Hi there,

First, let me say that the X-100A is somewhat restored: .047 caps replaced at the outputs and .033 in the tone control section. Power supply all rebuilt also.

The issue: the FM-200B sounds distorted when put at its highest level (10 on the tuner volume pot) but only when plugged in the X-100A tuner inputs. At first, I was thinking that the tuner (not restored, next on the bench once the X-100A will be up to the task) was dying slowly. Yesterday, I hooked the FM-200B to my Scott LK-72A and even at max volume, the sound was clear... Same on my Heathkit AA-32.

By curiosity, I did the test with a Sansui AU-888 tuner (with a volume pot) and it did the same: distorted on the Fisher amp. The only way to have clear sound is to put the volume on the tuners around halfway. Then, even if I crank the volume on the X-100A, there's no distortion. It's like the tuners are overloading the X-100.

There's no distortion at all with other sources like CDP or computer soundcard, only with tuners equipped with volume control (I tested with my cheap Realistic tuner - no volume control - and the sound is fine, undistorted).

Some things to put into the equation:

- the 12AX7's in the Fisher linestage are the original ones. However, it made no difference when I tried some old spares (Rogers) I have. Maybe all these 12AX7's are shot? I'll try as soon as possible with known good 12AX7's I have somewhere;

- the .047 coupling caps are mixed: on each side I put an ERO 400v with an Orange Drop 630v. I'll replace the ERO ones with Orange Drops ASAP but I doubt this is enough to make a real difference;

- almost all resistors are original (tested) but I plan to replace at least the 330k grid resistors and the 1k at the output stage. There are some resistors testing a little weird (with DVM, they ramp up slowly until they reach their values, like going from 85k slowly to 130k...). These resistors are located in the phase inverters section mainly (the 330k shows this weird behavior). The problem is I do not want to mess in this section because there are two pots to adjust for IM distortion and I do not have the equipment to do it... I guess that I could take a shotgun approach and replace all resistors in this section but I'll have to do IM adjustment after...:scratch2:;

- there are two ceramics (0.02) that I plan to replace with OD .022 ASAP but I doubt these ceramics are gone bad...;

- all power tubes (7189) replaced with brand new EL84 Russian military. Read correct at pins 2 and 3 (18v and 31-32v, Bias around -13/-14v). I'll try again with old Fisher tubes to see if it is part of the problem but I doubt also...

In conclusion (sorry for the loooong post), I guess I could live with it and adjust the volume on the tuner but sure these two components were used together by the previous owner with no issues at first. I could also take the shotgun approach and replace all resistors in the X-100A (matched at 1% on each side) but I'll have to mess with the IM distortion adjustment.

Could this issue be caused by a badly adjusted IM distortion pots? Bad 12AX7's? Something else? Weird that all other sources except tuners (controlled by their own volume pots!!!) sounds ok...

I know that I had a similar situation with an Heathkit AA-100 cured by the replacement of two 12AX7's with two 12AT7's. It reduced gain and eliminated distortion. In this case, it was the CDP distorting and the tuner was fine!!!

Thanks.

Patrice


**Schematic can be found here: http://fisherx100a.files.wordpress.com/2008/11/fisherx100ax1003.pdf

Not that it will fix your problem, but have you run the tuner through the AUX loop.:scratch2: Might prove something in your amp

I may be missing something... are you sure that the tuner isn't just overdriving the Fisher amp's preamp section at full output? If that's the case, the simple answer is "turn down the tuner output a little" :-)

Good advice so far.
Here's a couple more things.
1. Try adjusting the tuning knob while it's distorting. Is it off-center?
2. Pull the tube shields and tap each tube while the tuner is distorting and see if it gets worse. You could have a microphonic tube in the tuner.

Not that it will fix your problem, but have you run the tuner through the AUX loop.:scratch2: Might prove something in your amp

I'll try and report back.

**Oh, I just thought about it and remembered that I tried the Sansui tuner connected to a TAPCO mixer, the mixer going to the AUX inputs. This was yesterday... Same distortion. Had to turn down the volume on the mixer or the tuner to have clean sound.

I may be missing something... are you sure that the tuner isn't just overdriving the Fisher amp's preamp section at full output? If that's the case, the simple answer is "turn down the tuner output a little" :-)

Yes, like I said, the tuners (SS Sansui and tubed Fisher FM-200B) are overloading the preamp stage. Turning down volume (from 10 to about 6) to get similar volume as the Realistic tuner el cheapo, distortion is gone... It do not overdrive the LK-72 nor the Heathkit and Sansui SS amps at full volume however. Maybe the way the circuit of the Fisher is built?

Thanks to all.

Patrice

As a sidenote, the tuner was missing 5 knobs brass caps (6 total) and the amp was missing two but three of the are so rusty that I took another approach:

I bought a lot of cheap knobs sporting a similar design cap (silver instead of brass colored). Then, after some testing to remove the caps (screwdriver, knife, iron saw...!) I almost gave up. There was two much damage to the caps caused by the tools.

Two days ago, I've found the solution:

heating the cap with my 60w soldering iron until the caps fell up (the glue simply cease to do his duty at high temp). Then, I put Crazy Glue under the "new" cap and put it on the old knobs...

The only cap not replaced is the one on the big volume pot. I didn't found an exact replacement but I managed to somewhat clean the old brass cap and it is not so ugly looking...

Pics of the restored units will follow soon... Stay tuned.

**The tuner is also missing one of the "jewels", the one covering the "muting" function. Why "mute" a station on a tuner??? Anyway, I won't use this function so it's the only visual flaw.

**I also had to re-paint the slide switches as I put so much Deoxit on them that they discolored (the amp was resting on his side so the stuff slowly dropped on all the plastic recovering the switches... until I discovered the result. It was anyway too late!)... The switches are now working fine but if someone looks carefully at short distance, he could see the painting job! :D

Thanks.

Patrice

Your Fisher amp connects the tuner output directly to the grid of your first input stage without any kind of grid stopper resistor.....which is probably contributing to the overloading/mismatch. If it were me, I would simply turn the volume down on the tuner and live with it...that's what the control is there for. If it really bothers you, you could simply try adding the grid stopper resistor and see if it works better....add a 10K resistor tying pin 2 of V3 to the corresponding pin on your selector switch...and do the same for the other channel with pin 7 of V3 to the selector switch. Not sure why Fisher left this out on your unit....most other Fisher schematics have one (esp the original X100- the progenitor to your unit). Maybe they omitted it because they left out a gain stage in this amp (you only have half of a 12AX7 powering the tone control circuit). It is easy enough to reverse if you don't like it....

Your Fisher amp connects the tuner output directly to the grid of your first input stage without any kind of grid stopper resistor.....which is probably contributing to the overloading/mismatch. If it were me, I would simply turn the volume down on the tuner and live with it...that's what the control is there for. If it really bothers you, you could simply try adding the grid stopper resistor and see if it works better....add a 10K resistor tying pin 2 of V3 to the corresponding pin on your selector switch...and do the same for the other channel with pin 7 of V3 to the selector switch. Not sure why Fisher left this out on your unit....most other Fisher schematics have one (esp the original X100- the progenitor to your unit). Maybe they omitted it because they left out a gain stage in this amp (you only have half of a 12AX7 powering the tone control circuit). It is easy enough to reverse if you don't like it....

If I understand well: tying 10k resistors from pins 6 (rear and front) of the selector switch to pin 2 and 7 of V3 will do it. Correct?... or live with it and turn down the volume of the tuner...

I'll think about it but if too lazy, I could take the second approach! :D

Thanks for your enlightenment...

Patrice

... I forgot. Could I rewire the "phone" plug to have normal "L" and "R" in the headphone? I think the connection is backward. I saw something about it on AK for other brands too (Scott?) if my memory serves me well...

I have a Fisher FM-200b and never run it full volume into the my X202 because of a slight overdrive into the x202 preamp, and don't feel I've ever been missing anything. I really don't think there's a problem.

Good advice so far.
Here's a couple more things.
1. Try adjusting the tuning knob while it's distorting. Is it off-center?
2. Pull the tube shields and tap each tube while the tuner is distorting and see if it gets worse. You could have a microphonic tube in the tuner.

1. As I already said, no distortion present when hooked to a Scott LK-72A or an Heathkit AA-32.

2. Will try... Anyway, the tuner is next to rebuild entirely.

Thanks.

Patrice

I have a Fisher FM-200b and never run it full volume into the my X202 because of a slight overdrive into the x202 preamp, and don't feel I've ever been missing anything. I really don't think there's a problem.

Yes, I could easily live with volume down but was just wondering why I could drive it to max on other amps...

BTW, lovely sounding tuner. It sounds full and present (not the case for my Realistic el cheapo) on the Scott LK-72A! I just have to rebuild it because I lost a channel sometimes when switching stations. There's also a slight hum that could be heard in the headphones.

I'll also have to call my tube retailer to test all tubes (and some from other gear at same time).

I'll replace all power supply including the selenium (I'll use four 1N4007 diodes instead + adjusting the first dropping resistor as needed) plus all .1 caps here and there. Add to this some small electrolytics values (two 8 uf, 4uf and 1 uf). BTW, I bought 1uf@350 but it looks to be non-polarized, is it ok? The 4uf replacements are identified as «bi-polar», ok also?

As for the X-100A, the soundstage seems to be wider than my Scott LK-72. Awaiting the end of the day (after work) to hook up to my Wharfedale's Diamond 8.2. It will serves me as my home studio amplifier, instead of the SS Sansui I have now (this Sansui sounds also really good however but will continue his duty at work). As I will lose my 4 speakers system (the Sansui have A and B connectors), a 6BQ5 Clairtone console will drive another pair of speakers...

Thanks.

Patrice

The pictures, finally...

As you can see, the tuner is... ahem... a little dirty and maybe rusty at some place.

I'll clean it when rebuilt done.

Listening to the X-100A right now and, wow!

http://s720.photobucket.com/albums/ww209/PatriceBrousseau/The%20Fisher/