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Amp In Protection Mode

Rick79

Nov 26, 2014
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O.K
As I suspected ,the reading on R623 is indeed wrong,
there should be no DC current through it (hence no voltage on it).

Please take a good photo of of R623,C613,C617 and vicinity.
Please take a good photo, of the solder side of R623,C613,C617 and vicinity.

Could be dirt oxidation or corrosion .
If dirty clean it well.

If clean, take out C613(note it's polarization) and C617.
Power up and re-measure voltage between both legs of R623 if we get DCV of 0.0mv we have a bad cap ,
most probably the electrolytic C613.

At this stage,measure the voltage on both legs of R621 and trim it with VR601 to about DC 300mv.

OK with C613 out, R623 Reads 00.0mV
I took a photo of the reading of R621 in 200 mV range and in 2 V range.
I can only go up to about 60.0 on 200mV range. Should I use the 2 V range on my meter to trim the voltage?
 

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dorke

Jun 20, 2015
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Yes,
0.291V readingin on the 2V range is fine for now.
You did trim it ,right?

Please measure the DCV, with both probes on resistors legs, of R607 and R627(the lifted leg connected to GND).
and voltages on Q603 relative to GND.

C613 is faulty,you should get a replacement for it.
 
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Rick79

Nov 26, 2014
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Yes,
0.291V readingin on the 2V range is fine for now.
You did trim it ,right?

Please measure the DCV, with both probes on resistors legs, of R607 and R627(the lifted leg connected to GND).
and voltages on Q603 relative to GND.

C613 is faulty,you should get a replacement for it.

R621 won't go any higher than 0.291V when I turn VR601, if I turn the other way, the voltage goes down. Plus it was already at 0.291V before I tried turning it.
R607 reads 09.6mV
R627 reads .721V
Q603 reads...
47.4V ==>C
1.32V ==>B
.721V ==>E

I'll try and get a Capacitor Today.
 

dorke

Jun 20, 2015
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O.K. that is fine.

With C613 out.
Please move VR601 to about it's mid position as shown in the pic.
Leave R627 lifted leg connected to GND.
Install Q605 and Q607 in their location.
Measure the voltages on Q603 and Q607.

IMG_20160815_042934.jpg
 

Rick79

Nov 26, 2014
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O.K. that is fine.

With C613 out.
Please move VR601 to about it's mid position as shown in the pic.
Leave R627 lifted leg connected to GND.
Install Q605 and Q607 in their location.
Measure the voltages on Q603 and Q607.

View attachment 28552

OK I got a couple of 1000uf 6.3V capacitors today.

Q607 & Q605 are installed.

Q603 Reads...
47.4V ==>E
-45.6V ==>B
47.9V ==>C

Q607 Reads...
-45.5V ==>B
47.8V ==>C
-45.3V ==>E
 

dorke

Jun 20, 2015
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Rick,
Q603 readings make no sense at all.
Are you measuring with C613 out of board and R627 lifted leg at GND?
 

Rick79

Nov 26, 2014
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Rick,
Q603 readings make no sense at all.
Are you measuring with C613 out of board and R627 lifted leg at GND?

SORY, I got confused, that was Q605 readings. Yes,the capacitor is not installed yet and R627 leg is still lifted and Grounded.

Q603 reads
.726V ==>E
1.344V ==>B
47.4V ==>C
 

dorke

Jun 20, 2015
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O.k ,
Please re-measure Q605,Q607,take care of the correct pins labeling.
Try trimming the voltage on the Emitter of Q607 with VR601 to read about 1.6V
 

Rick79

Nov 26, 2014
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O.k ,
Please re-measure Q605,Q607,take care of the correct pins labeling.
Try trimming the voltage on the Emitter of Q607 with VR601 to read about 1.6V

OK. With R627 leg still connected to chassis GND, and C613 Still off the board.
Q607 reads...
-45.4V ==>E
47.8V ==>C
-45.4V ==>B

Q605 reads...
47.5V ==>B
-45.7V==>C
47.8V ==>E
 

dorke

Jun 20, 2015
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Please try trimming the voltage on the Emitter of Q607 to read about 1.6V
using VR601.
 

Rick79

Nov 26, 2014
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Please try trimming the voltage on the Emitter of Q607 to read about 1.6V
using VR601.

When I move VR601, the voltage reading jumps around high and low and the protection actually turns off and on. Do you think it just has to be cleaned with contact spray? Because I think vibrations might make it jump out of 1.6V
 

dorke

Jun 20, 2015
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Did you get a reading of 1.6V?
I guess you did if the protection turned off.
Probably needs cleaning,
but only with spray intended for pot cleaning, what do you have?.
 

Rick79

Nov 26, 2014
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Did you get a reading of 1.6V?
I guess you did if the protection turned off.
Probably needs cleaning,
but only with spray intended for pot cleaning, what do you have?.

I have the Radioshack Contact Control Cleaner I use to clean the bass and treble knob pots
 

dorke

Jun 20, 2015
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Don't now this one,but if you have used it before on pots and it did a good job ,go ahead.
 
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dorke

Jun 20, 2015
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Next step is re-soldering the lifted leg of R627.
Trimming the output voltage as close as possible to zero volts(on the junction of R645-R647(or the previously lifted leg of R627 if it is more convenient relative to Gnd.

Then insert a good cap in C613 location (take care of polarity).
Re-measure the output voltage to verify it's still as you trimmed it.
 

Rick79

Nov 26, 2014
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Next step is re-soldering the lifted leg of R627.
Trimming the output voltage as close as possible to zero volts(on the junction of R645-R647(or the previously lifted leg of R627 if it is more convenient relative to Gnd.

Then insert a good cap in C613 location (take care of polarity).
Re-measure the output voltage to verify it's still as you trimmed it.

I sprayed VR601 A few times already and worked it back and forth turning it several times, but when i try to get the voltage down, it only starts dropping down unless I have it right on the spot where it was before I ever touched it. When I turn it passed the middle to the right, it reads 44.4V all the way till i can't turn it no more. When I turn it past the middle to the left, it reads -46.3 all the way till I can't turn it no more. Shouldn't the voltage change while I'm turning it? I have to put it right in the spot where it was originally being really careful being precise and then the voltage starts to drop slowly and low enough that the protection shuts off but then it starts climbing back up on its own tripping the protection again. Is it still dirty? Is there contacts on the underside? Or is it because I have the resistor leg lifted and the bad capacitor still off the board?
 

dorke

Jun 20, 2015
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No,that is open loop behavior.
solder back the lifted leg of R627 and trim the output voltage as close as possible to zero volts(on the junction of R645-R647(or the previously lifted leg of R627 if it is more convenient ) relative to Gnd.
 
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Rick79

Nov 26, 2014
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No,that is open loop behavior.
solder back the lifted leg of R627 and trim the output voltage as close as possible to zero volts(on the junction of R645-R647(or the previously lifted leg of R627 if it is more convenient relative to Gnd.

OK, R627 leg back on and the new capacitor installed. 0.04 V is the closest I can get to 0 volts.
 

dorke

Jun 20, 2015
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Excellent !

Have you got a good replacement cap for C613?
If you have, insert it (take care of polarity) and re-measure the output voltage.
What do you get?
 
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