PG Sapphire 3.0x one channel low one too high (NO CONTROL w/Gain) - diyAudio
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Old 21st March 2012, 03:41 PM   #1
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Question PG Sapphire 3.0x one channel low one too high (NO CONTROL w/Gain)

I have a PG Sapphire 3.0x that I picked up at a pawnshop "As Is" because upon testing in store I notice one channel was very low while the other was very high, not to mention the gain didn't help at all with problem.


*What I've done so far*
-cleaned the pot with deOxit
-cleaned off thermal compound and added new Dow Corning 340
-re-solder RCA's, speakers & power terminal pins (they looked cracked/cracking when I got it and thought that might be part of the problem but no!)


Whats it doing
-with gain all the way down "low" channel is pretty much off, while "loud" channel is loud.
-with gain half way "low" channel is producing audio but still low, while "loud" channel got much louder.
-with gain all the way up "low" channel gets a little louder but no where near as loud as the "loud" channel.


I believe this might be a problem with the gain potentiometer, because when I compared the reading to the "FHZ" pot they where different. The FHZ pot is a 10k while the GAIN pot is a 20k.

With the gain all the way up, I did some readings and they seem out of wack. The highest resistance I got was 8k-ohms one one side and around 5kohms on the other side (this is a 6pin pot, all 6pins are in one straight row).

With the gain all the way down, I did some readings and they also seemed out of wack. The lowest resistance on one side was 1.5Kohms and on the other side it was 10ohms (am assuming this is the low channel since once the gain is all the way up it only goes up to 5k-ohms).



Any help is appreciated. I will post pics today of the gain pot and amplifier.
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Old 21st March 2012, 03:58 PM   #2
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Try to recap the set. If it is several years old, there are the possibility some electrolytic caps goes down, and or some resistor have modified its values.

Good luck.
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Old 21st March 2012, 04:22 PM   #3
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Your reffering to the input caps? This amplifier has several tiny caps. I do have most of them in stock though.

Thanks for your input.


I believe this amp is from 1996 era but not positive.
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Old 21st March 2012, 04:32 PM   #4
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I refer to all electrolytic caps. Generally they are rated to 5000 hours of use, but if the set has been unused several months, they normally decreases the cap value (uF), and or have more internal resistance and or becomes open/short circuit and or becomes deformed, in such case the most sure work is to replace all, or almost dubituous ones.
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Old 21st March 2012, 06:30 PM   #5
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According to my info on hand you should also check the mute J-fets Q110 and Q210, along with the crossover switch for bad or dirty contacts.

There are two 10ufd caps just after the gain pot, if you inject a signal to both channels at this point they should both be the same volume. If not then you possibly may have defects elsewhere in the main amp channel

Could you please tell me the DC voltage present on the speaker terminals with no RCA's or speakers connected to the amp and the amp turned on?...

The gain pot is 20K ohm and the crossover pot is 10K ohm. This amp is laid out a bit different on its front end. There is the standard input buffer then the hi/lo crossover for 4 op-amp stages and then the gain is in front of the final op-amp stage which connects to each main amp channel. Most amps have the input gain up front just behind the input buffer stage

Last edited by 1moreamp; 21st March 2012 at 06:35 PM. Reason: description
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Old 21st March 2012, 07:06 PM   #6
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According to my info on hand you should also check the mute J-fets Q110 and Q210, along with the crossover switch for bad or dirty contacts.


**I cleaned both pots (gain and fhz) and the switch with deoxit I believe the switch is good but I'll double check it by engaging it to see if it makes a difference in balance.**


There are two 10ufd caps just after the gain pot, if you inject a signal to both channels at this point they should both be the same volume. If not then you possibly may have defects elsewhere in the main amp channel

**I'll give that a try might be a little hard since the solder joints are below the circuit board (on those caps but I'll try it). Makes sense and that was my next shot to insert singal after the pot and see if its balanced, if same problem I was going to skip the caps or measure to see if output signal is the same on both caps.**


Could you please tell me the DC voltage present on the speaker terminals with no RCA's or speakers connected to the amp and the amp turned on?...

**Give me a few minutes and I'll post them right up**
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Old 21st March 2012, 09:07 PM   #7
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I did some more reading on the believe culprit pot (gain) and with gain looking at me I labled the 6pins that are in one row as follows : 1 ; 1a ; 2 ; 2a ; 3 ; 3a because thats how I believe the pot works I maybe wrong but when I did the same with the fhz pot they read identical.

Gain all the way down.
1 & 3 = 11K
1a & 3a = 9K
1 & 2 = 4.69K
1a & 2a = 4.69K
2 & 3 = 9.67K
2a & 3a = 10.31K

Gain set to middle.
1 & 3 = 18.35K -Loud channel prehaps?
1a & 3a = 10.97K -Low channel prehaps?
1 & 2 = 12K
1a & 2a = 13.7K
2 & 3 = 10.31K
2a & 3a = 18.77K

Gain all the way up.
1 & 3 = 16.6K
1a & 3a = 8.95K
1 & 2 = 10.3K
1a & 2a = 11K
2 & 3 = 9.67K
2a & 3a = 8.95K

As you can see the reading are not as one would think.


The outptu signal seems to come out of pin 3 & 3a from the potentiometer. I did see two capacitors behind the gain but their not electrolytic caps they are what I believe film caps labled "102J" then the signal travels though a 1.1K resistor then to pin 1 and 7 of U106.

Trace 3a is to pin 1 of U106 and trace 3 is to pin 7 of U106.


I believe the low channel is the 3a trace and the loud channel is the 3 trace on the gain pot. Just guessing because of the readings I got.
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Old 21st March 2012, 09:15 PM   #8
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Quote:
Originally Posted by 1moreamp View Post
According to my info on hand you should also check the mute J-fets Q110 and Q210, along with the crossover switch for bad or dirty contacts.

Are reffering to Q118 & Q218? I couldn't find Q110 nor Q210 on the board. Part number 111A P552AR on both Q118 and Q218 , they seem to be on the input positive (their middle legs I believe).



Just for kicks I checked the RCA positive and the jacket/ground and I had a very low resistance of 22ohms, on other amps that work I notice very high resistance of 2.2Kohm. DOES this mean anything???
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Old 22nd March 2012, 06:05 AM   #9
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Quote:
Originally Posted by jeanious2011 View Post
Are reffering to Q118 & Q218? I couldn't find Q110 nor Q210 on the board. Part number 111A P552AR on both Q118 and Q218 , they seem to be on the input positive (their middle legs I believe).



Just for kicks I checked the RCA positive and the jacket/ground and I had a very low resistance of 22ohms, on other amps that work I notice very high resistance of 2.2Kohm. DOES this mean anything???

Sorry my info is a bit on the blurry side so I was trying to read blobs of numbers. The J-fets used to mute the amps crossover output are J111 type devices by my info.
All vintage PG amps mute their main amp channels inside the main amp channels using opto-coupler output to a very special silicon switch device. I have never seen them fail in 20 years.

As for the caps I was referring to, I can read them plainly 10 ufd 35 volt electrolytic's and they are connected to the gain pots wiper terminal. C102 and C202 are the caps I am looking for, and they are the input DC blocking caps for each main amp channel after the input buffer ICs and the crossover IC's. If you get equal signals out the amp by appling equal signals in on these caps the main amps are clear of defects, and all of your issues are in the front-end somewhere. Apply the signal to the wiper side of the caps which should be the positive lead of each 10ufd 35 volt cap.
As for a cheap simple signal injection you can use just about any battery operated device as long as you make sure it does not have more then 2 or 3 volts music output, the lower the better. Even the RCAs from a HU from a car can be use, and I would use one of those cheap ground loop isolator transformers they sell for car audio so the signal source stays decoupled from the amps thus protecting both devices from any mistakes or missed failure issues. You only need a mono signal so only one output is needed so both have the same level into the amp channels. The RCA shield is your ground connection for this signal injection.

This like many other PG amps of this era uses opto-coupler feedback signals from the secondary / audio side of the amp back to the 12 volt side of the amp so with that said you should have absolutely no connection at all between these two different grounds. The RCA shield will not read any ohm value to the ground of the 12 volt source unless something is shorting the case to the audio side somewhere....

Let me know what you find or if any of what i posted is not clear.....
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Old 22nd March 2012, 02:57 PM   #10
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Also if you can post a pic of your gain control I believe there is a X PG tech selling them on ebay right now, but I need to see yours to be sure.
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