Brightness at test lamp very low. Can barely see filament.
Q7511
Base = .867 vdc
Collector = - .027 vdc
Emitter =1.01 vdc

Q7513
Base = - 0.00 vdc
Collector = - 2.288 vdc
Emitter = - 50.8 vdc

Q7512
Base = .634 vdc
Collector = - 2.288 vdc
Emitter = -1.04 vdc

Q7514
Base = - 50.9 vdc
Collector = - 2.288 vdc
Emitter = - 50.8 vdc

 

Measure the DC voltage at the bridge rectifier D1201:

pin-1 with respect to GND
pin-4 with respect to GND

It would appear that the bridge rectifier is bad.

 

I must be doing something wrong. I looked for your post, even logging out and back in several times. Just saw your post with time of 2:38 pm.

I measured the D1201

pin-1 with respect to GND =52.4 vdc
pin-4 with respect to GND =-52.2 vdc

Looks like the rectifier ok?

 

AC voltage between pin 2 and 3 =74.8 volts.

 

I measured the D1201

pin-1 with respect to GND =52.4 vdc
pin-4 with respect to GND =-52.2 vdc

Try to do some signal tracing. (I am going to round voltages to nearest integer.)
You should have these:
+B1 = 52V
-B1 = -52V
Q7511 Emitter = 52V
Q7512 Emitter = 52V
Q7513 Emitter = -52V
Q7514 Emitter = -52V

 

B1- = - 51 v
B1+ = .121 v

Q7511 Emitter = .121V
Q7512 Emitter = .121V
Q7513 Emitter = -51V
Q7514 Emitter = -51V

 

At the connector I get -52v for B1- and 52v for B1+. At the horizontal test ponits I get the readings in #106.

 

You have a discontinuity in the +B1 supply line.
Look for a bad solder joint at the jumper or a broken track.

 

Just found it. Bad solder joint where I lifted the B1+ jumper yesterday. Resoldered it.

Rechecked all emitter readings and B1 + and - readings as follows:

B1- = - 50 v
B1+ = 51 v

Q7511 Emitter = 50 V
Q7512 Emitter = 51 V
Q7513 Emitter = -50V
Q7514 Emitter = -50V

 

Good. Now do the voltage measurements at the power transistor terminals, B, C, E.
(Show all digits as reported on the DMM.)

This is the standard power on test you need to repeat.

LEFT === RIGHT
Q7511 = Q7512
Q7513 = Q7514

Report the brightness of the test lamp.

 

Very dim filament on bulb.

Q7511
Base = ..805
Collector = .404 v
Emitter = 49.0v

Q7513
Base = - .003 v
Collector = .045 v
Emitter = - 49.1 v

Q7512
Base = .742 v
Collector = .431 v
Emitter = 49.2 v

Q7514
Base = - 49.3
Collector = .437 v
Emitter = - 49.2 v

 

The voltages at the base are not right. We now have to look at the circuits driving the base.
We have to measure the voltages at the driver transistors. There are 4 for each channel for a total of 8.

LEFT === RIGHT
Q7503 = Q7504
Q7505 = Q7506
Q7507 = Q7508
Q7509 = Q7510

 

I found Q7503, Q7504, Q7509 and Q7510 on the board. Q7505, Q7506, Q7507 and Q7508 on foil side of board and were SOT-23s

LEFT RIGHT
Q7503 = Q7504
B= .865 v B=.823 v
C= -.776 v C=.133 v
E= -.896 v E=.129 v

Q7505 = Q7506
B= .891 v B=.180 v
C= .175 v C=.182 v
E= .160 v E=.000v


Q7507 = Q7508
B= .874 v B=-.892 v
C= -.124 v C=-.135 v
E= -.112 v E=-.124 v


Q7509 = Q7510
B= -.804 v B=-.813 v
C= -.112 v C=.002 v
E= -.121 v E=-.135 v

Collector and emitter readings on Q7503 were not stable bouncing around between -. 6 and -.9 v.

 

The format changed. I did not put these close together like this.

 

The format changed. I did not put these close together like this.

That's ok. The browser removes extra spaces.

 

Unplug the unit from AC mains.
Measure the following resistances in-circuit.
Measure with test leads in both directions.

LEFT === RIGHT
R7547 = R7548
R7549 = R7550

The expected value is 220Ω for all four.

 

These are really tiny on the foil side of the board. Just touched the negative lead of my multimeter to one lead and got a spark. Not sure what;s working now. These resistors are really smali ! I think my meter leads are even to big for the tiny spaces. Called my self being real careful. Well so much for that.

What is the possibility of finding a board like this somewhere? I would know where to start my search, since this unit is 90's vintage.

 

The resistors are SMD packages. Most meter probes would be fine enough for probing.
If you think that you have reached your limits then I will leave it up to you to decide when it is time to call it quits.

 

I'll let you know tomorrow. Let me think on it tonight. Thanks again for your help and patience. If decide to throw in the towel, I will try to find a repair facility somewhere locally here in southeast Michigan.

 

Was I supposed to have discharged capacitors as in a previous step before I started to measure resistance of the 4 resistors? I just unplugged the power from the 120 v source.
I guess I am not really ready to throw in the towel if you are willing to continue .Your time is important, and if you feel I have reached my experience limit, I can understand.

I have enjoyed working with you and getting the benefit of your knowledge. As I said before I am no EE and have a lot to learn. Thanks again for your time and patience.