tektronix 422 dead screen

[atxbyea]

Hi all
Just got a tek422 for free since it was marked as broken.

I think it is a great looking unit and I wanted to try to bring it back to life.

It powers on fine, and the uncal lights turn on when turning the knobs, but the display is dark, and I see no trace.

Illum scale does nothing, nor does intensity

Visual inspection of PSU and onboard circuit boards yield no interesting finds, can't see any onboard fuses or leaking capacitors.

[George Edmonds]

Hi

Need serial number as there are several versions.

G Edmonds

[atxbyea]

Hi George
 
100508 is the serial, thanks again!

[George Edmonds]

Hi

You have at least two faults, the scale illumination is probably no more than a couple of blown pea lamps.

No CRT display is a more serious problem, but first I must ask you if you are comfortable with and have the facilities to work with in safety circuits producing up to 8kV.

G Edmonds

[atxbyea]

I was suspecting the pots at first, i see they tend to go bad, so thinking about opening them up first and see if there is an issue.

I'm a trained vocational electronics repair tech with focus on Xerox and crt, but I took my degree 21 years ago, and haven't been practicing my trade, so... I'm not unfamiliar, but I'm not very confident either I guess

[George Edmonds]

Hi

I seriously doubt that it is faulty pots, maybe so in the case of the scale illumination, but the pea bulbs reaching the end of their live is by far more likely.

No CRT display can be caused for a number of reasons from an actual CRT failure.  Possibly the simplest first step is to check the CRT heater for continuity, the heater is on pins 1 and 14, you should check for heater continuity with the CRT base connector removed.

With the CRT base connector replaced power the scope up and check that you have minus 1400V on pin 3 of the CRT.  If at all possible this measurement should be carried out with an old style moving coil meter with an impedance of 20K Ohms per Volt such as an AVO8 or Simpson 260.

I cannot stress strongly enough that this -1400V supply can easily be lethal, unlike later scopes which use a high frequency low power oscillator to generate this voltage the 422 uses a transformer driven from the mains supply inverter.  DO NOT live probe this voltage, make the connections with the scope turned off and even then when powering up the scope to make the measurement put your left hand in your pocket.

Let me know what you find.

G Edmonds

[atxbyea]

Since I don't have a HV probe I went ahead and tested the heater for continuity.

By base connector removed I guess you mean the white round connector on the back of the CRT, this was removed.

Measuring pin 1 and 14 on the connector that goes from the PSU -> Unit I guess ?
If so there was no continuity on those pins.

Thanks again George.

[George Edmonds]

Hi

Were you measuring the CRT pins or the connector? you need the CRT pins.

If you have no continuity on those pins the CRT is dead.

G Edmonds

[atxbyea]

I was measuring the connector pins, those were the only ones I saw that were numbered, let me see if I can find the CRT pin numbers...

[atxbyea]

Can't see any pinout in this https://w140.com/tekwiki/images/8/88/070-0434-02.pdf nor any pin numbers labeled on the CRT, got any hints ?

[atxbyea]

Disregard my stupidity..... the pins are described on the connector itself...

It has continuity!

[George Edmonds]

Hi

That is good news, at least the CRT  has a heater and is probably OK

The bad news is that you must now measure the voltages around the CRT base.

G Edmonds

[atxbyea]

Unfortunatly my multimeters only go to 650v, and I have no high voltage probes

[George Edmonds]

Hi

What meters do you have, if digital they are probably 10M Ohm input so 20M Ohm, made up of a number of resistors, in series with meter probe will give you almost 2kV, accuracy not required, just an indication of voltage.

G Edmonds

[atxbyea]

Off the shelf random stuff, the one I have at my desk currently is a BS1901W

[George Edmonds]

Hi

That DMM is 10M  Ohms input so 20M Ohms in series will be fine

G Edmonds

[atxbyea]

Curious, since the power supply snaps on the back of the scope, how would I measure pin 3 on the crt with it on? Is that pin routed to the side of the unit somewhere accessible perhaps?

[George Edmonds]

Hi

I don’t have a 422 to hand and I must of last worked on one at least thirty years ago, all I am doing in suggesting pin 3 of the CRT is to repeat what is in the Tektronix service manual.  If you can locate the High Voltage Regulator board and gain good access to it the voltage is available to measure at a number of points, for example, either end of R389 a 221K resistor or one end of C828.

G Edmonds

[atxbyea]

thanks george

I'll try to get hold of a high voltage probe or a meter with higher input tolerance, I feel unsure about playing around with a provisonal high voltage probe


thanks again!

[George Edmonds]

Hi

If possible I would recommend finding an old style moving coil meter as all of the measurements on the schematics were taken using one of them.  Quite a lot of them were 20K Ohm per volt which is what Tektronix recommended and had a maximum DC voltage range of between 2kV and some even 5kV.

G Edmonds

[jdragoset]

Curious, since the power supply snaps on the back of the scope, how would I measure pin 3 on the crt with it on? Is that pin routed to the side of the unit somewhere accessible perhaps?

The indicator (instrument section with the CRT) has a plug connector to the power supply connection that can be released by loosening two Philips head screws, giving about 8" of cable length to connect the power supply for servicing.