Dual Rectifier no power, fuse not blown...

[MitchK]

Hey everyone, its been a long time since I've posted here but I have a question!

A friend brought over his dual rectifier so we could do some recording with it, while we were setting up a tone I switched the tube/diode rectifier from diode to tube and the power mode from spongy to bold, as I did this the amp lost power, I'm not sure which switch caused this because I did one after the other, but either way I did it with the amp on and not in standby, which had always been fine to do on my old triple rectifier. Regardless I figured it had just blown a fuse when I made the switch and went to replace the fuse but that was not the issue, it had not blown and I even took the fuse out and double checked with my meter.

So, I have no pilot light, no heaters on any of the tubes, inside the amp I measure 120vac to the primary of the PT, but nothing on any of the secondaries, this is with the tubes in and with them pulled. I'm very worried the problem is with the PT, however, I've measured the hi voltage taps and there is continuity, about 30 ohms resistance, and 14 and 16 ohms when each side is measured from the CT. I get continuity on the heater tap and the 5v for the rectifier tubes... I'm stumped! If it's the PT I don't see how I have continuity, even on the hi voltage I switched the amp to tube rectifier and pulled the tubes and I still have continuity. It's been a while since I've worked on an amp but is 30 ohms on the hi voltage seem about right? why am I getting no voltage on any of the secondaries... What else would it be than the PT? Help!

[Mark Black]

Do you get any continuity on the primary side of the PT? It should be pretty low resistance (less than 2 ohms most likely).

[Jazz P Bass]

Mesa Boogie Dual Rectifier Schematic

Here is the schematic, so we can follow what you are up against.
As posted above, go right on the power plug, you should be able to directly measure the primary.
If you do get a low reading, this will prove out the fuse, the switch & the primary winding.
The secondary reading of 30 ohms seems good.

[wagdog]

I have no experience with that particular amp.

With the amp unplugged and caps drained...

Looking at the schematic, I would agree that checking the primary from the plug w/the power switch on would be the first place to start. While you're doing that, toggle the spongy/bold switch a few times - maybe that switch arced and failed(?) - which side of that switch did you measure the 120v to the primary?

I would check both the power switch and the spongy/bold switch for proper operation, and make sure you have continuity from the plug end to the BLK, R/WHT and WHT wires on the PT - at least then you KNOW that AC is getting to the primary side of the PT and if you are getting nothing on the secondaries after that, then the problem (in my opinion of course) is probably in the PT. Is there an internal fuse in the PT?

[MitchK]

Thanks for your replies!

I've measured and I do have continuity on the PT's primary, when I measure from WHT to BLK/WHT I get about .7 ohms and from WHT to BLK about .9 ohms, so it definitely seems fine to me, and of course I see 120vac on the primary too. If it were the safety fuse wouldn't I show infinity/high resistance on either the primary or secondary? Or if the coils fused wouldn't I have continuity between the primary and secondary (which I don't)? Maybe I just really hope its not the PT and something more easily fixable

[Jazz P Bass]

Good point on really checking out the switches.
As to an internal primary fuse, that will show true if there is continuity on the primary winding.
My thoughts on flipping any switch on this amp (recto or spongy) with it fully powered is, don't do that.

[wagdog]

So w/100% certainty there is 120v on the primary, there is nothing on any of the secondaries?

OK. (and again, I don't have experience w/this amp, use your judgement here!)

Pull the rectifier tubes out, put the rect select ON the tube rectifiers - nothing on the RED secondaries?

Nothing on the BRN heater supply either?

You are measuring AC here, across the leads? (just checking).

Also, not seeing it on the schematic, but is there by chance an HT fuse in this amp?

Standby switch - check that too.

Other things I might glance at: R361 and R362 in the heater supply - just to make sure they didn't take a shot of HT.

If there is NOTHING on any secondary then I'm pretty stumped. Hope someone here has some ideas!

[MitchK]

Found the problem! I was measuring from the IEC and then all of the switches individually and everything looked just fine, however when I measuring across the fuse holder it was being a little tricky, I figured it was just some oxidation or something on the contact because everything looked fine and I just had to push a little with my probe to get a connection. Anyways as I sat perplexed I plugged the amp back in so I could re-read the voltage on the primary and viola the amp turned on fine this time. So since I had been fiddling with the fuse holder I took a closer look switched the power switch a few times and it was arching, one of the wires had come loose but was still in the hole in the terminal. Thanks so much for your help and pointing me in the right direction!

[MitchK]

Wagdog you actually bring up a good point about something that really surprised me when I opened this up...There are no other fuses in the entire amp!! Isn't it part of the electrical code that the HT must be fused? Either way I was pretty surprised to see that.

[wagdog]

I don't know electrical code - but I was under the impression that HT fuses are added to protect the PT in the event of tube failure.

Glad you got the problem sorted!

BTW; I agree w/JazzPB - I would not flip the rect/spongy switches with the amp powered on (as in fully off, not on standby, off).

[Prairie Dawg]

Great catch. One for the books.

[R.G.]

It's worth remembering, not just in amp repair but in any situation: if what is happening does not make any sense at all, it's an indication that you don't know all the rules that are being applied.

[Jazz P Bass]

Amen to that, brother.
And it can happen to anyone.

[Prairie Dawg]

It's worth remembering, not just in amp repair but in any situation: if what is happening does not make any sense at all, it's an indication that you don't know all the rules that are being applied.

+1. Of course it's not a total wash when you've learned what doesn't work-progress of a sort you might say. The indirect approach.

[jmaf]

I don't know electrical code - but I was under the impression that HT fuses are added to protect the PT in the event of tube failure.

And don't forget the OT. When a tube shorts, all you have between HT and ground are a few tens of ohms on the OT's primary winding.