Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: LooseChange on June 23, 2011, 08:03:37 pm

Title: Need some help with an Ashdown Fallen Angel (schematic included)
Post by: LooseChange on June 23, 2011, 08:03:37 pm: The fuses for the +12.6 and -12.6 volt power rail keep blowing. (See second page in schematic).
Tubes are removed.
I did find a shorted cap (C50) and thought I had it solved. But the fuses keep popping.

I replaced all three regulators and still blows. Replaced C50 and C52 (4700uf).
Until I replaced C50, the fuse blew quickly, on first power up. Now it blows on 2nd or 3rd power on.

I removed C51 and C42 (100n) and didn’t replace them, yet. Still blows.
I turned off the fan, which also runs on this power rail and I get a little more life out of the fuses.. But they still blow on subsequent power ups.

I am perplexed and I am looking for any suggestions.
Thank you!
Title: Re: Need some help with an Ashdown Fallen Angel (schematic included)
Post by: PRR on June 23, 2011, 10:03:31 pm: Replace D8-D11. Preferably with a 3A bridge assembly.

Replace the fuses with 10 ohm 10 watt resistors. See where it doesn't come up to 18V.

Mechanical shorts.
Title: Re: Need some help with an Ashdown Fallen Angel (schematic included)
Post by: LooseChange on June 24, 2011, 05:05:35 am: I would only read 18v before the regulators, right?

I did test the diodes in the circuit with my meter. The only strange thing I noticed was that D2 and D3 read funny in both directions. I replaced them. Outside the circuit they read fine.

FYI, The owner of the this amp says he left it on standby for an hour. Turned it off and back on and that's when the fuses went for the first time.

Thanks!
Title: Re: Need some help with an Ashdown Fallen Angel (schematic included)
Post by: DummyLoad on June 24, 2011, 06:59:44 am: yes -18V to pin 2 of 7912 and 18V to pin 1 of 7812. if C50 shorted D10 and D11 were likely compromised. look for something that's common to both rails - C59? check the OP-AMPs - one may have fried. as PRR says, it's likely molexia.
Title: Re: Need some help with an Ashdown Fallen Angel (schematic included)
Post by: LooseChange on June 24, 2011, 09:29:29 am: What a goose chase!
C59 was the final culprit. Tested with my meter, it's not shorted and seems okay. Why would this cause these issues?
Thanks!
Title: Re: Need some help with an Ashdown Fallen Angel (schematic included)
Post by: sluckey on June 25, 2011, 11:04:51 am: Speaking of goose chases, where is C59 on the schematic? I see 58 and 60, but 59 is hiding very well.
Title: Re: Need some help with an Ashdown Fallen Angel (schematic included)
Post by: VMS on June 25, 2011, 11:24:24 am: Quote from: sluckey on June 25, 2011, 11:04:51 am
Speaking of goose chases, where is C59 on the schematic? I see 58 and 60, but 59 is hiding very well.

It's with the diodes D8-D11.
Title: Re: Need some help with an Ashdown Fallen Angel (schematic included)
Post by: sluckey on June 25, 2011, 11:34:41 am: Thanks. I probably would have never found it. Especially in that location! I don't recall ever seeing a cap across the AC inputs to a bridge.