2235 AN/USM-488 power supply low voltages


richardallred@...
 

Im still struggling with this power supply

If I remove the A5 board from the scope the power supply voltages return to normal and can be adjusted to spec by adjusting R938. As soon as I plug the A5 board back in the voltages drop to the following levels: and R938 has no affect
+5.2=4.3
+8.6=7.4
-8.6=-7.3
+30=+25.8
+100=+87.8

Also when I adjust the Scale Illumination knob the with of the trace shrinks

Any and all suggestion would be appreciated

R A


richardallred@...
 

On Fri, Mar 5, 2021 at 09:30 PM, Charles wrote:


1N4735A
So I had one of those is stock. I installed and I still have low voltages across power supply and adjusting R938 has no affect on the voltages.

I checked (R937/938/939, VR943 ) Vr 943 was bad, but has been replaced. the rest were good

The scope does show a trace and the vertical seems to work , but the horizontal only takes up about 4 graticules, I can move it from side to side but is is jumpy and of poor quality.


 

How about a 1N4735A?


richardallred@...
 

It looks like vr943 is bad!

service manual says it 6.2v 5% 0.25m, do-7. mfr code 04713. mfr part number szg20012

any modern equivalent suggestion please

R A


 

On Fri, Mar 5, 2021 at 04:31 PM, Raymond Domp Frank wrote:


You may consider noting as exactly as you can the current -7.3V of the -8.6V
supply to be able to see if the voltage changes by slightly adjusting R938 and
carefully readjusting it to the current voltage afterward.

By temporarily re-adjusting R938 you'll likely be able to see if the drive level of the inverter is hard enough. All voltages being off by approximately the same percentage indicates there may be a drive problem, not a load problem.
If the 8.6V can be adjusted up and down, there's probably a reference- or compare-problem (R937/938/939, VR943 and possibly R943, which may have badly drifted).

Raymond


 

On Fri, Mar 5, 2021 at 02:18 PM, <richardallred@me.com> wrote:


Yup, I knew about that procedure, but was trying to avoid that until I was
sure that was it. It's my understanding once you do that you have to calibrate
the entire scope. Is that correct?

Yes, after adjusting the -8.6V you'll have to calibrate/adjust the 'scope for it to be as accurate as it is capable of.

Be aware that *everything* is connected directly to the mains, until after T948!

Since none of the voltages currently are ok, calibration will be off - if the 'scope works at all. The error is around 15%, which is far too much.

I don't think your problem simply is just a matter of adjusting the -8.6V, *unless* someone misadjusted it or any of the voltage-determining components have drifted. I'd start by checking R937, R938, R939, and the voltage across VR943.

Repeat: Be aware that *everything* is connected directly to the mains, until after T948!

It's a very good idea to fully calibrate/adjust a 'scope which hasn't been calibrated for a long time.

You may consider noting as exactly as you can the current -7.3V of the -8.6V supply to be able to see if the voltage changes by slightly adjusting R938 and carefully readjusting it to the current voltage afterward. The last calibration may have been done with a slightly different voltage than exactly -8.6V and this way, you may be able to restore that. If you find a problem and repair that, the -8.6V hopefully will revert to a voltage close to what it has been adjusted to during the last calibration.

Raymond


 

On Fri, Mar 5, 2021 at 03:35 PM, <richardallred@me.com> wrote:


adjusting R 938 has no affect on the -8.6 volt supply

R938 is the -8.6V adjustment pot...

Raymond


richardallred@...
 

adjusting R 938 has no affect on the -8.6 volt supply


richardallred@...
 

Yup, I knew about that procedure, but was trying to avoid that until I was sure that was it. It's my understanding once you do that you have to calibrate the entire scope. Is that correct? Also will that fix the issue of the trace only displaying on 2/3rds of the screen,

thanks Again

R A


Ozan
 

Voltages as follows


+5.2=4.3
+8.6=7.4
-8.6=-7.3
+30=+25.8
+100=+87.8
All the voltages are about 86-88% of their target value. Especially -8.6V is the reference for all other supplies so if it is off by some percentage all others will be off by about the same percentage. "Power Supply and CRT DISPLAY" adjustment section (page 5-4) of the manual gives the steps for R938 trim which should fix -8.6V and all other supplies if there is no other fault.

Ozan


richardallred@...
 

I bought this scope earlier this summer as new old stock, never powered on. Due to covid illness I never opened it until a week ago. The scope powered on ,but the trace only covered about 2/3s of the screen and was jumping all over.

After it was warming up only got worse, I decided to check the voltages on the power supply and all were low.

So I did a full recap (caps are still old even though the scope has zero hours) and also replaced the following:

VR 901
Q935
Q946, 947
Q9070
R888,889,890,891,892
R900
and the ac filter
all 3 rifa x class caps


Same problem still exist

Voltages as follows


+5.2=4.3
+8.6=7.4
-8.6=-7.3
+30=+25.8
+100=+87.8

resistance to ground;

+5.2=188oms
+8.6=129.8 ohms
-8.6=81 oms
+30=552 ohms
+100=16k ohms


also the trigger led blinks when the scope is on. I have been reading all the power supply post here and frankly, I'm stumped

This is my first attempt a troubleshooting a switch mode power supply. any and all suggestions would be greatly appreciated

Thanks in advance
R A