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800C Volume Pot problem.

I did get the program to work, although I am confused by the results. on this program, I was unable to turn the volume up past about 9-10 o'clock because the meter didnt go any higher.

more interesting than that was that there was a constant 2-3 db difference the entire measurement. I honestly expected more. It sure sounds like more. One other interesting thing was that the program sited inter channel leakage from the left to the right. at the highest level of measurment, there was leakage in both channels.

So, with my amateur analysis, I have a couple questions.
1. is there a way I can measure the volume up to at least half way on the dial? or will this blow out the soundcard or something.
2. this leakage thing. it reads even with the volume at 0. is this something I need to look into? at about 9:00, it shows leakage on both channels, although the db measurements dont show it becoming equal.

what next?? how fun to have a new way of looking at all this.
 
keithpgdrb said:
I did get the program to work, although I am confused by the results. on this program, I was unable to turn the volume up past about 9-10 o'clock because the meter didnt go any higher.

more interesting than that was that there was a constant 2-3 db difference the entire measurement. I honestly expected more. It sure sounds like more. One other interesting thing was that the program sited inter channel leakage from the left to the right. at the highest level of measurment, there was leakage in both channels.

So, with my amateur analysis, I have a couple questions.
1. is there a way I can measure the volume up to at least half way on the dial? or will this blow out the soundcard or something.
2. this leakage thing. it reads even with the volume at 0. is this something I need to look into? at about 9:00, it shows leakage on both channels, although the db measurements dont show it becoming equal.

what next?? how fun to have a new way of looking at all this.
Click to expand...

Right now let's just focus on the volume control. The amp will reach full power and start clipping somewhere after 12:00. To measure between 9:00 and 12:00 turn the soundcard output and input way down.

The way they did the switch, load resistors and headphone output design is going to generate crosstalk, no way out of it. However, the headphone channels are switched by design (no idea why). In order to fix this for the test you need to flip soundcard output jacks. That should stop it from complaining.

Run a loopback test first to be sure everything is working correctly. In this case you will want the channels oriented correctly (not flipped).

Here is an example from a 500c. I believe it was done during modifications so should not be used as any sort of baseline.

Also here is a loopback test from my soundcard.
 

Attachments

One thing before I measure again. I switched the wiring of the hp Jack so that the channels play on their proper side. Is this wrong? Do I have to switch them back? I'm a little confused as to what you mean by the channels being switched.

I don't see why it would make a difference. I switched the wires right behind the selector switch.
 
keithpgdrb said:
One thing before I measure again. I switched the wiring of the hp Jack so that the channels play on their proper side. Is this wrong? Do I have to switch them back? I'm a little confused as to what you mean by the channels being switched.

I don't see why it would make a difference. I switched the wires right behind the selector switch.
Click to expand...


It doesn't matter where or how you correct the channel issue. Just try each channel by itself and make sure that left is left and right is right. If they are switched RMAA will interpret this as inter-channel leakage.
 
drewbolce said:
It doesn't matter where or how you correct the channel issue. Just try each channel by itself and make sure that left is left and right is right. If they are switched RMAA will interpret this as inter-channel leakage.
Click to expand...

Sorry, I should have explained this better. the original wiring for the headphone jack had the channels reversed. I know this because when I would move the balance knob, the speakers would respond correctly, but the headphones would work in reverse. To correct this, I flipped the headphone wires that sit right behind the selector switch.

now, everything is playing correctly, right is right etc.. so RMAA should be reading correctly? right? I was just worried about changing the wires from their stock state. although I dont know why they would have purposely set up the HP jack that way. doesnt make much sense. I've read that its not uncommon, but not universal.

I'll be able to test again later tonight. any specific numbers that would be more helpful to report?
 
keithpgdrb said:
Sorry, I should have explained this better. the original wiring for the headphone jack had the channels reversed. I know this because when I would move the balance knob, the speakers would respond correctly, but the headphones would work in reverse. To correct this, I flipped the headphone wires that sit right behind the selector switch.

now, everything is playing correctly, right is right etc.. so RMAA should be reading correctly? right? I was just worried about changing the wires from their stock state. although I dont know why they would have purposely set up the HP jack that way. doesnt make much sense. I've read that its not uncommon, but not universal.

I'll be able to test again later tonight. any specific numbers that would be more helpful to report?
Click to expand...

The headphone jack on my Fisher was also reversed so you did good by flipping the wires.

Wow, RMAA is a wonderful program to analyze tube amps!
 
keithpgdrb said:
my wife agreed to let me use her pc laptop for the test. I tried to download the first program, and it fails while installing every time. I tried to get voxengo, but the download is an extension that you add to another program. what program is that?
Click to expand...

It's a VST plugin, usable on any VST capable sequencer like Cubase, Sonar, Reaper, etc...
 
Ok. I don't know what I'm seeing here. I'm in the process of uploading a couple videos of the programs meters while I was testing. I am getting some kind of spikes on the Right side meter. The spikes seem big, but are not audible when listening to music from headphones. I'll post the link when they are up. Geez.
 
Hey, Keith...

I'll wait for Drew to jump back in, but that reading on the right jumping around like it is makes me think it's cracks or other problems with the carbon element in that half of the volume control. More noticeable when you're physically turning the knob, but it would tend to settle in once you stop moving it.
 
Thanks Clay. Odd that those spikes are inaudible when listening to music. I tried to hover the volume knob in those problem areas. any other insights? My main worry is that its something else besides the volume knob. is there a measurement I can take somewhere to measure the actual signal going to the volume pot? to see if its equal?
 
drew, did I lose you? any last thoughts? I'm thinking of ordering up a new volume control from that guy if that would solve my problems.

although, in the big picture, I really havent had any unsolvable problems yet. Clearly a nod to the design. if I can even work on it, its got to be good.
 
keithpgdrb said:
Thanks Clay. Odd that those spikes are inaudible when listening to music. I tried to hover the volume knob in those problem areas. any other insights? My main worry is that its something else besides the volume knob. is there a measurement I can take somewhere to measure the actual signal going to the volume pot? to see if its equal?
Click to expand...

Maybe the spikes are a little bit of static or scratchiness that's either too high pitched or short in duration to notice with your ear.

It seems to me that with a mono source, both volume pots would get identical AC signal and you should be able to measure mismatching tapers by connecting your DMM, set to AC, across the left and right wipers. You could jumper the input lugs of the two sides to make sure both have the same signal amplitude. With perfectly matching tapers, the voltage difference at the wipers should remain zero as you sweep all the way up.

For this to have real meaning, you need to be aware of the measured difference compared to the signal at the wiper relative relative to ground. So if the wipers are out from each other by 0.05 volt and the voltage on one is 0.05 and 0.1 on the other, you have a pretty extreme problem. But if the difference is 0.05 but the signal on one is 0.45 and 0.5 on the other, the problem is not near as severe as the first case.

Sorry, I've never tried this test but it seems like it should work.
 
keithpgdrb said:
Thanks Clay. Odd that those spikes are inaudible when listening to music. I tried to hover the volume knob in those problem areas. any other insights? My main worry is that its something else besides the volume knob. is there a measurement I can take somewhere to measure the actual signal going to the volume pot? to see if its equal?
Click to expand...

Keith, the only way you could measure the output of the two sections of the volume control would be to use a constant voltage signal through each channel, such as from a signal generator. There's no way you could do it with music because each channel of a stereo signal is constantly changing. Even if you used a mono source to each channel, while both would be the same, trying to watch two rapidly fluctuating voltage readings to see if they're both fluctuating "identically" would drive you nuts.

It would make more sense to measure the resistances at matching positions for the two channels. Which you've already done.

How different is the volume level for each channel, using speakers? Iow, how much do you have to turn the balance control to even it out?

Keep in mind, no electronic components were "perfect", even when they were new.
 
Hey Clay, Thanks for the response. I think one thing that lingers in the back of my mind is someone elses report of the drastic improvement they got when they did indeed replace the volume pot with (I believe) the blue velvet pot. I know these things are old. and they do wear out. The question is, as they wear out, how much information are we loosing before we actually notice a problem. there may indeed be breaks in the path, but I wonder how much of the path has worn away? I'm no expert of course, but it does seem worth discussing I think. I'm surprised there are so few people who have replaced the pots. although it is a bit daunting trying to find the right part. If I hadn't come across the guy who can fix or rebuild the pot, I'd probably just deal with using the balance pot.

The sound seems to balance out pretty well somewhere between 10-11 o'clock. its not all the way down to 10, but slightly below 11 I think. probably not worth messing with actually. But, I'm in there fixing things Now. so I might as well consider just doing it.
 
keithpgdrb said:
...The sound seems to balance out pretty well somewhere between 10-11 o'clock. its not all the way down to 10, but slightly below 11 I think. probably not worth messing with actually. But, I'm in there fixing things Now. so I might as well consider just doing it.
Click to expand...

Agree with the underlined part.

I'd probably just enjoy listening to it for a while. You deserve it after all your hard work! :yes:

Then, you can continue your research on pot replacements, and if you decide to do it, you can finish it up when it starts snowing for real. :D
 
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