Marantz PM7000

stofbj

Member
2011-11-19 7:44 pm
Hi,

Would anyone have any idea why the 1k Resistor (3261) right from transistor 7261 would be smoking. The amp fails to power up with a lamp in series, the relay clicks on the 100W light flashes and then the relay and light turn off, I have recapped the main power amp section including the protection relay card, i have inspected the orientation of the electrolytic caps and inspected all the solder joints. Diode 6263 measures ok in diode mode, transistor 7261 measures ok out of circuit with hfe of around 520. Resistor 3263 (1 k at the other end) measures ok. Since I can not power up the amp with the current limitor its hard to take voltage measurements. But when powered up without the limitor R3261 smokes, turned it off very fast. My theory is lacking and would like to ask some more experienced Engineers or Technicians

Thanks in advance.
 
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stofbj

Member
2011-11-19 7:44 pm
No it was even worse, it did not power up at all, so don't think its related to the caps. I rechecked them all, and all are the right way around, proper value and properly soldered. I have also re-soldered the connector holding the relay board. I inspected all the solder joints under magnification and re-heated any bad ones, actually went over nearly every one on the main power amp section, cleaned all old flux with IPA.
 

stofbj

Member
2011-11-19 7:44 pm
I suspect Q 7261, I have an 2n5401 transistor at hand, will swap B & C to match the 2sc2240, Q7261 is measuring ok out of circuit in diode mode but am wondering if its leaking could it case the resistor to fry? Or could it be Q7265 leaking from emitter to base?
 

stofbj

Member
2011-11-19 7:44 pm
Sorry forgot to attach the schematic earlier
 

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stofbj

Member
2011-11-19 7:44 pm
Yes R3281 I could not read the PCB or schematic, I have downloaded a better one.
Ok I think I might have two separate issues, the smoking resistor might be fixed as I replaced the 1k (3281) resistor which has good continuity to emitter of Q7261 the other end of the resistor 3281 has continuity to the input resistor 3295, 3295 is also measuring ok, I replaced Q7261 to rule it out. But will not power it up without the limiter to test it out.



For the relay not staying on, when the 3 cables going to the main amp are disconnected the Power relay (not speaker relay) stays on as expected. I disconnected 3315 and 3313 on the channel that had the burned resistor and still the same so that rules out a nice portion of it anyway and will do 3316 and 3314 just in case its the other channel.

I downloaded a better schematic also see attached. I went over each component on the small little protection board, and re-soldered everything and all looks perfect. 7289, 7288 and 7287 might well be suspect at this stage, or the chip on the protection board TA7317P? I will test everything again, I checked all the diodes in diode mode and all are fine (well they could be leaky)
 

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stofbj

Member
2011-11-19 7:44 pm
Turns out that my bridge rectifier (4 pin) was intermittent bad, it is now solid, one rectified side is close to 0V and the other side is higher than it should be as a result, I have since un-soldered it and confirmed in diode mode that its bad. I have a new one on order, and a few 1k resistors and a few 1n4148 diodes as a few look suspect after the higher voltage on one channel.