Can one bypass a PS choke for the purpose of troubleshooting?

[Randall]

I have a VOX AC30 CC2 on the bench. It came in with both the line and HT fuses blown. Bringing it up on the bulb tester revealed an intermittent flicker that suddenly flashed. I eliminated all the tubes, one at a time, replaced the four 22/450v filter caps, tried a new 5AR4 rectifier. The filament and 5v taps look to be OK. Thinking it looked good on the bulb tester, I switched it on, smack there go both fuses again. I lifted one side of the choke and the primary side V's look good. The problem seems to be somewhere with the secondary side, which feeds the screen V's as well as the preamp HV. I reconnected the choke and disconnected the preamp feed, no change. With two EL84's in I have a solid plate voltage, but pins 9 slowly charge up to not enough, and then slowly charge back down again. I have also tried a new set of EL84s.

So here's the question: can one bypass a choke to determine if it may be the problem? I had a choke in a Fender Twin make a fool of my years ago, not wanting to go there again. I am suspecting it is shorting during switch on, maybe causing a big enough surge to take out the fuses. Is there a better way to troubleshoot a choke?

Here is the drawing, it's page 7.

http://bmamps.com/Schematics/vox/Vox...05%29%20SM.pdf

[Chuck H]

It should be easy enough to check the choke for anything that would cause a gross failure. Is the resistance from end to end approximately correct? Is there any continuity between the ends and the case, lams or chassis? I suppose there's a possibility that it's only arcing to chassis with HV on it.

Why didn't you flip it on with one end of the choke lifted? If it was the plate side that was lifted, and the fuses blew, that would eliminate the choke as the cause. Leaving only the PT or rectifier tube as possible causes. If the fuses didn't blow with the plate node of the choke lifted that would indicate that the problem is the choke or downstream.

[Dave H]

So here's the question: can one bypass a choke to determine if it may be the problem?

As a test you could disconnect both ends of the choke so there's no chance of any shorts and replace it with a 1k 5W resistor. That's what I used in my 'AC30' for years.

[Randall]

Two great responses and just the prodding I was looking for. Thank you.

I tried the 1K/5w substitute, no change. I lifted the plate side at TT21 with all four output tubes in place, and the HT fuse blew. Then I reconnected the plate side at TT21 and lifted the OT at TT19, and reconnected the preamp HT at TT4, which had been disconnected up until now. So now I've got screens, and preamp, no plates. Bring it up on the bulb tester, it takes a long time to come up, and when it does the bulb flashes, which I take as an arc, probably PT time.

Would anyone concur?

[Chuck H]

Two great responses and just the prodding I was looking for. Thank you.

I tried the 1K/5w substitute, no change. I lifted the plate side at TT21 with all four output tubes in place, and the HT fuse blew. Then I reconnected the plate side at TT21 and lifted the OT at TT19, and reconnected the preamp HT at TT4, which had been disconnected up until now. So now I've got screens, and preamp, no plates. Bring it up on the bulb tester, it takes a long time to come up, and when it does the bulb flashes, which I take as an arc, probably PT time.

Would anyone concur?

That's not exactly how I might have done it. You still haven't eliminated the rectifier or main filter caps as potential suspects. Lift the PT HV secondary. If the fuses blow it has to be the PT. If not then you probably have a shorted cap. I know you replaced the rectifier tube so unless there is something like charring on the board or socket a rectifier circuit failure doesn't seem likely.

[Randall]

Chuck, The first thing I did was replace the filter caps after eliminating a bad tube by swapping in fresh ones. And the customer bought a new rectifier before he brought it to me, and it still has the original problem. It only does this under load. If I pull all the tubes, I get all the V's, albeit high, and no fuse blowing. The way I see it now is, it blows fuses whether connected to the plate side, or the screen/preamp side independently, as long as there are tubes in it. SO, don't see why we should disconnect the OT. (little Enzo reference there). I think that eliminates the choke, tubes, filter caps, rectifier, OT. I'm going with arcing OT, as much as I squirm at the idea of ordering a $200 part without really, really knowing. Give me a dead shorted PT any day over an intermittent arcing one.

C'mon guys!.... who's with me?!!

[Chuck H]

Still hard for me to know what all was tested and what peripheral circuits may have been involved. I'll trust that your assessment of circuit isolation is accurate. But also as per Enzo, you can't always trust that new = good wrt to parts. Still, not likely. So just to be 100% before pulling the PT...

Do you have another amp around that requires similar HV and current such that you could jumper the winding from one amp into the other? Basically, if it's the PT HV winding and it only fails under load a good way to be sure it's not the load is to connect the suspected winding to a different load.

EDIT: It occurs to me that the alternative load doesn't need to be an amp. It could just be a cap and resistor in parallel. Maybe 100uf/450V cap and a 470r/20W resistor.

[Randall]

More good ideas, thanks Chuck. But, as it turns out the transformer is no longer available, and the next option is Mercury Magnetics coming in at $245 +shipping. That's going put this amp over $350, and I don't see that happening somehow on a Chinese Vox.

[Jazz P Bass]

You really do need to define what is wrong.

If it's blowing fuses, start at the power transformer, as Chuck stated.
Pull all of the tubes.
Me, I would not even try the 5AR4. The new ones (Sovtek) are crap.
For troubleshooting & testing, put in a few series diodes.

If the fuse holds at this point, it's down to a tube or cap.

[Tom Phillips]

I'm also with those that don't believe that there is sufficient evidence that the PT is bad. The fuse has never blown when only the PT is powered and the all secondaries are open circuit. Have you tried that yet?

The theory that the PT fault only shows up under load with this particular PT doesn’t seem credible either. I say this because, when the fuse has blown, it does so very shortly after power was applied. A mechanism I can think of that would make the PT itself short under load but not short without load would be heat related. However, based on your description, the PT didn't have time to heat up. Looking at the situation from another angle, I point out that the inter-winding voltages are actually higher when the PT is lightly loaded. If there was an internal fault causing intermittent acing, the probability of it showing up would be higher when the transformer was lightly loaded. This further suggests that the problem is external to the PT.

The bottom line is that I don’t suspect the PT itself until it blows fuses, draws excessive magnetization current or heats up excessively when it is powered in total isolation from the rest of the wiring in that amp.

I also emphasize, as has been previously point out, that we should never fall into the trap of assuming that a part is “known good” just because it was recently replaced.

Tom

[Randall]

OK then. When respected voices speak, one must listen. I took a step back, forgot what I thought I knew and will start again, learn, and be grateful for the opportunity and assistance.

I put back in the original Groove tubes 5AR4. I reconnected everything and pulled all the tubes. Amp powers up quietly and runs with all voltages in place. The only difference from when it came to me is four new PS filter caps. One difference I failed to mention previously is that before the standby LED is now working, where before it did not. It initially got past me that there is an extra winding that supplies the +/-15v supply that powers the back board, and this is where the ON and STANDBY LEDs get their power. The ON LED was working, but the STANDBY was not, and in this scenario now is. I'm not sure if this is relevant to my initial problem at hand.

My thought is to add to the load by inserting tubes one at a time. Would it be better to add them or put them in singularly to see if one is rogue?

Thanks guys.

[Jazz P Bass]

One at a time seems like a good plan.

It would be helpfull if you could monitor what the current draw from the mains is.

I use a variac & an ammeter.
When it is up to full voltage my Kill-A-Watt comes into play.

[Randall]

Well then, it turned out to be the rectifier after all! All this time chasing rainbows, when it was a misunderstanding after all. Customer told me he replaced the rectifier, so my mistake was accepting that as fact. When will I learn? It seems he bought a new GT 5AR4, but did not mention that he gave the amp to me with the old (and failed) Sovtek back in it. He did not know about the lines fuse in the inlet socket, which was blown. So I did my troubleshooting believing the rectifier was not to blame.

With the new rectifier, it comes up with no trouble, and the STANDBY LED now comes on after the tubes have warmed up. Now the problem is a wicked ripple in the PS. I suspect I may have to yet again replace the filter caps, which I dread because this is one of those amps made to be manufactured but not disassembled. It's a recipe for more trouble to lift this board, but I am determined to get to the bottom of this.

I dare not say how much time and frustration I have into this, but it is a new customer, as are all since my move to SWFL, so returning the amp working at a reasonable price should be worth something. If anyone has any comments about why this may have such a ripple, I'm all ears. Thank you!

[Jazz P Bass]

"Now the problem is a wicked ripple in the PS".

Which power supply node?

Is this with all of tubes installed?

[Tom Phillips]

Also verify the actual measured filter cap values and the integrity of each of their grounds.

Tell us if the ripple changes as you select between the "Vintage/Modern" Smoothing mode?
(Man! This amp has so many flippin' options. You better check that there isn't a "Normal/High Ripple" switch hidden somewhere on the thing.