Help Bring Long Dead M-80 Back to Life

[Semanon]

I have a Yamaha M-80 that I received in non-working conditions a couple years ago. It has not been operated in at least 15 years. It was pretty filthy and in poor condition.

• The DSBG had damaged parts on both channels.
• The left output transistors were shorted.
• Several of the 5W emitter resistors were shorted or
• The smaller emitter resistors on the APS were burnt.
• Several Zener diodes were shorted on the left channel
• A few small signal transistors were damaged.
• The meter board was a total wreck, with every semiconductor destroyed (some with legs burnt off), resistors fried
• Something had been spilled inside on the power supply board and filter caps.

it's been a lot of start and stop work, and it spent a lot of time on the shelf. However, after working through many of the issues and cleaning up the glue, dust, residue, spills, and other foreign objects, and replacing many components I'm finally at a point where it has been powered up with 2W, 47 ohm resistors in place of the output transistors.

However, the amp has a problem I need help with. The power supply voltages are off on the left channel which results in improper voltages. Here are my voltage readings:


Everything looks OK except for -LP. At -92 volts looks like the 33,000/56 cap might be shorted. However, if that was the case, I would expect -LB and -RP to be at -92 volts, too, since they all share the same bus. I pulled the blue -LP wires from the main amp board and the left channel APS and both were at -42 volts

The voltage at the resistors soldered to the Base & Emitter pads (in place of output transistors) are:


I triple checked pinouts and polarity for every device I’ve installed and looked carefully for broken traces, solder bridges, cold joints and solder joint cracks. I found several and repaired them, but still have the same issue.

Any help on what to do or check next will be welcome.

 

[Semanon]

Some pix of its original condition.


As found. No display.


Lots of Dirt


Something spilled


Very crusty


DSBG Damage


One side cleaned. Busbar corroded.


Debris field. Note legs eaten off transistor and capacitor.


Broken C-speaker connection plate

 

[avionic]

The left output transistors were shorted.

All 4 ?

 

[avionic]

Reflow all transistor solder connections that are not fastened to the heatsink. On the left channel APS board

Did you test the 4 SANKEN MT-200 APS transistors ?

 

[Arc deTriamp]

I have a Yamaha M-80 that I received in non-working conditions a couple years ago. It has not been operated in at least 15 years. It was pretty filthy and in poor condition.

• The DSBG had damaged parts on both channels.
• The left output transistors were shorted.
• Several of the 5W emitter resistors were shorted or
• The smaller emitter resistors on the APS were burnt.
• Several Zener diodes were shorted on the left channel
• A few small signal transistors were damaged.
• The meter board was a total wreck, with every semiconductor destroyed (some with legs burnt off), resistors fried
• Something had been spilled inside on the power supply board and filter caps.

it's been a lot of start and stop work, and it spent a lot of time on the shelf. However, after working through many of the issues and cleaning up the glue, dust, residue, spills, and other foreign objects, and replacing many components I'm finally at a point where it has been powered up with 2W, 47 ohm resistors in place of the output transistors.

However, the amp has a problem I need help with. The power supply voltages are off on the left channel which results in improper voltages. Here are my voltage readings:

View attachment 2374693
Everything looks OK except for -LP. At -92 volts looks like the 33,000/56 cap might be shorted. However, if that was the case, I would expect -LB and -RP to be at -92 volts, too, since they all share the same bus. I pulled the blue -LP wires from the main amp board and the left channel APS and both were at -42 volts

The voltage at the resistors soldered to the Base & Emitter pads (in place of output transistors) are:
View attachment 2374694

I triple checked pinouts and polarity for every device I’ve installed and looked carefully for broken traces, solder bridges, cold joints and solder joint cracks. I found several and repaired them, but still have the same issue.

Any help on what to do or check next will be welcome.

Take a close look at the left APS board. The APS board circuitry regulates -LP (and +LP). It uses the voltage at the output transistor base resistors (fed through R339 and 341) as references and maintains +/-LP a bit larger than those voltages, but never smaller than +/-LB. TR305, 307, 309 and 315 and components in their vicinity are suspect.

 

[Semanon]

All 4 ?

Thanks, Avionic.
Yes, all 4. Plus 2 of the 5-watt emitter resistors.

Reflow all transistor solder connections that are not fastened to the heatsink. On the left channel APS board

Did you test the 4 SANKEN MT-200 APS transistors ?

Will reflow. TR311 (2SC3298) and a TR309 (2SA1145) were shorted. I replaced them with MJE15030 and MJE15031.

Tested all 8 on the APS, no shorts.

 

[Semanon]

Take a close look at the left APS board. The APS board circuitry regulates -LP (and +LP). It uses the voltage at the output transistor base resistors (fed through R339 and 341) as references and maintains +/-LP a bit larger than those voltages, but never smaller than +/-LB. TR305, 307, 309 and 315 and components in their vicinity are suspect.

Appreciate the input, Arc.

I'll measure voltages at the four non-heat sink transistors and post them.

 

[Asmodeus2112]

Cool, good luck, looks like a fun project!

 

[Semanon]

I checked over the board and reflowed the joints with no difference in the voltages. Here is what I measured at each of the transistors on the APS boards. I've included the right channel transistors for comparison



The yellow cells don't come close to the expected voltages.

Just for completeness, I also measured the other 4 transistors



My next step is to measure the voltages per the schematic.

 

[Semanon]

I pulled the entire left side heat sink to do a full check again. Found 12 volt Zener D307 was shorted. Replaced it with little effect on the voltages, Just moved up a few hundred millivolts. Everything looks like it is installed correctly with no solder bridges or incorrectly installed components on the APS board.

 

[Semanon]

Cool, good luck, looks like a fun project!

Parts availability has made this less fun. The meter board was fully blown up.

Finding the 2SK68A for the meter circuit proved difficult. $7 each put the cost to replace them at almost half what I paid for the unit. Still have not sourced the TA7318P power meter driver, because it seems it’s only available from China from the auction site. I bought the LED drivers from my local electronics shop during the last week before they closed permanently.

It has spent many months on the shelf, and I’ve thought about selling it as non-working, mostly because I do not need another amp. But there is something about this one, and I’m so close….

 

[avionic]

TR309 , 315 and D309 are suspect.

 

[avionic]

Parts availability has made this less fun. The meter board was fully blown up.
View attachment 2375987 View attachment 2375988
Finding the 2SK68A for the meter circuit proved difficult. $7 each put the cost to replace them at almost half what I paid for the unit. Still have not sourced the TA7318P power meter driver, because it seems it’s only available from China from the auction site. I bought the LED drivers from my local electronics shop during the last week before they closed permanently.

It has spent many months on the shelf, and I’ve thought about selling it as non-working, mostly because I do not need another amp. But there is something about this one, and I’m so close….

There are probably a handful of bad LED's as well.

 

[onwardjames]

In for the ride. Should be fun!

 

[Semanon]

It hasn’t been all dreary parts sourcing, though. On the meter board 4 LEDs were dead. Per advice from @avionic, I moved good, existing ones from the top positions to positions further down. I then installed Kingsbright WP424SRDT. They were too bright. I could have left them that way, because if you are running your amp at those levels, a visual indicator might be helpful.


But instead I varied the current and found that they match pretty well when the new LEDs are driven at 4.5 ma. Color match is quite close.

Note the missing JFETs to the right of the meter. I hope I’ve solved that one, too.

EDIT: The second photo really does not do justice to the difference in brightness.

 

[avionic]

Is TR170 still good ?

 

[Semanon]

Checked TR309, RT 315, and D309. All good. Checked out of circuit with diode test and component tested. With those still out of the circuit I'm testing the traces to see if they match the schematic.

TR170 tests good. Took me a while to find it. I have not spent much time in that area of the amp.

 

[avionic]

D309

D309 is a zener. How did you test the zener voltage ?

 

[Semanon]

I measured the voltage when it was in the active circuit. It shows a 12 volt drop across the zener.

The voltages are not right, but what I've measured (and recorded)

Is there a better way?



I gotta buy a manual from Norman.

 

[avionic]

I gotta buy a manual from Norman.

If you do.. get the M-85 manual.. It would be closer than the M-80..