• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Yamaha M-80 Protection/Restoration

Before setting the idling current, you’ll need to put Tr143 and its peripheral resistor and capacitor back in … just a reminder.
 
Hahah yeah i threw them back in last night before i finished up otherwise i probably would forget. I also changed that cap while i was in there. Used a 100v 10uf i had.
 
So the resistor reads 10k on the color code so i’m going to assume it’s correct. I was easily able to set the bias to 15.5v HOWEVER, i do not get a bias increase of 31mv when going into class A. Hopefully this is related to my problem. I see the class A area on the main amp board. Should i start checking transistors there? It occurs on both sides. Right channel goes up 5mv left goes up 2mv when in class A.
 
RyeGuy859 said:
RyeGuy859 said:
. I was easily able to set the bias to 15.5v
Click to expand...
I hope you meant millivolts...
Click to expand...


RyeGuy859 said:
i do not get a bias increase of 31mv when going into class A. Hopefully this is related to my problem. I see the class A area on the main amp board. Should i start checking transistors there? It occurs on both sides. Right channel goes up 5mv left goes up 2mv when in class A.
Click to expand...
Check all 8 of the emitter resistors & base resistors preferably out of circuit..
 
I wouldn’t worry about Class A bias until the amp is out of protection in normal Class B operation. Do any of the voltages look different after setting the idle current?
 
yes i meant mv lol. I was hoping the Class A issues could be tied with the protect.

Voltages seem the same TR143 has E 89v C -.98 B 89.1v so roughly the same as before
 
Sparkplug said:
I wouldn’t worry about Class A bias until the amp is out of protection in normal Class B operation. Do any of the voltages look different after setting the idle current?
Click to expand...
I overlooked that minor detail.. opps ..

I've been slinging solder at and M-85 and a C-70...
 
+L.B voltage has to wait a second.

I pulled all 8 emitter resistors thought 2 tested short put them aside. Installed the rest, tested them again. No short. Put them in and now my dbt lights up bright.

So i believe At least one of my emitter resistors is shorted, but where it was it wasn’t causing a direct short. After i put it back in (in a different spot) it is now a direct short.

I’ll replace all 8 just to be sure. Sometimes my leads measure 2ohm sometimes they measure 0ohm. i should probably get a nicer DMM
 
Last edited:
RyeGuy859 said:
So i believe At least one of my emitter resistors is shorted
Click to expand...
Unlikely... The resistors are 0.22 Ω most all digital multimeters will read around .7 to .9 Ω. Subtracting your test lead resistance will get you closer to 0.22Ω
 
RyeGuy859 said:
Sometimes my leads measure 2ohm sometimes they measure 0ohm. i should probably get a nicer DMM
Click to expand...
Usually when emitters go bad they burn " open ".. Your test leads probe tips may just be dirty.
 
Last edited:
Mine reads .4/.3 on them all except one that reads .2/.1 then I touch my leads and Measure .1

Not sure why else I would have a direct short after reinstalling them. No solder bridges
 
Change them all, also make sure there’s no short with them removed somewhere else along the traces.

It wouldn’t hurt to get a nice DMM either, everything relies on accurate data.
 
After I pulled one of the pairs the short went away.

This is what’s strange to me about my readings
 

Attachments

  • 914DCB50-3E7E-4D6A-A162-574EE62F3411.jpeg
    914DCB50-3E7E-4D6A-A162-574EE62F3411.jpeg
    199.3 KB · Views: 3
  • 85BDE094-98BF-444E-A9AE-0AB50A2D6AE5.jpeg
    85BDE094-98BF-444E-A9AE-0AB50A2D6AE5.jpeg
    196.3 KB · Views: 3
  • E2FABC29-9FE8-4C77-A58F-A44923F24D4D.jpeg
    E2FABC29-9FE8-4C77-A58F-A44923F24D4D.jpeg
    187.5 KB · Views: 3
To determine a short, I’d use the buzzer function. To get that, in the same dial position, press the function key twice. The DM will beep if you have a short …
 
Sparkplug said:
To determine a short, I’d use the buzzer function. To get that, in the same dial position, press the function key twice. The DM will beep if you have a short …
Click to expand...
sadly my dmm buzzes with anything under 1ohm so they all read short that way :thumbsdown:
 
You must log in or register to reply here.
Back
Top Bottom