Re: Tek 2235 power supply

"bico959 via ElectronicsKB.com" <u37735@uwe> wrote in
7958851db54e4@uwe:">news:7958851db54e4@uwe:

Hi Jim, did you manage to get the download?

Per your last post,I downloaded the PS schematic;I was NOT going to try
downloading 70Megs of pdf on a 48K dialup.

Here is my latest
findings. I'm not over volting. When I measured on the correct ground
point, test point 950, test poin 940 never quite reaches 40 volts.
This would tell me that there is an over current problem however I
have lifted all the jumpers (+/-8.6, +5, +30, +100) and I removed the
voltage multiplier completely from the board and there is no change.
So is the over current on the primary side then? I went ahead and
removed the GP caps and put in hi quality Nichicon low ESR's in their
place. I have a good sawtooth wave at pin 5 of U930, oscillation is
visible through Q908 and the FET. The SCR doesn't have anything on the
gate. I can see the voltage drop at R926 (it measures 150k) when it
'chirps'. There is a short duration 5V pulse from the IC at pin 14
(the ref output) and I can see a small increase in the voltage at pin
16 also timed with the chirp. R907 measures good (.2 ohm). On the
other side of the transformer, each rail tries to come up gaining
about 3/4ths their normal values and then falling again. My probe hit
and killed Q930 and now I have to order a replacement for it. I see 1
volt at pin 12, no chirping and Q930 is resistive now...

Any help is greatly appreciated. I will paypal you a beer! :-)

~RB


have you replaced C925? 100uf/25v,IIRC,Low-ESR.
that is the housekeeping supply cap for the TL594.
it charges thru R926 to about 14-16V,which allows the IC to start,then a
winding of that inductor takes over supplying power to the IC,thru
CR920.Otherwise,the IC quickly draws down the charge on C925,and the IC
stops.

--
Jim Yanik
jyanik
at
kua.net
.

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