Fisher TA-600 Restoration

Get creative.

How's this for creative?

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The IBAM board is built, but not yet connected.

Please refer to the image below, and click here for a larger version.

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Question: Should R152 and R155 be removed and replaced with jumper wires? It looks like this was done in Larry's image posted in this thread. (Yes, I am aware that Larry's is a late version.) I need to know this before I proceed with the IBAM installation.

Question #2: Suggestions on lead dress of the wires coming from the IBAM board?
 
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I don't think the 10K would affect how the IBAM operates. Looks like its being used as a combination grid stopper / HF step network. Without knowing precisely why they put that in on one side of the phase inverter only, I'll take a guess and say it has something to do with amplifier stability at high frequencies. I'd leave it be. I believe Dave's work on developing the IBAM setup was done on an earlier model.

The late models don't have that, but they don't use a 7199 either.

For the wiring, mostly I'd say keep it away from any terminals that have high voltage as much as you possibly can.
 
Leave R152 and R155 as is. Gadget is correct in that they used them for HF stability. I just asked Dave about the disparity in the TA-800 between one side being 10K and the other side 1K in a pair. I'll quote his response here from the e-mail:

My question:
Dave. I'm possibly getting a TA-800 soon. Looking at the scat, it looks like a typical fixed bias 7591 setup. Looking closer at the parts list (particularly R157-160-162-163) the 1K resistors before the output tubes at the grids, I'm seeing parts list values of 10K for 1 of each pair. Can this be right?? Or did Fisher screw the pooch on this one and all 4 are 1K. I'm tending toward the latter.

Dave's RESPONSE:
I have no doubt that parts list is correct. This is an approach that Fisher used on a number of models to help achieve HF stability. By making one resistor larger, it very slightly reduces the HF response of that one tube (due to allowing increased effects of Miller), and in doing so can help to balance the response presented to the OPT, based on the internal capacitance that the primary winding represents to each output tube plate — which is often unequal.

Take a look at the original X-101ST, and the mono model 30s for example, where I think you’ll find the same approach was used. As OPTs became better and better in later years, such expedients were no longer necessary. In any event, it was in common practice for Fisher to use this technique around the time the TA-800 was designed, so I’m sure the parts list is correct.
END DAVE's RESPONSE.

As the TA-600 is the smaller sister of the TA-800 I would surmise the reason is similar.
 
OK - thanks Gadget and Larry - the resistors will remain.

Tonight after work, I started preparatory work to get things ready for the IBAM circuit.

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Since I had to remove some resistors, I just went ahead and replaced all of the resistors in the area of the output tube sockets.

I will next install the new 10 ohm cathode resistors, then I will hook up the IBAM wires and we'll see what happens after that.
 
Another question:

Could one of you walk me through the procedure to adjust the trimpots once the IBAM is installed? What (voltage/current) am I measuring for? Where?

From what I have found, it looks like I'm measuring millivolts at the four 7189 (in my case, 6П14П-EB) cathodes. What is the "sweet spot"? Is that all there is to it?

Some elaboration would be appreciated, thanks. I would like to know what I need to be doing once I finish the installation and fire it up for the first time.
 
Per Dave

During the course of this work, it was determined that the optimum output tube cathode current draw is 34 ma per tube.

Its slightly awkward to adjust this honestly. The way I did mine, I set the pots all at 1/2 travel, connected the - lead of the meter to the common line where all of the 10 ohm resistors come together feeding to the heater circuit, and then moved the + lead to pin 3 on each output tube, adjusting for 0.34 volts on each one. The reason it becomes awkward is that changing the setting on each trimmer will alter where the other tubes run, so you have to repeat the process a number of times. As you go through, it should be smaller and smaller adjustments until everything matches up. If it won't quite match, just get it as close as you can. The set of tubes in mine is not fresh and not remotely matched but they work out OK.
 
Thank you, Gadget. I was beginning to think I had asked a verboten question. :confused:

Well...I have good news and bad news.

Good news: The IBAM board is hooked up, as are the 10 ohm resistors.

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Probably while Gadget was composing his last reply to my question, I was searching this site via Google for answers.

I found this warning in another thread, so after performing the test called for in that thread, I put the original weak Fisher 7189 tubes in place, and set the Variac to 110 volts.

Bad news: My IBAM doesn't work.

The voltage across every 10 ohm resistor is fluctuating between 57 and 65 mV. And the trimpots have no effect. The sound is now distorted (probably, I suppose, because of the bias now being wacky?). Damn the luck.

I shall have to set this aside for awhile, and will return to it sometime next month. In the meantime, I would certainly appreciate any comments, suggestions, etc.

Edit: Forgot to mention...last night I cleaned the socket contacts of the V20 (7199) tube socket with DeoxIT and one of those tiny brushes meant to use between one's teeth. The left channel seems to be OK now (no longer suddenly popping in after a few seconds' delay).
 
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Mr.P -- With the readings you're getting, I suspect that there is an issue in the connections for the 8.2K resistor than connects between the the "top" side of the four pots, and the cathode buss, where the 10Ω resistors all join together. If the connections for that resistor are not in place, then the tubes will all be well over-biased (too little current draw), and the four pots will have no effect of the current draw. Also make sure that resistor is in fact an 8.2K resistor. If (for example) it is an 82K resistor, you will have the same problem as if there were a broken connection with the resistor.

Your installation looks great! Check the connections mentioned and I'm sure you'll have it going in no time.

Dave
 
Okay, this is really getting weird...

I doublechecked my connections and all are correct. I replaced the 8.2K 1/4 watt resistor with an 8.2K 1/2 watt resistor just for fun.

I then hooked everything back up and, after letting the TA-600 run for a few minutes, I found that the millivolt readings across each 10 ohm cathode resistor are changing with rotation of each respective trimpot. However, I'm not getting readings anywhere near 34 mV. More in the range of 330-370 mV on V15-V16, and 190-230 mV on V21-V22.

These readings were made with the original, old, weak 7189s still installed. I don't want to put the new 6П14П-EB tubes in until I know this IBAM circuit is going to work correctly.

I'm beginning to suspect my meter is not giving accurate readings in the millivolt range.

I'm going to ask a friend to bring over a known good meter this weekend, and we will try it again.

Otherwise...I found that the distortion went away after it played for a few minutes, and the left channel difficulties have returned but now seem associated with either the Selector switch, the Mono-Stereo switch, or both. I'll clean them again.
 
dcgillespie said:
During the course of this work, it was determined that the optimum output tube cathode current draw is 34 ma per tube.

34ma is what Dave is specifying for optimum cathode draw. 330 to 370mv divided by 10 ohm = 33ma to 37ma. It's right in the ball park on the one side. Swap the tubes side to side. See if they do the same on that side. If they do, pop the 6p14p-EV;s in, set them for 340mv and close it up. The weaker tubes are probably putting out as much as they can @ 190-230mv on the weak side.

You didn't forget:dunno: your glasses, did you????:idea: BTDT!:thumbsup:
 
Yes, I just re-read right before you posted, Larry, and caught my mistake. D'oh! :oops:

Good point re: the weaker tubes; all four tested extremely weak on my Heathkit TT-1.

I will do as you suggested...
 
Bingo, you were spot on, Larry. I switched the 7189s as you suggested and the low readings did indeed follow the tubes.

The new 6П14П-EB tubes are now in.

I set my Variac to 125 volts and was able to adjust three of the four to 340 mV. I can't get V16 below 352 or so mV. Close enough, I suppose. All four of the cathodes are now measuring 20.1 to 20.3 volts to ground.

I think that when I run this unit, I'm going to run it through a bucking transformer to cut the line voltage to around 110-112 volts. My line voltage here is usually 125 volts, with occasional 126 volt spikes.

For now, though, those new 6П14П-EB tubes do sound good, even on AM and through my tiny Minimus-7 speakers. :thumbsup: I can't wait to get this thing hooked up to my Bozak E-300s.

But I still need to install an inrush current limiter and re-clean the Selector and Mono-Stereo switches before I can button it up and call it done.

Many, many thanks to Larry, Gadget, and Dave for helping me get this TA-600 up and running again!
 
Good to hear.

1.2ma of difference is peanuts. Its less than 5% variance, nothing to worry about.
 
So please allow me to bring this thread to a close with this photo...

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This is the longest I have played the TA-600 since I restored it. It has been running for 45 minutes or so. No red plating (not that I expected any). My FM transmitter is coming in a bit fuzzy here in my home office, but other FM stations are OK. I always seem to have some difficulty with my FM transmitter in my home office, but in other areas of the house it is A-O-K.

It is as if this TA-600 and my Bozak E-300 speakers were meant for each other. Low lows and crisp highs.

As I have read elsewhere...yes...FM mono can and does sound good. :thumbsup:
 
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Looks good. I didn't realize these had a domed screen. Mine has no cabinet, and they don't seem to turn up that often so its not a look I'm familiar with.

You can add a multiplexer if you want. I believe its got the outputs for it. My late model one does at any rate. I'm actually running a Fisher MPX unit with it, though its not the proper one for the task. Mine is an internal MPX chassis from a 500B that I planted in a geiger counter case. Its ugly but it works fine.
 
That metal cabinet with its raised screen was one of the main reasons I bought this unit. That, and the front panel being in such nice shape. Not perfect, but very nice with only a couple small scratches.

I actually have an add-on multiplex decoder for it already. It came from the auction site and is solid state, but supposedly made for use with tube receivers that do not have a built-in multiplex decoder. I need to find a 9V power supply for it so I can try it out with the TA-600.
 
I agree that while the raised grill on the cabinet looks good that cabinet may not be original to the TA-600 or it's been modified. The metal cabinet for the 600 had louvers on the top back 1/2 like those on the hood of a 50's hot rod. I believe they called them french louvers. The TA 800 also used the same cabinet. I have a sneaky suspicion that the cabinet was made for one of the integrated amps that had the raised top OR a previous owner had the top cut to remove the louvers and had the grill installed in the louver place for more heat removal. In any event it DOES look good.

2 things I did to my TA-600, was (1.) cover the sides of the transformers that face the output tubes with Silver faced aluminium HVAC Tape. Helps cut down on radiated heat buildup on the trannies. and (2.) install tube retainers on the tube sockets for the EL-84 type tubes. They can have a tendency to lean excessively and the retainers keep them upright and tight in the sockets. You'll need to drill out the rivets, slip them over the sockets and either rivet them, or use 4-40-3/8" screws and nuts.
https://www.tubesandmore.com/products/retainer-el84-tubes
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Larry

I agree that my TA-600's cabinet is probably not original...but it sure dresses it up compared to the industrial cabinet with louvers!

After I bought my TA-600, I looked at several photos and as you pointed out, every TA-600 I saw that had an original cabinet had one with louvers.

I haven't forgotten about the aluminum HVAC tape. I just need to go get another roll - I had some but can't find it now. And thanks for the link and picture of the retainer. That is a very good idea!

I'm not sure at this point how much I will actually use this space heater, er, TA-600. It does not like computer audio for an AUX input - it distorts terribly. I haven't tried my turntable with it yet. But FM and AM sound great...it could benefit from a thorough alignment, though. Reception on both are weak up here in my home office. From what I have read, it should be better than this.
 
Try a 3 or 6 db attenuator with the computer input. It may be just too hot for the Aux. IIRC Aux, being a line level input, at the time was limited to 250mv or there about's. The computer output may be in the neighborhood of 1v-2v.

I concur the tuners could be out of alignment or could use a good tweaking. 1st thing to do is to make sure all of the RF/IF tubes are good. If they are good then I'd go with the alignment.
 
might also work to just turn down the sound level on the computer. Most computers I don't find to have a particularly hot output. I usually have to crank the volume on the amplifier even with the sound on the computer set wide open. I have not actually tried it on my 600 though.
 
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