"Don Bowey" wrote in message
...
On 4/16/08 12:11 PM, in article
, "EricM"
wrote:

On Apr 16, 12:30 pm, Don Bowey wrote:
On 4/16/08 9:59 AM, in article
,
"EricM"



wrote:
On Apr 16, 7:25 am, Heinz Schmitz wrote:
EricM wrote:
Here's a link to the schematic; after replacing CR1 and CR2A/B with
10 amp 600 V silicon diodes, when the relay closes to enable the 600V
plate voltage, the main power fuse F1 blows. I'm not sure I need
dropping resistors, because the output voltage on the 12.6 and -38
terminals is very close to what it should be. Is there something I'm
missing? http://img410.imageshack.us/img410/4691/1567pscb9.jpg

I'd disconnect the relay (S-1 and VS3) and comfortably check all other
voltages (value, polarity, ripple). If ok, I'd power off and check the
electrolytics in the 600 V circuitry (otoh, if they do not look
exploded, they are probably ok). Then the fault is somewhere else in
the device.

BTW, what is connectors 2 on CR4 and CR5?
Does somebody switch from 8 to 2 at some point?

Regards,
H.

This is also a problem. The original 1N1239 rectifiers only have
three pins on the bottom at locations 4, 6, and 8, and the 'key'.
There is no pin on the device at #2, but the two wires from the relay
switch pole that is NC (until the coil is powered) are tied to #2 the
socket. (It's not connected to chassis ground either, which might be
inferred by the schematic - they're just meaning that two wires are
tied to #2 on the socket, even though there is no pin on the 1N1239 at
that location.) I ordered newer replacements for these from
americanmicrosemiconductor.com - they were the only place on the
internet that had exact replacements for the '1N1239' Sarkes Tarzian
center tapped rectifier. HOWEVER, when the new replacements - that
look much more like a metal envelope vacuum tube than the originals -
they have all pins and are shorter, there's no 600V on the output.
Just a few millivolts of nothing. The newer units must be wired
differently than the originals. The 1N1239 was a replacement for the
5R4 vacuum tube rectifier; I also located another online vendor that
sells 'C-Cap' rectifier replacements and he states that his 5R4
replacement should work. I'm debating whether or not to order some to
test though, since the originals seem to be oddballs of some type.
And essentially what the relay does, is shunt resistor R5, as pins 2
and 8 seem to be looped no matter what position the relay is in. This
one is really a pain - I've never encountered a PS like this one...

All the "relay" does is to complete a low resistance path from the 600 V
voltage source, to the filter caps AFTER a time delay has (gently via
15k)
pre-charged the filter caps.

Disconnect the 600V lead from the PS to the Amp and see which way the
problem is. If the fuse still blows, trouble shoot the PS. You do have
a
voltmeter?

Yes, the weird part about this is that before doing any work on the
power supplies (i.e. everything working) the output voltages with no
load were around 17 on the 12.6 legs, -38 on the bias, 6.7 - 7.2 on
the 6.3 volt legs, and nearly 930V on the "600" output. I used a
Variac to bring things up slowly after doing the modifications to
check the output and it was nearly the same on all outputs (again, no
load). Hooked the cables up to the amp and used a Variac again, and
the 17 was pulled down to around 9.7 once under load, the -38 remained
about the same, the filament voltages were all within 6.3 - 7.1V, and
the plate side of the output transformer read about 500V, but once the
voltage got nearly up to full input voltage and the relay closed
(110-120) the 5A fuse popped. If I disconnect the 600V connection at
the amp - the fuse doesn't blow. The amps and components test fine
and no changes have been made there since everything was working.

One thought though, and I'm not sure if it makes sense or not; I
measured the resistance of the selenium I removed in the bias circuit
- which shares the "common" with the 600V supply - and it was about
1.4 Meg.

Measured in or out of the circuit? Forward or reverse? At what voltage
was
the resistance measured? It's been a while since I worked on anything
using
a selenium stack, but....... They age. The forward resistance goes up.
I
was not ever impressed with the reverse resistance, and 1.4 Meg might be
ok.
When changing them out with silicon, I always added a series resistor. As
I
recall, I commonly used about 300 Ohms for a 75 to 100 mA selenium
change-out, but 50 to 100 Ohms might be more appropriate for this PS


The resistance across the silicon diode I replaced it with
is only around 830K.

Forward voltage drop aside - is it possible that
since the new component doesn't have the same resistance
characteristic as the new that adding a "dropping resistor" per se
would increase the resistance and balance out the circuit? I've put a
1/2 watt 330K resistor in series with one of the 10A 600V diodes that
I used, and the resistance then measures around 1.4 Meg, but I doubt a
1/2 watt unit would stand up to the circuit load. I'd probably have
to have at least a 10W unit, and 330K in 10W is hard to locate. Does
this make sense? I'm running out of ideas with this thing... :/


The forward resistance of a typical silicon diode will be about 700 ohms to
1k ohm, measured on a typical analogue multimeter. Reverse resistance will
be, to all intents and purposes, infinity.

Reading again where you currently are, and how you got there, it seems that
before you started 'modifying' the rig, it worked. The only implication of
this is that the current problem must be down to something you have done
yourself. Looking at the sorts of values of resistor that you are trying to
work with, it is clear that your understanding of how this equipment should
work, is a little sketchy. Whilst tube based equipment is actually very
simple in design concept, and normally a breeze to troubleshoot, it is
actually quite 'specialist', and if you are not used to working on it,
regrettably, you may be getting yourself out of your depth on this one.

Unless you have a clear idea of exactly how to troubleshoot the problem, I
think that it might be time to abandon your efforts, and call in some
on-the-spot help. I'm not trying to deliberately put you off - or put you
down even - just offering some good honest advice with the experience of
many years at this.

Arfa