Sunn 200s multicap can, etc. replaced but showstoping problem persists

[kupervaser]

Jeff,
Given all the evidence. It seems as if the PT is OK so I wouldn't worry about doing the magnetization current test. That's just a little extra curiosity now. The fact that the PT doesn't heat up when left on for a long time tells us that there are no internal shorts.
Tom

Tom, I had the transformer on for about half an hour, let me test it for one or two hours ok?

What do you think about these voltages:

pin 2 - pin 8 of the rectifier tube - 4,64 VAC
Heater wires - 6,0 VAC
HV winding - 751 VAC

The first two are a little lower than should be, according to this website:
Power Transformer
Something could be wrong with the transformer if the heater is beneath 6,3VAC and less than 5 VAC on the GZ34 heters. However I've seen so many amplifiers already with the voltages a little off and they still worked fine and sounded good.

p.s. Just connected the DMM in series with the mains. During the switching I can see the current jumping a little above 1A and then it stabilizes on around 230mA. Turned the mains switch on and off a few times with the DMM in series and I think something went wrong in the DMM. There is no more continuity between the 2 probes when measuring current. I can see one 2A fuse in it so far, which is still good. SO now when i connect DMM in seres with the mains, the PT is not turning on. Will open up the DMM to check for another fuse or maybe burned components.

p.p.s. Yep, it is another big 3A 600 VAC fuse in my DMM which blew.

[Tom Phillips]

Tom, I had the transformer on for about half an hour, let me test it for one or two hours ok?

Leave it on until the temperature stabilizes (Usually ~4 hrs or more). It should only get ~20˚F over the room temp which just feels a little warm to the touch. But, if it starts to get hot then turn it off because it has failed the test.

What do you think about these voltages:

pin 2 - pin 8 of the rectifier tube - 4,64 VAC
Heater wires - 6,0 VAC
HV winding - 751 VAC

The first two are a little lower than should be, according to this website:
Power Transformer

The low heater voltages are suspicious for an open circuit secondary condition. This could be an indication of shorted turns in the primary winding. But it is still possible that the transformer was that way since new.

p.s. Just connected the DMM in series with the mains. During the switching I can see the current jumping a little above 1A and then it stabilizes on around 230mA. .

No smoking gun there. I just did the measurement on a Fender Showman PT and got 220 mA.

Turned the mains switch on and off a few times with the DMM in series and I think something went wrong in the DMM. There is no more continuity between the 2 probes when measuring current. I can see one 2A fuse in it so far, which is still good. SO now when i connect DMM in seres with the mains, the PT is not turning on. Will open up the DMM to check for another fuse or maybe burned components.
p.p.s. Yep, it is another big 3A 600 VAC fuse in my DMM which blew.

I recommend that you stop doing the power switch fast cycling. Even a known good transformer can blow the fuse under those conditions.

[kupervaser]

Right, thanks. Just had the transformer on for about 1 hour. It still feels cold, under the room temp I suppose.

I guess I will hook up the 5 VAC tomorrow and start testng.

One question: Why is there a Ground tap between the 6,3 VAC heater windings?

[Tom Phillips]

Why is there a Ground tap between the 6,3 VAC heater windings?

The center tap is used to reference the heater circuit to a specific voltage. Most commonly chassis ground (0V) or a 20 to ~50V positive voltage. The purpose is to reduce the background noise that would otherwise be present with a floating heater circuit. That's the brief explanation. The technical explanation of why this works is longer. A search for heater hum reduction will yield lots of information.

Cheers,
Tom

[kupervaser]

Ok so today I started putting all the secondaries back.
5,0 VAC doesn't blow the fuse
6,3 VAC doesn't blow the fuse
HV doesn't blow the fuse

Ok so I put in the rectifier tube and powered the amp, the fuse didn't blow. I put in the Power tubes in and the amp powers on as it should,the fuse doesn;t blow. I tried several rectifiers tubes but everything seems to be fine. I have tried to power the amp on a few times and it works well. It sounds great, no hums or hisses.
I played it for a while still good. At this moment I think either it were some bad solder joints or it is a very unstable transformer. I will try this amp during rehearsal from now, lets see what happens. I am kind of disappointed I have not been able to find the problem.

P.S. I also checked the voltages, check the schematic, for example on the 20uF capacitor where the schematic shows 320 V I measure 280 V.

[kupervaser]

Tried the amp during the rehearsal for a few hours yesterday. Everything was fine. Very strange.

[Tom Phillips]

Tried the amp during the rehearsal for a few hours yesterday. Everything was fine. Very strange.

You may never know. There might have been a little fragment of solder splash or wire that was cleared out when you did the last rewiring.
As discussed earlier, I think the fuses that blew during the switch flipping were a false alarm.
Glad the amp is working well now.
Tom

[kupervaser]

Well, yesterday played 15 minutes on it, than put it on standby, after 10 minutes wanted to play some more, flipped the stand by switch and the fuse blew again. I still think it is a unstable PT, the amp sounds great when it works.

[Tom Phillips]

The transformer was already connected when you were in standby mode and wasn't blowing the fuse. When you flipped the standby switch to the operate position you connected the filter circuits and the rest of the amp. It could be that you have always had more than one intermittent problem causing the fuse blowing. Troubleshooting intermittent problems is a real pain as you know. However, this last occurrence doesn't make me think that the PT is the problem.