Problem with Monarch 8 Series 8000 amp

[mamboze]

Hi,
This is my first post to the forum. I've got a Monarch 8 Series 8000 amplifier. It's a circa 1976 model (I think) and it's recently developed a problem. After about 30 mins of use, a series of crackles come on in the left channel, some of them are loud and there is a kind of continuous low level noise (not really a hum, rather like white noise). It is not affected by the volume control. If the amp is turned off for a few hours and then turned on the "performance" repeats itself. But if the amp is turned on after a few minutes then the noise is immediately apparent.

I last had the amp serviced about 2 years ago in Australia, with complete PCB resoldering and general service. But I am now living in Thailand and good quality service for audio seems to be hard to come by here. So if I could find out what might be the problem, I would be happy to try a repair job myself. My skills here are limited to being able to solder plus some very basic knowledge of electronics.

I've attached the power amp part of the schematic. The main reason I would like to save this amp if possible is that it fits into my rather modest system of a Yamaha CD player, a Yamaha T-7 tuner and Yamaha NS 555 speakers very well. It's quite substantially built with shielding over the input circuitry and elsewhere.

I've had the covers off and cleaned the PCBs and checked for poor solder joints but couldn't see anything faulty. The amp does have what I think is a speaker protection device in each channel. This thing has two flat strips of metal with what look like contacts enclosed in a small glass envelope. Maybe some kind of bimetal switch which heats up and opens (hopefully) when any DC is present. I did think that this might be the cause of the noise. But I haven't done a channel swap to check that yet.

I would be grateful for any help that forum members might be able to offer.

regards

Roy

[Leolabs]

First,check the semiconductors,then go for the passive components.You may try to get voltage reading on both channel to see any differences between them.

[jaycee]

It might be worth replacing the electrolytic capacitors in the circuit as unless they were replaced at the last service, they will be very old and knackered.

[mamboze]

Thanks for the suggestions. I'll try voltage measurement first as it is the easiest to do. I know that channel comparison is the way to go to isolate the fault but I really could do with some advice on where to start first. I'm familiar with the physical layout of the amp but not the details of voltages etc. Any general suggestions on how to go would be very welcome.

[Leolabs]

First go for supply voltage,then output offset,then bias voltage and so on.

[anatech]

Hi Roy,
That doesn't look like it's too bad a little amp. Absolutely, fix it.

That "white noise" sounds like a transistor breaking down. Old carbon composition resistors could make this sound also.

So, I'd replace all the electrolytic caps if I were you. Take a picture or four before you start in case a component spins in your fingers. Most these caps have a polarity.

After this, allow the amp to begin making the noise. With a can of freeze spray, cool one transistor at a time. Wait at least 10 sec between each attempt to give the spray time to work. A little frost is enough, and that takes a second to appear after you stop spraying. Start with the small transistors first (Q302,304,306,308). Then move on to Q316,318 2 and 4. If you suspect a transistor, test it by warming it up (noise starts) and cooling it down (noise stops).

If you feel the breaker is causing the noise, play low level music, a tone is better. Tap the device to see if the sound cuts out.

-Chris

[destroyer X]

This way you may find the defective component.

regards,

Carlos

[destroyer X]

This way you may find the defective component.

Do it with the amplifier switched on...no signal in the input..and listen to detect the noise..... wait component to absorb the heat from the soldering iron...listen at the speakers searching for the damaged part.

Of course sensitive stages will amplify the AC noise captured by your soldering iron metal chassis....when testing those stages reduce the volume for some seconds...remove the soldering iron and turn the volume up again.

regards,

Carlos

[anatech]

Hi Carlos,
I do this as well, but you must be very careful not to overheat the part.

Touch with a soldering iron the components leads

No! You must hold near a non-conductive part of the casing. Touching the lead may (probably) upset normal circuit operation. Many irons also have ground referenced tips these days.

-Chris

[mamboze]

Hi,
It's been a fair while since my last post (work etc intervening) but I do have some progress to report.
I used the freeze spray, as suggested, and when I sprayed two transistors (Q302 and Q304 in the schematic and red outlined in the pic) on the power board, making sure they had frost on them, the crackling noise stopped instantly.

And interestingly, the noise has not returned even after about 24 hours of continuous use or being left on. I used the amp both with the cover on and off - no noise either way. I ran the amp biwired to the speakers at around 11 o/c volume, the heatsinks got quite warm but no noise reappeared. I thought that the freeze spray might have affected a dry solder joint, but I couldn't see any sign of that. And, in any case, when the amp was checked out a couple of years ago, the tech resoldered all the joints.

So I'm wondering what might be the next step. Should I replace the 2 transistors on each side. If so, are these transistors sitll available (or could somebody please give their equivalents).

From my very limited understanding of amps, if the transistors were replaced it would be necessary to adjust the DC offsets and quiescent currents. Would it be necessary to replace the trimpots with sealed ones, for example, following the recommedations here http://www.tnt-audio.com/clinica/bias_e.html

Any comments on all this would be welcome.