Gain control on gen 1 Orion amplifiers

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Thanks Perry. I will do that, and use it for a few weeks like before and see if it starts cutting again.

Or, I wonder what you think about just replacing the muting JFETs? I think they are available for order, and I think the gain pot is good and installed well. The gain pot contacts (and the board in the area) don't look bad, feel loose, etc, and the gain pot I installed is new and the right part.

Thank you again.
 
Techs generally have to do things in an efficient manner. Eliminating all but one variable lets the tech determine where the problem is without guessing. I thought you said that the pot was scratchy when touched. If it's perfect, you can replace the muting transistors or simply remove them (temporarily) from the circuit to see if that makes a difference.
 
The gain pot is new and installed well as far as I can tell, but what happens is the output of one of the two channels drops to a minimum level sometimes when it is touched. Then it comes back, if you adjust it again or even touch it again the output will drop again, and so on. So I am confused. It is like the original problem I was having with it happens intermittently now.

It acts like there is a solder loose or something, but I really think there isn't.
 
I'm ordering 2N4392 as a replacement for the outdated 2N5639 present in the amp:

2N4392 Central Semiconductor Corp | Discrete Semiconductor Products | DigiKey

I think I will try simply replacing both (with new pot installed as-is) and test.

I had to remove another 250 hcca from the car due to noise. On the bench with no signal drive it sounds like one of the two 3300uF 16V (at the end of the board on the power supply side) is hissing, there is no obvious damage otherwise. So, I am ordering replacements for these as well:

TVX1C332MCD Nichicon | Capacitors | DigiKey

Any suggestions or comments? Thank you.
 
OK I can remove them and test at least until the parts come in.

The JFETs would only be "ON" right at start up, then turn "OFF" and stay off during normal operation, correct? I suppose the last time the issue occurred, I should have at least checked if the muting transistor on that channel had somehow turned on.
 
Thanks Perry I remember now this should be checked first. The annoying thing is, the problem is so intermittent that it might be a week or more before it shows up again. Is it OK to run the amp without the muting transistors indefinitely?
Hi, have you replaced the caps 470 uF after the leveling capacity (the four axial caps)?
For Orion moon & stars is the first thing to fix, maybe it's the cause.
 
I have had this amp hooked up and used constantly for a few days now, just as I left it before, and the problem hasn't reappeared yet. When it does, and it will, I will check if either muting transistor has come on by measuring gate voltages, and update.

On the other amp, I replaced the two power supply 3300uF 16V caps and the problem persists. When powered up, without any signal input, it puts out steady high-pitched noise. When I connect signals, both channels play but are very noisy. There are no obvious signs of damage, the gain pot has been cleaned, I'm not sure how to proceed. Perhaps I will start by making a new thread about it.
 
Sorry, maybe i don't write exactly.
I say the two caps 16v/470 uF downstream the leveling caps (those axials).
These ever are the first fixing in my Orion moon & stars, and cause the problems you tell.

Ah yes I thought you might have meant those.

You are right, in the second amp that has noise now those have not been replaced, thank you very much for the tip. Probably should just replace them.
 
Just to update ...

I have had this amp running sound at my desk for a week or so, with gain set maybe 90%.

It generally works fine and sounds great, but ...

If I nudge the gain pot package so as to stress the contact points in a certain way, one channel will drop to minimum output, meaning what it would be if the gain was set to minimum.

I checked the muting transistor gate voltages while doing this, and found them to be stable, at low voltage, around -15.8 VDC, meaning neither seems to be turning on when one of the channel output drops.

I monitored the gain pot resistance while recreating this symptom, and found unstable results. The pot resistance on the affected channel goes from a low value (corresponding to the high setting) to erratically high values or even open circuit.

So it seems that in replacing the gain pot, I have gone from a chronic symptom (one channel always stays minimum), to a mostly avoidable issue where the symptom only occurs when I physically press on the gain a certain way. Perhaps this could be eliminated by improvement of the soldering, or perhaps there is some contact issue with the traces in the board. Will investigate later.
 
Are the muting transistors in or out of the circuit?

They are still in, I haven't altered them at all as of yet.

If I press on the gain pot so as to cause the problem (one channel will minimize, then come back up when I remove my hand), both muting transistors seem to remain off (the gate voltage doesn't change on either, remains around -15.8, indicating they are both probably off as they are supposed to be).

Maybe I should replace the caps with the correct 470uF, check the soldering again on the gain pot contacts, remove the muting transistors, or replace the muting transistors (I already have replacements on hand).

The current symptom seems to indicate there is some contact or solder issue, but I'm not opposed to just removing the muting transistors to see if it helps. I'm just running bookshelfs 4 ohms, not worried about the thump.
 
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