Digging a little more. Appears the power amp boards are powered solely by the unregulated supply. J702 is more about providing power to downstream boards (PX01) So, if you lift +ve/-ve rail connectors J706, J707 on one channel and note any improvement in DBT, repeat on other channel, first step is to identify the faulty channel.
Marantz 2500 repair help
- Thread starter quadmatrix82
- Start date
Hopefully lifting J706,7 will identify the faulty channel.
Some other things to try on faulty channel, DBT power up, main caps should be bleed down after each.
- lift J706 only, replace
- lift J707 only, replace
- lift J704 only, replace
- lift J705 only, replace
Note if dbt bright or not
Some other things to try on faulty channel, DBT power up, main caps should be bleed down after each.
- lift J706 only, replace
- lift J707 only, replace
- lift J704 only, replace
- lift J705 only, replace
Note if dbt bright or not
Ok so resurrecting this thread as I’ve run of ideas, issue still remains one channel won’t bias below 40mv and the other won’t bias above 15mv, the dim is pretty bright I think too bright. Turning the scope on or off seems to make no difference to the dim brightness. Tests are done with the 150watt dim bulb.
Outputs are new mj21993/4,
Qm03-4 mje15032-33
Q705-9 ksa1381/ksc3503
Q711-2 ksa1381/ksc3503
Bias transistor mje243g
Dc offset and bias remain steady
Any ideas what else I can check to track down this issue??
Outputs are new mj21993/4,
Qm03-4 mje15032-33
Q705-9 ksa1381/ksc3503
Q711-2 ksa1381/ksc3503
Bias transistor mje243g
Dc offset and bias remain steady
Any ideas what else I can check to track down this issue??
Case is live, mica washer used with non-conductive paste?
Thankyou for everyone’s help on this one, I thought I’d share an update on the progress made, good news is that dim is now very low - I removed the plastic sockets that the bias transistor and qm01/2 sit in and hard wired them and reflowed the differential transistors.
Using a 300w globe or just a variac up to 220v, I still cannot get bias above 20mv or so on each channel, I’ve replaced resistor r721 previously from 180 to 330 ohm, is it worth increasing the value here higher? Or do I need revisit transistor substitutes? Bias and offset and fan speed stable for an hour of testing Thankyou
Using a 300w globe or just a variac up to 220v, I still cannot get bias above 20mv or so on each channel, I’ve replaced resistor r721 previously from 180 to 330 ohm, is it worth increasing the value here higher? Or do I need revisit transistor substitutes? Bias and offset and fan speed stable for an hour of testing Thankyou
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If bias is stable and DBT (appears) not significantly bright then time to remove DBT, first rest bias to 0-5mV(ie, low).
Remove DBT then variac up while monitoring bias on the problematic channel, if all ok then try slight adjustment of bias just to confirm stability, if all ok then power down and then mains power up and set bias as per service manual.
Remove DBT then variac up while monitoring bias on the problematic channel, if all ok then try slight adjustment of bias just to confirm stability, if all ok then power down and then mains power up and set bias as per service manual.
Thanks mbz, both channels seem to be in the same boat, at around 200v on the variac I couldn’t get past 20mv max turn out on the pots, seemed to be no different with the dim or variac when on 200vac, I will test the bias stability with variac only to see if any different my initial test was with 300w dim for an hour and was steady the manual calls for 25mv
Just so I understand, you are you trying to make an Idle Current adjustment? You have your DMM on the correct test points, either JX01-1 to JX01-4 (L) or JX02-1 to JX02-4 (R) shown for the appropriate channel and speakers disconnected and volume turned down to minimum? You are adjusting R716 on the P700 board, correct.
Hi Dankik yep this is correct currently can only reach 16mv on one channel and 25mv on the other at max trim pot adjustment however r721 currently modified to 330ohm instead of 180ohm I am going reset to original and recheck
The following comments are pure speculation since I have not worked on a 2500 nor own one.
If I follow the signals from PX01 to P700, pin 1 goes to the base of QM08-QM12 (red line) and pin 4 goes to the emitter of QM05 (blue line).
You are trying to adjust R716 to get 25mV between these two points. Does that make sense... maybe?
EDIT: Sorry I made an error in tracing the signal from J702 to J703 (red line).
On this thread it suggests to check the diode stack. Maybe yours is bad?
https://audiokarma.org/forums/index.php?threads/marantz-2500-problem-setting-idle-current.847327/
If I follow the signals from PX01 to P700, pin 1 goes to the base of QM08-QM12 (red line) and pin 4 goes to the emitter of QM05 (blue line).
You are trying to adjust R716 to get 25mV between these two points. Does that make sense... maybe?
EDIT: Sorry I made an error in tracing the signal from J702 to J703 (red line).
On this thread it suggests to check the diode stack. Maybe yours is bad?
https://audiokarma.org/forums/index.php?threads/marantz-2500-problem-setting-idle-current.847327/
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Yeah, the diode string is probably the problem and that fiddling with the resistors is just a "work around". Question is that would replacement with 1N4148 or whatever be any better than what's loaded, would have to measure the Vf over the expected current range.
Somewhat odd that both channels are behaving the same, the ok channel is almost at the limit of the VR while the notok channel falls short. This triggered the thought that the replacement bias transistors had slightly different Vbe characteristics so the resistors needed a slight tweak. Note the bias increases as the VR resistance is decreased.
The bias diode are both nos stv-4h the originals had similar issues but still have them if needed to change back
So not good results with the 2500 unfortunately, I replaced the 330ohm resistor back to 180ohm reassembled on both channels
Dim bulb is pulsing and looks like bias over 100mv briefly regardless of adjustment I quickly switched it off
Dim bulb is pulsing and looks like bias over 100mv briefly regardless of adjustment I quickly switched it off
Trimmers should be both preset to minimum bias current=max resistance, then power on and adj.
I think it's bad practice to try and set the bias while on DBT, ok, I understand that you are trying to prevent a meltdown.
The pulsing reads like a significant current draw then a partial collapse of the driving force(voltage), so less push, less current and the psu recovers and then cycle...
Assume you are still using the 300Wdbt.
Is there anyway to completely disconnect one channel then set the bias on the other to confirm no fault. Schematic suggests this would be difficult.
I think it's bad practice to try and set the bias while on DBT, ok, I understand that you are trying to prevent a meltdown.
The pulsing reads like a significant current draw then a partial collapse of the driving force(voltage), so less push, less current and the psu recovers and then cycle...
Assume you are still using the 300Wdbt.
Is there anyway to completely disconnect one channel then set the bias on the other to confirm no fault. Schematic suggests this would be difficult.