ANOTHER dead valvestate !

[Valvehead]

could be---there are 2 caps right after the bridge, c40/ c41 . i might have something close i can clip in temporarily

[Valvehead]

c40 41 are good. swapped them out and still have a huge short

[Enzo]

OK, we are spinning our wheels here.

If you disconnect the two hot wires from the transformer, we have no short, right? Connect them back to thge board - short. Now look on the schematic above the power transformer. There is an autotransformer for the tube voltage. Disconnect R114, which feeds it. Short or no?

Ther are only so many places the supply can load down. That autotransformer is one. ANother is the tube heater supply, look near the autotransformer for two resistors. Disconnect them. ANy help? All you need do is unsolder one end and lift it up off the board, you don;t need to completely remove the part. And the only remianing branch for current is through the main rectifier bridge. You could remove the bridge rectifier and power up. Of course the amp won;t work, but do you still have your "short?" Once we determine the branch that is causing trouble, we can determine just what in that branch is bad. SO we need to determine by elimination of it is the autotransformer, the heaters, or the main bridge power circuits that are affected.

I asked earlier if your output transistors were shorted to their heat sinks - missing insulating wafer behind them. I don;t recall getting an answer. SO with power off, measure resistance to ground from the center leg of each output transistor. This SHOULD give the same reading as the + and - terminals of the bridge rectifier, or in fact across the main filter caps. Are either of these main power supply rails shorted to ground?

Things that short do not ONLY short to ground,, your main V+ could be shorted to your main V-, and neither to ground. So check that. What do you see?


Have we determined that the VERY important TR9 is not OPEN?

[Valvehead]

sorry enzo, i know im a p.i.t.a .....

ok here we go...

lifted R114 - short

lifted 2 res. next to the auto trans. = short

i lifted the 3 chips off the heat sink to be sure they arent shorting to it. no change. and yes--they have the plastic wafers under them.

tr9 tests good--plus its new

checking +VE to -VE -- not shorted together


the +15v and -15v rail is not shorted together

and yes , when i unhook the power trans the short goes away.

[Enzo]

So your problem lies on the path with the main rectifeir bridge. Did we ever power up with the bridge not installed? That SHOULD be essentially the same as having the transformer wires off the board, but if there is some sort of short ON THE BOARD, then we'd still have it with the bridge removed. <aybe not very likely, but we need to eliminate anything.

I seem to recall 15v problems. Did we measure resistance from -15 to +15?

Is this a one sided board or a two sided board? In other words are there copper traces on both sides, or are they all on one side, and on the top are all the parts and bare wire jumpers where needed.

If this is a single sides board, we might be able to find jumper sires or other places we can disconnect the power supply circuit from the amp circuit. In other words we might leave the rectifier bridge and the main filtrs C61,62 but break the path from them over to the output transistors. Looks easy on paper, might not be easy on the board itself. If nothing else, we could remiove all the output transistors and fire it up, that is close to removing power from the amp .

[Valvehead]

FROM +15V TO -15V i get 20K .

the pcb is 1 sided

i tried removing C61 / 62 from the board -- still a short.

i need to call it a night enzo---ill try removing the bridge, and also cutting the traces after c61/62 tomorrow night.

THANK YOU for helping !!

[g1]

You keep referring to a short. Can you specify where you are measuring it or do you just mean the limiter bulb is bright?

[Enzo]

I am assuming he just means bright bulb.

[Valvehead]

yes--the limiter bulb in the ac line, is 75 watts and very BRIGHT when on

[Valvehead]

ok... i cut the trace from the bridge to TR8 = no short !

[g1]

Not trying to toot my own horn here, but more so the importance of re-reading through these threads for things we may have forgotten or missed. I try to re-read entire threads to refresh my memory if there is a time lapse.
Back in post #10 I said that TR8 was turning on hard and causing your bright bulb. So this is still the case. Being that all the transistors have been replaced, you need to check resistors, and check your work for solder bridges etc. Also that TR9 is properly isolated from the heatsink. Probably something in the bias circuit like a bad connection.
I'm pretty sure if you reconnect TR8 and put a short across TR9 you will have a dim bulb.

[Valvehead]

the 3 chips on the heatsink are still up in the air..... TR8/9/10 ....not touching the metal at all. I will get out my magnifier glasses tonight and go back over everything . All the resistors in the output area (if you look at the scem) , have been checked out. Plus the larger 3w/5w ones are new .

[Valvehead]

after another pcb inspection.... i didnt see anything wrong , solder , etc...

[J M Fahey]

the 3 chips on the heatsink are still up in the air..... TR8/9/10 ....not touching the metal at all.

That's a GREAT way to burn them.

All heatsinked parts are so for a reason.

[Jazz P Bass]

Just a thought.
Where are the voltage measurements?
This is not that complicated of an output stage.
I would go through , with the limiter on, & measure every transistor, Base, Emitter & Collector.
Do that, scan it & post it.