Newly-Acquired Fisher 400

[Dave451]

Hey, Folks. Back with new Fisher gear and very happy about my latest acquisition, an apparently late model (S/N 77XXX) Fisher 400 receiver in excellent condition with a full tube complement. The unit is quite clean (pictures are pre-cleanup, as-received), the faceplate is nearly perfect (including the knobs) and the chassis has no rust or corrosion whatsoever.

The phono and tone control 12AX7's are all Fisher label Telefunkens and the 7868 output tubes are two GE's and two Fisher Labels. Underneath, everything looks to be completely stock with only one obvious issue: one side of the power switch failed and both the transformer primary and accessory outlets have been wired into the other switch contact, which seems to be working well at this point. They didn't re-solder the shield cap back on, so I'll be taking a closer look at the switch to see what is what (and will add a CL-80 thermistor in any case). Both dial lights and the magic eye tube are working and the dial cord looks almost new. All controls work very smoothly.

I decided to power it up very slowly on my Variac after checking continuity on the power and output transformers, ESR checks on all the electrolytic caps (all checked good with no physical signs of distress or leakage) and a careful visual inspection. I was rewarded with what appears to be a completely functional receiver: audio is fine, knob controls are quiet except for a little bit of noise in the level control and switched circuits work fine. Stereo multiplexer is working and no hum at all in FM, phono, tape, and aux modes.

Operated it a couple of hours while carefully checking for heating of the electrolytics (none) and any other kind of distress to the components or tubes (none evident--no smoke!). Voltage checks were spot on for the B+ and bias supplies (line voltage set at 117VAC). Tube voltages for the output tubes were almost eerie in their consistency. All four 7868 plates were 398VDC, all screens were 361VDC, and the control grids were all at exactly -17VDC (out to two decimal places!). What a fine job of precision construction!

I'm thinking over how to proceed and reading up on all the fine restore and upgrade work that has been done on these. One question: the bias bridge is definitely not a conventional selenium unit (see picture). In fact, the parts list shows it as a silicon unit, so does it need to be automatically replaced like the selenium units? As per above, the bias voltages were spot on. Voltage right out of the bridge was -25.6VDC.

Also, one of the cores on the bottom of the front end (see picture) is turned way out. Thoughts on this? Receive seems to be OK so far.

Any thoughts/guidance on these things and general restoration are most welcome. I'm reading a lot of excellent past threads on restoring/upgrading this receiver. Thanks! Dave

 

[dcgillespie]

Nice looking unit Dave!!

The original silicon bridge rectifier is just fine, unless you simply want to replace it as part of a rebuild, but I would not. There is 0 history of the original silicon based part ever failing........

That is the oscillator coil slug, and it is very normal for it to extend quite notably out of the RF "can" as shown when the unit is properly aligned.

Good luck with it!

Dave

 

[Dave451]

Thanks, Dave. The 400 is in such good shape, I almost hate to touch it. I'll leave the bridge and thanks for the feedback on the oscillator coil position--it just made me wonder. So, I'll likely do a re-cap, CL-80, and the usual thorough cleaning of switches and controls and pots, along with a mild cleaning of the chassis (it doesn't need much). I do wonder about whether to change the electrolytics. They all seem to be working properly and there is no hum anywhere and they all check out for low ESR. I know the accumulated wisdom is to change them, but I left all but one section alone in my X-200 and have had no issues whatsoever. Dave, do you have an opinion on the Ero Fol II's? Again, most say change them all and I did for the X-200 (with excellent sound). I'll think that over. I'll also do the cathode resistors and likely screen protection resistors. I was a little surprised to see absolutely no adjustment for bias (truly "fixed" in this unit). I am debating your bias control recommendations. Thanks for your response and your body of excellent work on the 400 for me to study! Dave

 

[dcgillespie]

I have never seen any Ero cap fail or even show any leakage, and therefore never automatically change them -- unless a client wants them changed.

None of the Fisher vacuum tube stereo receivers had a bias adjustment on them, so its a good idea to at least install one and use a quad of well matched output tubes -- or go with one of the individual biasing modifications. Either way however, you will always achieve the best performance using a well matched quad of tubes.

Dave

 

[Dave451]

Hey, all. Voltage check detail on the 400 below. Have been doing a lot of reading about restoration and improvement of this. Voltage checks as follows with line voltage at 117VAC out of the Variac (schematic values in parentheses):
B+ Power Supply:

B+ at C56+: 403 VDC (405)
B+ at C56-: 202 VDC (198)
365V Source: 361 VDC
320V Source: 331 VDC
290V Source: 294 VDC
210V Source: 204 VDC
166V Source: 175 VDC
182V Source: 185 VDC

Bias Supply:
Exit Bridge Rect: -25.6 VDC
Post R35: -17.6 VDC

OUTPUT TUBES: (all the readings were pretty much identical, so only showing single value):
Plates: 398VDC (405)
Control grids: -17.0VDC (-17)
Screen grids: 361VDC (365)

I would consider all this well within tolerances--all in all, very well-behaved circuits. PS resistors all quite close to spec. Transformers running only mildly warm. Electrolytic caps all cool.

Sound checks on (marginal) bench speakers good. Ran the unit for about 2 hours--no problems observed. Tuner seems to be working well on weak signals in the basement.

Measured power output of the 7868s to onset of clipping at 1kHz and saw 23.5 WRMS (13.7 VRMS) on each channel. Line voltage for this check (no Variac) was running high at 121.6VAC. B+ voltage was 421VDC (up as you would expect with higher line voltage), output tube plates were 416VDC and control grid bias voltage -17.8 VDC.

Took it up and hooked up to my Klipsch KG4's and the sound was quite good, especially after a little operating time. Tuner is quite sensitive, even with just a clip lead on one terminal.

So, a couple of questions at this point:

-Does 23.5 WRMS per channel seem OK for this unit, or a little low? My X-200 gets about 32 WRMS with 7591's, but is no doubt a hotter amp. Specs for the 400 say 30 WRMS per channel with one channel driven (as I was testing). I'm wondering if the output tubes may be a little soft. No way of measuring tube current, as you all know, without cathode resistors.
-The tuning eye closes up nicely on strong signals, but appears to 'modulate' a little with voice peaks. Is this normal?

Thanks! Dave

 

[dcgillespie]

Dave -- Regarding the power output issues, this is a can of worms I opened up some time ago, which resulted in this thread that you might find of interest. My first post starts to address some of your questions:

http://www.audiokarma.org/forums/index.php?threads/modifying-the-fisher-400-with-efb.509090/

It is normal for the eye tube to modulate slightly when turned to the FM Stereo mode when the modulation is strong -- which is almost always, on almost all stations today.

Dave

 

[Dave451]

Dave, thanks as always for the thoughtful responses. The linked thread is most interesting and very on-point for me. You have to wonder at such a change late in the model run (as mentioned, mine is a 77xxx S/N--have you heard of later?). Any thoughts as to why? Economy? Tube life?

I'm studying this with great interest as I develop a refurb/mod plan for this unit, along with all the other information from you and Larry. This is a very nice unit, so I'm taking my time on it. More to come...

One other question (I asked on a Tube Audio thread, but I'd love to hear your input): My CD player audio output spec is 2.0VRMS into 600 ohms, and the audio input spec for the 400 is 280 mVRMS for full output. On high dynamic range CD's, the audio gets brittle/harsh during the loud passages even with the volume well down and I suspect over-driving of the early stage amplifiers. A fellow suggested in-line attenuators, which I may try to deal with this. I've often read in your descriptions of the Fisher amps that the early stages are driven flat out, so I wonder if excessive input level is the problem. Any thoughts on the problem and potential solutions much appreciated! Dave

 

[dcgillespie]

If your CD player is able to deliver and output of 2.0 vac RMS, that equates to a peak output of nearly 3 volts (2.828 volts). Typically, the input stage should be able to handle that input level, although it is near the upper limits for sure. But before you go to the expense of trying attenuators, try this: Plug your CD player into the Tape Monitor input jacks, but leave the Tape Monitor switch turned OFF and the selector switch set to the Aux position. Now note the sound quality to see if it is improved. In this scenario, the signal of your CD player is now applied to the input stage through the internal attenuation network that is connected between the Tape Monitor and Aux/Tape input jacks within the receiver. If it does improve the sound, then adding external attenuators would be a good way to address the problem. Using this little trick will help you determine if that is the way to go.

I hope this helps!

Dave

 

[Dave451]

Dave, it is indeed 2.0VRMS and 6V AC Peak to Peak. I did try your suggestion and it seemed to help, but now I am paranoid about my right side speaker. Using the tape monitor input also gave a more reasonable volume level setting on the pot, instead of having to keep it cranked down to less than '9 o'clock.' I'll check everything out, but may invest in the attenuators. Thanks for the tip!

Alas, I was also hearing some distortion on FM stereo on the 400 despite center tuning to a strong signal. No noise on mono, so I'll need to do some tube checking and maybe a basic front-end alignment as part of the work. I'm thinking about the 'no instrument' peaking of the IF and RF stages, ala the KM-60 manual.

Dave, I am reading through all the threads you've done on 400 output transformers, EBA, IBAM, and general upgrades on the 400 to develop a plan.

 

[Dave451]

Well, this is interesting. I read the two threads started by Dave G on Fisher 400 output transformers, noting that Dave's late model 400 had higher input impedance transformers with serial numbers T1020-116-1/2AX, and the EFB parameters were beneficially adjusted to deal with these transformers and their circuits. So, I checked my 400 S/N 77444 and my transformers are T-1020-116-1/2 (no AX!--see photo). So, 'what's a mother to do'? Dave, does this mean I have the lower impedance transformers and should use different EFB set up than your late S/N unit? When I get this one back on the bench I'll check some component values (particularly the feedback resistors and caps) versus the new and old schematic and post here. This one does have the finned silicon bridge and the extra IF/Limiter stage and as per above, and I was only able to get 23.5 WRMS per channel at 1kHz, but this look like it meets the spec for the earlier units per Larry's post? I did note that harmonic distortion looked very low on my FFT scope output at 23.5WRMS, but I didn't take note of the levels--just a really low second harmonic peak and nothing much at all above that just before clipping.

 

[dcgillespie]

Dave -- This has come up before where another late model unit also had the non-AX transformers, and to confuse things further, were still shown to in fact have the higher impedance primary as I recall. Larry would know for sure since he is the official site record keeper (Yes Larry -- that was a promotion!). Check the resistor in the NFB networks: Low impedance transformers used a 3.9K resistor while high impedance transformers used a 2.7K.

With the power output levels you got, you very likely have the higher impedance transformers. The THD levels are low when you only drive one channel. Against other units, they're still even respectable with both channels driven -- although distortion does rise significantly in that scenario.

If the FM distortion is only present with the unit set to the FM Stereo setting, then more likely its your MPX decoder that needs some attention.

Dave

 

[Dave451]

Hey, Dave--thanks. I'll check the components and report back here. I'm going to try to determine a turns ration and, therefore, impedance for the OPT primaries and will come back with that as well. I suspect your are right, and these are high impedance regardless of S/N.

As for stereo noise, I don't have the instruments (at this point, at least) to check and align the multiplexer. But I did use a highly advanced troubleshooting method and wiggled the three tubes in the sockets of the MPX and things are quiet now! Stereo indicator is working fine. We'll see if it holds up, but I know the tube sockets need cleaned and I always do this when refurbishing. Also, will check the tubes of course.

Dave

 

[larryderouin]

Thanks Dave. I'll take the Promo even if the pay does stink. Check the Late Aug. to Mid Sept 400 threads. It was either right before or righyt after my back surgery.. That guy had a 70xxx series IIRC.

 

[Dave451]

OK, guys. I checked the components in my S/N 77XXX 400 and the feedback RC components are consistent with the higher impedance (10200 ohms) output transformers; that is, 2.7K and 560 pf. I also did a turns ratio check and using that calculated the impedance at 10500 ohms, so there you go. I have a late run unit with no "AX" suffixes on the OPTs, but it has the higher-impedance transformers, consistent (as you said, Dave) with the 23.5 WRMS per channel max output. Therefore, I should be able to follow your EFB setup for your late model 400, Dave, as you did it in that thread.

One other question (for now, at least!): For the output couplers between the PI's and the 7868's, why is one of the 0.047 uF caps rated 630V and the other in the pair rated at 400V?

 

[larryderouin]

I get the feeling FISHER realized there wasn't any point to having the suffix on the late late serials as they were near end of productionand no other versions were to be produced. It probably cost them per letter or number on the side shells.

I don't recall ever seeing or hearing of any serials in the 80's.

 

[dcgillespie]

Dave -- The coupling cap voltage rating was purely an economic decision that (gasp!) actually made sense.

The cap on the plate of the phase inverter potentially sees the maximum unloaded level of B+ applied to it, along with the negative bias voltage on the other side of it at the first moments of turn on. This combined level is sufficient to clearly exceed safe surge levels of a cap rated for only 400 vdc. Therefore, on the plate side of the inverter, a 630 volt cap is used to handle the turn on surge. Of course, after the phase inverter tube warms up, the operating voltages are such that a 400 volt rated part would be just fine -- except for the turn on surge.

On the cathode side however, no such surge exists, as there can be no cathode voltage until the phase inverter tube warms up -- and when it does, it will only rise to its normal operating level with no surge to contend with. Therefore, a 400 volt cap is just fine for the cathode position.

The move saved a few pennies -- but Fisher did this trick in virtually all units (amplifiers and receivers alike) that used the Eurofoil caps, so given the literally thousands of units produced, the pennies add up!

I hope this helps!

Dave

 

[Dave451]

Makes perfect sense, Dave--thanks! The X-200 did the same thing (I used 630V for both when re-capping). I guess that's an advantage of tube rectifiers, vis-a-vis surge.

Onward with the re-furb work. I ordered the can caps today from Hayseed Hamfest (nice Fisher 400 kit). After extended play, the four section 20/40 was heating up a bit more than radiant heat and the 2W resistors on the bottom would account for and the 20 uF section ESR increased to 32 ohms after a few hours operation. Signs of leakage now in the others as well, including the hot can C56, so out they all go (including the 1000 uF bias supply caps). Pulling a parts list together for the other mods now.

The multiplexer definitely needs work. Still getting distortion on stereo audio (none in mono). I replaced V100 and V101 in the multiplexer with new 12AX7's and the tuning eye closed up considerably more, but still a good bit of modulation of the eye on audio peaks and distortion in stereo. I don't have a wide band multiplex generator for alignment, so I'll have to think about how to proceed.

Do the PS doubler diodes usually need replaced? They seem pretty robust, but I have a few 1N4007's on hand.

Dave

 

[larryderouin]

The originals were 250v for the P.I. side, and 400v for the plate side. The originals in your's are Ero-Foils , but some late models got those Axial Film Caps that were turquoise in color. like the one in your mpx. FISHER swapped over to them in 1966-67as they were changing over to solid state.

I'd change the diodes out along with the resistors between can cap sections. Makes for a more stable and robust P.S.

 

[Dave451]

Thanks, Larry--will do. Right now I'm juggling all the upgrade/restore posts for the 400 (lots of great stuff!) to develop my plan and parts list without getting everything screwed up. Included is your thread on restoring the 400 for Tom. I'm going to try to hang onto the Ero's, but will need to test for leakage to be sure. No signs of problems so far after a few hours of operation (unlike the electrolytics), but we'll see. I'll do cleaning and maybe have a look at the power switch while I wait for the parts.

 

[larryderouin]

Like Dave, I've never found a leaky Ero-Foil, but with the seriously small difference between the operating voltages and the max on the cap so small back then and the increase in today's wall voltages,the differences become even smaller and are sometimes reversed. So there are instances where a higher voltage rated cap is required.

I normally replace the installed e-foils with good reading higher voltage rated e-foils from previous overhauls if I have them. Not always with E-Foils, as some people want a different cap in it. So if you do replace them, hang on to them.