bosoz blowing fuses

[cowanrg]

sigh. im always having problems it seems...

well, im testing my bosoz right now. its pretty much all built, just not in a case yet. however, everytime i test it, i blow a fuse.

i have done some troubleshooting and found that its not the preamp (when i remove it from the chain, and just have the power supply board and xformer, it still blows), and its possibly not the power supply board (the transformer alone blows fuses!).

so, i think after 8 fuses and a lot of head scratching, im using too small fuses? i am using 4A 125V fuses. however, as some of you know, im using a HUGE transformer for this little pre-amp. i have an 800VA transformer powering it. it seems that turning it on and off makes the fuse blow. but, when the pre-amp is on, the initial inrush current blows the fuse. im all out of fuses at this point. should i try it direct and see what happens? its only when i turn it on and off quickly that i get a blows fuse with just the transformer connected.

if i have the transformer and the power supply connected only, and have a meter on it, i get 80.5V (which is what its supposed to be), but the fuse blows JUST as it gets up to 80.5V.

what size fuse should i be using? i get a new digital camera (5.0 MP sony!), so i can take pics if needed.

[Cobra2]

but wit that large trafo, you need SLOW fuses, and probably more than 6Amp.

Arne K

[Blues]

check for shorts...check your semiconductors if they're mounted right (rectifiers and trans regulators).

[cowanrg]

yeah, im pretty sure they are. any huge risk of just NOT using a fuse right now to test? im all out of fuses, and certainly dont have any bigger than 4 amp.

the power supply runs for a second, the transformer runs 50% of the time, as long as it was already "charged" from the last turn on. (but if its cold, or was sitting overnight, it will blow a fuse...)

i just dont want to replace a circuit board.

[eLarson]

Maybe a light bulb temporarily wired in series after the fuse as a make-shift inrush suppressor?

[cowanrg]

hum...

i guess i could try that. but if there's a problem will i be cleaning up glass?

[cowanrg]

i think i failed to mention that after i get this working, i will finish my aleph2's! so, if anyone wants to see them done, you can help me out a bit

[cowanrg]

so i think it was just the fuse size... i tried it without a fuse, and it seems to work just fine.

however, im getting another issue...

my voltages are pretty far off on my power supply...

i read the manual on passdiy and am following the directions pretty closely. i did what nelson says to test it loaded (put a 15kohm resistor across the positive and ground and negative and ground) and measure the rails and the regulated rails.

i am supposed to get +-80v unregulated and +-60v regulated. this is a dual mono, so on PSU1 i am getting 84.8V rails (within reason), but +40.23 and -40.00 on regulated rails. (i should be getting +-60v). on PSU2 i am getting 85.3V rails (close enough i guess), but -55.3v and +47.7 on the regulated rails. these arent very close, nor do they make a whole lot of sense.

i spot-checked a few of the zeners and they all seemed to be getting about a 6.2V drop across them. the only thing that i can think of would be a bad transistor or a bad zener, only because thats what has happened to me before help would be greatly appreciated.

[BobEllis]

The zeners should be 9.1 volt types. looks like you got some wrong parts in there.

Your rail voltages make sense when you consider a 5% tolerance with a couple of 9.1 volt types sprinkled throughout. Output voltage will be the total of the zener string minus the gate-source voltage of the pass transistor.

your unregulated voltages are high because you aren't loading that monster transformer down much. watch the heat sinking on Q101 and Q102.

As for your fuse issue, a CL-30 in series with the primary and 6 amp slow blow fuse ought to do the trick.

[cowanrg]

ok, that makes more sense now...

in the papers for the preamp (which i am re-reading and trying to understand), it states that there is a 3.7v drop across the gate to source, and there are 7 9.1V zeners (63.7v), so it should give 60v. hum. im going to measure my drop from gate to source, and see if its 3.7. and ill take out some of the zeners and measure them again.

im pretty sure they are 9.1, thats all i have in my shop. i thought i had the wrong value before, so i bought some more and tested them to make sure. i have like 50 of the things in the shop, and they are all tested at 9.1v (or they were).