no output tubes, dum.y loads, bring it up to 117VAC.
2. DBT at 121VAC (wall) with output tubes, dummy loads.
And if all goes well:
3. Hook up some speakers and see what she sounds like.

I'll have to use a power strip since the on/off function in the volume knob hasn't been resolved. I haven't gotten a response to multiple emails to Mr. Oppat. I'll try calling the number listed on his site if things work out well tomorrow.

 

[RS Steve]

Nice progress! Those cans can be a daunting task for sure, and I too have learned over time better ways to remove them without causing damage. I noticed that the power switch is missing in action unless my eyes deceive me. I know in the past when learning some of my restoration skills that you should keep most replacement resistors fairly close to original wattage except for a few of the high wattage units that are known to fail. I imagine this is so because you want the resister to fail fairly quickly if something in the circuit goes bad, thus not causing a fire or more damage upstream.

 

[klamsoss]

1 of the 4 original Fisher labeled Sylvania 7868s was cracked when I got the receiver, as was one of the Fisher labeled Sylvania 12ax7s. The two Fisher labeled Telefunkens are in good shape and test good (strong life, no shorts, no leakage on a Sencore TC-114 that is rebuilt and calibrated. It's not a mutual conductance tester or a curve tracer, but it has proven to be a reliable piece for me), and the other 3 Fisher labeled Sylvania 12ax7 test good as well. The remaining 12ax7 tubes consist of a MIJ Raytheon, a red-label GE and a Dastrom labeled Mullard (short ribbed plate) all of which are in great condition as far as I can tell. I pulled the Mullard to set it aside. I have another single Mullard, same description, that might match up well. I put in a pair of Westinghouse (USA) long black plate tubes for V7 and 8 (which are the phono stage tubes) for right now. These Westies are very strong on my simple but reliable TC-114, and worked satisfactorily at actual operational voltages in all positions on my Fisher X-100-B. Actually, I liked them for the phono stage until I tried out the tung-sol USA pair I have in there now.
I replaced the missing 7868 with an RCA from my stash. I know that this quad of output tubes is unlikely to be well-matched, but for basic operation check, it should be fine.
I replaced the Fisher labeled 6HA5 and 6BA6 in the tuner section because they tested bad, but I kept them in case these were false negatives. The replacements are a Japanese Raytheon and a USA Sylvania, respectively, which I understand to be new old stock (they also test as such). The Fisher-labeled Telefunken 6AB4 in the tuner section tested well and is still in place.

The DBT steps were uneventful except that I was initially confused and thought there was a short because the 60w bulb barely dimmed at all. This is likely due to the fact that 60 watts isn't much compared to the idle current draw and the presence of the CL-90. I replaced it with a 100 watt bulb and saw a more pronounced (but still not as dramatic as expected) dimming of the bulb.
My home-brew DBT stack includes a current metered outlet so once I established that the unit didn't have a severe fault, I plugged in the output tubes, and switched to the metered outlet. With or without signal coming thought (FM or AUX) this seems to idle around 1.15 amps. Can anyone weigh in on the current draw of a healthy 400?

Baseline:
It works.
There was no drama whatsoever on startup.
The front lamp on the right seems to be intermittent, and I don't see much of a "backlight" effect at all.
The stereo beam tube/indicator does.... something... but it at least works. There is some drag when adjusting the tuning knob (decades of crud on every pulley?).
The sound from the right channel is weak... almost non existent, and overall, the sound from the left is also nowhere near as robust (in terms of tone and volume) as the X-100-B. I am suspicious of the 3 original Fisher output tubes. I did not test the headphone output.
Both channels display some level of distortion. I didn't push things much because the intent today was to establish that this works, has no severe internal electrical faults, and establish the baseline operations.
AUX works, FM stereo, Filter, and Mono all yield sound, but it's not clear yet if the functions are doing exactly what they should be.
The treble control works at least somewhat, at least on the left channel. (right is too quiet to tell for sure).
The level from AUX appears to be consistent with the FM level, so that would suggest the weak output is unrelated to either of those inputs, and theoretically more likely to be in the voltage amp or beyond (in terms of the signal path).
I have not yet tested the phono section.
The balance control appears to work for the left channel, but it remains to be actually tested directly for both channels.
The volume knob feels like it is filled with molasses, but I do plan to replace it eventually, anyway.

I plan to put in a set of acceptably (but not especially) well matched output tubes and try things out again tonight. Testing will be documented in my personal notes on the unit, including all of the tone controls, the phono input, tape head input, loudness and high filter switches, headphone output, and I will hook up a better antenna than the folded wire type I use for my basic testing. I'll also move some tubes around to see if I can identify the channel imbalance thing. I have enough strong old stock American tubes to outfit this thing, so I can DEFINTELY eliminate the issue if it is just a tube. After I do tonight's tests, I think the next thing will be to install 10ohm cathode current resistors, the 100 ohm screen stability resistors, and re-test. From there I'll start to address the distortion and output issue (if it is not resolved with the reliable output tubes and other fiddling with tubes and functions).

A few pics of the top side are included to show the Hayseed Hamfest kit installed, and that the chassis on this one is in pretty good condition. I think a lot of the dirt and tarnish will come off with some ammonia. function before form, though. I'll get the electronics working the way I want before I undertake the cosmetic restoration.

 

[larryderouin]

You're drawing approx 140W at idle. Full load is about 180W. Sounds about right for a 400. Mine draws just over 1.4A on startup surge then draws down to just over an amp. That's an Early and a Late version. 75W to 100W is just about ideal for DBT lamp.

 

[klamsoss]

Tonight I replaced the output tubes with a quad that I matched up on the X-100-B. They are within 4 mA of eachother and I know they're reliable so I'm just setting the Fisher-branded sylvanias aside for now. Once I have everything working right, I'll put the Fisher branded tubes back in and see what kind of current they draw.
I also switched the Fisher/Sylvania phase inverter 12ax7s left to right. Then I figured I should give it a go and see if anything was improved.
I power her up, run 1kHz into AUX and I'm rewarded with tone from both speakers, much less distortion, and more output. Output is not even on both sides, but we're making progress...

Could it be a dirty socket in the PI section that cause the right channel not to work well initially? Could there be some gumming up causing the imbalance? I cleaned the tube sockets when I picked this up over last fall, but I know enough to know that more than one cleaning can be the ticket to success. I spent another chunk of time cleaning all the individual pin sockets with these little flosser brusher things... I've attached a picture of them. These work reasonably well, but I'm looking for something better, still. Eventually I will follow up with the deoxit gold protectant on the tube sockets.

Anyway, after a bit of drying time I put all the 12ax7s back in their original spots and tried again. Unfortunately, the right channel disappeared again. I hooked my bench meters up to quantify that the output was uneven and my ears weren't playing tricks on me. Okay, so I guess it's not the sockets, and I can conclude that one of these Fisher Sylvanias isn't quite up to the task of phase inverter duty. Like with the output tubes, I figured "let's set these aside for now and just put some robust tubes in there that I know can carry that load" and in went the short plate GEs. This brought both channels back, but there is still a big imbalance. Now the right speaker is much louder. And I'm also getting distortion in the left speaker, especially in the lower frequencies. We'll circle back...

No headphone output... hmmmm that seems weird, but I'm just going to put that on the list and come back to it.
The panel light on the stereo beacon side comes on when you press the tuning knob in, so I'm thinking there is just a loose contact in there.
The phono section yields no output of music, just a 60Hz hum, which is notably louder in the right speaker when on "stereo phono" and then equalizes in both channels when switching to "mono phono"

I tuned up an FM station, using balance control to get even output from the speakers, and switched back and forth between mono and stereo. The stereo reception is good! In fact, I sat and listened to a few minutes of my local classical radio station. They broadcast a high quality signal which sounded beautiful even on just my little bench speakers. Switching from Mono to Stereo added detail in terms of hall reverberations in the cathedral where the recording was made, and showed that the upper octaves of the organ were to the left of the hall while the lower notes seemed to swell from below the right speaker and drift into the center of the soundstage. It sounds a LOT like the FM-100-B/X-100-B combo, which is a great thing!
AND the stereo beam tube seems to be working, too!

Next thing to do is figure out why the left and right don't match. I'll start with the voltage and tone tubes. I'll do the same thing I did with the PI tubes: first switch, see if it changes anything, then swap completely with known strong tubes, and see if THAT does anything. I'm less hopeful that it will resolve the issue. Assuming it does not, my normal next step would be to break out the oscilloscope probes....

EDIT:
I now realize that the imbalance could be due to the PI adjustment. I assume one would make an adjustment whenever they change tubes?
I have made no adjustments to either side, so I think I will actually start there, based on guidance from @larryderouin on the now-archived PI adjustment thread here: https://web.archive.org/web/2017031...ck-and-easy-400-phase-inverter-adjust.559415/

If the phase inverter adjustment doesn't clean up the unequal output, I'll do some tube swapping, then progress to signal tracing.

QUESTION: I have either read or been told (or both) that I should not use an oscilloscope with my X-100-B because the ground is not actually ground and I'll let the magic smoke out of one or both pieces of equipment. Is the same true for the 400?

In lieu of the 'scope, I think I can use my twin bench DMMs and just compare the voltage, right? I won't get the waveform, or be able to track where the distortion is starting, but I should be able to find the level imbalance, and with any luck, that may also be the source of the lower frequency range distortion in the left channel.

After that, I need to figure out what the issue is with the phono stage. Could be as simple as a bad solder or broken wires. Again, normally I'd input a test tone and trace but if I can't use the 'scope, I'll have to get creative since I'm not sure my bench meters will reliably read a <5mV signal on each channel. They certainly can go that low, but it's the very low end of their range.

Way past my bedtime.