Tek Model 305 problem


wallace.gibbons
 

Greetings,
I have a 305 scope on the bench with a small problem. There's a small pulse
type waveform riding on top of any waveforms being displayed. It can be
seen with another scope by probing the vertical output amplifier outputs to
the CRT, and the vertical outputs from the main board.

If I disconnect the input to the vertical output amp board the trace is a
nice flat line. Good focus, very sharp line.

I assume from this that the errant signal is coupling into the vertical
channels on the main board. Probing the power supply outputs with another
scope has proved that only one supply was "noisy". I saw the pulse type
noise on the +150 volt power supply. I changed the two 10uf filter caps to
no avail. The +150 volts really isn't used in the signal chain but I
thought I'd eliminate it as a source.

The plus and minus 3 volt supplies look clean.

Has anyone seen this on a Model 305?

I know it's not a top of the line scope, not worth a whole lot, but it's a
friends and it's good enough for him to work on guitar amps and such.

Thanks,

Wally Gibbons
North Logan, Utah