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Fisher X-100A little misbalance between channels

Patrice B

Patrice B

Super Member
Hi,

I solved a similar issue a few weeks ago (with a Scott LK-72A) but on my Fisher, I'm not sure that's exactly the same thing.

AC voltage at speakers out has a difference of .02vac at low volume level and it become .45vac at half-volume...

Resistors were checked and found to be well-matched (I heard that resistors used in Fisher gear were of better quality than Scott's one).

Do you think I'm dealing with a volume pot lacking precision? Something else.

All important caps (coupling) replaced and power supply rebuilt BTW...

Thanks.

Patrice
 
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Greetings from Toronto.

It could be volume pots as these are known to be variable. The first thing is to measure channel balance with the pots at full. If they don't balance at full volume then you would need to check for resistor matching or even tube gain between channels.

If you get good balance at full volume and poor balance at mid volume settings then this is definitely a volume pot issue. This can be fixed too (I've done this on a Mac C20). Take the channel with greater volume at the half way point and hang a high value resistor from the pot wiper (middle terminal) to the ground end of the pot (lower right terminal viewed from the back?) The resistor might be in the 100k to 1 meg range, you will have to experiment. Generally the resistor will pull the volume of middle settings down, have no effect on low settings and next to no effect on high settings. This makes it suitable for matching up a ganged pot.

Good luck,
David
 
Thanks Dave,

I've sent a reply yesterday but my ISP seems to have serious problem...

I resume:

There's another Ak'er (seta1) helping me with some suggestions too:

- replace some caps and resistors at specific points (loudness/tone/phase splitters/feedback);

- verifying AC balance and adjust it if necessary;

He also mentioned to me that the volume pot could be part (or responsible) of this behavior.

Sure, if the bits replacement doesn't lead to success, I'll try this "add-a-resistor" trick.

Thanks.

Patrice
 
If it's not too far off, and if the misbalance is linear across the volume spectrum, meaning that the right channel drops at low volume and comes up more as the volume knob goes up it's not linear across the volume spectrum, I've been known to adjust the balance pot to where it sounds center, pull off the balance knob and reglue it with rubber cement with the pointer being straight up.
Depends on how far off it is though. The vol and bal pots work in tandem, so please, continue your discovery process first.
audiodon657
 
gugusse said:
Hi,

AC voltage at speakers out has a difference of .02vac at low volume level and it become .45vac at half-volume...

Patrice
Click to expand...

One other thought is that .02 vs. .45 doesn't necessarily mean a drift in gain. When the signal is smaller (lower volume) then the voltage difference should be less. You need to think in percentage terms. For example if high volume control setting gave 10 volts in one channel and 9.5 in the other, then if a moderate volume control setting gave 0.1 volts for the first channel and 0.095 for the other, that would be perfect pot tracking. Even though we are comparing 0.5 volts difference to 0.005 volts, in both cases the second channel is 95% of the first and a slight balance shift would make perfect balance for both high and low levels.

Regards,
David
 
speaker dave said:
One other thought is that .02 vs. .45 doesn't necessarily mean a drift in gain. When the signal is smaller (lower volume) then the voltage difference should be less. You need to think in percentage terms. For example if high volume control setting gave 10 volts in one channel and 9.5 in the other, then if a moderate volume control setting gave 0.1 volts for the first channel and 0.095 for the other, that would be perfect pot tracking. Even though we are comparing 0.5 volts difference to 0.005 volts, in both cases the second channel is 95% of the first and a slight balance shift would make perfect balance for both high and low levels.

Regards,
David
Click to expand...

Yes, would be fine like this but... I finally noticed that about 12h30 - 13h00, the left channel now gets more voltage, around 1 AC volt more than the right.

The difference first mentioned was in favor of right side.

To resume, between 0 and half-way, right starts from 0.02 v to 0.45 over left approximately then suddenly, at higher volumes, the left side is higher of 1 AC volt...
 
gugusse said:
To resume, between 0 and half-way, right starts from 0.02 v to 0.45 over left approximately then suddenly, at higher volumes, the left side is higher of 1 AC volt...
Click to expand...

Hi Patrice,

Again, it is hard to make sense of a voltage difference. The difference will get bigger because the signal is getting bigger.

You need to think in percentage terms, or better yet, in dB. Do you have an analog meter with a dB scale? Better to be able to say that: "left channel is low by 2dB at 10 O'Clock swinging to right channel louder by 3dB at 3:00 O'Clock." (Or similar.) You can also calculate with any scientific calculator: dB = 20 x log of (left volts divided by right volts).

Without this it is very hard to quantify if it is a significant variation or not. I certainly wouldn't be surprised if an old stereo pot had 2 to 3dB of variation from the top to the 9:00 O'Clock position. (An expensive Alps would be much better.) Turning down towards the off position you can see even worse imbalance, so you should always try and use stereo pots in the middle of their range.

I do understand that you are saying the volume swings louder in one channel to louder in the other, but not sure what "suddenly" means.

Hope that makes sense.

David
 
speaker dave said:
Hi Patrice,

Again, it is hard to make sense of a voltage difference. The difference will get bigger because the signal is getting bigger.

You need to think in percentage terms, or better yet, in dB. Do you have an analog meter with a dB scale? Better to be able to say that: "left channel is low by 2dB at 10 O'Clock swinging to right channel louder by 3dB at 3:00 O'Clock." (Or similar.) You can also calculate with any scientific calculator: dB = 20 x log of (left volts divided by right volts).

Without this it is very hard to quantify if it is a significant variation or not. I certainly wouldn't be surprised if an old stereo pot had 2 to 3dB of variation from the top to the 9:00 O'Clock position. (An expensive Alps would be much better.) Turning down towards the off position you can see even worse imbalance, so you should always try and use stereo pots in the middle of their range.

I do understand that you are saying the volume swings louder in one channel to louder in the other, but not sure what "suddenly" means.

Hope that makes sense.

David
Click to expand...

I konw that's far to be scientific...

What I mean by suddenly is that there's a point on the pot where the advantage (in term of AC voltage measured at speakers screws) turn drastically in favor of one of the channels. It was before an advantage of channel B and then, channel A win by a much larger margin...

No, I do not have a DVM with a dB scale BTW.

Thanks.

Patrice
 
Okay, its a little hard for me to diagnose from afar. Are you saying that gain in one channel "jumps" or the other channel's "stops" increasing?

If it is a gentle swing from one channel being larger to the other being larger then I still suspect the pot balance. If gain somehow "jumps", then that is a very different problem. Could be some strange bias shift or premature clipping or some other very nonlinear effect. Do both channels sound clean as the gain shifts?

Any other ideas, guys?? (I'm stumped.)

David
 
speaker dave said:
Okay, its a little hard for me to diagnose from afar. Are you saying that gain in one channel "jumps" or the other channel's "stops" increasing?
Click to expand...

It seems to jump suddenly but my test wasn't exhaustive (rotating the vol. pot, measuring, rotating, measuring,... With a 440 Hz test tone). I'll have to use alligator clips on the speaker screws and look if it is really a sudden jump. This by rotating gently the volume pot and looking for the values on the DVM.

AFAIK, the area where it shifts looks to be really small, so maybe I prematurely concluded that it is a jump...

speaker dave said:
If it is a gentle swing from one channel being larger to the other being larger then I still suspect the pot balance. If gain somehow "jumps", then that is a very different problem. Could be some strange bias shift or premature clipping or some other very nonlinear effect. Do both channels sound clean as the gain shifts?

David
Click to expand...

The sound remains clean when the channel A take the lead...

I'll do further testing when coming home...

Thanks.

Patrice
 
Well, I don't know how I measured first... but forget about the aforementioned "jump" in readings.

AC voltages:

Channel A going from 0 to 5 on the scale (5 corresponding to halfway): 0 - 6.50 AC volts

Channel B from 0 to 5: 0 - 6.83 AC volts

The ranges used for listening are between 2 and 4 and the comparisons are shown here:

A, at position 2: .305 AC volts
B, at position 2: .381 AC volts

It means a differential of .075 AC volts, nothing to worry about

A, at position 3: 1.809
B, at position 3: 2.16

A differential of .3 AC volts. Now, it's more pronounced but I don't know if I have to care about it...

A, at position 4: 5.13
B: 5.61

Now, it's around .5 AC volts...

But at 5 (halfway), this difference is now .33 volts!

So, maybe a pot slightly imprecise, a component that drifted in value, something else.

Just to be sure, I'll follow the suggestions seta1 gave to me and replace some specific components (in loudness circuit, feedback circuit, maybe tone control circuit, some resistors and caps in the phase splitters section).

Then, if the difference remains, I'll just have to live with the mistracking (or follow the advice of Audiodon657) or add a resistor.

I have good faith that replacing components should help a bit (like it did on my Scott, bringing back a difference of more than 1 AC volts to around .1 volts).

Thanks to all.

Patrice
 
Hi Patrice,

As I was saying it isn't the difference but the ratio, or better yet, the dB difference that counts.

Lets convert these numbers to dB:

Pot at 2, .381 volts vs. .305 volts. The B channel is 1.9dB louder.

Pot at 3: The B channel is 1.5dB louder.

Pot at 4: The B channel is 0.8dB louder.

Pot at 5: The B channel is 0.43 dB louder.


At higher levels where your pot is less in circuit your channel balance is very good (better than 1/2 dB). Gain between channels, not counting volume pot, is about as good as you could expect.

As you turn the pot down there is a little shift in balance, due to the pot, but never more than 2dB. This is not a great amount, is probably just perceiveable as an image shift, and probably was within spec for the part when manufactured. If you want to improve it you could try the resistor trick mentioned above, but at 2dB I would live with it.

Outside of the pot tracking there is no indicator that you amp has any significant imbalance. No reason to adjust any parts outside of the volume pot.

David
 
speaker dave said:
Hi Patrice,

As I was saying it isn't the difference but the ratio, or better yet, the dB difference that counts.

Lets convert these numbers to dB:

Pot at 2, .381 volts vs. .305 volts. The B channel is 1.9dB louder.

Pot at 3: The B channel is 1.5dB louder.

Pot at 4: The B channel is 0.8dB louder.

Pot at 5: The B channel is 0.43 dB louder.


At higher levels where your pot is less in circuit your channel balance is very good (better than 1/2 dB). Gain between channels, not counting volume pot, is about as good as you could expect.

As you turn the pot down there is a little shift in balance, due to the pot, but never more than 2dB. This is not a great amount, is probably just perceiveable as an image shift, and probably was within spec for the part when manufactured. If you want to improve it you could try the resistor trick mentioned above, but at 2dB I would live with it.

Outside of the pot tracking there is no indicator that you amp has any significant imbalance. No reason to adjust any parts outside of the volume pot.

David
Click to expand...

Well, I measured again today and there's finally a real imbalance... sometimes... between position 4 and 6.

In this area, A channel is in average 1 AC volt over B. By looking underside, I saw what's happening. When crossing this region, I could visually see the center lug of the right side of the pot moving...! This is accompanied by a "scrtchhh" in the test tone.

The pot was deoxited a lot of times, without improvements.

I look if I could find a replacement as it looks this area of the right pot is kind of worn probably.

Anyway, as you said, the difference (apart the worn area) is negligible.

You know, if I hadn't measured the voltages, I wouldn't had probably not noticed the difference by ear!

But being a little (too) meticulous and suspicious, I had to do it to verify that it worked normally.

BTW, do you think I could replace this pot with one of lower value? I ask because I have a dual-ganged pot of 100 or 150k somewhere in my pieces. It comes from a Realistic receiver and it has a turn-on switch and a loudness tap.

Thanks.

Patrice
 
Worn pots can be a problem. From your description it sounds like you are saying the wiper (part that turns and contacts the resistive track) wobbles or rides over some bumps in the tracks?

Cleaning is about the best you can do. With some pot designs you can bend the wiper a bit and get it to follow a fresh path on the track.

If it really bad then a donor replacement might be the answer. (Make sure it is good before you go to the effort of changing!) Pot values (100k, 200k) should be reasonably close (say +-20%) unless you are desperate just to keep something running. Are there any suppliers of modern pots out there of the right spec??

David
 
Oh, thanks also for the calculations... I tried to calculate yesterday with web calculators but rapidly gave up!!!

As for the components replacement, I've done some today. 4 resistors and 4 caps, thinking that it could help but obviously, as there's no problem (like you helped me realize:D), no improvements was made.

Latest: re-done the AC measurements between 4 and 6... and no problem. Maybe something to do with the fact that the case is open, the amp is resting on the top, anything else,... when I measured previously.

7.33 for A and 7.12 for B, looks fine, sounds fine!!!

I have to stop fiddling with electronics and listen to the music instead...

Doctor, where are my meds???:smoke:
 
speaker dave said:
Worn pots can be a problem. From your description it sounds like you are saying the wiper (part that turns and contacts the resistive track) wobbles or rides over some bumps in the tracks?

Cleaning is about the best you can do. With some pot designs you can bend the wiper a bit and get it to follow a fresh path on the track.

If it really bad then a donor replacement might be the answer. (Make sure it is good before you go to the effort of changing!) Pot values (100k, 200k) should be reasonably close (say +-20%) unless you are desperate just to keep something running. Are there any suppliers of modern pots out there of the right spec??

David
Click to expand...

I just saw this thread :http://www.audiokarma.org/forums/showthread.php?t=268056

If it really bothers me, yes, I could bend the wiper... but how to do it??? With a pair of pliers? Dismantling the pot entirely?

Thanks.

Patrice
 
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