Oscilloscope. To repair or not? Fixed.

[clay1905]

The 'scope has been diagnosed and repaired. I'll briefly cover the main points-

The faults:

All the resistors in the voltage divider were open, including potentiometers.

Some (most) of the resistors in the intensity control circuit were open, others were out of range. These resistors are in an area on the horizontal board where a red, and a white with red trace wire are connected, rear R/H corner, and cover an area about the size of a matchbox.

The high voltage was chronically low in some circuits, but not others. This was quite a mystery until a sneaky 0.02uF 1500 volt capacitor linking cathode (CRT pin 3) to the external cathode input terminal was checked. Follow the yellow cloth covered wire. This capacitor is in the area for the voltage multiplier, under the socket end of the CRT and directly behind the banana sockets on the rear.

The resistor from cathode input to ground was shorted. This was at first thought to be the shorting link, but tested faulty when disconnected.

There is a 200K ohm resistor inside a transparent sleeve that is connected to the centre leg of the front panel intensity pot, alongside a white with red trace cloth covered wire. I would strongly suggest testing this resistor as well. All the failed resistors were in high voltage areas, and this resistor is as well.

Schematics will be posted on the web soon. As soon as I have the U.R.L. I will publish here.

Once all the faults were attended to, a trace appeared. Wobbly at first, but over time has settled down to a nice crisp straight line.

Thanks to all those who constructively commented on this subject. Learning the basics of diagnosis has been one steep, but very enjoyable learning curve.

Regards,
Clay.

P.s.
If you are inexperienced around high voltage, be very, very careful. I blinked at the wrong moment, and I'm up for a new DMM. 

[tautech]

Congrats.
Sorry to hear about your DMM, that was bad luck. In the future sharpen the points and even consider sleeving the shank just leaving the point uncovered.
Your experience on this scope with the failed dividers and other EHT part is very typical of old CRO's, they're some of the parts of the scope that are under the most stress.

[John Carlaw]

Good to see it is working.
Just some history.
These were common as university undergraduate lab project instruments in the early 70s,
Pretty sure it was the Jayem insignia, there were a couple of similar ones.
At least at UNSW, a dozen in a lab for physics/engineering labs, not top of the line but not bad until they let you loose on the expensive gear  .
In the late 70s , around 1977 ?? most were replaced and auctioned off.
Got one cheap at auction for $100  that is $530 in 2016 money and it seemed a decent price by comparison to new gear at the time.

[Housedad]

Wow,  I remember all the surplus stuff back then.  That was when (1978 or so) That I got my first oscilloscope.  A surplus Tektronix 535 with a dual amp drawer and a rolling stand.  I think I paid around $50 for it.   Ah, memories.

[Gadgetman]

Hi pricemp. I know, this topic is old and I am not sure you will even read it. I am in the process of trying to restore the 555 Oscilloscope too. I was given it for free and it is in much worse shape than shown in the photos attached by Clay1905. A lot of dust inside and slightly rusted outer cover. I cleaned most of inside - at least to a workable level. It turns on alright, but no trace as yet on the CRT. I replaced already dead 12AU7 tube with the spanking brand new one. The trace is still not there, so I have to hunt further. I tried to locate a schematics for it (I have the user manual but without schematics). You mentioned that you have it. Could you P L E A S E make it available?