Issue with Gould 1421 scope

[dougietamson]

Has anyone seen this problem or has an idea about a fix?

The scope displays a signal then it fades out and comes back. Sometimes it just slowly fades after 5 mins and doesn't come back.

I've added a sequence of screen shots that shows the various levels of the signal trace.

Also a short video here of the issue:
https://youtu.be/lMSuWSWfSOE

Doug

[RogerEvans]

The deflection sensitivity doesn't change during the fading so the CRT cathode voltage would seem to be stable. The problem is most likely to be in the Z axis circuitry that controls intensity and unblanking of the trace. I believe that one of the Gould models has a hybrid method that uses capacitive coupling to the CRT to deal with the 'fast' unblanking (maybe faster than 1 usec) and an optocoupler that deals with the slow part of the blanking and the front panel intensity control. If that is the case and particularly if the optocoupler is socketed then that could be a prime suspect.

I am not at all familiar with Gould and hopefully someone with a better knowledge will offer some input. A schematic of the Z axis would be useful.

I am guessing that the traces you show are in X-Y mode (they look like Lissajous' figures). Is there any difference in normal Y-t display mode and at fast and slow sweep speeds?

Regards,

Roger

[dougietamson]

Here's a sine wave on dual channel mode.

https://youtu.be/TptW-B5kRRs

[dougietamson]

And a longer recording showing fading and blooming

https://youtu.be/EDLgO9eBsik

[Chris55000]

. . . There's a manual available for the OS1420 which will be close enough to your OS1421, only trouble is, it's a grotty Mauritron one!

OS1420 Manual :–

https://www.google.com/url?sa=t&sour...D8OfPVChPpsTmO

. . The main transistor that controls the CRT cathode/grid voltage is the PNP device TR715 connected between the CRT grid/cathode circuit – it is actually an emitter–follower from TR703/TR704 but the way Gould's diagram is drawn doesn't make it obvious!

. . .DO NOT attempt to measure TR715 emitter voltage/point 733 with a meter connected to chassis, as you will bias the CRT on very heavily and destroy it!

. . .Instead, I would replace TR715 and check it's emitter/collector grid–return resistor R743, and I would also throw out the two diodes here, D705 and D706.

. . The CRT bias transistors, TR703 and TR704, also form part of the EHT stabilising network and intermittent leakages here will cause erratic intensity fluctuations – replace them both along with R715 and R716 if you're in any doubt about them, using metal–glaze VR25/VR37 resistors for replacement purposes!

. . .The "SET INTENSITY" preset R713 will certainly cause this sort of fault if it develops a dud–spot as it couples back from the collector of the bright–up amplifier series driver stage TR701/TR702, whose collector is fed from the 210V HT+ line via R558, R573, R574 and R575.

. . .If you're really unlucky you might have an intermittent fault in the thick–film e.h.t. sampling/regulator network RN720 (section "D" of this between pins 6 and 7 is the e.h.t. regulator feedback resistor) – it IS safe to measure the stabilised –1850 V e.h.t. supply at tag 715 to see if the e.h.t. is fluctuating!

Note the focus control R744 and the two resistors across it, R714 and R715 connected to tags 715, 716 and 718 need looking at if your focus is drifting in and out as well!

Problems with the bright–up amplifier itself are either due to TR701/TR702 or it's collector resistors R558, R573, R574 and R575 as mentioned earlier, in a bad case you might have to replace the high–voltage coupling capacitor C702, but I would look at all the transistors, diodes and resistors I mentioned first!

. . .If it looks like you're still getting fluctuations in this circuit after you've checked or replaced all the above components as needed, you might have a fault in the thick–film network RN720, but all its elements are accessible at both ends separately, so you can always try measuring them as best you can and substitute the nearest suitable fixed resistor to prove the point!

Chris Williams

[Chris55000]

PS!

. . .The actual bright–up amplifier stage is simple in operation – bright–up/unblanking pulses come from IC501 pin 11 in the Time base/Trigger circuit diagram, Fig. 8, and are fed by R701/C718 in parallel, (C718 is a select–on–test "speed–up" capacitor, nominally 22pF), then fed to TR701 via R703. The base voltage of TR701 is adjusted by the front panel "INTENSITY" control R745, connected across the +11 V and –11 V rails, this control therefore adjusts the standing collector voltage of TR702 by virtue of it being in series with TR701, so that between the unblanking/brightup pulses, TR702 collector sits at such a potential that, when added to the charge on C702, (initially set by the preset "SET INTENSITY" control R713), maintains the CRT at cut–off between the pulses!

. . .During the unblanking/bright–up pulse interval, (the pulses are negative going) the transistors TR701 and TR702 conduct less heavily, therefore the collector potential of TR702 is more positive with respect to chassis, and as the voltage on C702 is nominally constant, the effective C.R.T. grid potential with respect to chassis follows the positive rise of TR702 collector potential during the unblanking/bright–up period and therefore the CRT is unblanked!

. . .It is very easy and safe to check if brightness fluctuations are occuring here, check the positive voltage at TR702 collector ONLY!

Chris Williams

[dougietamson]

Thanks Chris, very nice write-up.

A bit more info, from a cold start (12 degrees in the worksop today) the scope works as normal for the 1st 7 mins or so, the display is stable and I can adjust the intensity to my preference.

Then the random intensities started and the other symptoms seen in the videos.

Eventually the display faded out completely.

I let it 'cool' down for 15 mins and powered up again, the time to the issues happening was shorter.

Doug

[Radio Wrangler]

Originally Posted by dougietamson

Here's a sine wave on dual channel mode.

https://youtu.be/TptW-B5kRRs

That makes things a lot clearer.

Firstly, the whole picture is breathing linearly. This says the velocity of the electrons passing the deflection plates is changing erratically. or the acceleration after the plates

Secondly, there is loss of focus coincident with the breathing, which also fits with potentials changing in the electron gun.

Z-mod and blanking try to not affect the accelerating potentials, but only affect the beam current by relatively small changes in cathode-grid potential.

I'd want to measure EHT voltage and check it's stable. I'd go looking down the CRT bias chain and look to see if there's a resistor or capacitor going iffy. There could be something braking down, maybe a surface discharge.

There will probably be a tripler on an EHT winding to make the high positive voltage for the PDA this could give breathing.

But with the picture fading right out and not breathing too far, I'd reckon the prime culprit is in the gun bias chain or the blanking/z-mod system.

David

Edit I had a shiny new Gould 3000 scope as my bench scope for a year as a student, and liked it. Experience with Gould scopes over the years has not been much, but the EHT and tube power circuits seem to have been a weak point of the brand.

[dougietamson]

Originally Posted by Radio Wrangler

That makes things a lot clearer.

Firstly, the whole picture is breathing linearly. This says the velocity of the electrons passing the deflection plates is changing erratically. or the acceleration after the plates

Secondly, there is loss of focus coincident with the breathing, which also fits with potentials changing in the electron gun.

Z-mod and blanking try to not affect the accelerating potentials, but only affect the beam current by relatively small changes in cathode-grid potential.

I'd want to measure EHT voltage and check it's stable. I'd go looking down the CRT bias chain and look to see if there's a resistor or capacitor going iffy. There could be something braking down, maybe a surface discharge.

There will probably be a tripler on an EHT winding to make the high positive voltage for the PDA this could give breathing.

But with the picture fading right out and not breathing too far, I'd reckon the prime culprit is in the gun bias chain or the blanking/z-mod system.

David

Edit I had a shiny new Gould 3000 scope as my bench scope for a year as a student, and liked it. Experience with Gould scopes over the years has not been much, but the EHT and tube power circuits seem to have been a weak point of the brand.

Thanks David, I don't have any high voltage probes or high voltage resistors to make a divider.
I've got some 10M and 1M normal voltage resistors available, I don't know how long they would last if I made a quick voltage check.

[Chris55000]

. . .To make a suitable high voltage probe, obtain nine 10 M VR25 or VR37 metal–glaze resistors, cut their lead out wires to ¼" long each and then solder them end to end to form a long series chain, solder one end of the chain to a thin piece of brass rod filed to a point at one end furthest from the resistors, then solder the other end of the resistors to a length of heavily insulated cable terminated in a 4 mm plug.

. . .Insert the completed assembly in a piece of insulated tubing about 1" or 25 mm diameter, of suitable length to extend over the resistor chain plus 1cm at the ends, then fill round each end of the tube round the brass probe end and outlet cable end with RTV non–acetic acid silicone rubber compound to provide a layer about 20 mm thick at each end of the tube, and leave the sealant to cure!

. . .Once the sealant has cured, obtain a piece of adhesive–lined heat shrink tubing just slightly greater in diameter (30 mm "unshrunk"), place one piece over the probe exit end of the tube and another piece over the cable end of the tube, and shrink them in place with a hot–air gun or hair–dryer).

. . .The 9 X 10 M resistors, together with the 10M nominal input resistance of the DMM, provide a safe high–voltage probe up to 30 kV max. reading!

. . .To use, simply locate a high–tension point in your Oscilloscope (the +210 V rail will do for this) and measure the voltage on this rail with a simple direct test lead on the 1000 VDC range, then replace it with the high–voltage probe made earlier – if the probe resistors are reasonably accurate, the DMM should read "0.210" on the 1000 V scale, therefore a reading 1.000 equates to 1,000 V at the probe tip!

. . .If my explanation isn't too clear I'll produce a diagram tomorrow!

Chris Williams

[Chris55000]

. . .Correction to my post above for making a high voltage probe :–

a) Make up the chain of 9 X 10M VR25/VR37 type series resistors, then ADD 1 X 100 k further in resistor in parallel with another value to make an equivalent parallel resistance of 90.1 kohms at the end of the chain AWAY from the high–voltage probe end ;

b) Solder a length of good quality audio screened cable across the 90.1 K bottom resistor of the divider chain, connecting the junction of the last (ninth away from the probe) 10 M resistor and the 90.9k final end resistor to the INNER of the screened cable and the BRAID to the free end of the 90.9 K resistance combination ;

c) Also solder 0.5 m of BLACK 16/0.2 connecting wire to the BRAID and the junction of the 90.9 K resistor free end, secure this wire to the screened audio cable with a small cable tie ;

d) Fit the completed assembly into the tube and seal and cover with heat shrink tubing as per my previous post ;

e) At the free end of the screened cable, solder a RED 4 mm plug to the INNER and a BLACK 4 mm plug to the BRAID – these plugs will go into your DMM ;

f) Fit a good quality BLACK insulated crocodile clip to the free end of the 16/0.2 insulated wire leading out from the probe to form the earth connection to the Oscilloscope circuit ;

g) Test as before using the +210 V rail on your Oscilloscope, you should now get a reading of 0.210 if your resistors are reasonably accurate ;

h) If you prefer you can shunt the 100 k resistor with a 20 T 1M0 trimpot, this will allow you to adjust the probe to give an exact 100 :1 division ratio to suit your DMM you are using !

. . .If the probe is made carefully using new VR25/VR37 divider resistors, and a 1 W 100k lower resistor, with 1000 V at the probe tip, the open–circuit voltage at the ends of the 4 mm plugs will only be 1.0 V maximum and therefore safe to touch if left unconnected to the DMM !

Chris Williams

[WME_bill]

Advance/Gould 1421.
I have a manual if you are still looking. I will have to scan it.

High Voltage Probes. Much less complicated than you might think.
I use a string of say five 10M resistors with 50k at the bottom in a plastic tube and put a digital voltmeter (10M input) across the 50k.
The secret is to recheck the calibration, by measuring one of the high voltage rails on the oscilloscope or TV with and without the probe.

Work out the ratio, and either set it up on your calculator, or vary the 100k to give a convenient multiplier. I think I read this advice in a HP leaflet.
Normal metal film resistors are quite good enough. There is no need to go for VR25/37 metal glaze. They will not alter in value under voltage stress in occasional test probe use.
If you are wanting to monitor high voltages continuously and do it every day, then Chris55000's proposal is the long term solution. But for 10 minutes a couple of times a year, my method has done me well for years.
wme_bill

[Diabolical Artificer]

Here's a HV probe I made years ago for someone on the forum, made out of stuff out the box of useful stuff, poly pipe & heatshink tube.

Andy.

[John_BS]

Doug: if you neeed any VR37 HV resistors please PM me.
John