Randall short

[jmaf]

so test with the secondary un plugged from pcb ?

Yup.

Take it off the PCB and test the transformer with a current limiter bulb on the primary. Turn it on, test the secondaries for proper AC voltages. Does the bulb light up? It shouldn't. If it lights up there's something wrong on the PT and you've isolated part of the problem.

[Valvehead]

Yup.

Take it off the PCB and test the transformer with a current limiter bulb on the primary. Turn it on, test the secondaries for proper AC voltages. Does the bulb light up? It shouldn't. If it lights up there's something wrong on the PT and you've isolated part of the problem.

the trans is fine--no short

[Valvehead]

When you say "the short went away", you measured with a meter or the bulb limiter just went dim?

bulb dim

[The Dude]

When you pulled the diodes, you also removed the path (V+ & V-) to the output transistors, so the bulb would be dim even with good diodes. My bet is that the short was still there and you didn't measure it. My bet is also shorted transistors, missing isolation washers, etc., especially given your readings in post #10. Something is funky in the output section.

Edit: Another thought: Also verify that outputs are in the correct positions. It's a complimentary pair. Make sure Q14 is 2SA1941 and Q15 is 2SC5198. If they were installed in the wrong positions, they would have blown quickly.

[Randall]

Hey, who are you calling short?

[g1]

Just because the bulb is bright does not mean there is a short. An output transistor turning on hard will make the bulb bright. In such a case, you can do all the resistance checks you want but you may not find a short.
Talking about a short here may just lead to confusion and mis-diagnosis.
Do you have the speaker or load connected, and does the bulb go dim if you disconnect the load/speaker?

Edit: Of course, anywhere you measure 0 ohms with your meter it is fine to call that a short.
But don't call a bright bulb a short, it can be caused by other things and will just confuse us.

[The Dude]

g1,

FYI and the way I read it: In post #10 the readings indicate short(s). Presumably continuity tests, but I could very well be wrong.

[g1]

I kind of assumed that too, but am not so sure now. So I figured it would be best to ask for more specific language.
I had figured the title of the post referred to a resistance measurement, but I don't think that was the case.

[DrGonz78]

C14 and C15 positives are going right to ground

This would definitely imply that resistance measurements had been made... Also, C15 positive end is Ground. So we need confirmation how that C14 shorted to ground was measured. Also, here is a better schematic that might be very similar to the RG75B but you will need to change the component names to match function. For example, ZD3 is now D14 on this example of the circuit. This one has way more detail and is just drawn better IMHO.

EDIT:

there is no short with the trans sec. un hooked

You later did even more testing of this to prove the PT is not shorted out. So PT is not the problem.

i pulled D10-D13 out of circuit---the short went away , but all 4 test good on diode test and are not shorted. I put them back in.

So the short is after the rectifier section.

i measured the legs of Q12 /13/14/15 to ground

Q12 all 3 go to ground
Q13 2 legs
Q14 2 legs
Q15 all 3 legs

What does Q12 and Q15 measure out of circuit?

[jmaf]

Make sure those caps aren't charged, I don't see leak resistors on the RH100 schem.

A few volts on the PS caps will make your ohmeter go to 11

[g1]

Thanks for clarifying those readings Dr. Gonz.
I'm still a bit confused about some of the "shorts" so hopefully we can always refer to bright bulb as "over-current" rather than short.
If C14 plus end reads short to ground, I don't see how disconnecting the PT secondary can remove that short (when diodes tested good).
The issue of something changing when the PT is disconnected should be an aid to troubleshooting, but not if it's just that the bulb goes dim.

[Valvehead]

I kind of assumed that too, but am not so sure now. So I figured it would be best to ask for more specific language.
I had figured the title of the post referred to a resistance measurement, but I don't think that was the case.

im sorry ..

when i was powering up , the bulb went super bright and a very loud hum out of the speaker

and the V+ rail is going right to ground measuring resistance

[Valvehead]

I can pull Q12 - Q15 and measure

would it help to pull those 1 at a time and power up, seeing if the bulb lights up ?

the 2 big caps , C19., c20 (3300uf) are still out of the pcb , i didnt put them back in

[52 Bill]

I can pull Q12 - Q15 and measure

would it help to pull those 1 at a time and power up, seeing if the bulb lights up ?

the 2 big caps , C19., c20 (3300uf) are still out of the pcb , i didnt put them back in

I know that you are trying to approach this repair in the correct manner, but please do not try to use the limiter as a measurement/testing device. It's only there to save you from wasting fuses and burning up new parts when you miss something that is still wrong with the amp.

The amp will not work without the main filter caps. If they tested fine, put them back in, being sure to have the polarity correct.

If in fact both Q12 and Q15 read all three leads to ground, then there is a problem with the transistors or the circuitry around them or both. Test each transistor in, and or out of circuit the way that you should test a transistor, by reading from leg to leg to see if any have zero or near zero readings between legs.

If your meter has a diode test function, use it. You should test all of the transistors and diodes in the power amp. If nothing else, it will help teach you how a transistor tests in circuit and how to spot parallel circuit paths that can cause incorrect readings.

Look at Q14 and Q15. The Collectors of Q14 and Q15 are connected to the speaker output by way of 0.22 ohm resistors. If you have a speaker connected to the amp, then there is a low ohm resistance from the Collectors to ground via the speaker circuit. If you read resistance from the Collectors to ground you will get a low resistance reading. As long as the transistors are okay the power supply voltages will not reach the speaker output bus. If one of these transistors is shorted Collector to Emitter, then there is an almost direct connection from the power supply to ground through the speaker.

So, having a speaker plugged into the amp right now is not going to help, so keep it unplugged until the amp is turning on with no voltage on the speaker output.

[Valvehead]

ok thanks Bill-

caps back in , speaker unplugged

i had time to test a little today, i pulled Q15 out , it tests 0.8 - 1 ohm on all legs to legs , on diode test all legs/all ways test 0v

i had another new 5198 so i installed it

still have a problem and R18 gets real hot--real fast , if thats a clue

i will pull Q12 later to test and continue on