Tektronix 1731 Waveform Monitor

[vampi-the-frog]

Hey gang, I recently got a pair of Tek 1731 and 1721 waveform monitor + vector scope. However, they have a bunch of problems:

• The waveform monitor (1731) won't turn on. There is a clicking sound coming from the main transformer, and no lights turning on.
• The vectorscope (1721) doesn't always turn on, sometimes it comes on after a while, there is a clicking sound in this one too.
• Some of the vectorscope knobs are broken, but relatively easy to fix. There is an interesting rotating control for the phase, which is a sort of strange 3-way potentiometer. I'll post pictures when I perform the repair on this one.

For now I'd like to concentrate on the 1731 waveform monitor. Here are some links I've found, which have been useful, but not 100%:

• http://uk.tek.com/datasheet/1720-series-1730-series
• Instruction manual from Tek official site
• A scan of an older manual, which seems to correspond, but no schematics or troubleshooting steps are included

I own the 1731, which is the PAL version (1730 is the NTSC version). The serial no is E202089. The reason I'm mentioning the serial number is because the second link in the list above is a fairly complete manual, but it's for "SN B070000 and above", which means it's a different schematic for the PSU at least. The component numbers don't match.

The third link, which is a scan, corresponds, there is some text near the end that explains how the power supply works, and the component numbers match, but unfortunately there is no schematic or troubleshooting steps.

Btw, the power supplies are identical on the 1730 and 1720 series, so a manual for 1720 would work too, as long as it's not for "B0X0000 series and above" like they are on the tek site.

So I need either help fixing it, or a manual scan. I've found some manuals on ebay, but they're for >$50, and I paid about $75 for both devices. I know I got ripped off, but it's more fun to fix it than to send it back. 

Anyway.

I've measured the voltages at +5, +15, -15 and +40V, and they're around 1V, so 10 times as low. I've taken a look at the PWM chip and its +Vcc looks like a 10Hz spike on the scope. On the other device (1720), which eventually turns on, the supply for the PWM chip is a constant 12V, and the PWM output looks like 50% duty cycle, I guess a few hundred kHz. I've read somewhere that the clicking is actually the supply going into some sort of undercurrent protection mode and shutting down and turning on again.

I've replaced most of the electrolytics caps too, btw.

Anyway, I'm attaching some pictures, and if anyone can help with some troubleshooting, that would be great.

Thanks!

[PA0PBZ]

Looking at the pcb it seems that there should be a jumper wire for either 110 or 220V operation, but there is nothing there.
Having said that, it doesn't look like anything was removed so it could be a red herring, but it seems a bit strange.
The clicking sound is the smps going into protection, either because of a voltage or current failure. If you have the expected voltage on the cap after the bridge rectifier it's time to start measuring semiconductors on the secondary side.

[vampi-the-frog]

The 220V jumper is not necessary. Those two points are not connected to anything. In the manual (third link in the list above) it says

Operating voltage is selected by the position of W476, which is soldered in place for 120 V input and removed for 220 V input.

I'll do those checks and reply with the results, thanks.

[vampi-the-frog]

Here's an update.

- I've checked the voltage after the diode bridge, it's 315V (input is about 220).
- I've checked all the transistors in the secondary side:

Q344    2N3904
Q137    151-216-M823
Q238    2N3904
Q237    151-216-M823
Q313    2N3904
Q413    0749-M833
Q518    2N3904
Q516    2N3906
Q419    151-0350
Q519    2N5551

They're all Motorola branded. I've found some info about the ones with strange markings as well, but they all seem to have base in the middle of the TO-92 capsule, and they all check out seemingly fine.

There's a TIP31C with a clip type radiator on it, and a power diode with another clip radiator on it, both are fine.

I've checked some diodes as well, and some resistors. There's a larger diode on the HV transformer that doesn't register on the multimeter in any mode (diode check mode or resistor mode). However, this behaves the same on the other PSU board, so I don't know if it affects it in any way. It says ASSY MEXICO around it, and along it it says 0409 and 8840 (the latter is probably the date). So far this is the only possibly faulty component I've found.

Both boards have clicking, but the other one eventually starts up.

I've also tried switching the power boards around, and the device started working (very crisp image btw), but only after some tries. Sometimes it starts to click, and eventually it starts up normally. Then again I turn it on and it goes into click mode. Currently I can't get it to start again.

At some point I've replaced the last large electrolytic capacitor and it seems the clicking is of a lower frequency, but still there.

I'm not sure how it's supposed to behave when there's nothing connected to the output, but I noticed that shorting the +15V line makes no difference in the clicking, and neither does adjusting the +5V ADJ pot.

Anyway, what can I do next?

[helius]

Just spotted the Schaffner line filter: don't they always fail? Check the output waveform.

[vampi-the-frog]

Just spotted the Schaffner line filter: don't they always fail? Check the output waveform.

You mean right after the line filter? I dunno if my scope can measure voltages that high, and I don't have an isolation transformer. Is there something else I can try?

[TiN]

Just replace it!

[vampi-the-frog]

Well I've tried just bypassing it, and it made no difference, so it's not the line filter. Any idea what I could try next?

[vampi-the-frog]

Here are the parts I've tested. Green are tested parts, magenta are replaced capacitors. The dotted component on the left hand side is the weird diode I was telling you about. Should I test the transformers? If so, then how does one test a transformer?

[SeanB]

See the 1M carbon composition resistor in the corner? Check it has not drifted high, and have you changed that low voltage cap near U455. Also all the small electrolytics on the primary side, and any resistor over 100k.

[PA0PBZ]

Well I've tried just bypassing it, and it made no difference, so it's not the line filter. Any idea what I could try next?

The problem with these filters is that they all die, spitting goo all over the place inside your gear and it's not fun to clean that, so that's why you should just replace it.

[vampi-the-frog]

@PA0PBZ: that sounds like a good idea, but what should I replace it with? Should I look for an identical one or what?

@SeanB: Turns out my cheapy multimeter can't measure beyond 2M ohm, so I can't measure the one on the top right (brown, black, blue = 10M), but the one on the top left (didn't know which corner you were referring to) reads 1.7M instead of 1.5M (brown green green). Also, if I'm going to replace these resistors, what power rating should I look for? I can't just search by diameter. I'm replacing the small caps now.

[vampi-the-frog]

Actually, replacing the small caps in the primary side fixed it. There is now only one click on startup, then it works fine, voltages measure fine. I'm going to try the same on the other one and see if it still misbehaves. I'll post a picture with them working if they work.

[PA0PBZ]

@PA0PBZ: that sounds like a good idea, but what should I replace it with? Should I look for an identical one or what?

The short answer would be: one that fits. Otherwise search on this forum for schaffner, there are some links to the parts, but I'm not sure if it is the right one.
What you need is FN 326-1/02, maybe google for it.

[vampi-the-frog]

Thanks. Well, they both work now, the 1731 Waveform Monitor and the 1721 Vectorscope. The vectorscope has some broken knobs that I'll attempt to fix once I can find some materials, and I need some more power cords so I can power both of them and the monitor so I can take a picture of all four (game console (don't have a signal gen yet) + monitor + 1731 + 1721).

Thanks for the help guys.

Lessons learned:

- when replacing electrolytic capacitors, replace ALL of them, not just the big ones.
- clicking in a SMPS means it's going into protection mode
- Tektronix equipment is pretty nice (these are my first Tek devices)
- After checking the caps, check for drifting resistors
- After a rectifier bridge, the voltage is about 220 * sqrt(2)

[crispy_tofu]

Glad you got it fixed! 

[dave_k]

I might be able to fix you up with some spare knobs .. at work we are junking a whole bunch of Tek WFM's and a couple of Vectorscopes.

[vampi-the-frog]

thanks dave, that would be pretty sweet! see PM

[SeanB]

Good to see it works again. the small caps are very necessary, they along with the 1M5 resistor ( and those little transistors in the area) are a start up supply, so if they hgo low value or high ESR the supply cannot start up., as as soon as the main controller switches on the power device the capacitor voltage drops below the UVLO voltage, and it switches off the device till the voltage is high enough again. It needs enough charge on the capacitor to do around 10 cycles of switching before the auxiliary winding can generate power for the chip, as it first has to charge up the secondary side capacitors to around half voltage.

[vampi-the-frog]

Well, when I stuck them back into the rack chassis, the WFM worked fine for a while, later on I adjusted a potentiometer CRT BIAS, that adjusted the brightness (rather, I set the intensity range that I could control with the INTENS pot), then later on I heard some clicking or sparking and it started turning off then back on. After a bit of looking it seemed that one the wires from the transformer next to the HV transformer, the coil with plastic insulated wire, was touching the aluminium chassis and sparking. Which is strange because the wire is insulated. There was also light coming from one of the lamps next to the HV transformer (visible on the upper left, above the 3 big white caps). I pushed the wire in slightly so now it doesn't spark, but now the intensity is high, focus is poor, and moving the intensity knob doesn't do anything. If I move the knob quickly, the intensity goes down slightly for a second and goes back up. If I enable the line select mode, the intensity knob works in reverse, but there is a very bright dot next to the OSD.

There are also a couple of potentiometers on the power supply board, CRT Bias and CRT Focus. turning CRT Bias seems to have the same effect as the intensity knob, perhaps less visible.

Guess it needs more fixing now any idea what I could test? I'm sure it's obvious for anyone with experience.

BTW, I know that the transformer with MURATA on it makes the kilovolts for driving the CRT, but what does the one with Tektronix on it do?

[dave_k]

The transformer creates high voltage AC, the "MURATA" module rectifies it and turns it into DC. It's listed in the parts list as "MODULE,HV:7.5KVAC IN,15KVDC OUT,POTTED MODULE".

[vampi-the-frog]

Thanks, I guess that makes sense. I found it strange that it would spark through the insulation, but if it's about 7.5kV, that seems more plausible. What I do find strange is why they would build it so the wire comes so close to the chassis. Maybe I didn't see it right? Maybe there should be an extra insulating plastic sheet which I dont' have? Either way now I have to fix the brightness issue.

Perhaps change the caps in that area again?

[dom0]

I found it strange that it would spark through the insulation, but if it's about 7.5kV, that seems more plausible. What I do find strange is why they would build it so the wire comes so close to the chassis. Maybe I didn't see it right? Maybe there should be an extra insulating plastic sheet which I dont' have? Either way now I have to fix the brightness issue.

It shouldn't spark. The EHT wire is probably damaged (mechanically, e.g. small cracks in the insulation or chemically).