[DaleHCook]

What make and model of power supply are you using? Is it regulated? Have you tried measuring the current drawn by the radio?

[462ron]

Hi R3, just glancing over the schematic,if C1 and C2 are good and they appear to be by the voltage you are getting on TP 18&19, then I would check X6, the output transistor next, the big one probably mounted on a heat sink. It would appear that if it were shorted, it’s the only transistor that would pull enough current to melt wires as it only has a .68 ohm resistor on the emitter and the output transformer on the collectors to limit current draw, so if shorted would have the ability to put a heavy load on the 12.6 volt supply. The other transistors appear to have a decent amount of resistance in their circuits. Not saying you shouldn’t test those but X6 would be my first one to test and they are known to short.

Ron

[morzh]

Is your power supply adequate? Perhaps it cannot source the current required.
Or...bad transistors.

[DaleHCook]

Perhaps I am missing something - which volume (AR-28 or AR-33) has the 6TPZ?

[R3Sons]

Thank you all so much. First the power supply: "KORAD KA3005P - Programmable Precision Variable Adjustable 30V, 5A DC Linear Power Supply Digital Regulated Lab Grade". I've done a lot of things with this power supply, electroplating, battery charging, prototyping projects. Always been very reliable. I thought it might be the problem, so I tested it on a few things with a lot of draw. Rock steady. I limited the draw to .5A. When I raise the limit the radios will draw much more. Not instantly, but gradually.

Thank you ron. I can't fit this into my transistor tester, so here's the multimeter diode test results:
B-E OL
E-B 0.14
B-C OL
C-B 0.13
E-C 2.6
C-E 2.8

Dale, I will try to visually confirm which one.

Thanks again.

[R3Sons]

Here's the readings from the AR14 (No markings on the one from the 5TPO) on the 6TPZ:
B-E OL
E-B .14
B-C 2.9
C-B .13
E-C .15
C-E 2.9

[morzh]

1. Have you tested out of circuit?
2. If yes, any transistor not showing E-C as OL is bad.

From your measurements, all your transistors you showed heea, are bad.

Best way to test:

analog meter. Set on Ohms x1.
N-P-N transistor:
Red is plus, black is minus.
EB - RED-BLK - open, no movement at all
EB-BLK-RED - conducts well.
CB - RED-BLK - open, no movement at all
CB - BLK-RED - conducts well.
EC, CE (colors don't matter, but test both ways) - open, no movement at all.

[R3Sons]

Yes, I tested them out of the circuits. I posted my measurements where the first letter was red and the second was black, so B-E would be B red E black. I should have mentioned that. This doesn't look good then. I removed X1 and tested it on my transistor tester and it seems ok. Should I remove and test the rest, or try testing the circuits without x6?

[morzh]

I would test all, but the X6 is the main suspect.

The output is the autotransformer, and it also could potentially have suffered due to being exposed to large current.

Now. When you have X6 removed, test for the current draw again; see if the radio now stops drawing excessive current from your power supply.

[R3Sons]

X6 is out. Only pulling 3mA now, 220mA with the light on.
Test point 18 is 12.2
Test point 19 is 12.06

[morzh]

Well...taht's a start.

[R3Sons]

Does anybody have a line on replacement resistors?

[radiorich]

R3Sons,
I buy my resistors from either digi key or mouser !

Sincerely Richard

[R3Sons]

Thank you Richard.

[462ron]

RS3, that output transistor X6…AR14, can be replaced with a GE-3 or ECG-121, probably others too.

Ron