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| | |-+  Hum caused by fried resistors?
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Author Topic: Hum caused by fried resistors?  (Read 4795 times)
elliptic
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« on: March 08, 2014, 07:34:13 PM »

I was playing with some friends last weekend at fairly low volumes and after about 20 minutes my Dizzy 30 made some horrible noises, then went silent, at which point I realised there was smoke coming out of the input jacks and wafting out of the back.  I had recently re-wired the speakers (a Celestion Blue and a G12H30) to be parallel instead of series by having a cable from each of them and using both of the speaker plugs, as according to the schematic these are in parallel.

And yes, I changed the impedance switch to 4 ohms.  I had been playing at home like that for a couple of weeks and everything sounded fine.

I thought that maybe I had misinterpreted the schematic and had blown the output transformer.  I wondered whether it was the whole thing or maybe just the 4 ohm output, so I unplugged one speaker, switched to 8 ohms and turned it back on.  Surprisingly it worked, which was lucky because I had a gig that night, which also went fine, even at medium volumes, although there's been a low level hum that I don't think used to be there.

Meanwhile, during the week some new EF-86 valves I ordered on eBay months ago finally turned up, so I've been swapping them around all week and having fun.

I wanted to see if I could figure out where the hum was coming from.  After eliminating the pre-amp as the source I decided to try swapping some output valves around (I have some JJ's that came with it and some Sovtek EL34Ms which I think sound better, but they all worked fine as far as I could tell.)

After trying a few combinations suddenly there's more bad noises and silence and smoke.  Fearing that the output transformer had gone for good I opened everything up, and the picture is below.  Can't see any scoring anywhere, what seems to be fried are the resistors on the output valves on one side (circled).  I also noticed that there is a brown wire that is lodged between the pins of one of the output valves (arrow on the diagram), and that it looks like the insulation may have melted.

This was built at Ceriatone, not by me, and everything else looks great, but surely that wire's not supposed to be there?  I'm in Australia, what are my options for getting it fixed?



* DC 30 problem.png (1420.31 KB, 1044x783 - viewed 452 times.)
« Last Edit: March 21, 2014, 03:27:25 AM by elliptic » Logged
elliptic
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« Reply #1 on: April 05, 2014, 08:10:06 AM »

So I replaced the four fried resistors and all was well for a few weeks, then last night a different pair fried.  Replaced them, and then this afternoon one of the original replacements blew again.

I'd switched a few things around as I went (like the rectifier tubes, I was using a pair of old ) and I came to the conclusion that the real culprit was the output tubes. Replaced the latest resistor (this time it was just the 1.5K 1w, not both it and the 100 ohm 3w), replaced all the output tubes and now the hum has gone, it was all back to normal for about 10 minutes except that now the master volume pot seems to be screwed, various dead spots between off and 10 o'clock and generally scratchy sounding.

It looks on the schematics to be a dual-gang 2.2M, can anyone confirm this please?

Beginning to wish I'd just dropped the extra $$$ to get a real Matchless...
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elliptic
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« Reply #2 on: April 05, 2014, 06:09:39 PM »

Never mind, looked a bit more closely at the schematic, dual 250K, but log or linear?
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cmoore
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« Reply #3 on: April 09, 2014, 02:01:54 AM »

It looks to me like the Cut Control is 250K.
The cross-line master appears to be 1M with a Push-Pull...On and Off switch incorporated.
Maybe your bad Power Tubes zapped it pretty bad when it took out the Grid and Screen Resistors.?
best
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