I need some advice and insight from the amplifier gurus in this group.
I have a Heathkit SB-220 which I purchased two years ago. I upgraded the
linear with Harbach's mods (Sorry Rich) as follows:
1. Replaced the capacitors with new capacitors and bleeder resistors.
2. Replaced the existing rectifier board with a new board. This board
includes the replacement of the zener with 8 - 1N4005's.
3. Installed the Parasitic Suppressor Kit.
4. Replaced the meters.
5. Replaced the Fan.
6. Installed the inrush current limiter upgrade.
7. Installed the Soft key switching module.
8. Installed a 'home-made' H.V. fuse using #30 wire .. 1/2" long.
You must be thinking ... why on earth did he do all this stuff?
Well I like the SB-220 and it was less expensive to buy the amplifier and
make the mods instead of buying a new amp.
Bottom line is ... economics?
Here is my problem. About a few months ago I was operating normally and the
H.V. power supply failed.
There was no "flash" or "Bang" ... just quit. I have had the "flash" and
"bang" when I first fired up the amp when I bought it and operated it
without any of the mods being installed. That's one on the main reasons why
I initially installed the Parasitic suppressor kit.
I opened up the amp and found that one of the 8-1N4005 bias diodes had
failed. It was quite obvious ... charred and black circuit board at the
diode location. The H.V. fuse was still intact. I called Harbach and he
couldn't think of anything obvious that would make the diode fail and he
concluded that possibly the diode may have been faulty.
I replaced the diodes and operated normally for a few days and the same
thing happened. A bias diode had failed again.
It should be noted that I always operate the amp in CW/Tune (1kW) position.
If I operate the unit at SSB (2kW) the H.V. Fuse will open with a "flash and
a bang" ... yes ... even after the mods.
I haven't HI-POTTED the tubes yet but they look clear (no blackening).
I'm sorry I'm so long winded here but this is my first posting to this group
after monitoring it for a while.
Does anybody have any idea why the bias diodes would fail?
Thanks,
John (VE7JDB)
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