Blowing Fuses on Power Supply, No Load (F5)

[vdi_nenna]

Had built a F5 with an Antek AN-4218 18+18 400VA toroidal. It worked for a few months, then one day it started humming. After a while longer, one channel smoked a tiny bit. Might have been a jfet.
I took it apart and started over with new FETs, but then the fuses started blowing.

-Removed the circuits and ran the power supply with bridge and CRC filter only. Still blowing fuses.

-Removed the CRC filter banks and the fuses stopped blowing . Voltage after the bridge recs show 16.9v on all 4 bridges.

-Tried installing new bridge rectifiers, no good with CRC filter installed.

-Removed the NTC thermistors (CL-60), no good. They tested out of circuit like new thermistors. Put the thermistors back in.

-Tried 3 and 5 amp slo-blow fuses, no good.

-Built a new CRC filter bank, but still fuses blow.

The PS circuit includes a 15a line filter. Is it possibly causing a problem?
With the CL-60's, could there still be too much in-rush current? Should there more be in series or parallelled?

Not sure whatelse to try. Was going to try a simple 10k uF filter bank, but not sure if it's a waste of time and should be looking elsewhere.

Anyone have an idea what should be tried next?

Thanks in advance,

Vince

[nigelwright7557]

I would look again at the CRC.

Unless the bridge has gone and is feeding ac into the capacitors.
Have a look on a scope.

[vdi_nenna]

Cant check AC off a bridge with a DMM?

It's not bad to use a scope directly on voltage?

[nigelwright7557]

Originally Posted by vdi_nenna

Cant check AC off a bridge with a DMM?

It's not bad to use a scope directly on voltage?

A DMM might read ac as dc.

It is fine to use a scope to measure voltages directly.

[AndrewT]

turn all the output bias setting VRs to minimum resistance.
Plug in your F5 to a bulb tester.
Now you can safely measure voltages @ the mains transformer, @ the PSU, @ the supply rails @ at the components.

But do remember that the mains side (primary) is still Live

[dave_gerecke]

Hello Vince, are you sure about the voltages on the transformers? You say that the output is 18v+18v. I am assuming AC, thus when this is rectified, you should see around 24-25volts DC on the output of the Bridges. If you are seeing 16.9v, you either have transformers with 12VAC secondaries, your incoming power has an issue, or the transformers have an issue. You may want to double check the output voltage, and the voltage rating of the transformer. You also have to consider the loading of the transformer. If the rated output voltage is at full current draw, then the unloaded output voltage will be higher.

Peace,

Dave

[vdi_nenna]

Hi Dave. Thanks for the reply.

I was thinking yesterday that maybe there is a short in the windings? Too much current draw pops the fuse? The transformer is new, but doesn't mean it can't be defective.

The transformer secondaries are 18v-0v and 18v-0v. Green is 18v, blue is 0v.
I have the 18v and 0v going to the ~ ~ of the bridge. The plus out is 16.9v.
I was under the assumption that there is around 1v loss due to the diodes.
Is this correct? See jpg for schem of power supply being used.

Just though of something. I'm using 1 large transformer, but using 4 bridges feeding 2 filter banks. When I originally wired the amp, I alternated the rectifiers on each side of the transformer windings. When I re-wired the amp, I used 2 bridges on the same side of the transformer windings. While testing with a temp filter I continued to use 2 bridges from side of the transform.

Could this be drawing too much current??

I would have been so easy for me to just use 1 bridge from each side of the transformer's 18-0/18-0 secondaries, but did not try it.

Man, if that's its...I'll $#!+ !!!

I'll try it tonight. Will take 2 seconds. Jeez!

Thanks for jarring that loose!

Vince

[dave_gerecke]

Hello Vince, you are correct in the loss of approx 1 volt across the diode bridge, but you also have to take into account the rectifying action. This turns the AC into a pulsing DC. The pulsing DC should be the AC voltage times square root of 2. For and 18VAC input, this is around 25.5 volts DC, not counting any voltage drops across the diodes. Thus it should be around 24 volts after the rectifier. Disconnect everything from the transformer secondaries and measure each of them with no load for AC. Keep in mind to follow AndrewT's advice and put a 60w lamp in series with the primary. The lamp will act as a current limiter. Assuming that the transformer is rated for the 18VAC near the full VA rating (i.e. 18VAC with full current drawn) then the voltage should measure a bit higher than 18VAC with no load.

Peace,

Dave

[vdi_nenna]

I connected the filter bank with a rectifier from each secondary of the transformer.
No blown fuse and 25.5v (exactly) out of the filter bank.

I think that was it! Just mis-wiring, unless you think I should check something else.
Next challenge will be to connect the repaired amp circuit.

Thanks Nigel, Andrew and Dave!

Vince

[dave_gerecke]

Hey Vince, if you are now reading the correct voltage out of the PS, sans amp load, I would suggest hooking up the amp channels, one at a time, and see what happens. I would suggest using the light bulb current limiter on the primary side, and hook up one channel at a time, and let it settle if it seems ok. Then after a 1/2 hour or so, adjust the the channel per Papa Pass instructions in the F5 documentation. Once that's done and things seem to be behaving correctly, hook up the second channel and do the same procedure for it. Once you have both channels setup and nothing is failing, recheck all of the voltages, etc. per Papa's instructions. If things look good, check the DC on the output of the amp, check the speakers to make sure they don't have any shorts, and let it run!

Peace,

Dave