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T912 storage oscilloscope - No 2.7 KV high Voltage


Attilio
 

Greetings to all,
I have an old T912 that no longer shows traces on the CRT screen.
I checked and the 2.7 KV on the cathode of the CRT is missing. By disconnecting the diodes CR463 and CR465 from the T460 HV transformer, the negative rectified T460 output voltages are present (- 120 Vdc and - 220 Vdc are respectively - 150 Vdc and - 210 Vdc), by reconnecting CR463 and / or CR465 these voltages drop around - 14 Vdc.
I tried CR463 and CR465 diodes and apparently they look good, the T460 transformer I don't know how to test it not having the dc resistance values ​​of the windings.
On T460 I have between pin 9 and pin 10 about 145 ohm and between pin 1 (gnd) and pin 8 about 142 ohm.

Do you have any suggestions / advice for me to try to fix it.

Thanks
-- Cheers
Attilio


unclebanjoman
 

Hi Attilio,

Check for Q458.
I've repaired a T935 some weeks ago. Symptoms: -2000 V sometimes disappeared.
Turning the scope off, waiting a few seconds, then turning on again, sometimes it worked sometimes not. Then he died completely.
Pulled out Q458: was almost short.Measuring the resistance between C and E showed a few hundred ohms.
Replaced with a BD410 I had on hand, the scope started working properly again. Original transistor was TIP50. It was badly fixed on the heathsink so it probably got hot and stressed over time.

Max

P.S. i'm from Italy.


Attilio
 

Thanks Max,
I also from Italy, near Varese (Lombardy).
But when I disconnect the diodes CR463 and CR465 of the high voltage, the voltages -120 Vdc and -220 Vdc return to almost correct values, I deduce that Q458 oscillates and is not shorted or interrupted.
Anyway, I will replace it for test.

-- Cheers
Attilio.


Attilio
 

Greetings to all,
i replaced Q458 but nothing has changed.
I checked the voltage and frequency on the collector of Q458 with the HV diodes CR463 and CR465 disconnected: the frequency is 54 kHz and the voltage around 336 Vpp.
I also measured the voltage + 120V UNREG and it is +151 Vdc; on the collector of Q446 there are 5.3 Vdc, while on the emitter of Q454 there are 5.99 Vdc, the voltage + 8 V is + 7.98 Vdc.
On the output line from the T460 transformer I replaced a 0.1 uF capacitor towards ground but the CR468 diode continues to boil, if I remove the capacitor the diode remains cold and the voltage at - 140 Vdc (instead of -120 Vdc).
If I connect the HV CR463 and / or CR465 diodes there are no - 2.7 KV and the voltages of -120 Vdc and -220 Vdc drop to about - 14 Vdc.

I don't understand where the fault is, maybe it is the T460 transformer that is fault ?

Thanks for the attention

--Cheers
Attilio


unclebanjoman
 

Do you tried to disconnect the rear socket of CRT? Just to be sure there isn't some internal short.

Max


Roger Evans
 

Attilio,

You say 'CR468 diode continues to boil', which implies that something is drawing too much current from the -120V supply. Which capacitor did you change? Can you measure the voltage across R469 and maybe check that the value of R469 is reasonably close to 1.2k? If this excess current is going to the storage board (rather than a leaking C469) then something will probably be very hot. It is not safe to touch components while the board is powered since there is 220V (and +335V!) in places but you can check that the -120V and -220V (and also +335V) have decayed a few seconds after the scope is turned off and then try touching a few components. I would start with Q942, Q944, Q946 and Q948.

Be very, very careful about not touching the parts which have high voltages (ie more than 50V) on them!

Regards,

Roger


Attilio
 

Yes Max,
I disconnected the CRT socket, but without changes.

Thanks

--Cheers
Attilio


Attilio
 

Hi Roger,
the CR468 heats only with the C468 condenser connected, if I disconnect it the CR468 remains cold.
I wanted to add that I measured the voltages at pin 2 and pin 7 of the T460 transformer with respect to ground and I find very high voltages (on pin 2 there are 840 Vpp at 53 kHz and on pin 7 there are 484 Vpp at 53 kHz).
From pin 2 through the CR468 diode the - 120 Vdc are generated and from pin 7 through the CR467 diode the - 220 Vdc are generated, 840 Vpp to generate the -120 Vdc seem too many! I suspect an insulation problem in the T460 transformer (?).

--Cheers
Attilio


Roger Evans
 

Attilio,

I am writing this on the phone and I can't easily get schematics at the same time - so no part numbers!

When you disconnect the diodes that generate the CRT cathode and grid voltages (-2.7kV) you effectively disable the negative feedback from the end of the focus resistor chain to the power transistors that energise T460. So the oscillator is running at maximum drive trying to generate -2.7 kV and in the same way is producing too high values for -120V and -220V. When running normally with negative feedback the transformer turns ratio ensures that if -2.7 kV is correct then so are the other voltages.

To me it looks like T460 is OK and so is the circuitry that drives the power oscillator. You have a problem with a failed diode or capacitor on either the CRT grid or cathode supply. If you replace just one of the supplies then the voltage across the two neons causes them to conduct and a fault on either supply will cause the oscillator to stop working.

I am afraid you may have check individual components to find the failure. Quite often a leaking diode or capacitor looks OK with a DMM but the leakage shows up with a high voltage around 1kV. I bought an insulation tester as used by electricians for checking house wiring.

I can go into more detail when back at a decent sized screen!

Best regards,

Roger


Attilio
 

Roger,
thank you very much for the explanation, I had sensed that the feedback could raise the output tensions. On the HV secondaries I have disconnected all the 4 KV capacitors and the connections to the CRT, but with CR463 and / or CR465 connected to the T460 transformer the other voltages (-120 V and - 220 V drop to - 14 V).
I will try to replace the CR463 and CR465 diodes, but at the moment I don't have them in the house, they are 10 KV - 10 mA diodes.


Roger Evans
 

Attilio,

If you are buying spare parts it would be wise to get some 5nF (0.005uF) high voltage ceramic capacitors. It will probably be easier to find modern parts rated at 5kV rather than 4kV. Also have a close look for any signs of tracking around the high voltage area and the focus resistors R444.

Regards,

Roger


Attilio
 

Thanks Roger,
I have the 5 nF 6 KV capacitors.
I will check the condition of R444 (focus resistors).
I will purchase the new 10KV - 10mA diodes.

Regards,

Attilio


Trevor
 

Attilio-

Please note that the two diodes should be "Fast Recovery" (100ns) since the frequency is 50 kHz or so.

Trevor


Attilio
 

Thanks Trevor,
yes I knew, the frequency at the T460 transformer is 50 kHz.

--Cheers
Attilio