Trying to fix a Marantz SR-2000 receiver

Hi all, I am trying to fix this receiver, I replaced the hybrid STK080G power amp for the right channel, after having checked the voltages on the pins. Now this power amp goes red hot when powered on. Any ideas what/how to check ?

Thanks

Francois

The driver ... the circuit that feeds the hybrid.

Thanks Charles, I checked the voltages(DC) on all pins on both hybrid power amps and they are the same. I swapped the input to the hybrid power amps (left and right channels) and I still have the same power amp getting very hot. Not sure what to check next.

Without speakers connected,of course?

Speaker connected (switch on or off for them), right channel power amp still very hot. The sound is good on both speakers but I cant leave it on for long, I am afraid to fry the power amp ($20.00)

If the sound is good, but the hybrid is frying, then I would suggest that it is hooting, maybe as high as RF. You need to get a 'scope on the output to check for this. There may be a Zobel network ( R & C in series ) on the output that was damaged by the previously faulty hybrid. Alternatively, there may be a defective decoupling cap on one of the bypass pins of the hybrid, or an open or high ESR supply decoupling cap, and this is allowing it to hoot, which may be what caused the demise of the original hybrid, and would ultimately do the same to the replacement, if left.

Arfa

'Hooting', you say. I suppose by that--and your description of the solution--you refer to high-frequency oscillation.

Hooting...that's a new one on me; but I like it.

jak

Hi all, after swapping a couple of components (caps, and 1 transistor) between the faulty right channel and the working left one) I found out (with a continuity tester) that pin 5 of the hybrid was floating (not properly soldered to the land pattern). After having corrected that, it is working fine now (maybe a little bit hotter than the left channel but not by much). I will check Arfa suggestions when I get a scope. For now music sound very well. Thanks all François

Oh, ok. Perhaps it's a funny British thing !! It's a term that was taught to me when I was an apprentice nearly 40 years ago, by my apprentice mentor, and I've always used it ...

Your assumption of its meaning Jak, is basically correct. It is a parasitic oscillation of a stage which may be, but is often not, detrimental to the basic operation of that stage. The OP's amp is a classic example, if that is indeed what the problem was - pins 3,4 and 5 of that hybrid are all up the front end, and typically have caps connected to them. The stage basically carried out its job of amplifying ' low freq ' music perfectly well, but was also outputting an additional signal either at an ultrasonic, or low HF frequency, probably at close to full power, depending on how good a bandwidth the output stage has - hence it gets very hot. I don't know if you remember the old Texas Instruments SN76003 / 013 / 023 series of power amp ICs. They had a sort of " fan " shaped hetsink bonded to them. Well those used to suffer from it badly, if the supply decoupling wasn't perfect.

I've also had genuine oscillators in mixers that have suffered from parasitic oscillations. Their primary function has been basically fine, except that every other radio in the vicinity is going bananas, due to the high amplitude 'hoot' that is also occuring.

Sorry if I've caused any confusion to anyone. I'll stick to ' contemporary ' terminology in the future ... !!

Arfa

I used a Studiomaster desk that just sounded like utter sh*t. I couldn't get a decent mix out of it to save my life, although the rest of the PA system was really excellent. I couldn't put my finger on it, but something was just not 'right'. I subbed in a cheap Sunn console, and the whole system came to life!

Turned out the Studiodisaster had parasitics (up into AM radio frequencies) in almost every stage.

Hooting is fine. I like it, but it must be a British thing...like 'valve'.

jak

Yes, that's the sort of thing. I've had similar where a high amplitude RF signal is being created in an audio stage, and rectified in the following stage due to its huge amplitude driving that stage to non linearity. The DC then produced at the base of the transistor, upsets the bias, and bingo, hefty distortion.

Arfa