Having trouble with high voltage fuses on Laney chassis rebuild

[amptweeker]

I replaced the PCB in a Laney AOR 50W amp, and have rewired the whole amp
This is not my first amp build, I am trying to troubleshoot the high voltage power supply.
I have tried swapping almost everything in the amp including subbing in a power transformer
but I am starting to wonder if how I am testing is invalid and would blow the fuse anyway.
I started removing things from the circuit to eliminate components..

Right now the amp has been reduced to a bridge rectifier, a HT fuse 1/2 amp
and 1 cap, that goes to ground, ( with no load on it)
Is this not a bomb? I mean can you troubleshoot like this or should you at least have
say a 220K resistor going to ground, I have not been able to keep that fuse from blowing
I have gone through too many fuses to count, would that cap just draw current and charge like mad
and blow the fuse anyway? ( I also tried a different cap to make sure the cap was not shorted)

I really would like to see the power supply work with minimal load on it and not blow the HT fuse..
What would be a good way to test this? The fuse is after the bridge and yes I have also swapped out the bridge

Sorry if I rambled I am trying to catch you guys up on this build, but right now the amp is not really involved

[Chuck H]

It might be a good idea to use a current limiter or Variac to avoid blowing the fuse too quick for testing. Something is drawing current. No current, no blown fuse. If you're down to the PT, the diodes and a cap, and you've subbed the PT and the cap, the only things left are the diodes, a miswire or that you have either two bad PT's, caps, or both.

[Enzo]

Hi
Look up "light bulb limiter" then make one and use it.

If you are blowing high voltage fuses, are you using slow blows? Fast blows might be popping simply from the charging surge.

Are you sure everyting else is unhooked? Like is the OT center tap still connected?

[amptweeker]

Hi
Look up "light bulb limiter" then make one and use it.

If you are blowing high voltage fuses, are you using slow blows? Fast blows might be popping simply from the charging surge.

Are you sure everyting else is unhooked? Like is the OT center tap still connected?

Thanks for the replies, Chuck and Enzo, Everything is disconnected including the OPT CT
I wondered the same thing if the cap draws too much current and pops the fast blow fuse
I have seen these light bulb limiters, I guess its time to build one, so is it OK to test with just one cap?

[Mick Bailey]

Can you do a quick sketch of how it's wired? I may be wrong, but didn't think the AOR50 had a CT on the original PT.

[amptweeker]

schematic

Can you do a quick sketch of how it's wired? I may be wrong, but didn't think the AOR50 had a CT on the original PT.

See attached PDF there is no center tap on the Laney that I could see,

[Chuck H]

Can you do a quick sketch of how it's wired? I may be wrong, but didn't think the AOR50 had a CT on the original PT.

We've been discussing the OPT CT so far. If the PT does have a CT it certainly shouldn't be grounded with a bridge rectifier.

EDIT: Simulpost. Disregard

With no fuse in place, is there correct voltage at the rectifier output AND is there any continuity between the capacitor + end and chassis or circuit ground?

[eschertron]

Thanks for the replies,
I wondered the same thing if the cap draws too much current and pops the fast blow fuse

If you are not using slo-blo (time delay) fuses, then you must do so. Fast fuses WILL pop on the initial power on surge.

[R.G.]

+1
Time delay (slo-blo) fuses are needed as the power-on surge into the first cap is big. Well, really it's BIG, not big.

There are other ways to fix this issue, but time delay fuses are what are used in commercial amps.

[amptweeker]

I thought it might be something stupid like the fuse itself, this is why I started this thread, you guys are great, and I dont mind feeling stupid
I will see what they have at rat shack, they only had the fast blo in the small values, I did have the amp running once before the rebuild and I thought it was a fast blow fuse..

[52 Bill]

Wouldn't it be better to have the fuse after the first filter caps and before the OT center tap? I would guess that a fast blow fuse would work fine there.

[Jazz P Bass]

Here is the AOR 50 schematic.

[amptweeker]

I agree about the fuse location , I really just left the fuse where Laney had it, this is killing me now because I am at work today, and cant try a slo blo fuse until tonight
I had a feeling that cap would draw enough to pop that fuse.. with nothing being shorted or anything, I would still like to build the current limiter and get a variac..

[52 Bill]

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

[eschertron]

So the primary side fuse is correctly-rated, and it does not blow? Only the HT fuse? Good.

Thought I'd share this; how I built my light-bulb limiter. I bought the duplex outlet (mounted it in a box) and split the outlets by breaking the jumper tabs on the side of the assembly. Then I wired the sockets *in series* so that I can plug a lamp (light bulb holder) into one and the amp into the other. I don't have to dedicate a whole lamp to the device.
A bonus is that I can run a loop of wire in one socket (or a 10X loop, as the need arises) and use it to quickly check current draw with my non-contact ammeter.
Easy to make, easy to store, and NOT a uni-tasker!