PS120? Brand X O-scope issue

[DavGoodlin]

I have this neat little scope with a Dumont 3" CRT, 6GH8, 12BH7 and 6GM6 in the vert section
All seems ok but the vertical seems to deflect about half a cm with 6.3 VAC on the most sensitive rahge.

My first move without a circuit schematic was to check power supply voltages and caps. The B+ from the 6CA4 rect tube is about 400 v, so that cap was a prime suspect and the 1k resisitor 325v to the next section was getting hot. Two new 450 v e-caps, no improvement.

[DavGoodlin]

The 1963 version of my RCA receiving tube book has a typical oscilloscope schematic posted. I know its an RCA model WO-91, and the vertical section tubes are close to what Im looking at. A check of the tube voltages may turnip a clue.

[MickM]

Hi;
I found the schematic and have attached it.

Mick M

[MickM]

The zip file was too big.
So I sent the gifs.

Mick M

[MickM]

Here is the rest.

[DavGoodlin]

Thanks Mick, That helps me undertsand how the stages are laid out. My voltage checks have turned up nothing too abnormal. The 6GH8 pentode section seems to be full conduction but the bias resistor is OK.

The front panel reads "professional wideband oscilloscope PS-120" but its certainly not a Sencore, which popped up on a Yahoo search.

[DavGoodlin]

After eliminating 8 tubes and capacitors as a possible cause, I decided to get smart (like Don Adams) and fire up the old VTVM I took VTVM readings of DC voltages on the 6GH8, 6GM6 and 12BH7. I swear by a VTVM versus my Fluke DMM for working on tube gear anyway

The first two tubes seemed to be biased OK, the 12BH7 seemed to be in a state of extreme cutoff on both triodes, output stages driving the deflection plates. I pondered the only reference I had, the RCA WO-91 schematic.

Enjoying the warmth of the attic bench, the snowfall and time off work...then
suffering an attack of Technician's A.D.D., I grabbed another recent acquisition, Triumph model 830 Oscillograph Wobbulator. A very interesting unit from 1943, I figured it was an easy fix. I reflexively started re-capping it until it responded with basic functions in a similar manner to the one Phil Nelson donated to a Navy Museum. Richard Sears also wrote a nice piece on it in Wordpress. I was suddenly able to understand the signal path of a scope's vertical input with this classic unit, so I then grabbed my faithful vintage scope, the RCA WO-91B to do the actual tracing - measuring.

Tracing the test signal through the stages of the PS120 and got as far as the grids of the 12BH7 with an amplified sinewave. Then nothing at the plates. The tube was definetly in cutoff, but why Bias network resistors on grids were OK. On to the cathodes, grounded through two 6600 ohm "power" resisitors, so what is the chance both are bad??? They were odd wirewound "sand-sticks" and appeared to be corroded where leads attached.

They both measured OPEN, mystery solved! I located two 'dog-bone' 4 watt 6.6 K resistors in my parts drawer.
The power supply and horizontal stage, I got the last two wax caps hiding on the sweep selector
PS120 horz.jpg
The vertical stages
PS120 vert.JPG
A simple unit showing a simple sinewave
PS120 working 12-31-12.JPG
The Triumph and PS120 display composite video from a VCR
830 and PS120 display video.JPG

[mlmcleod]

Does anyone have the tube layout for this device?

[MadMan]

It could be carefully inferred from the wiring diagram, in a pinch.

[maxhifi]

The way that power transformer is mounted, is something else!

Nice little scope - I use a tube scope all the time too, mine is an old Stark. Not the most portable piece of equipment, but it's sure helped me to figure out a lot of different problems over the years.