Marantz 250 help needed

[dr*audio]

He did try a germanium diode and the voltage drop was too low to allow the amp to bias up. If I recall he replaced the outputs with On-Semis, and the difference in the new output devices could be why the germanium won't work. I say go with the 1N4003. If that doesn't work, there may be another problem, like a driver transistor not working like a transistor anymore, or an open resistor. Check the bias pot, too, in case it's open.

 

[ESTC]

I have a Marantz 240 Amp which has high DC (+58V) on one channel. Output transistors Q802/803/804/805 all seem OK. Looking for help to point me the right part of the component/ciruit to look at.

I have replaced some burnt parts of the output protection relay K301 and R557, C520, R529.

Thanks in advance

 

[dr*audio]

If you have properly tested the output transistors with a DMM, then it appears you are missing the negative drive, for the negative half of the amplifier. The first thing to check, though, is the regulated negative supply for the current source for the diff pair: check the voltage on both sides of R518. On one side you should have -58V, on the other, about -15V. If the -15V is low, replace CR502 or C504 or R518 could be open. If the -15V is ok, then you need to follow through the negative half of the amp; starting at the output with Q803 and Q805 and then work backwards toward the frontend, always looking to see if each transistor is biased on, and conducting current. Also, the output transistors share a 0.1 ohm resistor, R532. If that were open you would have no negative drive and a large positive offset.

 

[ESTC]

Marantz 240 Problem

Hello Dr.

Thanks for point me the right (-ve) direction. I did a bunch of mearsurements as suggested as below. It was tough that the component topology do not quite match my copy of the service manual.

R-518 reads -19.1/-59.3 which mimics R-517 which reads +19.3/+59.4. Therefore, the current source regulation seems OK.

Q-801 and Q-511 are tested OK with DMM.

There is a bunch of abnormal +58VDC on the -ve side of the board, including R-532, R-538, casing of Q507 and Q508. The later two are puzzling. Do they indicate that the Q508 is open or not turned on?

Transistor voltage measured:
B C E
Q 508 -57.6 +56.4 -58
Q 506 -58 -57.6 -58.5
Q 503 +24.6 -18.3 +19.2
Q 504 -17.5 -18.3 -18.3

Q 508 Collector voltage looks abnormal and it runs hotter relative to Q 507.

Appreciate further advice.

ESTC

 

[dr*audio]

Q507 is on the positive side, and if the collector is reading a high positive voltage it could mean there is a short on the positive side, or it could mean Q508 is not being turned on. Measure the voltages on Q508, Q506, Q503, Q504. Write them down; B=__C= __E=__
Put a piece of electrical tape on the meter probe so only the very tip is exposed, in case you slip you won't short something out and release the magic smoke...

 

[ESTC]

I have edited the previous message to show transistor voltage measurements. Hope there a silver bullet among them?

ESTC

 

[dr*audio]

Q 508 is clearly not being turned on.
Q503 should be turned on and it's not. the +58V on the output should be feeding back to the base of Q502 and turning it on, which should pull the voltage down on the collector of Q502. Measure the voltages on Q502, Q501 and Q520.
It is possible that Q504 might be shorted from Collector to Emitter. Check it with a DMM on the Diode setting, (power off) with the emitter or collector desoldered. It should read infinite from C-E. If it's good, don't forget to resolder the pin you disconnected before making any more measurements.

 

[EchoWars]

The 2N3584 (NPN) and the 2N6211 (PNP) can be put together as a makeshift complementary pair.

As far as the Toshiba specs, I'm not anywhere near my main system ATM, so what you can find through Google is pretty much the same as what I can presently do.