Tektronix 7704A fault

[delaitt]

Hello

I have a Tektronix 7704A which has developed a fault.

The fault can be observed on either timebases (left or right). The fault goes away if i move the trace to the left.

I assume the fault is related to the electronic in the 7704A chassis rather than the actual timebase (left or right).

Has anyone observed this kind of fault?

Thanks

Thierry.

[tony brady]

Hi Thierry

none of mine have done that ( so far!) but I agree with you the fault is in the mainframe, not the timebase plug ins - this would be easy to prove by just having one plugged in and trying it in the A and then the B bay.

I would start by looking at the X amp first, there are a couple of resistors under stress from the 150V line, R582 & R592 - both are 2K61, they might be worth measuring first and go from there. With the power off for a couple of minutes it might be worth pushing the transistors home in their sockets as they can become loose or dirty

Tony

[6AL5W-Martin]

in normal mode the beam can never go back again to right. But I see that here !
So I think there is a bad deflection plate in the CRT (not sure)

At first: please swap the connections of the both horizontal deflection plates and look if the problem is then at the same - or at the other side.

greetings
Martin

[trh01uk]

Martin,

you said "...in normal mode the beam can never go back again to right..."

True enough if the waveform on the x-plates is a rising voltage, i.e. a sawtooth. But there is no reason at all why a faulty timebase cannot deliver a falling voltage - so yes, the trace can definitely go backwards under fault conditions.

I would say either the timebase or the x-amplifier is bursting into spurious oscillations. Use of another scope to pinpoint where this is occurring would be helpful - if one is available.

Spurious oscillation is often caused by failed decoupling capacitors. It may be quicker to swop out capacitors than trying to test each one individually.

Richard

[trh01uk]

The fact that the fault goes away if you move the trace to the left tells us that the oscillation is level sensitive. Probably because a semiconductor device has to be in a certain state to start oscillating.

Richard

[Radio Wrangler]

Select X=Y mode and feed in a sinewave from an audio generator to both X and Y inputs, adjust the the X sensitivity to fill the screen and see if it still misbehaves at the right side of the screen. This will eliminate the timebase from the hunt.

David

[PJL]

Just before it bursts into oscillation the Y amp is folding back then the oscillation appears on both X and Y. I would therefore start with the PSU. Does this model have tantalum capacitors? If so they should be replaced in any case.

[Radio Wrangler]

I think dead decoupling capacitors may be where this is headed. Going through everything on the boards with an ESR meter is one approach, but the scope still has more to tell us about probable location of the prime bad one. Mind you, with a load of similar parts of similar age, a general replacement programme can be labour-saving in the long run.

David

[dave cox]

I would also check the supply rails.
For the X output stage they will be quite high +/- 100V or so, IIRC.
In X/Y mode you could just use the POSITION controls (rather than feed in a signal) but I seem to recall the 'X channel' is actually a second 'Y channel' fed through the 'trigger' circuit of the time base plugin then routed onto the X-plates so it doesn't eliminate the time base. Alternatively a second vertical amp plugged into a right hand slot might be more conclusive ...

dc

[RogerEvans]

I think your pictures are consistent with the fault just being instability in the X amplifier. The Y output IC is fed by the +50V rail and the X output stage is all discrete and fed by +100V and -15V. There are two highly stressed capacitors (probably Tantalum) C4373 and C4394 which have around 86 volts across them according to the manual (look for BAMA if you need to find it online), or the fault could be in the 100V supply.

If you don't have another scope to probe around the X amplifier try using a very slow sweep speed and if the instability persists use an analogue multimeter to probe for the fault, this is very much second best since it assumes that the instability will cause some change in the average voltage.

Best of luck,

Roger

[Refugee]

Have you worked out what frequency the noise is producing?
It could be as simple as a voltage regulator producing low output with mains ripple on it.
Just sweep the audio oscillator and time base until you can see the ripples.
If it is close to 50 or 100hz it will most likely be a power supply problem.

[dave cox]

At 200uS/div I doubt its ripple !
I didn't find a manual for the 7704A but if its like the 7704's horizontal drive then one failure mode I have seen (on a 7603) is when one half of the deflection drive is AWOL. The other half will try and do double duty (due to feedback) but obviously it won't be able to drive the beam to full deflection both ways. Possibly, as it reaches the rail it goes unstable ...

dc

[karesz*]

Dave,
KO4BB has mans for both versions.
Regards-Karl

[RogerEvans]

Also on TekWiki (w140.com)

Roger