1959 Jason kit OG-10 oscilloscope now restored

[howard]

Hello again,

Good news, restoration work on the Jason kit OG-10 oscilloscope is finished and it works, in fact it works well despite its 48 years. Ron Bryan restored this scope and here in his words is the work he did to it.



The Jason OG-10 is a simple 2.75" AC coupled single-beam oscilloscope kit, with a 2MHz -3dB bandwidth and a 10Hz to 100kHz timebase. As found, the scope had a cut off mains lead and the case screws were missing but otherwise it looked complete, with all valves in situ. There was plenty of evidence of previous repair/restoration, with two silicon rectifiers replacing the original selenium unit and some replacement high quality 1% resistors and polyester capacitors. Some modifications had been incorporated, a fuse had been fitted in the EHT supply and wire wound resistors had been put in series with the tube and valve heaters, to reduce the filament voltages to around 6V rms according to my meter.

Separate HT and EHT transformers are used in this Jason design, which on the face of it owes a lot to the Heathkit OS-1. A megger check confirmed that the transformers had not failed, but before powering up, an attempt was made to re-form the HT and EHT electrolytics. A gradually increasing voltage from a 100V power supply in 10V steps was applied and the leakage allowed to reduce to around 1mA before applying the next increment. Mains was then slowly applied via a Variac and a slightly dim trace appeared, which would focus quite reasonably. The timebase had a non-linear sweep on the lower ranges, and the trigger was weak but otherwise it seemed ok.

The capacitors on the two lower ranges of the timebase were leaky and so both the sweep (0.05uf and 0.005uF) and flyback (0.02uFand 0.002uF) timing caps on the timebase switch were swapped. This sorted out the non-linear
sweep.

An EF80 in the differential Y amp output stage decided to fail at this point, causing the trace to disappear from the screen. It was swapped with a good used one.

The HT was low and the HT transformer seemed heavily loaded. Most of the problems seemed to be with the 32+32uF reservoir and smoothing electrolytic, so this was swapped for a new JJ cap from Savoy Hill (£7 +£1.50 p&p).

A few other suspect capacitors were changed, a leaky 25uF electrolytic coupler in the Y amp cathode follower, a 0.01uF coupler in the sync amplifier input and two series connected 0.1 uF 500V caps in the blanking circuit for a 0.047uF 1000V polyester. The last two caps were so large they were seriously restricting airflow through a large ventilation hole in the chassis, so the smaller replacement sorted that out. A large ropey looking toffee coloured 0.22uF 750 V DC Wima capacitor decoupling the EHT on the brightness control was changed for safety's sake, although a 250V Megger check showed it to be blameless.

The X and Y shift circuits had been modified from the original Jason (and Heathkit OS-1) circuit. I tacked in components to revert the Y shift to the standard arrangement, but immediately found out why they had been changed - when the Y shift was adjusted, the original shift circuit suffered severely from a terrible 'overshoot and drift back problem', caused by long time constants in the differential amplifier DC conditions. This is exactly what you don't need when using a 'scope, so I quickly re-incorporated the modifications.

Finally the cut off mains lead was replaced with a new three core PVC cable and the case screws replaced with new chrome 1/4" 4BA pan head screws obtained from Margnor (Fasteners) Ltd (20 for 30p), a local company dealing exclusively with fasteners.



Pics below of the scope now, and the traces on the scope taken in the dark over a 3 second period, showing a nice straight line across the screen after switch-on and then a stable calibration trace, and also a pic of the replaced parts. All the controls now work perfectly too.

This Jason scope is a nice little thing, there's no rust on it anywhere and it's in excellent cosmetic condition and we feel it was worth restoring, and Ron has done a splendid job on it too ........... even if it is only suitable as a museum piece !

Howard

[Brian R Pateman]

Well done!

These were never "state of the art" but they were all many people had available when they were new.

Although I have no experience of the Jason, I don't remember the OS1 having an overshoot problem.

Good to see pictures of it working.

Regards,

[XTC]

Originally Posted by howard

This Jason scope is a nice little thing, there's no rust on it anywhere and it's in excellent cosmetic condition and we feel it was worth restoring, and Ron has done a splendid job on it too ........... even if it is only suitable as a museum piece !

Impressive work. I'm not sure it's only a museum piece. Even if you don't normally use a scope for restoring radios there are odd occasions when it's nice to have one, and in any case, it pays to fire them up every few months.


Pete.

[Mike Phelan]

Nice job, Howard, and still a useful and neat tool.
I notice the electrolytic bottom left has "Jason" printed on it, and as for the Wima bottom right; just mention "Grundig tape recorders" !

[howard]

Thanks all,

Ron did all the work on this Jason so deserves all the credit for this restoration, I just obtained the new parts for it. We did consider repairing the 32+32uF smoothing capacitor with 'Jason' printed on its can but we don't have the tools to cut into one of these to replace the caps inside, besides the new JJ cap is identical in size and fitted perfectly. That WIMA capacitor had had plastic padding shoved into holes in its case as it was falling apart yet it was still working fine !

I will fire it up from time to time to keep it in good working order.

Howard

[ronbryan]

Brian said:

Although I have no experience of the Jason, I don't remember the OS1 having an overshoot problem.

I have drawn up the modified Y shift circuit that had been incorporated into the Jason OG-10 and have included the original circuit for comparison, in case anyone is interested.

When you analyse the original circuit (2. in the diagram), the C5 - R11 time constant is less than 200mS, which is a lot shorter than overshoot that I observed, so maybe there is another explanation. Possibly my too-rapid tweaking of the pot caused a large excursion of the cathode bias which took a long time to settle. This may have been made worse because C7 (0.01uF) was inadvertantly not fitted.

It is also possible that a shift control induced change in HT was fed back to the previous AC coupled stage which then bounced. The HT distribution is different on the Heathkit OS-1 compared to the Jason OG-10. On the former, both X and Y deflection amps are on the same HT rail, with the preceding Y amplifier and timebase separately fed. Unfortunately, I've returned the 'scope to Howard now, so I can't investigate further.

The modified circuit holds the EF80 grid at a fixed potential close to the mid-point of the shift pot, rather than referencing it to the cathode. Apart from taking a little more current than the original, it seems to work very well and truth be told, there must have been a reason for the original owner incorporating it in the first place.

Ron

[Brian R Pateman]

Hi Ron,

Very interesting. Looking at both versions I think that they are both valid and a sound enough circuit. I agree with your assessment of the time constants and I wouldn't have thought that they should be a problem.

I wonder if the ubiquitous EF80 is the root of the problem here? Although they were one of the standard workhorses I wonder if the inter-electrode capacitance on certain valves may have been a problem?

When I started out in T/V 40 years ago I remember my old boss saying "EF80s never go but they sometimes do some by funny things!"

I have a large number of NOS and used EF80s and a fair few ECFs as well so when I have my new workshop sorted out in about 2 months I may knock up a tes circuit and fiddle with some values. The HT rail may have an effect but I wouldn't have thought so.

At this rate I'll probably end up building a Jason. (As if I don't have enough scopes!)

Any way I suspect that the OG10 would still be a handy instrument for quick checks around AF stages at the very least.

Regards,

[Alf Fisher]

Hi Ron,
I can see how the original circuit suffers from the overshoot and drift back problem as mine does it well! If the shift pot is moved, then by cathode-follower action, the DC voltage at the cathode will follow and so too will the DC voltage at the bottom of the 1M. The time constant of the 1M and 0.22uF is what gives the drift back whilst the 0.22uF either charges or discharges to restore the balance. To a large extent the whole circuit design tends to cancel out a shift adjustment in the first place because the voltage on the grid of the EF80 will tend to follow whatever the shift pot does. I think the modified circuit overcomes the problem nicely by maintaining the EF80 grid at a fairly constant potential. Well done.
I've now found out that mine has a problem to be sorted out one day - the HT transformer has shorted turns on half the HT secondary.

Alf