QSC 850 help

[Woody]

Hi i just got my AA degree but now that im actually working on stuff. im trying to put together everything that was cramed in in 2 years lol. Everything works when your in school. .
Im working on a QSC 850 amp. Right channel is shorting out. i hooked up light-bulb as to quit popping fuses. if I unplug the board short goes away. I took the board out and tested every component (transisters, resisters, caps)and touched possible cold solder. All seem to test good. I took out the bridge rectifier and and it appears to be fine. Any ideas where to go next would be appreciated. Could the rectifier test good but go bad under load is my next thought?

 

[dorke]

Woody,
"Right channel is shorting out"

What do you mean it is not clear :
how do you know it is the right ch.?
with speakers/without speakers?
with input signal/without signal?
low volume/high volume?

photos would help a lot...

 

[Woody]

The amp works on the left chnl or chnl 1 . I can run sound thru chnl 1 . chnl 2 shorts and blows fuse. there is a fuse for each side. Unit shorts with or without speaks plugged in. with or without input plugged in. heres a pic. The right side of pic is chnl 2

 

[dorke]

Hope you did the transistor test in "diode" mode test.
How did you check the caps?
The big caps should be discharged before you do ohm/diode test
use a 1k resistor to discharge them(check for no voltage on them with DVM).

Test speaker out to GND ,ohm test (not diode test!)
Check Q203-Q210 on the heat sink in diode test.
Check D217-218 in diode test.

If all the above are fine.

You can work with the light bulb(or variac).
Since you have one good ch and one bad ,
you can do a direct comparison of DC voltages:
Start with (-VCCB and +VCCB ) vs (-VCCA and +VCCA)
Check VBE voltages of Q205 vs Q105 and Q206 vs Q106.
Check voltage on U201 pin 7 to GND vs U101 pin 7 to GND,
Let us know what you find.

 

[Woody]

Hope you did the transistor test in "diode" mode test.
How did you check the caps?
The big caps should be discharged before you do ohm/diode test
use a 1k resistor to discharge them(check for no voltage on them with DVM).

Test speaker out to GND ,ohm test (not diode test!)
Check Q203-Q210 on the heat sink in diode test.
Check D217-218 in diode test.

If all the above are fine.

You can work with the light bulb(or variac).
Since you have one good ch and one bad ,
you can do a direct comparison of DC voltages:
Start with (-VCCB and +VCCB ) vs (-VCCA and +VCCA)
Check VBE voltages of Q205 vs Q105 and Q206 vs Q106.
Check voltage on U201 pin 7 to GND vs U101 pin 7 to GND,
Let us know what you find.[/QUOTE

I disconnected transistors, and tested in diode mode. caps I pulled all the way out and tested in cap mode. shook them also couldn't here any clunking so they weren't dried up (the big filter caps). I will check dc voltage as you described.
Thank you so much ill repost after I do it.

 

[Woody]

Yes test was done in diode mode. I removed the transistors and caps when tested. caps were tested in cap mode. I also shook them to listen if they were possibly dried up. (for clunking in big filter caps). I try testing as you described the DC voltage. Thanks for the input. Let you know soon as I'm done. I know I got to be over thinking this .

 

[Woody]

Hey guys I finally got back to it. I had other stuff to finish up.
I checked U101 and 102ic chips. It appears they are sapose to be ne5332 and what I have in there in 5532. I'm gonna check and see if its an updated number and the dats sheet for the correct part or replacement. either way 102 tested bad with the chip tester I have and 101 was good.
I ordered the correct part anyway just because.
ill let you guys know.
Thanks

 

[davenn]

I also shook them to listen if they were possibly dried up. (for clunking in big filter caps).

not a reliable test for dried caps. They can dry enough to totally loose capacitance long before they ever became rattly

A 5532 is a dual low noise Op-Amp
I have no idea what a 5332 is ? ... cant find any listing ? .... 5332 could be a typo and it really is supposed to be a 5532 ?

either way 102 tested bad with the chip tester

bad chip tester ???
tell us more


Dave

 

[dorke]

Woody,
Dave is correct ,the IC is NE5532 it is U101 and U201 in sockets.
There is no U102 on the board and NE5532 is non-existent .

What IC tester are you using ?


From the SM:

 

[Woody]

yes you are correct I wrote it wrong. the chips U101and U201 are correct and I replaced the bad one. one tested good the other bad. If I unplug the connection at E-201 which goes to the output board from the main board short goes away. (see 850edit pic) I high lighted. so I'm thinking short may be in the output board itself or the transistors (sc5200).
My chip tester is a cool little unit. You plug the chip in and it tells you the model number of chip and which pins are shorting or open if any. it doesn't have all chips in it but about 2/3rds of them I check are in there. if it doesn't find it, it just says not listed. Ill take a pic of it and post it tomorrow when I go to my shop. I think it was only about 50bucs or something on ebay I got for the heck of it and its actually come in handy.
thanks guys

 

[dorke]

Disconnect both E200 and E201.
Measure resistance between JP15 and JP16
to verify short is on the output board.

Measure resistance between E200 and E201 as well.

.

 

[Woody]

ok theres no short on the the output board. When I test E201-E200 on the board(there on 2 separate boards. I have no resistance
Ok ill see if I can explain this well enough. I disconnected everything off of board A wich is E101. and I disconnected the wires going to board B. I have a direct short between E101 and E201 if I lift the board off the chassis it goes away. so I'm thinking there is a short direct to ground from board B.

 

[dorke]

Well,
E101 and E201 are both P1 GND(from the sch.)
-they are the same point,no problem here that is the way it should be.

Looks to me like they shouldn't be connected to chassis GND.
Can you trace that on the boards?

maybe post some good pictures of the boards?

 

[Woody]

ok I pulled board B out and removed all transistors and checked every component on it. All is good. As a test I took the power supply from channel A and plugged it into chnl B It works, and no short. So on the power supply I checked the 2 b-rectifiers and they test good. chnl B has the correct dc volts but as soon as its plugged in it shorts. Could the rectifier just be bad under load. Or possible output from the transformer. A and B from transformer are separate wires coming out.

 

[dorke]

If I understand correctly:
1.P.S- A works perfectly fine with both PA-A and PA-B(each one at a time).
2 P.S- B doesn't work with PA-B.
have you tried P.S-B with PA-A?
3.You are doing all the above tests by merely swapping VCC_A pair with VCC_B pair
at the P.S side.
While all boards are at their locations "fastened with all screws and connectors"?

 

[Woody]

that is correct B shorts outs on either A or B. But unplugged the voltages are the same. (within a few volts).

 

[sndscientist]

assuming this is an rmx850 i have a few questions for you. first you measured both sets of dc voltage going into the output pcb correct? did you measure both in reference to ground or just across the two? if i remember correctly the rmx series uses 70v rails. judging by what you typed out so far it sounds to me like a diode in a rectifier shorted. it will still give you close to the same voltage open circuit but the ac ripple will wreak havoc on the capacitors and voltage regulation ics. me personally whenever i replace capacitors in amplifiers i just replace the rectifiers with higher current ones in the same package for good measure.

 

[Woody]

assuming this is an rmx850 i have a few questions for you. first you measured both sets of dc voltage going into the output pcb correct? did you measure both in reference to ground or just across the two? if i remember correctly the rmx series uses 70v rails. judging by what you typed out so far it sounds to me like a diode in a rectifier shorted. it will still give you close to the same voltage open circuit but the ac ripple will wreak havoc on the capacitors and voltage regulation ics. me personally whenever i replace capacitors in amplifiers i just replace the rectifiers with higher current ones in the same package for good measure.

 

[dorke]

that is correct B shorts outs on either A or B. But unplugged the voltages are the same. (within a few volts).

Disconnect the 3 points shown in the pic.
In this way you can do a complete diode test of the disconnected bridge.
You should get very clean 4 good diodes
(forward and reverse).What do you get?

Another thing would be to check the transformer secondary for A and B the AC voltages should be the same.
what do you get?

 

[Woody]

assuming this is an rmx850 i have a few questions for you. first you measured both sets of dc voltage going into the output pcb correct? did you measure both in reference to ground or just across the two? if i remember correctly the rmx series uses 70v rails. judging by what you typed out so far it sounds to me like a diode in a rectifier shorted. it will still give you close to the same voltage open circuit but the ac ripple will wreak havoc on the capacitors and voltage regulation ics. me personally whenever i replace capacitors in amplifiers i just replace the rectifiers with higher current ones in the same package for good measure.