Mystery vintage oscilloscope

[djrm]

   

I got this oscilloscope from a radio rally sale a few weeks  back, just got around to looking at it.
I found a couple of faults already and its now showing a trace.

I have no information on it, there is no makers name I can see but I think it is probably a commercially manufactured product.
It could be a kit but the wiring is all nicely laced up. 

I haven't managed to work out the purpose of the buttons on the front, some affect the Y gain, others are marked with sync and plate.
Has anybody seen anything like it before?

What would the date of manufacture be, 1950 perhaps?
I'm familiar with this kind of thing, I made something similar when I was at school.
I don't have the one I made but over the years I have acquired some replacement vintage 'scopes.

   
   
   
   
   
   
   

Kind regards, David.

[Amie]

Hi David.

The knobs look very Cossor, but the rest doesn't look like that kind of quality or heft

.....

[djrm]

Thanks Amie. Looking closer I think the front panel may have been repainted which could explain the hand written control legends. I'll shine a bright light on it and see if I can see anything peeking through. David

[ppppenguin]

Probably home built. possibly modified from war surplus?

[Jem]

It looks like a conversion of a Government Surplus WW2 indicator unit, possibly an ID6A or similar Loran unit. Similar knobs were used on various equipment including the R1155 receiver!
John

[Andy M0FYA]

I don't believe it is a conversion of any of the WW2 indicator units, but a competent home-build from scratch using surplus components as they were readily available.

Andy

[djrm]

Although it looks similar to the indicator units I have seen its unlike any in detail. Time base and power supply for instance. I wondered if it had been made in the same factory as an indicator unit. I agree it could be a competent home built, done to a very high standard in my opinion. David.

[djrm]

I have been tracing out some of the circuits on the 'scope. it uses a three valve puckle timebase, feeding a long tailed triode pair tube driving amp.
The Y amp is a single valve feeding a cascade pair of  pentodes to drive the plates.
By operating the push buttons on the front the Y amp can be bypassed and the input signal fed directly to one of the Y plates.
There is no provision for trace flyback blanking, unusual as it is easy to add with the circuit used.

What I can't understand though is the addition of some diodes in the Y amp plate bias network, I have never seen anything like it before.
Can someone suggest what is the purpose of the diodes. they look as if they would perhaps do some kind of level shift or clamping.
The diodes are a point contact variety of some kind, there are no markings.
See attached photos:

   
   
   

KInd regards, David.

[ppppenguin]

My first guess on the diodes is that one of them will allow the shift control to work with less lag in one direction, by reducing the time constant wrt the coupling caps. I've no idea why this should be a good thing.

I also wonder if it's to provide a lower impedance path for any stray current on the Y plates. It's possible that a very small current will flow from the beam to the plates in normal operation. A current will definitely flow if the Y shift is at an extreme and the beam hits a plate.

[djrm]

Thanks Jeffrey, your ideas sound plausible. by the way, the diodes are fitted only to the X axis plate circuits.
Today I have improved the HT smoothing and tried the Y circuits with and without the diodes. Having them fitted does improve the Y shift response.
A side effect due perhaps to the resistors on their way to infinity and differences on the diode characteristics is that the trace centre line moves as the signal level is increased. This doe characteristic does not completely disappear with the diodes removed but it is less noticeable.
I also linked the timbase pulse to the Z input, this gives a nice trace without the flyback being visible.
It maybe worthwhile changing the high value resistors and capacitors, maybe even replacing the diodes, or I could just leave well alone.
All the critical components seem ok, some even look to have been changed already by the PO.
I wonder what vintage those diodes are, there is a right hotchpotch of components, some are marke AM. The rectifier valves are 4 pin pre-octal types.

Kind regards, David.
   
Excuse the scrappy notes...  shows the valves used, approx measured in circuit resistor values, CRT divider chain and EHT supply.