GOULD ADVANCE SPLIT BEAM OS260 SCOPE - CANNOT FOCUS SPOTS

[rgarnett1923]

Hi All,

I have a Gould Advance Split Beam scope which I cannot get to focus.  The scope had some power supply issues. In particular, the PDA EHT voltage regulation was faulty.  Someone had tried a repair but put the HV regulation transistor in with the collector and emitter transposed. In addition one of the HV caps on the voltage quadrupler was loose.   I got all that fixed and going and adjusted all of the presetting pots to get the voltages on the tube within spec.  I checked all of the voltages for ripple and stability and they were ok. (I have a 42 kV Pintek scope probe and some 1500 volt probes for the lower voltages in addition to my multimeter.)  I also checked the electrolytics and meggered HV caps.  They were good. 

The only thing left that could be at fault was the tube, an E14-100GH. Maybe it is gassy?

You cannot see the getter coating of this tube because the aquadag coating extends to within 2 cm of the tube base.  However, on turning off the lights and looking down the tube from the base there was no apparent blue glow around the accelerator that would indicate poor vacuum.

I thought maybe the tube had been knocked and an electrode had come loose so I pulled the tube and gave it the old tap test with it pressed up against my acoustic transducer (ear hole).  There was resonance, but no rattling. Shaking the tube did not elicit any loose particle noises so I gave up on that theory. Turning the tube with the base down did not cause any foreign material to drop to the base.

I then thought maybe there is something caught between the cathode and grid 1, a possibility as there are pretty fine clearances between them. So I ran the tube heater at 8 Volts for a short time and tapped and vibrated the tube. That was no good either.

There is a small magnet on the base of the tube with wrap-around pole pieces.  This magnet is used to even up the brightness of the two beams presumably by moving the beam vertically in relation to the beam splitter plate.  I tried this in numerous positions, but this was no good either.

I am at a loss to know what to try next.  I can't believe the tube is faulty.  The instrument has never been badly handled, the tube is well protected from assault by the mu-metal shield. The tube is a Phillips made in Holland which were constructed to a very high quality and were exceptionally reliable.

I also checked the tube connector. Some of the gripper pins were a bit soft so I tightened them up so the socket was a lot firmer on the tube and I am confident I don't have a problem with connections.

Has anyone had any experience with this problem, or know anything about these split beam tubes?

I would appreciate such assistance greatly.

[tautech]

What's on the scope inputs for that display ?

[Chris56000]

Hi!

Instructions & Service Manual:–

https://www.kevinchant.com/uploads/7/1/0/8/7108231/gould._os260._service_and_operating..pdf

CRT Data Sheet:–

https://frank.pocnet.net/sheets/011/e/E14-100GH.pdf

Remember that the voltages given on the CRT data sheet are given with respect to the CRT heater/cathode common pin (pin 1), NOT the chassis–earth terminal of the oscilloscope!

Can you get the timebase and Vertical Amplifier to function correctly? If the timebase and/or Vertical Amplifier are faulty the spot will be concentrated over a small area of the screen and the design range of the intensity control may not be enough to cut off the beam fully!

Go through the general instructions for setting up a waveform on the scope and then go through the CRT setting–up adjustment procedures again – if you've still no luck I'll get back with more tips!

Chris Williams

[rgarnett1923]

Hello, thanks for your question. 

The time base (X Plates) were switched to external and the Y amps are set to ground input at max V/div.  I checked the ac voltage on both the x and y plates and they were zero.  The plates are driven by push-pull amplifiers with the mean dc bias voltage on the plates set at the mean plate potential. This is set by the astigmatism control.  I checked all the power supplies to the X & Y Amps and they were spot on with very little ripple (mV). To check the deflection plate voltages I used my scope with ch 1 & 2 on dc with and subtracted the two channels. I also used my multimeter on AC with a 1uF cap in series, just in case the scope measurement was flawed.

I don't think that the deflection plates have any modulation on them that is broadening the spot.

 

[tautech]

Why would you set the timebase to external ? 
How can you then know if the timebase even is working ?

Typically the vertical plates when centered have the same voltage WRT ground on each plate and ~60V DC.
I think to find where the unwanted vertical is you'll need to trace back and forth the signal path to and fro from the inputs.
This can be done with a sine wave or the probe cal input waveform.....shouldn't be too hard as you have another scope.

[rgarnett1923]

Hi Chris,

Thanks for that.

The Y Amps and timebase are both working fine.  You can display waveforms with no problems with the exception that the trace is smeared mostly in the horizontal direction.

I understand the referencing of the tube electrode voltages.  I measured with respect to ground, then put the voltages in a spreadsheet (which I have attached) and referenced them to the cathode.

Nothing really stands out as a problem when I looked, but maybe I missed something. 

I have attached three screenshots. 

When I was setting up the scope for the screenshots I noticed that the channel 1 trace was truncated if I move the Ch 1 Y pos upwards.  The channel 2 trace can be moved full over the screen without any truncation.  This truncation of the channel 1 trace is affected by the beam split magnet. You can turn the magnet in the direction that brightens beam 1 and this also reduces the truncation of the channel 1 trace but does not get rid of it completely. With the magnet in this position, the channel 2 trace is too faint for practical use.  Moving the magnet makes no difference to the focus.  It looks more and more to me like the splitter plate has moved out of position somehow.

[rgarnett1923]

I di this so So I could see the spots without any x-y modulation.  Like you I thought the X-Y plate mean voltages might be incorrect, but they are operating at a voltage close to the accelerator electrode. Their exact value can be changed with the astigmatism pot, but the focus cannot be corrected by, changing this pot along with the brightness, focus and geometry pots.

I have posted the tube voltages to this forum about 5 minutes ago along with some screenshots.

[tautech]

OK, now scope the timebase ramp/sawtooth output, it should be perfectly linear at any timebase setting.

There's also a fault finding section on P14/42 with several measurement values for you to check. Be sure to set the controls as described for results to be valid.

[rgarnett1923]

Hi,

I carried out checks per pages 14/42. All OK.

The Timebase as measured at the X plates, both individually; cyan and green traces and the difference red. Looks OK, very linear and the mean voltage is around 104 Volt which is pretty close to G5.

RJG

[tautech]

Hi,

I carried out checks per pages 14/42. All OK.

The Timebase as measured at the X plates, both individually; cyan and green traces and the difference red. Looks OK, very linear and the mean voltage is around 104 Volt which is pretty close to G5.

RJG

Great, now check the ramp again at the timebase output and compare for linearity against those at the plates.
If both the source and plate outputs look clean proceed to tracing signal fidelity from inputs to vertical plate outputs.
Some study of the schematics will decide the best test points so document them and the results.
It might just be that a channel has taken an overvoltage event and it's hurt a signal path stage.
Good hunting.