Marshall Haze 40 shorting out?

[Jazz P Bass]

What I meant by checking the power supply is "all of the rails"
The B+, all of the preamp plates, the 12Vdc regulator & the 5 volt regulator.
I must apologize again , as I do not work at Marshall.
Where is R73/R91 in relation to the schematic?
And what pray tell measures 175Vdc.

[ricach]

Where is R73/R91 in relation to the schematic?
And what pray tell measures 175Vdc.

FX Loop page 2, all the way to the right top - PREAMP_HT. Schematic sez to expect 200V, I'm getting about 175

I'll check the whoe power supply tomorrow, run out of time tonight.

[Enzo]

That waveform kinda looked to me like a classic example of a weak filater cap causing a ssupply to drop below its regulator threshold. Is that series of spikes at 120Hz? Or 100Hz in the rest of the world.

Page 6 is the FX card, no got that?

Preamp HT comes from upper left page 3. Whatever voltage that is becomes the 200v rail on page 2.

My spider sense is itching that decoupling caps like C14, upper left page 3, or C49 by R91 on page 2, might not be doing their job.

[ricach]

I'm having trouble understanding what's going on with the OT. The center tap is the source of voltage to power the preamp rail? How is it getting the voltage to do that? Or a better question is where does W13 and W11 go (on the power supply page after the diode rectifier)

[52 Bill]

I'm having trouble understanding what's going on with the OT. The center tap is the source of voltage to power the preamp rail? How is it getting the voltage to do that? Or a better question is where does W13 and W11 go (on the power supply page after the diode rectifier)

You're looking at it the wrong way, the main power supply's first stop is the OT center tap. Then the voltage is reduced and sent on to the rest of the amp. The voltage is going into the OT not coming from the OT.

[Jazz P Bass]

To connect the B+ W13 & W11 may go to a choke coil, although I do not see it.
Unless it connects to W1 & W2 (page 4) which is a fuse.
Like I said, The Maze.

[ricach]

......

My spider sense is itching that decoupling caps like C14, upper left page 3, or C49 by R91 on page 2, might not be doing their job.

With Enzo's spider sense usually being on the mark I investigated those caps a little more. They did appear suspect so I replaced them. I've run the amp for several hours now and things seem to be fixed. Even the cracks and pops that I experienced occasionally seem to have disappeared. Thanks Enzo.

[Jazz P Bass]

Glad to hear that you found something wrong.
Enzo rules.

[ricach]

Glad to hear that you found something wrong.
Enzo rules.

And Jazz, thanks for the time you put in as well. Don't go too far, I still think there are issues with the mute and efx loop bypass switching. I havn't messed with them again since the cap change so we'll see if those are fixed as well.

[ricach]

And Jazz, thanks for the time you put in as well. Don't go too far, I still think there are issues with the mute and efx loop bypass switching. I havn't messed with them again since the cap change so we'll see if those are fixed as well.

Unbelievable. I put several hours of playtime on the amp as well as 10's of hours that I left the amp on. Decided its time to put the chassis back into the enclosure and button things up. 10 minutes of playing and its back!!!! Not in the mood to put it back on the bench just yet.

[ricach]

I'm back to working on this amp. Along with the caps I replaced, I also was going to replace C31? It is a 47uf 500V radial cap. I couldn't find a source for that and since replacing the other caps seemed to fix the problem, I didn't bother. So that is where I started this time around, and just by grabbing hold of the cap, it produces the problem. So now I'm back to trying to find a source for a 500V radial. Any suggestions? I know I can just try and resolder the connections, but while I'm in there, I might as well replace.

[Jazz P Bass]

UCY2H470MHD6 Nichicon | 493-6864-ND | DigiKey

[ricach]

UCY2H470MHD6 Nichicon | 493-6864-ND | DigiKey

Thanks Jazz. That was the last place I checked before giving up. Not sure how I missed that.

[ricach]

I replaced the caps and now appears we're on to something different now. The howl can now be controlled with the master volume (VR7B) on channel 2 which definately isolates its origin to the preAmp. The guitar signal remains intact up to Pin 1 on V2A which is the last tube in the preamp section, and becomes a very high voltage 50cycle sine wave at Pin 8 on V2B. Working backwords on the B+ rail I find the Diode Bridge putting out a perfect sawtooth wave. (Power Supply page 5). The secondary coming out of Xformer (W12 and W14) that feeds the bridge is a sine wave but the tops are perfectly clipped at the very tops and bottoms. I took a break and went to eat dinner. When I came back I was hoping to look at the Diode Bridge while the amp was functioning properly. However when powering up the amp, the Xformer vibrated furiously for a second before the HT fuse blew. Don't have a replacement fuse right now. WTF happened?

[ricach]

Did a little research on the vibrating transformer and it appears the sawtooth wave might have been an indication of things to come. Somebody discovered a bad diode in his bridge rectifier caused his transformer to vibrate before blowing a fuse. I suspect this is just a symptom of a real problem elsewhere but I will check/replace diodes before proceeding.