Howdy,


A few of random suggestions...

The MJ2955 is a PNP and the 2N3055 are NPN.
The pass devices are NPN that make an emitter
follower output which provides current gain.

Load the supply until the voltage drops.
Then measure the base voltage at each 2N3055.
The bases should all be the same, a base to
emitter voltage drop higher than the output
voltage. One of the pass devices may have
failed or the drive current could be insufficient.

I'm guessing without a schematic but the MJ2955
is probably boosting the LM317's output and may
also be part of an over current foldback
protection circuit. Check the collector voltage
it should be nearly the same as the 2N3055 base
voltage. If it's much higher that points to a
problem with the pass devices. If it's not then
there's not enough loop gain.

You may find a high power low value resistor near
the MJ2955 for current limiting, check the value
of that too. If it's damaged or drifted high
you could be hitting the current fold back
protection.

The 21V bulk supply will sag if there is a bad
rectifier so a check of that under load is a good
test. If it's much below 18V that could be too
low. The LM317 needs about 3V head room and the
2N3055 will need a Vbe drop.

I think if the bulk cap had dried up you'd hear
hum even at lower load current.

You can trouble shoot this with a voltmeter,
stage-by-stage, once you load it until it's
exhibiting the problem. Once you identify the
failing stage a careful visual inspection followed
by some resistance and junction checks should
solve the puzzle.


73,
Grumpy



"Larry Gagnon" wrote in
newsp.uj37o6zpaiss39@localhost:

Hello. I have a homebrew linear 13.8V 20A power supply I have been
using for some years to power an IC-735, a 2M rig and some
accessories. The PS has these characteristics:

- well filtered (35000uF)22AMP, 21 volt DC output from diode bridge
circuit - LM-317 regulator circuit (as per LM-317 datasheets)
- MJ2955 NPN feeder transistor from regulator driving 4 - 2N3055 PNP
power transistors in parallel
- large heat sinks (I don't think heat is a problem)
- good air circulation with fan

I have the regulator set to 13.7 VDC out. Lately however, I have had
some problems with voltage drops: while drawing 20 amps the voltage
drops to about 11.3 volts and I have had reports of a rough CW signal
as a result.

I have tested the reg circuit and even replaced the LM-317 to no
avail. I cannot see where the problem lies, unless perhaps the MJ2955
feeder transistor is not working as it should? (I don't yet have a
replacement transistor to test that idea, hence this post).

Any ideas greatly appreciated.

Larry VE7EA