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Old 1st November 2012, 04:33 PM   #11
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Good tip, thanks Kevin! I'll have to see if we have any incandescent bulbs left.... I'm guessing that compact fluorescent bulbs might confuse the issue
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Old 1st November 2012, 06:55 PM   #12
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Pssst...hey buddy - I know a guy who knows a guy who can get those contraband bulbs...but it ain't gonna be cheap!
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Old 2nd November 2012, 12:51 AM   #13
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Actually, now I think about it, there are still "appliance" bulbs of the incandescent type available, for oven lights etc. - might even have one or two stashed away somewhere.

Ed
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Old 4th November 2012, 01:42 AM   #14
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Progress today! Also some pictures that I'll resize and post later.

New drivers and outputs were fitted to the damaged channel, and I decided to test at that point. PSU fuses for that channel were replaced (the other left dead) and the amp run up with a voltmeter on the output. Nothing scary on the Fluke, so I connected a speaker... All quiet, and the only signal I had available was my finger on the input, which produced a healthy bzzzzzz. The other channel tested similarly :-). So far so good...


Then, of course, I decided to work on the other channel, again with new outputs and drivers - to keep things consistent. On completion I went to repeat the test above, but when the power was applied and the PSU circuit operated there was a loud POP and a blue flash near the board on which I'd just worked, with one PSU fuse blown. Imagining a total loss, I started to probe. Outputs OK, drivers a bit funny but not shorted, sure enough, I found one of the drivers to have a short circuit to the heatsink, thanks to a metal burr. Miraculously, this was a device whose collector was direct connected to the PSU, so the fuse was the only casualty! A lucky escape indeed! After careful reassembly, the unit powered up fine, and showed operation in both channels. Tomorrow, I'll complete reassembly of the chassis, and try it with music. Quite a day...

Cheers,

Ed
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Old 4th November 2012, 01:54 AM   #15
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Congratulations. Nothing like a little good luck. I used all mine up on my first transistor amp (ST120), the one after that was a 15 month horror flick.
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Old 4th November 2012, 04:07 AM   #16
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Thanks! I may have bought the luck... Humbled am I to say that I recently sought professional helpto get our Adcom monster amp fixed. It had lain unprobed for a couple of years, with no time to investigate it, so I had a local shop look it over and carry out repairs. It was good to get it back in our system and just too flipping good to lose by neglect. This Dynaco project was of a much simpler nature, even including the mishaps, and much easier to work on mechanically. Can't wait to see how it sounds

more tomorrow, hopefully I'll find time to sort through the pics

Ed
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Old 4th November 2012, 03:13 PM   #17
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If you want a winter project, or the ability to drive 2Ω, convert it to a ST416. This involves drilling the heatsinks to accept 2x the number of outputs.
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Old 4th November 2012, 06:12 PM   #18
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Thanks djk - I actually have the original paperwork explaining the modifiation.

UPDATE: today I was all set to try the amp (speakers, & my phone running Pandora for a quick trial). Alas, during the night, another of the drive transistors had shorted to the heatsink. This time I was not so lucky with the current path - at least one driver and one output has been destroyed.

I'm highly annoyed by this, because I double checked every mounting carefully yesterday after the previous incident, and all were good. A fresh order for new parts has just been completed, including new insulating bushings for the drivers.

Music will have to wait until next weekend
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Old 4th November 2012, 09:18 PM   #19
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Not wanting to leave anything to chance, all the drivers were removed again from each board. Two problems are evident

1) The bushings are a tight fit in the new devices, and can crush instead of fitting neatly into the mounting tab

2) There were burrs on the heatsink, though I thought I'd eliminated this AND also on some of the transistors. Evidently just enough in some places for the two unwanted edges to meet...

Now these causes known, I can go about eliminating them properly. I wonder if they were he original source of trouble?
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Old 4th November 2012, 09:42 PM   #20
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Hi,
Normally when I mount transistors to the heat sink I double check resistance between the pins and the heat sink to make sure there it is not a short with the heat sink. it is save some transistors and time.
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