Tektronix 7904 - UK Vintage Radio Repair and Restoration Discussion Forum

gerar2182 · 26th Apr 2019, 10:28 pm

Hi to all.
I have a very strange fault in this scope.
After some minutes of working, the screen start to shrink ciclically.
Then stop for some time, and fault appear again.
Some idea?
Thanks

trh01uk · 27th Apr 2019, 7:49 am

My first guess is that it is a supply voltage that is collapsing. Possibly due to some other fault that is causing an overload on that supply rail.

I would suggest hooking up as many DVMs or similar as you have to each supply rail. Check firstly that the supply rail voltages are all in spec when the scope is operating normally. Then wait for the fault to appear and see if any supply rail is affected.

After that, it will be a matter of chasing the fault through the system.....!

Richard

RogerEvans · 27th Apr 2019, 11:06 am

I totally agree that you begin with the low voltage power supplies. If all the low voltage supplies are affected in about the same ratio then the fault is probably in the inverter part of the power supply which is a quite common failure in the 7904 due to ageing capacitors and sometimes failed diodes. If yours is a 7904A the power supply is much more reliable. Working on the inverter is difficult in terms of access and involves working on circuitry directly connected to the mains supply so it is not recommended unless you are confident of your safety.

If the low voltages are all OK the probable fault is the 7X multiplier U1615 that supplies 21kV to the PDA electrode at the front of the CRT. The characteristic of a failing PDA supply is that the display shrinks by exactly the same amount in X and Y, becomes dimmer and slightly defocussed. A low PDA voltage can sometimes be due to tracking of the 21kV across dirt and grime, turn off and wait several minutes before trying to clean it.

A fault in the CRT cathode supply (-3kV) usually causes the cathode voltage to drop and the display expands in X and Y. It is possible that a failure in the cathode regulator circuit could let the cathode voltage go high and the display would shrink, probably by no more than 10-20%. There is a test point in the regulator circuit (TP1635) that should be very close to 0V if the regulator is working properly.

You can find the PDF manual online at w140.com/tekwiki.

Regards,

Roger

Craig Sawyers · 27th Apr 2019, 11:39 am

Join Tekscopes https://groups.io/g/TekScopes currently 7000 strong, and super helpful.

Someone will have seen this with the 7904.

Craig

26th Apr 2019, 10:28 pm	#1
gerar2182 Retired Dormant Member Join Date: Mar 2019 Location: Hurlingham, Argentina. Posts: 1	Tektronix 7904 Hi to all. I have a very strange fault in this scope. After some minutes of working, the screen start to shrink ciclically. Then stop for some time, and fault appear again. Some idea? Thanks

27th Apr 2019, 7:49 am	#2
trh01uk Octode Join Date: Nov 2010 Location: Ross-on-Wye, Herefordshire, UK. Posts: 1,654	Re: Tektronix 7904 My first guess is that it is a supply voltage that is collapsing. Possibly due to some other fault that is causing an overload on that supply rail. I would suggest hooking up as many DVMs or similar as you have to each supply rail. Check firstly that the supply rail voltages are all in spec when the scope is operating normally. Then wait for the fault to appear and see if any supply rail is affected. After that, it will be a matter of chasing the fault through the system.....! Richard

27th Apr 2019, 11:06 am	#3
RogerEvans Hexode Join Date: Oct 2014 Location: Wiltshire, UK. Posts: 382	Re: Tektronix 7904 I totally agree that you begin with the low voltage power supplies. If all the low voltage supplies are affected in about the same ratio then the fault is probably in the inverter part of the power supply which is a quite common failure in the 7904 due to ageing capacitors and sometimes failed diodes. If yours is a 7904A the power supply is much more reliable. Working on the inverter is difficult in terms of access and involves working on circuitry directly connected to the mains supply so it is not recommended unless you are confident of your safety. If the low voltages are all OK the probable fault is the 7X multiplier U1615 that supplies 21kV to the PDA electrode at the front of the CRT. The characteristic of a failing PDA supply is that the display shrinks by exactly the same amount in X and Y, becomes dimmer and slightly defocussed. A low PDA voltage can sometimes be due to tracking of the 21kV across dirt and grime, turn off and wait several minutes before trying to clean it. A fault in the CRT cathode supply (-3kV) usually causes the cathode voltage to drop and the display expands in X and Y. It is possible that a failure in the cathode regulator circuit could let the cathode voltage go high and the display would shrink, probably by no more than 10-20%. There is a test point in the regulator circuit (TP1635) that should be very close to 0V if the regulator is working properly. You can find the PDF manual online at w140.com/tekwiki. Regards, Roger

27th Apr 2019, 11:39 am	#4
Craig Sawyers Dekatron Join Date: Nov 2010 Location: Oxford, UK. Posts: 4,992	Re: Tektronix 7904 Join Tekscopes https://groups.io/g/TekScopes currently 7000 strong, and super helpful. Someone will have seen this with the 7904. Craig