Gould 4074 Repair - Assistance Required

[LandorG]

Hello,
I bought an old Gould 4074 from ebay and I'm trying to repair it.
This led me here. I already read the other threads about similar issues.
I'm not that much of an electronics guy. Maybe you can give me some assistance.

When I first plugged it in the fan did not start and smoke came out of the PSU. It all went very fast 15 seconds but a lot of black stinky smoke. I did not see anything on the screen.
I think the fan was not connected because of a little damage from delivery. But I am not sure if it was the reason for the fault.

I found 3 resistors that burned as you can see on the images.
Image 1
Image 2

I repost the PSU circuit from WME_bill here. (Thanks to him for the sm)
In the picture in the attachment i marked the burned resistors. I will replace them but I think they are not the only thing damaged.

I unsoldered the capacitors around the burned resistors and tested them with my multimeter they seem ok (6.6nF instead of 4.7). I checked the resistance of the transformator (P9-P10) it is about 130 Ohms. Is it ok?

Should I just replace resistors replug and try again? I read here in the forum that the PSU needs some load. Should I connect everything? Any suggestions on what to do next? I have the Service Manual and a Multimeter for the testing.

Thank You in advice
LandorG

[dragonser]

Hi,
I may have read the diagram wrong ( and it is late so don't reply on my opinion alone ! ).
I think that the resistors you have shown are on the eht winding of the transformer.
So to be able to test the components fully you would need to test them at more that the voltage that the multimeter you are using can give. Of course if the rectifier diodes show as short circuit on the multimeter then they would need changing.
it is possible that the capacitors may test ok at the volatge the multimeter uses 9 volts ? but break down at a higher voltage.
the invertor circuit ( from the pdf ) that drives the transformer looks quite complex.
I wouldn't power up the power supply when it is connected to the scope, I think you risk too much change of damaging the rest of the oscilloscope.
It should be possible to use some correctly chosen resistors to act as a load for the power supply to be able to test it correctly.
But please do be careful working on the eht side of things, if you are not used to high voltage equipment.
Hope fully other people here who have worked on a similar scope can give better advice. ( and correct me if I have forgotten anything ).
regards Peter B

[MotorBikeLes]

Is this a heater to kathode short on the CRT? Or the tube base?
If not, check all the small C's and Di's adjacent to the burned resistors. It could just be one or more of those gone short.
C33 would obviously cause the damage if that was S/C.
Les.

[LandorG]

Hi thanks for your replies.

I'm not sure about the heater cathode short. I measure 14 Ohms between pin 1 and 2 on the crt (heater - cathode) but I'll have to dismount it to get sure and reach the other pin.
What does that mean to me?

Thank You LandorG

[WME_bill]

Gould 4072. This is an extremely complex oscilloscope, and represents a real challenge to get working, even for an experienced engineer.
1.The power supply is a switched mode, of the usual extra complex arrangement which Gould seemed to like.
2. Are the other power supply rails working, the +16, +5, +12 etc. If they are not, you must look to the primary side of T2. But see about the fan at 12 below.
3. Be very careful of the primary side as it is all floating at mains or 600v. Lethal to work upon. You will need an isolation transformer, a fully insulated meter and ideally a battery operated scope.
4. If it is just the EHT circuit, the three burnt out resistors are presumably carrying too much current. So check the high voltage capacitor C33 4kV, the 12kV rectifier diodes and all round the regulating transistor Q15, BC556, the zener diodes and the capacitor C29 10kV. Look also at the base feed capacitors and capacitors.
5. Testing these high voltage components needs something more than an ohm-meter tester. Use a megger at 500V or rig something up to produce over 100V, as faults in high voltage components often do not show up at low voltages.
6. While testing with the high voltage, check out the components around the blanking circuit from C35 and the focus chain from C37.
7. I suggest the CRT heaters is a secondary issue. Can you see the heater glowing through the base of the tube. That will be quite good enough.
8. The tube is a simple mono accelerator: Valvo (German Philips) 10D18 or Brimar D18-180. Details on Frank Philipse Tube data site or I have it.
9. Heater is 6.3V, 100ma (when hot 60 ohms, with cold resistance much lower and so not very helpful).
10. There is a resistor of 100k from heater pin14 to cathode pin2. Resistance from pin1, heater to cathode should be say 80 ohms, as it is measuring through the EHT transformer and the two resistors R61 and R62 = 30 ohms. Assuming the wiring actually follows the sequence in the circuit diagram. Measure this resistance at low voltage, as there are two back-to-back 6.2V zener diodes across the heater.
11. You should not operate a switched mode power supply for long without a reasonable load. I'd guess 200ma across the 12V and 2A across the 5V, but you have table in the manual.
But a short run into the normal oscilloscope load while you are measuring the power supply voltages will not do any more damage than has already been done. There is an overheat thermostat - S2- which cuts out at 90C.
12. The fan is 12V, so if it smoked, it suggests that the 12V rail is hopelessly too high, and that your major problem is failure of the main switching regulator. (Usual fault, just to cheer you up). Solve that before wasting time on the EHT arrangments.
13. If you are worried that you might damage the cathode ray tube, unplug it. The currents are so low that the EHT circuits will all operate correctly, and you can measure the voltages within 10%. Make up a high voltage probe for your meter. I have posted suggestions and so have others.
14. MotorBikeLes and Dragonser both talk sense. I hate working on these switching power supplies. I have a 4072, and several Philips PM3262 and PM3311 waiting me getting up my courage.
wme_bill

[LandorG]

What I first wrote was wrong. The 14 Ohms are between Pin 1 and 14 (the heater pins) and no connection with the cathode. I guess CRT might be OK.

Thanks for your answer Bill. I'll have to check the local hackspace to see if they have some test gear or a power supply which I can use.

I'll report back if I know more.

[WME_bill]

Gould 4072/4.
First check the voltages on all the power rails.
And report them back to us. They should be within 10% at this stage for initial assessment.
You already have a meter which will do that. Then we will know the measure of the problem and can assess what other test equipment you may need. wme_bill