Installed capacitor backwards and it blew replaced it and no sound.

[petehall347]

what is written on q742 ?

 

[hogdaddy]

NEC
A985A
Q52S

 

[petehall347]

that one is BCE left to right legs down .
the 5.2 at collector should be negative .you wrote positive .

 

[hogdaddy]

Yes it's negative sorry.

 

[petehall347]

its not that then .
back to the drawing board so to speak

 

[hogdaddy]

A few days ago I accidentally hooked up the M-90 amp to the C-90 tape 2 jack instead of control amp jack and did hear some sound but volume had no effect.

The control input is right next to tape2.

 

[xlbee]

@hogdaddy
Have you tried all of the inputs to see if any produce sound?
If you have audio coming into CD or Tuner, do you get the same audio out of Tape 1 and Tape 2 (volume control will have no effect here)
Have you tried turning on and off the mute function?
Are the tape monitor switches off?

@petehall347 How do we trace an audio signal through the circuit with no scope? Be nice to know if there is audio on one side of relay RY409. Maybe we at least measure the voltage across the coil?

 

[petehall347]

i haven't got the manual yet .. i would first want to know if the relay clicks in or not .
if relay ok then look to see how the input switching works .

 

[hogdaddy]

I have tried CD and Tuner did not try TT or tape.
I have tried the mute switch many times.
Tape switches are off.
Where do I measure voltage across coil on relay?
The switching seems to work well at least the light do.

I tried out my turntable and no sound there either.

Use DMM check volts on 1 and 16? 12v?

 

[petehall347]

right across the diode sat next to it .

 

[hogdaddy]

I had a major setback while being careless I smoked the transformer after shorting it out on chassis.
Gonna swap it out with the spare. Wish me luck.
As much as I like this preamp, it's a pia to work on.

'

 

[hogdaddy]

After getting a better look at that board (I pulled it completely out) I can see clearly where it got warm previously as in before the transformer damage.
It looks to be concentrated around the transistors with the tall heat shields attached to them. Thinking I bumped them too many times and loosened the solder joints on the transistors.

Does it really need those heat shields? Some don't have them. Maybe cut them down shorter or not attach them to circuit board just to transistor. It's nearly impossible to solder them back in anyways. Maybe glue?

 

[quaddriver]

heat shields....heat sinks? may I see a pic?

 

[hogdaddy]

I guess heat sinks would be proper term.
I think the tall one was loose before I even worked on unit.

 

[xlbee]

@hogdaddy did you ever measure the voltage across the diode next to the relay?

 

[hogdaddy]

Yes it was either .5 or 5.2 I believe.

 

[xlbee]

Yeah, you would need to measure it in both directions. In one direction it should be less than a volt. The other way it should be around 12 volts. If it's not then that is why you have no audio.

 

[hogdaddy]

So just the diode would be bad in that case?

 

[xlbee]

No. It would be likely something with the relay driver circuit (Q719 and Q720). You could bypass the relay as a test but measure the voltages both ways across the diode first

 

[hogdaddy]

I finally figured out how to make the schematic pictured by cutting edges and joining pages with scotch tape.
Still working on getting unit back together.