Author Topic: Too much heat from Marantz PM4200/N2B  (Read 984 times)

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Offline etet

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Too much heat from Marantz PM4200/N2B
« on: September 01, 2019, 05:52:02 pm »
I have this amplifier for repair.
I found scheme for Marantz PM4200/N1B from
Heat comes from C4467.
As i notice it has higher base voltage.
I move back and back and notice that problem start
from first transistor after IC AN7062N - A1268.
On second chanel base ot A1268 has 1.7V, but on chanel
that produce a lot of heat A1268 has 2-2.1V on his base.
Where to continue to check for fault?


Offline Chris56000

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Re: Too much heat from Marantz PM4200/N2B
« Reply #1 on: September 22, 2019, 08:20:42 pm »

I suspect you didn't get a reply because you didn't quote the circuit component reference nos., and nobody could work out where exactly you were referring to - never mind tho', I can!

This circuit is a direct coupled high-voltage complementary-symmetry class AB output stage with a high-voltage dual power op-amp IC31 (AN7062) providing the main amplifying stage!

First of all, make sure the emitter protection resistors R359/R361 aren't damaged or high value, replace these if they are, check the bias resistor R353 (330R 1W), then I'd consider replacement of IC31 itself - it isn't horrendously expensive:-

If replacing IC31 doesn't help, you'll have to go through the transistors Q349/Q351/Q353/Q355/Q357 to see if you've got a leaky or defective one!

Don't forget to reset the quiescent current ("Idling Current") using VR31 as given on page 4 of the Service Manual after repairs are complete!

Chris Williams

« Last Edit: September 22, 2019, 08:25:16 pm by Chris56000 »
It's an enigma that's what it is!! This thing's not fixed because it doesn't want to be fixed!!

Offline shakalnokturn

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Re: Too much heat from Marantz PM4200/N2B
« Reply #2 on: September 23, 2019, 08:52:49 pm »
Though I can't be bothered to go and look for the schematic right now and can't be sure what's used in this model also beware of thermal compensation or bias voltage "double/triple" diodes and the quescient current trimpot itself, they are often causes of the current going too high.

Chris forgot to mention, if you haven't already done, check the solders there should be dozens that need reflowing if this amp hasn't been butchered before....

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