NAD 3150 Repair experience? advice needed

[mbates14]

Does anyone have experience in the NAD 3150 repairs?

Someone brought me one of these to fix, the protection light is solid.

Found the left channel putting -46V DC on the outputs.

So, i did a quick static diode check of the outputs, drivers and etc.. and they all appear to be fine.

Found the schematic, and did some quick voltage measurements. the bases of both outputs have -46V on them. both bases of the driver transistors have -46 on them as well.

the -46VDC follows all the way back up to the heatsinked Q607 emitter lead. where it reads -46.5VDC

the base reads -46.7vdc. while its collector reads 51.8VDC POSITIVE voltage. Q605 looks to be darlington paired to the Q607 driver.

the base of Q605 reads 51.2vdc (varies depending on when i decide to turn the power on, but not by much).

Base of Q601 reads 51.4vdc and emitter reads 51.9vdc. the emitter is tied to the collector of the pre-amp transistor Q603 and the darlington configured driver, which was 51.2vdc.

the base of Q603 has the audio signal via scope. the emitter lead reads -2.7vdc and has NO signal. the good channel, i have a signal here at the emitter being the same signal at the base. unfortunately, this isnt the case in the bad channel.

So i removed Q601, Q605, and Q603 to be tested externally on DMM diode check. they are all fine, no reverse leakage, or cross conductance leakage between C-E.

Did an hFE test, the beta varied betwee 150 to 215 on the transistors.

Q607 and Q613, Q611 driver transistors statically checked ok out of circuit as well. but i could NOT check the hFE beta because it wouldnt physically fit in the meter's transistor socket.

I checked all the resistors and diodes inside the channel as well, and they all appear to be right on target. no opens, or out of tolerance resistors. pots are OK too.

I have never seen this type of amp circuit before. its awkward. im used to the differential style power amplifiers. this one is "different". But from looking at it, you would think that Q607 has to conduct to be able to "balance" the output, as Q607 appears to be the only way for positive voltage to get to the output/driver section. however the output/driver section seem to have full negative voltage. with no positive reinforcement.

Any ideas? It has me totally stumped, unless something is breaking down on full voltage.

[sakis]

that might be an option some transistors may check fine outside and brake down under voltage

start playing around transistors Q605 Q607 that is easy to replace or interchange from channel to chanel

double verify that rail voltage is proper ( since more than one exists )

regards sakis

[mbates14]

i managed to fix it... the cap was shorted right off the emitter of the input transistor. and it took out a resistor in the bias circuit.

replaced both, amp played perfect...