477 Max low volume left channel

[jukenorman]

Pete,
I really don't know at all, clearly things have gone downhill across this thread. I don't have the diagram here, I don't have a pdf of that amp only a paper copy which is not to hand at the moment. Are the DC voltages to the amp OK? If so, are the power transistors OK? Something is very obviously amiss, especially since what was a working channel no longer functions!

Norman.

[prime]

Does the power for the driver boards come directly from the power supply or does it come from the rectifier, seems strange that both boards in either channel show no millivolt power. If it does come from the rectifier i will need to test it.

[jukenorman]

Pete,
I cannot believe that you're messing around with that amplifier without a diagram, that is totally alien to me! There is a picture of that section showing the power to the amp.

[prime]

I have the service manual , i also have alot of schematics but not the page you sent me showing clearly -32 vlts. All power is good, i have been able to set one board and get sound out of that channel, the other board will not set, i compared the good board with the bad, removed all components one by one on each board, they are all the same. Im thinking there's something wrong with the board itself, no obvious crack.

[jukenorman]

I read through the thread again last night and it's not easy to follow exactly what is going on. So give us a reminder of what you've done exactly especially these questions.

Does the good driver board work on both channels (proving that the driver board is the problem and everything else is functional)?

Which driver board did you replace the driver transistors on, the good or the bad one?

I take it that you have checked for poor solder joints, the mounting plug for the driver board is especially prone to that?


Norman.

[prime]

Hi Norman, i checked all solder joints ,im quite sure that is not an issue, I replaced the Q11 and Q 12 on each board, I replaced the PEC - 8 on both boards, I removed each board from the amp and checked all components, i tested power supply. when i started it was the left channel that wouldn't work, I tried the left channel board in the right side still no sound , so i figured it was the board. I tried the right board in the left channel and no sound so more than just the board. After replacing parts i installed the good board in the left channel and adjusted pots as per manual and got sound in left channel, i put in the previously bad board in the left channel, set as per manual, connected jukebox to dim watt, to see if there was an issue and although the light did not show a short, a loud sound came out of the speaker, i replaced the good board and tested with dim watt no issue but when i used full power now volume was at full, although it is set to minimum, Now both boards make a loud noise on dim watt power. I tested the pot and get 100 ohm to 62K ohm on both pots so i dont know what can cause the volume to go to full on its own, I removed the amp i can see no issue with wires touching , not sure what has changed. I will retest the transistors but if both board test ok again this must be something else. By the way i tried dim watt power with pick up removed same loud constant noise.  ps checked all of the transistors the spec are the same as before i installed the boards, also i had tested all of the pins on the new pec-8 to make sure the readings were the same as each board and they are still the same. It doesn't appear to be the boards causing this issue.    

[prime]

Good news, readjusted the good board and i have music on the left side again, the other board will not adjust, i took out both trimmers and tested them, they tested good, evey component matches numbers with good board, must be a crack or something with that board, or a part is affected under load.. I think if i can find another board, with proper settings box should play.

[jukenorman]

I still don't get this - if all that is faulty is one driver board, why can't you get the good driver board to function in the right side (other) channel?

Norman

[prime]

I found the issue there was two main ones, when removing the speaker connector ,the right feed wire pushed back a bit so when i tried the good board on the right side it was a bad connection, after putting the good board on the side that didnt work left side, i thought it may have damaged something on the board so i never focused on the connector. After my last post to you i realized i had re flowed all solder joints, checked with my meter on beep to make sure of connections and checked all components, now thinking it was a broken trace i decided to use the ohm setting to compare each connections resistance, and to my surprise i found an area between two pins on the same line or trace that on the good board showed .02 ohms but on the bad board showed about 5M, i scratched off a little of the board on each end and soldered a jumper wire, tested again and got .02 ohm, put it in the left side of the amp and was able to set the R25 to o, on both boards i can only get the R33 to .08. I put the good board back in the right side after adjusting and noticed a little sound coming out so i played with the connection and found it to be the issue. Would definitely spend more time checking not only solder but board in general. Also sadly i also had a damaged transistor Q11 on the bad board that lead me to believe it was something further down the line that damaged it, like a faulty component. ( spoke too soon ,this just doesn't make sense, i had both channels working, i played a couple of songs set ballance and sound was perfect, I later went out and tightened the bolts holding the amp and reconnected the ground wire from pickup, i turned on the box to try another song and i got the same loud blare coming out of the speakers. Dont know what could have changed over 30 minutes) I removed the formally bad board and tried to re adjust R25 to confirm 0 vlt, I now get no reading from that trimmer, tried in the other channel ,no reading, tried the other board ,both channels, no reading, checked for power to boards , get 32 vlt DC.  Checked both board components with values for each component i had written down, no change including the PEC-8  . Checked connection from test 1 to PEC-8 good,  followed to pin eleven good down to R28  good.   what ever is stopping me from re adjusting has to be the same thing thats causing a loud blare through the speakers when the boards are installed.. 

[prime]

Pin voltages. 1-0 2-5 3-35 4-6 5--35 6-0 7--35 8--35 9--7 10-0 11-7 12-0 13-35 14--35

[jukenorman]

That all screams - or should it be hums - bad solder joints to me!

[prime]

I dont think its a solder issue, Ive been checking all voltages and have definitely confirmed rectifier is good, ive been testing voltages at transistors to see if i can isolate the issue, I have found on both boards there is an issue at Q11 I should be getting 1vlt at base, 32 at collector and 4.6 at emitter, unfortunately im getting 34vlts at all three posts . Will be testing Q8and Q9 tomorrow to see if voltages are correct.

[jukenorman]

That would probably indicate that Q11 was toast!

[prime]

I have 4 replacements for Q11and Q12 so i removed Q11 and compared it to the new ones, almost identical readings, I tested Q8 and Q9 . On schematics it shows Q8 base to be +32 I get -7.8 on Q9 it shows +1.5 I get -7.8, something is causing all of these improper readings. All testing on DC voltage

[jukenorman]

32VDC (34V in your case) from the power supply is being presented to the collector of Q11. If you're correctly measuring the voltage on the base and emitter at 34V, it's hard to see how Q11 could ordinarily be OK - but what do I know?

Norman.