Help needed to stop blowing fuses

[Paul Joppa]

Next step is the third paragraph in my previous post.

[Loquah]

OK. Tested with a slow blow fuse with no joy, but it allowed me to here an electronic sound which seemed to come from the tube socket nearest the HP jack.

There was a lot of excess solder on the socket pins prior to my efforts to suck away as much as possible and I noticed that the tube doesn't seat properly in that socket (it sits about 1-2mm off the chassis plate). I'm wondering now if there's some arcing or something going on in the socket. Would that cause excess current flow?

The good news is the tube is still fine (especially because I'm testing with my own tubes because he didn't have his handy), but I can't see any visual clues of where the problem lies and I'm not exactly sure what I should be testing for.

Paul, I just re-read your post and realised you are suggesting powering up without the tubes installed. I'll try that when I get back home after a short errand and get back to you.

[Loquah]

No tubes. Blown fuse.

This time I used a quick blow fuse so I couldn't hear any noises, but with no tubes installed there is still a very definite problem. There is a visible layer of flux on the ceramic between A1 and A8, but no solder and 280k ohms so I don't think that's the issue.

[Paul Joppa]

Great! Now we know it's not the tubes or the primary circuit, so it must be one of the power supplies.

It is not that uncommon for a bit of solder to wick down into a tube socket pin, blocking the tube from fully entering, so your approach of sucking as much of the excess out as possible is a good one. Pay special attention to pins 7 and 8; they are the heater and if they are shorted together that will indeed blow the fuse. Also, since it's happened a few times, make sure the sockets are correctly oriented with the notch at the center towards the power transformer. A few people have mounted them 180 degrees rotated by accident!

That said, if you don't see the problem there I'd test the high voltage first, just because it's easiest:

There is a single wire at power transformer terminal 6. Disconnect it temporarily; this will disable the high voltage power supply. If it still blow fuses then the problem is in the filament power supply or wiring. If not then we'll look into the HV supply.

[Loquah]

Thanks Paul.

I disconnected the black wire from transformer terminal 6 (it connects to terminal strip contact #1 at the other end). Still blowing fuses so the issue is obviously in the heater circuit.

I've fully rebuilt the C1-5 terminal strip since the amp started blowing fuses so I'm confident the fault isn't on these contacts, but could be a faulty diode or could be at the other end of one of the connections coming from C1-5.

I'm guessing I should disconnect the heater wire next to isolate C1-5 from the tubes?

[Loquah]

Disconnected heater wire from C1 & C2. Still no joy.

I'm going to do another visual inspection for shorts. I have tested the diodes in situ, but not sure if I need to remove them for accurate testing. I'll await your guidance with my sincere thanks.

[Loquah]

Sorry for the multiple posts - I'm trying to cover as much as possible to minimise delays due to our time zone differences.

I decided to remove and test the capacitor from C1-2 in case it was blown during initial attempts to power up, but it tests the same as a new one using the diode function on my meter (i.e. counter climbs gradually to about 1500 before reading infinite)

[Paul Joppa]

Wow - you've done some good guessing!

If I am reading your posts right, it is now down to the heater winding on the transformer and the recifiers/resistor/capacitor.

I'm sorry to say this because it will be a pain, but you should remove the diodes from one of the transformer terminals, 4 or 5. If I were you I would remove the from both and unbolt the power supply terminal strip. Then see if you can blow the fuse - it is extremely unlikely that the transformer winding is internally shorted, but we need to be sure. At this point, only the transformer primary (which you have already confirmed is correctly connected) is connected. In the unlikely event that the fuse still blows, the transformer is suspect - but that winding is only 0.095 ohms and you probably can't measure the difference between that and a lower resistance.

Assuming you do NOT blow the fuse this time, then the power supply that you pulled off, and probably have in your hand, is somehow shorted or incorrectly wired or has bad components. I can't see enough in the pictures to tell what is going on, and you may find that a bright light, magnifying glass, and a lot of patience is needed even if you have it in your hand. But that's why I suggested removing it all.

Again, if it were in the Bottlehead lab and my job was to fix it, I would just throw is away and build a new one. But you would have to request and wait for the parts, so make your own judgement about how much time to spend puzzling it out.

[mcandmar]

Wow there isn't much left now.  Just a thought, double check the black jumper wire on terminals C1 to C4 in case its on the wrong terminals.

[Loquah]

Thanks Paul. I'll do that now and maybe rebuild the C1-5 strip yet again. I have all new components (some of which I have used in the current setup depending on the visual state of the old components. I'll use all fresh components this time around.

Mark, thanks for mentioning the jumper because I don't know if there is one!? But I'm not near the amp to check so it might just be my memory - I'll go looking specifically.

[Loquah]

Mark, I might owe you a beer! Just checked and the C1-C4 jumper is missing. I noticed that nothing was connected to the capacitor at C1 when I removed it for testing, but was so focussed on testing it that I forgot to check the manual to see if it was somehow correct.

I'll add the jumper and test as soon as I can. Could that have caused the blown fuses do you think?

[Loquah]

Culprit found. The transformer itself is the issue 

I'm assuming this means a replacement is required?

I'm attaching photos of the exact configuration when the transformer blew the last fuse (and also made an obvious, but very brief buzzing sound)

[Loquah]

Further to last post. Visual inspection shows no signs of shorting with excess solder, etc. Could this issue have been caused by powering up in the original, faulty configuration? Or could it be caused by excess heat applied to the transformer? There's no signs of heat damage to the plastic at the base of the terminals, but I'm trying to work out what to tell my friend. "It's busted and we don't know why" is always an option, but it'd be nice to let him know some semblance of the cause of his troubles and if he has to buy a new transformer (i.e. if it's not considered a random product fault) it'd help to explain what he did wrong.

[Paul Joppa]

Contact Eileen for a replacement transformer. Tell her that PJ says it was bad from the start.

Looking at your most recent photograph, I see three wires on terminal 5 and one on terminal 4. There should have been two on each. It's a manufacturing error - the first time I've seen this.

[Doc B.]

I'm sorry, I don't think we can deduce that from the photos. Looking at the shelf full of PT-7s we have, there is an extra turn of one of the two wires attached to terminal 5, and that could be what we are seeing in the photo. I'm late the party, but has anyone suggested measuring the resistance of the primary and secondaries now that you have them all disconnected from the circuit?