PG Sapphire 3.0x one channel low one too high (NO CONTROL w/Gain) - Page 2 - diyAudio
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Old 22nd March 2012, 04:03 PM   #11
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Sure. I'll also take pics of the circuit board so you can see that there is no 10uf cap after the gain pot (unless am looking at it wrong, dont know exactly which one is the wiper on this pot).

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Click the image to open in full size.


Now there is two 10uf caps on the input signal right after the RCA's (behind the RCA's).


When I also stated on post #8, I meant the "OUTPUT" RCA's had 22ohms between the ground and center/positive pin, NOT THE RCA INPUTS.

Last edited by jeanious2011; 22nd March 2012 at 04:13 PM.
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Old 22nd March 2012, 06:07 PM   #12
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Well I am reading directly off of a PG schematic for the SA3.0X and it certainly does have a 10 ufd 35 volt cap attached to each wiper of each gain control.

The numbers are a bit blurry but I can make out C102 and C202 as the caps that attach to the wiper for each channels gain pot. The caps connect to U101A Right channel and U101B Left channel.

The primary reason why you might be getting 22 ohms on the RCA output is because those J111 j-fets are connected to them and ground for a mute control of that output and they would be conducting to ground. So 22 ohms looks OK to me when the RCA outs are muted.

The guy on ebay is selling the gain pots for the M and MS series amps but if you contact him he might have the one you have disassembled.

The main reason I don't look at the gain pots so hard on a PG amp is because I have never had to replace one in 20 years so forgive me for thinking your problem might be elsewhere. My methods are about as old school as this amp. My document is dated 1994 with rev's from 1996.

You have fixed your focus on the gain pot and the tests I asked about will rule out all other possibilities. PG is not built like most other amps as they use fuse resistors that blow open without even a hint on the outside that they are damaged. The tests for DC offset and simple signal injection will tell me that they are OK and not the source of the weak signal out the amp. Once I know the main amp channels have not had any failures I would then digress to the front end of the amp. As I said in 20 years I have never had to replace a gain pot on any PG amp. This does not mean that they can not fail, but it does press me to look elsewhere for issues first. If you like you can PM me and we can talk directly about this amp in email.
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Old 22nd March 2012, 08:12 PM   #13
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I can change the caps, Perry also stated the pot looks fine and since you saying it they usually (or very rarely) fail then I'll get my mind off of that. I did put it back together and thats how I got those readings.

I got plenty of caps in stock so I can replace as many as I have to. Its just the power supply caps I dont have or the bigger ones.


I'll check on it right now, and let you know. Thanks.
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Old 22nd March 2012, 08:14 PM   #14
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Also if you could email me those schematics I would greatly appreciate it.
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Old 22nd March 2012, 08:55 PM   #15
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The caps are likely fine. They are a test point if you should decide to inject a signal to the main amps to test them for correct operation. So I would leave them alone for now since we still don't know where your signal loss issues is.

I agree with Perry the gain pot looks fine inside. So here we are again looking for the source of the issue. As I explained earlier this amps front end is setup differently then most as the gain control and the final op-amp stage is the last thing before the main power amp channels. So there is a input buffer, and then a hi/lo crossover before the gain control and it output buffer to the main amp channels. Most amps have the gain up front. this one has it out back of the front end circuitry. just a little different then most other amps.

I don't have your email address so maybe you could PM me and we continue this there.
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Old 22nd March 2012, 10:30 PM   #16
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PM sent.
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