Marshall Superbass with mystery short in power supply. Exorcist required.

[christarak]

Just a thought. The stacked caps are wired in series watching polarity. In other words negative of one cap to positive of the other. The generic dual 50+50 cap has them in parallel, not series. So both negative sides are wired together. There is no way to wire a single dual 50+50 cap for series. Could this be something we did here?

You're exactly right Enzo. Each cap can has its two 50uF sections wired in parallel and then seriesed (new word) with the next 50+50. I double checked, but I see where you thoughts are going.

[christarak]

Yes, a picture of the main filter caps wiring would be helpful.

I'll do that G1. I have stared at those caps for so long, I'm afraid I might wear them out. A fresh set of eyes might see the problem (although it existed before I swapped out all the caps). Now that I have installed 2x220K totem resistors across the two seriesed 50+50 caps, I might try running each 50+50 cap alone to see if the fault is limited to one position/cap. I'll send photo and report on that test later (my) this afternoon ... unless you shout at me and say "do NOT run the series caps on their own...even across totemed (another new word) resistors, or you will...."!!

[olddawg]

It will work with a less filtered power supply. Maybe just disconnect some caps and resistors to isolate the problem. Worst case a loud hum unless you mess up the bias or something. I also might try using a slightly higher amp fuse in this cases. Btw.. are all of your fuses from the same batch? I’ve gone crazy a few times from a box of “bad” (underrated) new fuses. Especially Sloblows.

[Helmholtz]

Now that I have installed 2x220K totem resistors across the two seriesed 50+50 caps

Balancing resistors are not required as the CT connected between the caps takes care of voltage balancing. (Of course the CT must not be grounded!).

[christarak]

It will work with a less filtered power supply. Maybe just disconnect some caps and resistors to isolate the problem. Worst case a loud hum unless you mess up the bias or something. I also might try using a slightly higher amp fuse in this cases. Btw.. are all of your fuses from the same batch? I’ve gone crazy a few times from a box of “bad” (underrated) new fuses. Especially Sloblows.

Thanks Olddawg. I hadn't even thought of the fuse. Because my Limiter is protecting the fuse, I haven't thought of replacing the customers original fuse. I suspect its not the cause here because my Limiter glows brightly.

I will take your advice and start disconnecting the 50+50 caps and see if one of the new ones has failed, or if it highlights a pre-existing failure point.

As Saint Arnold said, "I'll be back" with my findings. Thank you for taking the time to help.

[The Dude]

Hi Dude. Using Fluke 87V, the rectifier output (HT) to ground was 74K, which is also the top of the totem capacitors, so I measured the mid-totem to ground: 1M !!! I discharged the caps and tried again. Similar. I measured the capacitance of both 100uF caps (50+50 dual cap cans) and they both read at around 130uF. I'm not sure if I can trust my readings while in circuit.

So I thought "bugger it. I'll put two 220K totem resistors across the capacitors". Same. Short circuit appears when closing the Standby switch.

I'm stumped Dude.

Did you measure that with the HT fuse out? 74K is a clue. You shouldn't get 74k there. It should go all the way open as the caps charge via your meter. You have only a cap and 2 diode cathodes at that point with the fuse removed. You must have either leaky caps or diodes. Lift the + side of the cap that goes to the rectifier and remeasure. That should tell you if the resistance path is via diodes or caps.

Edit: Also double check diode polarity and that they are installed correctly.

[christarak]

Balancing resistors are not required as the CT connected between the caps takes care of voltage balancing. (Of course the CT must not be grounded!).

Oh. I didn't think of that. Thank you, Helmholtz.

When I get back this afternoon I plan on removing each of the two 100uF (50+50) independently of each other, and then totally. Should I leave my balancing resistors in place while I do this test as each capacitor will only see half the rectified DC voltage? I don't want to risk putting the whole DC voltage across one cap. Is there any potential harm in leaving the 2 x 220K totem resistors permanently? Vielen dank.

[christarak]

Did you measure that with the HT fuse out? 74K is a clue. You shouldn't get 74k there..

Excellent sleuthing, Dude. I checked with fuse In, Fuse Out, Standby Open and Standby Closed. I'm away from my notes, but I think you're onto to something.

You must have either leaky caps or diodes. Lift the + side of the cap that goes to the rectifier and remeasure. That should tell you if the resistance path is via diodes or caps.

Edit: Also double check diode polarity and that they are installed correctly.

I have checked the diode and cap polarities, and I'm confident they're OK, but I will check again. When I get back to the workshop I will lift the + side of the cap and report back to you. (I'll be back on the tools after you're asleep - 7 hr time difference between SD and Sydney).

Thanks Dude.

[Chuck H]

Please see post #28. You'll probably have the problem isolated in twenty minutes or less.

[g1]

Please see post #28. You'll probably have the problem isolated in twenty minutes or less.

He has the HT fuse out (#5). So that would leave 6 & 7, yes?

[Chuck H]

He has the HT fuse out (#5). So that would leave 6 & 7, yes?

Ah... Then yes. Thank you. Not sure how I missed that FROM THE FIRST POST!

[christarak]

The Solution

[ATTACH]56788[/ATTACH]

I have what should be a very simple problem, but its driving me nuts. It came in with "Replace HT fuse holder" and "keeps blowing fuses".

I am not sure where is the best place to post the solution to a Problem. Right back under the original post? Or right after the last post?

I think I would have gone nuts if it wasn't for you guys coming up with your brilliant suggestions. In the end it was G1 who alerted me to a possible short at the junction of the 2 x100uF totem caps, and Helmholtz who said I didn't need to use totem resistors because of the CT of the HT windings performing that role. Enzo also had me double checking my wiring with the parallel-series-parallel configuration of the caps....

And there it was! I did it. I created this problem, or at the very best I copied the problem created by the Tech before me when replacing the filter caps (the amp came in with this problem). I had mindlessly connected the yellow CT wire to the + of the top totem capacitor instead of its negative (ie I should have connected it to the centre of the totem). My regret was writing down the connections during the process instead of taking photos with my camera.

Along the way, I learned some very very clever problem solving procedures, including "how to check if the switch shorting to the chassis", the "build an external bridge with no caps", "don't trust a component just because you put in a new one".

Even though you guys are on the other side of this planet, it felt like I had a dozen eyes looking over my shoulder and giving me ideas. Wow. That's an amazing feeling when you work on your own every day. THANK YOU all.

[Chuck H]

Even though you guys are on the other side of this planet, it felt like I had a dozen eyes looking over my shoulder and giving me ideas. Wow. That's an amazing feeling when you work on your own every day. THANK YOU all.

I love it when I get a real corker. I fuss, stamp my feet and pace around. My wife has learned to recognize this as my sort of fun But I can only enjoy it because I know that the solution is only as far as this forum If I actually thought I was completely on my own I'd be overwhelmed and frustrated. And there have been several times when I was in a hurry or simply out teched by a matter that wasn't optional and the guys here have always come through for me. Usually revealing some mistake or misconception on my part This forum became and has remained integral with my avocation and how I enjoy it. For more than twenty years!

So I know how you feel.

[Greg_L]

The knowledge base here is pretty damn exceptional.

[Helmholtz]

Yeah, I suspected a CT wiring error. Connecting the CT to either B+ or ground would cause the same symptoms by partially shorting the HT winding.
This will make 2 of the diodes get very hot (and eventually fail) and cause a strong increase of PT primary current.

Glad you fixed it.