Tektronix 422 with No Power

[KYBOSH]

Picked up a Tektronix type 422 oscilloscope locally for a few bucks last year.
It wouldnt power on at all so I stowed it away under my bench for later testing.
Today I remembered it was down there and pulled it out to see what was what.

I checked for continuity and everything checked out.
There was a burned smell and a resistor was replaced with a temporary replacement (see yellow box in picture).
The burned smell is very pungent/acidic and I am not familiar with these vintage electronics to know whether I am smelling the remains of the replaced resistor or something else.
From my google search I learned this power supply has safety mechanism to prevent it from being turned on without a load. I wanted to hold off on bypassing anything before I did more spot checks and got some advice.

I turned my attention to the transformer. The smell was just lingering and I though it might be from the transformer.
First I tested the primary coils in circuit (see pink arrows in picture) with my Anateck Blue Rink Tester. First time I am using it so I am no expert but from the readings the transformer is reading as bad/shorted.
Again I don't have much experience with this tester and Im not sure what a good transformer of this age/type should read but if it is bad what might my options be?

The thermal cutout (red circle) isnt reading continuity on the leads and that's a problem for sure but I dont have a replacement and dont want to bypass it incase whatever caused to to die is still present on the power supply.

Is there anything else you guys suggest I try or look at?
Bypassing the no-load safety would involve injecting voltage directly to the primary of the transformer and reading the voltages on the secondary.
Not sure what bad could happen to the unit by bypassing the thermal cutout either. It would be inaccessible while I have the power supply back on the scope so wouldnt be able to keep an eye on anything while I was powering the unit up.

While this is common I am not to excited about such an undertaking bcos of this unit's age.

Any tips or advice would be greatly appreciated!

[KYBOSH]

Okay I went ahead and bypassed the thermal cutoff/fuse and the unit powered on.
I figured with the power supply out of the case the temperature of the unit couldnt rise fast enough for me not to notice it.
The cutoff likely failed due to age and stress.

So the scope powers on but nothing is on the screen.
One battle won and another one begins.
I'll keep this thread posted of what I discover.
I'll try to replace parts with OEM if they are available and reasonable in cost.

[redwire]

Red circle... to me that's not a thermal cutoff, it's a TO-66 packaged power transistor! Like Q637. Proof is the high value base drive resistors that also connect to it, and three connections.

The electrolytic and tantalum capacitors need to be checked, they are pretty old. 1969 was a while ago. I wouldn't blame the transformer quite yet.

edit: I couldn't find the schematic for the AC power supply board with the 600's reference designators. It's not in the service manual.

[KYBOSH]

Red circle... to me that's not a thermal cutoff, it's a TO-66 packaged power transistor! Like Q637. Proof is the high value base drive resistors that also connect to it, and three connections.

The electrolytic and tantalum capacitors need to be checked, they are pretty old. 1969 was a while ago. I wouldn't blame the transformer quite yet.

edit: I couldn't find the schematic for the AC power supply board with the 600's reference designators. It's not in the service manual.

Thank you for the reply redwire

My apologies and you are right about that being a transistor and not a cutout.
Not sure what I was thinking there.
Its labelled Q657 and its has to do with the display's lights.

Specific to the AC supply is https://w140.com/tekwiki/wiki/File:Tek_422_acps.png

The schematics for this unit can be found on its tekwiki page:
https://w140.com/tekwiki/wiki/422

You are definitely right that the caps (and even diodes) need to be checked and i hope to do that today.

[petehall347]

q657 supplies the +12v

[redwire]

Someone has replaced the R657 15Ω resistor for the graticule lamps. The lamps must have shorted out to GND, which would overload the 12V rail and make mucho smoko.
Problem is the +12V regulator depends on the -12V and +55V rails. So those have to be working first.
I would be using a variac to power it up and troubleshoot the power supply BUT with the HV DC-DC converter disabled so it does not roast trying to start - lift a leg on R689 1Ω which feeds Q675/Q685, also check those two transistors.
Or maybe a dim bulb to power it (with HV DC-DC disabled).
At this age I would suspect the electrolytic capacitors, the transistor sockets can also make a bad connection.

Older Tektronix is a challenge to troubleshoot and repair. Try to have fun and learn lots.

[KYBOSH]

petehall347 You are spot on. Thank you for that.

redwire Forgive me but I do not see a 1Ω resistor (R689) anywhere on the board. Would you lend me an assist and point it out? Ive been going mad looking for it.


I did some spot checking of components last night and just about everything checked out.

The capacitance/ESR of the Large Caps were as follows:

C689 - 113µF/0.28Ω
C652 - 2482µF/0.08Ω
C611 - 2616µF/0.08Ω
C642 - 55µF/0.55Ω
C665 - 111µF/0.68Ω
C666 - 376µF/0.12Ω

I checked most of the resistors and found only 1 which may have off a bit (1KΩ which i believe is R624) but when I lifted the left to check it out of circuit it broke.
I didn't use much for when pulling the leg so i think it may have been physically damaged.

I checked the power transistors (Q657, Q685, Q675, Q637) and they all checked out as good. Although Q637 (right below the dodgy 1KΩ resistor) wouldnt register with my meter until I took it out of circuit.

I would have to bridge the safety interlock (pins 13 & 24) to get the PS to turn on without the indicator unit attached.
There is simply no way or room to do any troubleshooting with it attached as its sandwiched together.
I dont have a variac (yet) but I am familiar with a light bulb as a current limiter if this is what you mean by "dim bulb".

[redwire]

Sorry I got too many threads I'm involved with, did not mean to leave you hangin.

OK so how I would approach fixing this PSU is to first disable the HV DC-DC converter driving T801 which connects at P701/J701. Disconnect power to it by lifting a leg on the 1Ω resistor R689, connects to 100uF cap C689. But where is this resistor, clearly stolen. Or on the other side of the board? It might have smoked and somebody cut it out. I would follow the (+) of C689 100uF cap to the 1Ω resistor. C689 surely dead dried out.
I wonder if that is the brown things on the board edge that look like a wirewound inductor (super fragile! leave them alone they will crack in two) There's other 1Ω resistors I can't find like R614, R615, R651 to see what they look like.

OR can you power up the PSU board with J701 (rest of scope circuits) disconnected? It looks possible? But mains safety interlock is there. Hmmm. Tek did not want this or something.
Just to confirm the +12V, -12V, +55V, -110V, +95V rails are working, hopefully using a variac because the rails will all be high due to no load, and we don't want caps going bang. Only the +12V, -12V are regulated.
Only power it up with a lightbulb and fuse present on incoming mains.

See pic, is something missing there or parts on other side?

[petehall347]

is this cracked ?

[KYBOSH]

is this cracked ?

It has physical cracks/chips/defects on it yes but it reads continuous when I checked it with a meter. Im not exactly sure what the name of this component is TBH but there are 3 or 4 of them around the perimeter of the PCB.
The one by the serial power connector is in worse shape than this one...

[KYBOSH]

is this cracked ?

Also the line in the center is a filament/wire. It wraps around one leg several times crosses over the brown body wrapping it once then terminates by wrapping around the other leg. It feels like a rudimentary fuse of some sort but its likely resistor of some sort. I'll take its measurement tomorrow to see if I can identify it via the schematic.

[redwire]

Schematic shows three 1Ω resistors but did not see them on the board. I would say Tek used these small phenolic-body inductors instead. Didn't have ferrite beads back then. For filtering RF.
The inductor windings appear on the outside body I think and look like a bump or crack but it's magnet wire. They are too fragile to take out I would leave them alone.
Another way to disable the HV DC-DC is short the E-B junction of both transistors Q675, Q685 to keep them off.

You can also just test all semi's on the board using Diode-Test with a multimeter, it's a couple minute's work and saves troubleshooting time.

[KYBOSH]

Sorry I got too many threads I'm involved with, did not mean to leave you hangin.

OK so how I would approach fixing this PSU is to first disable the HV DC-DC converter driving T801 which connects at P701/J701. Disconnect power to it by lifting a leg on the 1Ω resistor R689, connects to 100uF cap C689. But where is this resistor, clearly stolen. Or on the other side of the board? It might have smoked and somebody cut it out. I would follow the (+) of C689 100uF cap to the 1Ω resistor. C689 surely dead dried out.
I wonder if that is the brown things on the board edge that look like a wirewound inductor (super fragile! leave them alone they will crack in two) There's other 1Ω resistors I can't find like R614, R615, R651 to see what they look like.

OR can you power up the PSU board with J701 (rest of scope circuits) disconnected? It looks possible? But mains safety interlock is there. Hmmm. Tek did not want this or something.
Just to confirm the +12V, -12V, +55V, -110V, +95V rails are working, hopefully using a variac because the rails will all be high due to no load, and we don't want caps going bang. Only the +12V, -12V are regulated.
Only power it up with a lightbulb and fuse present on incoming mains.

See pic, is something missing there or parts on other side?

Schematic shows three 1Ω resistors but did not see them on the board. I would say Tek used these small phenolic-body inductors instead. Didn't have ferrite beads back then. For filtering RF.
The inductor windings appear on the outside body I think and look like a bump or crack but it's magnet wire. They are too fragile to take out I would leave them alone.
Another way to disable the HV DC-DC is short the E-B junction of both transistors Q675, Q685 to keep them off.

You can also just test all semi's on the board using Diode-Test with a multimeter, it's a couple minute's work and saves troubleshooting time.

Thank you for this explanation redwire
Didnt see it yesterday.

Firstly, I already tested C689 (along with all the other major caps) and it is surprisingly okay (cap=113µF; ESR=0.28Ω)
Taking petehall347's cue I went and checked out these brown things and 2 of them read 1.2Ω.
The other one (R651 on the bottom left of the board) is dead (reading -4.3 MΩ and fluctuating).
Cosmetically it looks in better shape than the other 2.
Thank you Pete for dropping me that bread crumb.
These are the only unidentified component that are present in a quantity of 3 and the fact that the 2 good ones read within tolerance (20%) of the 1Ω we are looking for.

I verified this by tracing the PCB track from the C689 to the mystery component and I got continuity. That 'resistor' (R689) is reading as good.

The spot which you circled in blue is where one end of some filter caps (C611, C652) are soldered to from the other side.
The other end of C652 connect directly to the blown brown resistor/inductor (R651). R651 leads directly to the blown 15Ω resistor (R657) that was replaced by the previous owner.

Since R651 is toast I think there is no harm (no choice really) in replacing it with a new/contemporary resistor as was done with R657. I have read about replacing old components with new ones in these vintage machines and how it throws things like calibration out of whack but I think that maybe more relevant to the indicator unit than the power supply side.

Shorting Q657 was how I got the PSU to show signs of life (back when I thought it was a thermal cutout Im ashamed to say!).
Dont have a variac (yet) so I'd have to hold off on that part of the testing. Will see if I can find one locally and proceed as recommended.

Thank you guys very much. Learning a lot here. Old time board are a lot different than the new ones of today. Takes a bit of getting used to!

[petehall347]

suppose you could try a 1 ohm resistor after checking for shorts on that line

[redwire]

Your sequence of events does not make sense
If R651 was open circuit, shorting Q657 would do nothing - there is no +19.8V rail feeding it with the resistor open.
Anyway, check Q653, Q654, Q657 because we don't want the +12V reg malfunctioning and giving the scope 20V or something.

For a R651 replacement I would put in a 1Ω 1/4-1/2W resistor with ferrite bead on the leads if possible. Ideally fusible resistor because it seemed to do that job here.
Or use a small inductor of a few uH and 1Ω DC resistance.

[petehall347]

maybe conformal coating or something screwed up the reading . even a broke meter lead can send you down a rabbit hole . if it is really open then something maybe made it that way . use metal film resistor if needed they dont set on fire .

[KYBOSH]

Thank you Gents.

Before I got a chance to read your last responses I went ahead and put a 1Ω (1.2 to be precise) 1/2 watt resistor on there to test it out.
The meter came on just as it did with the E and B of Q657 shorted.
It had lights on everything but there was no illumination on the CRT.
Funny thing happened when I cut the unit off though.
I saw a FLASH of the signal trace on the screen.
I repeated this again and saw the same thing.
This indicated to me that the CRT was actually on.

I read that a common problem with these units were the knobs (potentiometers) so I took the Intensity pot apart and cleaned out the corrosion/oxidation.
I put di-electric grease on and reinstalled. I still didnt see anything so I thought to adjust the horizontal and vertical controls a bit and turn the unit off and on so I could get a glimpse of where the trace was (or shall i say supposed to be). I adjusted it to near center as I turned the knobs and turned the unit on and off again. Once I did that I cleaned the intensity knob again and made sure I positioned everything correctly (might not have done that right the first time). I turned off the lights and got a very faint signal in the center. I tried adjusting the intensity some more but it was still pretty faint. About half the intensity of what I would expect. I started playing with the other control knobs (focus, astigmatism or scale illum) and nothing happened until i adjusted the scale illumination knob. The first thing that happened was the trace looked significantly better. The second thing that happened was the power supply started smoking! I quickly turned it off and had a look. It was the same resistor that was changed by the previous owner R657. I was burning up. It only started doing that after I adjusted the SCALE ILLUM knob.

This unit needs serious work it seems.
Also the horizontal and vertical knobs are not synched to what's going on with the screen.
Most of the knob's travel took the trace off screen.
I assume someone made some blind adjustments along the way.

I have the instruction manual which is VERY in-depth and I think I start reading it to get a better understanding of this unit and oscilloscopes in general. Its 200+ pages of technical stuff so Im not going to be master overnight. It also helps to have a working unit when learning from a manual but it should be interesting.

Not sure where to go from here but first thing Im going to do is replace the parts that have been replaced with spec components. Then Im going to try to turn my attention to some of these knobs which seems to be a major issue with these scopes. If anything that will at least eliminate them as a source of the problem.

Any other ideas you guys might have i'd greatly appreciate it.

Edit to Add:
Just came across the AC Power Supply Manual and found the specs for the components:
R651 is a 1Ω (±10%), 1/4W, Wire Wound resistor.
R657 is a 15Ω (±5%), 1W resistor.