Smoking resistor

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Hi,

I'm having trouble with my old Marantz pm-42 amplifier that I'm hoping to get some help with.
For the last while I have been getting distortion in the right channel and decided to see if I could fix the problem myself - getting this serviced would probably cost more that the amp is worth.

Anyway, having ruled out input or speaker issues, I determined that the issue is in the amp itself and opened it up.

I checked the output transistors and driver trans with a multimeter and the readings seemed consistent on both channels.
I couldn't see anything obvious like burned components or cracked solder joints, so started by reseating all the ribbon cables in case there was a loose connection.

When I put the amp back on, it started smoking from the right sided heatsink.

I reseated the cables again and everything seemed ok when I switched it back on but then a 330 ohm resister started to burn.

I've checked the board and I've noticed one of the output transistors on the left channel has a cracked solder joint on the collector leg - presumably, this happened when I reseated the ribbon cable.

I've ordered some new resistors and plan to re-solder the transistor but understandably I am apprehensive about switching it back on.


Here's a pic of the inside, I have circled the burned resistor (R764) and the transistor with a bad joint.
AMP — imgbb.com


The service manual can be found here
https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Marantz-PM-42-Service-Manual.pdf


Any help anyone can offer would be much appreciated.

Paul
 
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When working on this you should use a DBT (dim bulb tester) which can avoid major burnups and semiconductors from going pop.

The transistor you circled is on one channel, and the resistor is on the other. The transistor is used as a Vbe multiplier (it sets the bias current) and any problems there could cause failure of the output transistors in that channel.
 
If R764 smoked then you have excessive current in the LEFT hand channel. The transistor you have circled is in the right hand channel.

It's likely that you have damaged output transistors and/or driver transistors in the left channel now. How did you test the transistors ? They need to be tested out of circuit to get any meaningful reading.
 
Q751 that you circled is in the LH side of the schematic. Follow from J701 (Marked L, LG, R, RG) to help identify sides.

I would focus on the components in the area on the attached image. Something is likely allowing a short to ground allowing more than 1w to go through R764.

Components are best out of circuit when testing - Diodes, resistors etc can be tested with 1 leg lifted.

Good luck!
 

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Quick update.
I replaced R764 and tested remaining resistors and transisters and all appeared ok.
Fixed solder cracks on the D1508 transisters on both channels.

I didn't adjust bias settings as I haven't replaced any output trans and don't trust the accuracy of my multimeter.
Connected to DBT and switched amp on. Bulb flashed briefly and then went dim.
Left amp on for about 10 minutes with no problems.
Measured dc voltage on speaker terminals, got .7mV on left channel and .1mV on right.

Measured dc voltage bias on left channel (good channel), reading fluctuated between 1 - 12mV
Right channel gave similar readings.

Would it be ok to try speakers now?
Thanks
 
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That sounds promising. If you keep the DBT in circuit when testing with music then be aware that the supplies will collapse very quickly as you turn the volume up... so just play it very low at first.

If all is good then link the bulb out and test on full mains.
 
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Its normal for the bulb to glow because the amp is pulling more current as it delivers real power to the speakers.

You've ruled out speakers by swapping them around. The only 'common' issue that crops up generally is tarnished speaker relay contacts but typically this causes distortion at low volume levels and temporarily clears as you crank the level up. Worth checking but I don't think that will be the problem here.

I would check very carefully that the DC conditions really are correct around the output stage because a failed component there could still leave the amp working. Checking that each pair of drivers and pre-drivers is conducting correctly would be the first step. Each B-E junction should have approx. 650mv across it and each transistor pair working back from the output transistors should see a progressive rise in the voltage between their bases.

Example:

1.3 volts across R763

2.6 volts across R761

3.9 volts across C751

A scope check would be the proper course of action really in a case like this.
 
Thanks Mooly, I'll check the relay today.

Testing the voltages when the device is live worries me if I'm honest - I wouldn't feel comfortable at all.
If it's not the relay I might have to call it a day and accept the distortion.

Tell me, do most dodgy transistors overheat?
I suppose I could touch around the board to see if there are any obvious hot ones.
 
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Most (but not all as it depends on the circuit and failure mode) of failed transistors actually run cool/cold because they tend fail to a low resistance state.

All the amplifier circuitry is isolated from the mains voltage via the mains transformer although DC voltages of - or + 40 volts would be felt under certain conditions, and particularly if you had wet hands/fingers that completed a conductive path to elsewhere.

Holding isolated meter leads, one in each hand, is absolutely fine, and the golden rule (to make a habit) is to work with only one hand if you are actually touching stuff in the amp with your fingers while its on. Keep the other hand out of the way completely.

Touch 40 volts with a finger and you won't feel anything, touch it with a damp finger while grabbing hold of the chassis with the other hand, or while standing on wet concrete in wet socks you will definitely feel it... and we've all done it ;) and you learn not to do it again :D

The real danger is having current flow across your chest and that is why the one hand rule is so important to get into as it develops a lifelong habit to keep you safe.
 
Think I may have found the problem.

Just checked R760 which is a 3 legged resistor marked RGC33 0.18ohm KX2 Noble.
Got a reading of 0.4 ohm on legs 1-2 and OL on 2-3. No continuity between 2-3.

On the good channel the same resistor gives a reading of 0.4 on both legs which seems a little low, should I be getting 0.9? Getting continuity through all legs
 
The resistor has failed open circuit, probably when the original transistors failed. You will need to replace it.

You can simply get two 0.18 ohm resistors rated 3W (e.g. Farnell order code 1903853) and join two of the legs together. This will give you the same arrangement as the original device.
 
Thanks lads

I've checked R766 and R774 and got 2.4ohms so that looks good.

Removed bad dual resistor and got following readings on transistor checks.
Checked all output transistors, b-c and b-e and got a reading of 500 or more from base to collector, base to emitter

Now tested transistors from C-E (firstly Black probe on C, Red on E and then reversed them)
A1264
C-E got a reading that starts around 560 and keeps going up.
Similar result on good channel.

Reversed leads and got instant OL on bad channel but stable reading of 994 on good channel.

D1508
C-E = 1874, reversed = 1785
Similar reading on good channel 1880, 1540.

C3181
C-E = instant OL, reversed = starts at 600 and continues to climb.

Good channel C-E = 989, reversed = starts at 600 and continues to climb.

What this means I've no idea:confused:
 
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