Beckman 20 MHz 9202 Oscilloscope Repair

[ericwilk]

Beckman 20 MHz 9202 Oscilloscope Repair

Hi there. I just acquired one of these and the unit powers on but no traces. When you try to add a lead, ground and get a sweep it pulsates the display up and down (the readout) but no trace. Tried internal calibration and audio signal. Fuse is fine obv.

The vibrations did seem to occur when I had the cycles per time set to harmonics of 60 Hz which seems suspect (60hz wasn't an option). The PSU is PT-5500M and although the chassis is grounded directly to the AC in ground (and many board points use that) there is a green lead on the transformer that registers nothing with a continuity test (green into power board, chassis, etc.)

I'm assuming a subset of components uses this lead as a ground... I'd assume the green on the PSU is ground but I don't want to guess and potentially start a fire. I could solder this to the chassis though. The 110/220 switch seems to be modified. Nothing is hooked to green, it looks like white joins with green explicitly via a rubber-sealed connection (which I did not open so who knows if that's the culprit) but regardless it still should have shown on a continuity test.

Any ideas? I'm not an oscilloscope expert. I could always play with the 1,000+ servos, instead

 

[ericwilk]

Brown is common orange hot. I have a crappy multimeter but it didn't blow. (AC ratings are high though. I destroyed the thing with 2 Li+ cells in parallel which is < 2A). I see no way green isn't ground. Let's try this... I just need to find the right gauge wire.


Also, will test continuity on the other wire with the same marking just in case (sanity check).

EDIT -- I'm an idiot and could have been electrocuted. Hot to ground will produce a current :V

Glad I thought of that. Tracing out the wires is tough since there are so many taped and my meter is a piece of crap. I'm guessing the wires labelled 0 are ground but I don't want to guess.

 

[davenn]

before you go any further, and destroy the unit or zap yourself ... do some serious searching for a service manual or at the very least a schematic

in this section of the forum and down the page a bit, there's a whole list of service manual sites ... one of them may be able to supply the manual


cheers
Dave

 

[ericwilk]

Okay, got the schematics. The outlined part is the transformer I'm talking about. The rest is the power board (looks like AC to DC converters and to TV)

http://i.imgur.com/mQOigcZ.gif

For each connection there are 2 labelled the voltage (110 or 117) and one labelled 0.

I would think the one labelled 0 would be the ground but it's going to the main switch which makes no sense.

Here are more pics for reference (may want to go bottom-to-top):

http://i.imgur.com/7XT36xA.jpg
http://i.imgur.com/tfaC25Q.jpg
http://i.imgur.com/j0KP2aa.jpg
http://i.imgur.com/QHapxbh.jpg
http://i.imgur.com/d5Ftm0t.jpg
http://i.imgur.com/4dT22lD.jpg
http://i.imgur.com/6KlGJsF.jpg
http://i.imgur.com/zNpelJP.jpg
http://i.imgur.com/haFkJsP.jpg
http://i.imgur.com/sYT6a9j.jpg

The covered leads on the PSU read "0 V"

Oh, and sorry everything is upside down but I have the unit turned over to get underneath.

Thanks in advance!

 

[ericwilk]

Actually it doesn't look like the PSU is even grounded.

I'm ruling out noise by isolation from the AC source then calling this quits for a while.

If anyone knows what else could cause this, PLEASE let me know ^^