Randall short

[Valvehead]

Have a rh100 that has a big short to ground somewhere.

i think the RG75 scem is the same amp

Initially I found D3 and R19 were bad , replaced them

i Had some transistors here so i replaced Q12 thru Q15

still a big short somewhere but i cant find the bad guy...

C14 and C15 positives are going right to ground

Can someone give me some starting points ?

[DrGonz78]

Well C15 positive side of that cap should be to ground, but yes C14 Positive side should not be at ground potential. I would guess first that C14 is shorted out completely and remove the wiring to confirm it. Also, suspect C11 right there next to it in parallel and that one could be shorted. That is where I would start. Also, I hope you are only applying power to this amp via light bulb limiter or variac(one with ammeter).

Edit: Next in line to check would be the insulation on Q14, mica insulators might be damaged shorting to ground. That is a guess. Or even Q12's case is the collector and the mica is damaged shorting the collector to ground.

[Valvehead]

Q12 and Q14 are brand new

yes==i have a bulb limiter on it

i will check the other stuff

[Valvehead]

i pulled C11 , c14 , c15 out of the board...also pulled the output transistors off the heat sink (Q12-15) .....still a short

[DrGonz78]

I don't see anywhere on that schematic where the transformer feeds into a circuit for the lower preamp power voltages, only the main power rails. My thought there is that preamp supply could have the short to ground, but the schematic is very poorly drawn. I would disconnect the connection that feeds the rectifier portion of the circuit and turn it on with the light bulb limiter connected to see if it is pulling current or if the bulb goes dim.

[jmaf]

First things first, D10 through D13 are your main suspects.

[Valvehead]

ill check D10-D13

yes if i pull the 2 main power wires the short is gone

[DrGonz78]

By looking at the schematic that is posted I don't see how it is possible for D10-D13 to be shorted to ground. Not saying that is not what is happening here, but just that the schematic is at best questionable.

[jmaf]

ill check D10-D13

yes if i pull the 2 main power wires the short is gone

Test the transformer on its own using a current limiter on the primary. High power solid state PT secondaries are not high resistance, that may be skewing your DCR reading.

@DrGonz78 I went by the schem OP posted. If that section of the PS is shorted first thing I'd check after the transformer is the rectifier bridge.

[Valvehead]

there is no short with the trans sec. un hooked

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.

i pulled C10 out---still has a short.

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

[jmaf]

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.

Look for solder overflow touching neighboring contacts or something nasty going on under the rectifier. If the short goes away when you remove the rectifiers that's partly where you need to focus. Make sure the PT secondary isn't shunting your reading.

[Valvehead]

Look for solder overflow touching neighboring contacts or something nasty going on under the rectifier. If the short goes away when you remove the rectifiers that's partly where you need to focus. Make sure the PT secondary isn't shunting your reading.

so test with the secondary un plugged from pcb ?

[The Dude]

Is there a center tap on the transformer secondary winding that is connected to P1 and P2 that is not shown on the schematic?

[Valvehead]

Yes 1 black wire goes to P3 GND ...center tap i assume

the 2 blue wires go to P1 / P2

[The Dude]

......i pulled D10-D13 out of circuit---the short went away......

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