Denon DRA-550 - DC on speaker outputs

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Recently I bought this receiver from 1986, but didn't go well the first contact with it. All functions are ok, but no sound came from the speakers. After this, I opened the unit to find some causes for the problem. Both speaker fuses (4a) were blown, so I replaced both and nothing happened, but now there is a 50v dc voltage on both speaker outputs.

I've already checked the 4 transistors (2x 2SC2579 and 2x 2SA1104) but those are ok. Also measured the resistors and other small transitors in the amplifier section, but none of them were bad. No bad solder points or burn evidence are visible.

Any idea of what can cause this problem?

Thanks in advance.

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The receiver has conventional power amplifiers with an emitter-follower output stage. It's difficult to imagine then, how you could measure a full 50V DC at the output without at least one of the output stage transistors (which includes the driver transistors) being shorted, likely from collector to emitter. How are you testing them? Are you measuring them in-circuit by comparing voltages like Vbe, Vcb, Vce for example? Is the 50V measured wrt ground?

The power supply for this model should only be about +/-45V. Does it look like someone has being messing with this receiver and perhaps made some unwise substitutions before you? There seem to have been several types of power transistors fitted to this model but we can only assume they are all in the list shown on the schematic. Denon DRA-550, DRA-550 SM Schematic Diagram 1

You might start by checking that the power supply voltages at the main electrolytic caps is correct. This will at least provide a reference for nulling any reading errors that your meter might have.
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There are a lot of possibilities here ... are both plus and minus supplies present on the board? Is there a break in a wiring harness or connector? Is there a shorted capacitor? Are the driver transistors shorted? Work backwards from the output, checking as you go.
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Those 4 transistors I measured in circuit and I get consistent readings(0.5 ~ 0.6v from base to emmiter/collector and open line when inversed polarity) on all 4, but then in the emmiter-collector I measured 1.5 v or so on all, so I decide to get them out of the board to check better. And those readings from base to emmiter/collector were the same, but now open line on all emmiter/collector readings, so I guessed those are good.

Some resistors and one diode seemed to be bad in-circuit but when removed, were ok.

I measured this 50v dc with the ground in the chassis of the unit and on all speaker outputs.

This one is European model so is a 220v, and the schematic I use, is the same you post above, from the 120v US model. I'm not sure if there is any difference on the aplifier section between this 2 models... :confused:

The unit seemed to be untouched, so I don't think anybody try to fixed it.

I will check capacitors trying to find some faults.

Thanks again.
If the output's at the rails one possibility is the feedback loop is broken. Unfortunately many components are in the feedback loop and any one of them failing might do this. Bad solder joints and hairline cracks on the PCB can also have this effect.

I'd work from the input section forwards, checking voltages.
The front end of this amp (all the way to the VAS) runs off +/-15V from discrete sub-regs toward the lower right of the schematic -- TR526 and TR527. Their Collectors have fusible resistors, R584 & R587, 220R 1W -- famous for popping. If the -15V is down might give your symptoms.

Check those rails for overloads, too, before you have to replace the same resistor twice. (Or 3 or 4 or 5 times;))

Let's hope it isn't an open zener (not unusual on stuff this old) -- then you may have a 4558 and some other little stuff to replace.

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