Trying to get a Yamaha M-80 amplifier going again!

bmanone

Active Member
I picked up Yamaha M-80 off Craigslist a couple of weeks ago in non-working condition. Got it on the workbench and found:
  • A blown 10amp fuse,
  • Transistor (TR114), resistor (470 ohm), capacitor (C160) and diode (D120) all gummed up with glue (replaced these parts)
  • Output transistor (TR137) shorted replaced all the output transistors in that channel (TR131, TR133, TR135 and TR135)
  • Driver transistor TR119 bad, so I replaced these transistors (and complementary PNPs) in both channels (TR119, TR120, TR125, and TR126)
  • Also replace all the electrolytic capacitors (except the 4 large filter caps on the power supply)
  • Replace two 0.047uf, 400v capacitors (C324 and C331) on the Power 1 board (caps were out of spec – measured 0.020uf) with 0.050uf, 630v capacitors.
Now we’re go for power up with a 60 watt light bulb in series, bulb is bright for a second or two, then dims to a low glow, five or six seconds later the amp go out of protection mode. I check for dc offset voltage (less than 10 mV), then hook up test speakers and MP3 player. The amp is singing and sounds good with no distortion or noise that I can hear. So I listen to it for 30 – 45 minutes and decide to go for power up without the light bulb in series. The fuse blow and the amp won’t power up. So I replace the fuse and put the light bulb back in series, the amp powers up just as before and plays again just like before. So what causing the short, is it the something on the Power 3 or 4 boards?
Thanks for reading and for any suggestions.
 
That and what did you do about idle current- you only mention DC offset? If the thing is a on a 60w bulb it is quite limited to how much current it can draw. On full mains it may be pulling more than it should.
 
It's fuse F301 (it not labeled on the board, but it has a red and white wire going to it). I believe the amplifier is an R model with a switch for multiple voltages (I have it set on 120V) The idle adjustment was at 0 mV(both channels), I was able to adjust both sides to 8mV (requirement per service manual is 10.0+/- 1mV). The funny thing is while I was adjusting the idle the light bulb was going from bright to dim and the amp was going in and out of protected mode and display was going on and off.
 
You can't adjust the idle current properly on a DBT with a 60w bulb, as soon as you put it onto mains it may draw that much more current- this is a big amp, adjust it on a 150 or 200w bulb first- set it to 5mv and try it on the mains. Could be what blew the fuse the first time. If the fuses survive the mains, dial it up to 10mv- you're staving it for current then blasting it on full current with the pots turned up doing it that way. I know yamaha's have a pretty sensitive protection circuit on them.
 
bktheking,

I did as you said, except the highest watt light bulb I had was a 100w. It seems to have worked, now the amp is working with out the DBT in series. I dialed it up to 9 mV and cycled the power switch, the fuse held:thmbsp: Now I need to reassemble the chassis and try again.

Thanks for your quick replies and help!
 
Avionic,
Thanks, I checked the service bulletin. My serial number is above 2401, so I didn't have to make the updates.
 
Pictures of my repair

I took pictures periodically during my repair work I thought you guys might want to see: Part 1
 

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More pictures part 2

I have higher resolution and more pictures if anyone needs them.
 

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Have to love a happy ending. Congratulations on getting your Yammie singing again! :thmbsp:
 
Meter Level Adjustment

Now that this amp is working well again, I'm trying to perform a Meter Level adjustment. The manual says to use VR501 and VR502 to adjust the L and R channel levels, no problem finding those on the Meter board. My problem is I'm not sure where to measure the output voltage "f=1KHz and V=34.5V with NO LOAD". Is the voltage measured at the speaker out terminals?:headscrat
Thanks!
 
After replacing C164 with the correct bipolar 47uf, 25v capacitor, I had to reset the idle adjustment back to 10 +/- 1mv (the capacitor replacement caused it to go high in both channels). So then I do the Auto-A operation confirmation, the voltage is suppose to increase to 130 +/- 13mv, mine only went 80mv in the right channel and 79mv in the left channel.:scratch2: Should I be concerned, is the manual value wrong?:headscrat The amplifier runs and sounds great to me now with Auto-A "on" or "off".
Thanks!
 
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