Having trouble with high voltage fuses on Laney chassis rebuild

[amptweeker]

Still blowing even the slo blo 1A

So the schematic shows a 500mAT fuse, so slo-blo it is.

I tried at lunch time today first I got brave and hooked up the whole secondary with a 1/2 A slo blo
crack one fuse down..

then I went back to just the cap even tried a 1A slo blo

crack another fuse down..

so I tried putting a 220K resistor across the cap, this did not blow the 1A slo but it bent it real bad
and there was nothing in the way of DC voltage either

I was starting to think there was a center tap for that transformer but then I realized when
I tested the external transformer that I had no center tap there either for sure

I am pretty sure there is no center tap and that this amp used to have a bridge as you can see on the schematic

I am stumped I think I need ghost busters and the demon to be removed that keeps blowing fuses..

[amptweeker]

I guess the one common thing in this has been that first filter cap, I can try a new cap in there with the slow blo fuse, I am starting to think the chassis is not at ground
there has to be something obvious I am missing here

[eschertron]

I guess the one common thing in this has been that first filter cap, I can try a new cap in there with the slow blo fuse, I am starting to think the chassis is not at ground
there has to be something obvious I am missing here

So with the fuse OUT, what are you reading at the fuse holder? It will be a FW rectified voltage, and will have an AC and DC component. Can you select AC versus DC on your meter, or does it auto-detect?

edit: There's certainly something pulling the voltage down. It sounds like the cap, but I'm stumped why a 220k resistor *across* the cap would prevent the effect from happening.

[amptweeker]

I will do some more checking tonight, thanks so much for your patience and the replies..

[Enzo]

First the cap, are you saying you have not tried a differnt cap yet? That should be done immediately.


Center tap. I see your repeated reference to a center tap on the power transformer, and there is not one, as you have verified. But i brought up center tap way back in reference to the OUTPUT transformer. And it would connect to the power supply at that cap. We want to make sure that the wire to the output transformer is no somehow still conected.

[Mick Bailey]

Is the cap oriented correctly?

[amptweeker]

Center Tap of the output transformer is not connected

First the cap, are you saying you have not tried a differnt cap yet? That should be done immediately.

I had tried a different cap , however at the time I was burning through the fast blow fuses, I have not tried a new cap since we
switched to the slow blow, so I think that is worth burning a fuse to test with a new cap again with a stronger fuse.


Center tap. I see your repeated reference to a center tap on the power transformer, and there is not one, as you have verified. But i brought up center tap way back in reference to the OUTPUT transformer. And it would connect to the power supply at that cap. We want to make sure that the wire to the output transformer is no somehow still conected.

Sorry to be so confusing, I have a tag baord with large eyelets some un-used where I have 2 wires the fuse output wire
and the first filter cap the output transformer and wires to the choke and the rest of the amp are in a nearby eyelet not
connected to anything,

This amp had a blown output transformer when I got it, I changed that so the OPT is new but it is not connected right now.


and yes the cap is polarity correct it is a large LCR can,

[gui_tarzan]

They like using diodes, don't they?

[amptweeker]

It Was that first filter cap!! I replced it with a 33uf 450V and the fuse held I got about 417 Volts DC, so
I clipped it into the rest of the amp , installed the power tubes, and the preamp tubes, the amp stayed running
I have other issues to attend to like super low volume and bias etc.. but this was the deal
When I tried a new cap I had the fast blow fuse, so I had eliminated that filter cap as the source of the problem
so it took me quite a few more fuses to figure it out thanks to you guys

Cheers
now I get to hear my preamp finally!
well after some more trouble shooting anyway

Thanks again

[amptweeker]

They like using diodes, don't they?

Sorry who likes using diodes you mean diode clipping or ?

[Enzo]

I think he refers to the double diodes in the high voltage rectifier.

[gui_tarzan]

Yessir. Not that there's anything wrong with that, it was just an observation.