Vintage HP Oscilloscope 1220A - No power.

[michalism]

Hello all, and happy new year!

I took my chances and bought this old oscilloscope on an Ebay auction. The seller clearly mentioned that it is not powering up, but it was only 10 Euros, and I was the only bidder, so here we are now.

The first thing I checked when I received the unit, was to measure the resistance of the two power pins, and sure enough, I get no reading, so I guess the primary of the transformer is fried.

I will take the cover off, and see if there's some kind of a thermal fuse that may be tripped off, but apart from that, do you have any other ideas?
Is there a way to save this unit, or should I just donete it to a museum?

thanks in advance!
Michalis

[michalism]

I pulled the tranformer out of the chassis, and tried to take a better look at things. Took some pictures also.

I was under the impression that the power from the IEC connector goes directly into the primary, but this does not seem to be the case. Based on the schematic, and my primitive investigation, the power is fed to the primary of the transformer after it goes through the main power switch, a capacitor, and the voltage selector switch. Here's the relative page from the service manual


Now, when I opened up the unit, I saw two connectors attached to the tranformer. The smaller one, takes the line votage from the power connector and the voltage selector switch, and takes it to the main power sitch at the front. The second connector leaves the transformer and goes to the main board.


So, repeating the measurment of the primary with the power switch at the "ON" position, I do get a resistance value around 50 Ohms, which I suppose is ok.

Some more pictures in case they help.

[michalism]

ok...
News update...
The oscilloscope is working fine...Haven't changed anything; I just put the transformer back in the chassis, and powered the unit up to start measuring voltages and do some troubleshooting.
Once I turned the power on, the power led lit, which was a pleasant surprise since that was not the case this morning...
I checked the 4 power lines (-/+ 12V, +95V and +220V) and they're all fine...so I waited a little and after a while the green dot showed up to my delight!
Still, I need to run a few tests to make sure everything is ok, but at least the main functions are there.

Thanks for the attention