Gould OS3000A Repair Help

[frankplow]

Hi, I'm just getting into electronics and recently bought a Gould OS3000A CRO off eBay. It was listed as untested, and I was willing to have a go at repairing it if neccessary but I'm at loose ends. There's a PDF copy of the manual for the Gould OS3000 online (http://www.kevinchant.com/uploads/7/1/0/8/7108231/gould.__os3000._service_and_operating..pdf) but not the 3000A. The differences between the two models are supposedly pretty minor according to some forum threads online, but I have noticed a couple differences. If anybody has a copy of the OS3000A manual, a scan would be much appreciated.

The scope displays a dot with no horizontal deflection from the sweep or horizontal shift knob. The vertical deflection appears to work fine on the first channel (a square wave results in two dots with correct seperation) but not on the second, and both channels can be shifted with their respective knobs. There is also no intensity control or graticule illumination.

From the symptoms, I suspect the main problem lies in the horizontal amplifier board. All the power rails read a little high, but nothing that I think should be catastrophic. The scope's manual lists a few typical voltages, so I've checked all of them: TR953 & TR954's collectors read -13.1V and -14.1V respectively, versus the -8V the manual says should be expected. The collectors of TR956 & TR957 (which are connected to the horizontal plates) are both at 155V, versus the typical 60V. The manual also gives expected voltages for TR968 & TR969, but I can't find these anywhere on the X-amplifier / interconnect board, so this may be one of the differences between the OS3000 and OS3000A? I've found posts online saying that Advance did change some transistors when remodelling scopes, so this may be an example of that.

I've replaced the electrolytics on the horizontal amplifier board, however the old caps didn't show anything out of the ordinary on my component tester. I also desoldered the HV transistors (TR256 & TR257) and tested them as they areheatsinked and seemed the most likely components to fail, but they also didnt show anything strange on the component tester. None of the transistors are socketed, so ideally I'd like to have the problem a little more localised before I go testing every component.

Any help on this would be much appreciated. I have access to a working oscilloscope to test if needbe.

Thanks in advance.

[orbanp]

Hi Frank (if that is your name?),

The lower voltages on the collectors of TR953-954 could turn off the final transistors, this could be the explanation why the collectors of TR956-957 are at the supply voltage and not at +60V.

One approach I found very helpful to debug such circuits is to model parts of the circuit and see where is the discrepancy.
To get going, modeling the driver diff-amp stage, and the final stage, would give you the insights to the circuit.
You also need to find out that at what DC potentials are the "RAMP" and "SHIFT" inputs.

EDIT:
This site has a manual for the 3000A scope:
https://elektrotanya.com/gould_os3000a_sm.pdf/download.html
Interestingly, the horizontal deflection board is the exact same version as in the plain 3000 scope!

Good luck, Peter

[orbanp]

Here is a quick simulation of the DC operating points of the horizontal final amp.
Hope it helps.

Peter