Looking for some component info for a Fisher TA-800

Dave Perkins

Dave Perkins

New Member
I have been working through a Fisher 800 (the first generation TA-800) on my bench and it's mostly working pretty well, but I have one problem yet to resolve. Fortunately the volume control and power switch (a known weakness in these is OK and the power is now switched with a SS relay. But, to cut to the chase, the two halves of the balance control (R128) do not match. One reads roughly 260 K ohms while the other reads about 360 K ohms. I've used De-Oxit etc. to no avail and it seems to track OK, but to get the output volume to balance I have to put the control way off center. Annoyingly the parts list in the service manual does not list a value for any of the three sections of this control.

So, to get to the point, can anyone tell me what its resistance is supposed to be and is there ANY hope of finding a suitable replacement part anywhere?

And, finally, if there's a better forum than this for this sort of information that's active (I found some long dormant ones when looking around) please let me know about it.

Thanks in advance for any & all replies.
 
Hi Dave -- Welcome to AK!

The value of the Balance Control sections are 300K each, while the value of the Center Channel Output control is 1K. Your chances for replacement boil down to finding a good one in a junker, with junkers being pretty rare. If you use external resistors to pad the two sections appropriately, you'll be able to re-center the control with a balanced output.

You'll always get your most responses for Fisher equipment over on the AK Fisher forum, which is quite active. I did a fairly involved restoration of one of these models which is documented over on that forum, and should be easy to find with the search feature by plugging in TA-800.

Good luck with your Fisher!

Dave
 
If you want this moved to the Fisher forum just say the word.
 
dcgillespie said:
Hi Dave -- Welcome to AK!

The value of the Balance Control sections are 300K each, while the value of the Center Channel Output control is 1K. Your chances for replacement boil down to finding a good one in a junker, with junkers being pretty rare. If you use external resistors to pad the two sections appropriately, you'll be able to re-center the control with a balanced output.

You'll always get your most responses for Fisher equipment over on the AK Fisher forum, which is quite active. I did a fairly involved restoration of one of these models which is documented over on that forum, and should be easy to find with the search feature by plugging in TA-800.

Good luck with your Fisher!

Dave
Click to expand...
Thank you SO much for the detailed reply. In fact, my next step (today) is to pad out the balance control (although I was just going to guess at the proper values) to verify that nothing else is going on. If that's it, I'm not sure what I'll do in the long run. I could live with having the center channel control as a little used hidden control if all I can come up with is a ganged pot for the balance so I my be headed that way.
 
Tinkerbelle said:
If you want this moved to the Fisher forum just say the word.
Click to expand...

YES, please do. I'm new to audiokarma and still stumbling around.

Just by way of explanation, I'll mention that I'm a retired EE and long ago tube hacker (dating back to the 1960's) and for several years now I've been into antique radios as a hobby. But lately I'm branching out into vintage audio with a preference for tube based gear. I have a couple more restoration projects and a couple built from scratch amps in my mental queue, so I expect audiokarma will be a good resource. If you have any other forums that may be of interest to me, please let me know.
 
Welcome to the Fisher forum, Dave. I'm an amateur myself with only a 500-C and Marantz 2265B. I love listening to the Fisher. There are some really great people on this forum who I refer to as gurus. I imagine once they read your questions, you will get some very helpful responses that will get your balance control the help it needs. Stick around. It's a great place to be. Thorne
 
hm, 3 section, I don't think the one on the TA-600 will be the same. I have a parts 600. Depending who made the control you might be able to source some replacement carbon tracks from donor controls and rebuild yours though. Also possible you might be able to fake in some resistors to get things centered, though thats a bit of a kludge.
 
Correct, as I suggested as well. But even if pad resistors are used, the control will still work fine for its purpose -- and particularly so since the control would stay in the centered position nearly 100% of the time.

Dave
 
thornev said:
Welcome to the Fisher forum, Dave. I'm an amateur myself with only a 500-C and Marantz 2265B. I love listening to the Fisher. There are some really great people on this forum who I refer to as gurus. I imagine once they read your questions, you will get some very helpful responses that will get your balance control the help it needs. Stick around. It's a great place to be. Thorne
Click to expand...

Thanks, I'm looking forward to any and all exchanges.
 
Thanks to all who've responded so far.

Actually my situation got a little worse as of this morning when I dug into it more.

It turns out that BOTH the balance and volume control pots have issues. The balance control reads (roughly) 280 K and 380 K on the two pots (supposed to be 300 K) and the volume control reads around 430 K and 630 K on the two pots (supposed to be 500 K) and one side doesn't track smoothly even after cleaning.

I didn't find this until I'd gone all through it replacing tubes, recapping, adding a SS relay to handle the power switching and tweaking the B+ to get it into spec.... kinda bummed out right now.

I know about the guy on Ebay who's selling new volume controls for the 800 B/C but he doesn't stipulate the end to end resistance and what he shows doesn't obviously physically accommodate the wafer switch for the AM section (I've messaged him) . Have any of you used his part and do any of you know if the original VC shaft just passes through the switch section? I've not yet tried to remove & disassemble it.
 
Dave -- Just because the two volume control sections don't measure the same doesn't mean they won't track the same, as it is the rate of change within the total resistance that each section represents that determines how well the sections track. Now granted, unequal total resistance may and can cause other controls to then cause an imbalance, but that can be dealt with (again) external pad resistors. I have had fair success using external padding resistors on volume controls to at least make horrible ones tolerable, and tolerable ones pretty good.

In my experience, Mark only has the parts for Centralab's general replacement line of components -- not those they built as OEM pieces for Fisher.

Dave
 
When I say it doesn't track I based that comment on having watched the reading on an ohmmeter from the wiper arm leg of the pot as I ran it through its range. There are some definite jumps and drop outs. As I was working on this and got it sort of working, I'd noticed that I had to have the balance control off center to hear it, and it did sound pretty decent to my old, shot ears, so I figured I'd find a bias / gain issue with some pre-amp or amp stage. But it's come down to these controls (darn it!).

Do any of you have the service manual / parts list for an 800 B or C? If so, do those versions use a 500 K volume control? It would also be interesting to know if the original's taper is linear or logarithmic. I'm guessing it's log just from the visual results of reading the better (smoother) side of this one.
 
Dave -- All volume controls are of a Log or "audio" taper. Regarding the channel imbalance, did you check the cathode bypass electrolytic caps in the line stages of both channels? If they are still original, they are famous for going open, causing the gain to drop when that happens.......

Hopefully Larry will be along to give a clue as to where to get the service manual. He is the font of all Fisher knowledge regarding that information........

Dave
 
I have the factory service manual for the TA-800 which is the unit I have. What I was asking about was to see if the volume controls in the later B/C versions were the same or different. It seems like 90% of the stuff I see offered in support of the Fisher 800 is for the B/C models which are, apparently, preferred.

Regarding the caps. Yes. I replaced every electrolytic cap in it before I fired it up. Including all of the bypass and PS filter caps. It was interesting figuring out how to do that for the power supply in the available space.

Thanks for the info on the control taper. I figured it was log. I have encountered some linear ones but that was in very old (pre-war) radios.
 
The controls on the newer versions would be electrically compatible, but not mechanically compatible. As you know, the volume control assembly in the TA is complex with the AM band switch attached to it, where as in the later models, it is simply the volume control by itself.

Dave
 
dcgillespie said:
The controls on the newer versions would be electrically compatible, but not mechanically compatible. As you know, the volume control assembly in the TA is complex with the AM band switch attached to it, where as in the later models, it is simply the volume control by itself.

Dave
Click to expand...

No doubt... Someone offering replacements would at the very least have control sections with the right values and I may consider trying to use them... I have a stash of old IRC and centralab pots and parts that I need to dig through to see if I have anything that'll work. I know it's an outside shot so at this point I'm just grasping at straws.
 
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