VHT Lead 40 Blowing Internal Fuses

[mikeboone]

Guys,

I have a VHT Lead 40 that is blowing the internal line fuses on a low voltage tap (24VAC) that feeds two 7815 voltage regulators. The attached schematic is from a Lead 20 but I am fairly certain this part of the circuit is the same for both amps. I did a quick check of all 4 bridge rectifier diodes with my fluke and all seem to test normal. I am not exactly sure how to test/verify proper function of the 7815 regulators short of swapping them out. None of the electrolytic caps show any signs of swelling or leaks, the amp is fairly new.

I lifted the violet taps on the secondary and the trusty bulb limiter goes dim on power up.

I just need a little help figuring out how to troubleshoot this voltage regulator circuit.

Thanks

[audiopete]

Pull them and see if the amp still sucks current. Your current limiter will avoid blown fuses. If it still shorts, pull the rectifier next. After that, check C53. Somewhere there you should be able to power up or the short is further down the line. Check caps after the voltage regulators.

[Jazz P Bass]

That secondary power supply makes my head hurt.

[mikeboone]

Well, the regulators are good, the diodes in the bridge rect are good. I was really hoping after all that something would be amiss in the heaters for the preamps, like a bad tube. Too easy, right? No joy.

There's all sorts of shit being powered from there. I am just taking a look for a friend before he sends it back to them for repair.

[J M Fahey]

Please post the full .pdf here, because you are clipping it at inconvenient points, leaving out hidden connections we'd love to follow.
What JPB said in other words.

Back to that supply, it has 2 15V regulators stacked one on top of the other, to get +15V and +30V .

Just thinking aloud, some of this has already been mentioned, you may have 1 or 2 bad regulators, shorted D5 to D8, shorted C52/C62 or something shorted downstream ... what I ignore for now.

If you have a couple car lamps, 12V , 12 to 15W you can connect them instead of FS5/FS6 , consider them local lamp bulb limiters.

Awaiting that .pdf

EDIT:
pull all 4 12 AX7 and now that you are at it, also the EL84 .
We need none of them and in fact the 12AX7 might be part of the problem.

Use the 12V lamps as local limiters and measure the supply, both raw 40V (and non indicated 20V) , which by the way is what gets IN the 78xx regulators, and what comes OUT of them.

Is there a connector or some other way to separate regulated PSU from its load?

[mikeboone]

The full schematic is in my original post

[mikeboone]

Attaching again, in case it didnt load.

[mikeboone]

Ok... I don't have auto lamps but i do have the amp plugged into a limiter bulb.

I pulled the preamp tubes out and the bulb definitely gets much dimmer, a dull amber which I thought was OK. The regulator voltage was 14 (reg1) and 29 (reg2) outputs. I got brave and moved it over to regular power and it blew the fuses.

I then pulled the power tubes as you suggested. No preamp tubes installed. Once again the bulb was dim but much dimmer. The regulator voltages were 15 and 30, respectively. I moved it over to regular power and it stays powered up. The power tubes look rough, lots of carbon buildup. I put a new set of JJ tubes in, re-installed the preamp tube and put it back on the limiter bulb. Back to same loaded down scenario.

The screen resistors are not charred and they measure at or near 470 ohms, in circuit.

I also lifted the OT from the power tubes, no change.

[The Dude]

With the tubes out, check your bias voltage. Maybe you lost bias.

[mikeboone]

With the tubes out, check your bias voltage. Maybe you lost bias.

The bias voltage is around -13vdc. The schematic calls for -45 to -56 but I don't think that's right. I worked on a fixed bias Mesa DC5 and the bias voltage was around -16vdc with a similar plate voltage around 360vdc

[The Dude]

The DC5 schematic I just looked at shows -49VDC for bias. I've no reason to believe the schematic you posted is wrong. Is it clean? How much AC do you have at the bias TP? There's only a handful of components in that bias supply. It wouldn't hurt to check them.

[mikeboone]

The bias tap has 35vac. There is -42vdc just on the other side of the rectifier, past the filter cap at R112. At R111 there is -13vdc.

R112 (47k) measures 46.2K. R111 (15k) measures 15.1k. R110 measures between about 30k and lower depending on the pot, they're in parallel. C80 isn't bloated or shorted. Perhaps it's funky and loading the circuit?


The 180k resistors R76 & 77 measure 179k each and all of the grid resistors are at or close to 1.5k.

BTW, the Mesa amp I referred to was a DC3, not a DC5. Sorry for the confusion.

[g1]

Set the bias for most negative voltage you can get and see if that helps.
I don't think the -13V bias is that far out though, seems ball park for EL84's at that plate voltage.

[g1]

Have you tried with everything connected except the red winding of the PT?
If it still blows, then bias and power tube issues are not suspects.

[52 Bill]

Set the bias for most negative voltage you can get and see if that helps.
I don't think the -13V bias is that far out though, seems ball park for EL84's at that plate voltage.

What is the actual voltage on the output plates in this amp? The schematic for the 30 may not be the same as the 40.

When the fuse blows, is it after a warm up or does it blow quickly? Are you using the correct fuses?