HeathKit - Bell & Howell Schools Oscilloscope

[FlyingHacker]

In these two photos the top trace is again the left hand (input) side of R336.

The bottom trace is the Gate of Q317.

In photo three (the first photo in this post) the input frequency is tuned such that the scope under test is working correctly.

In photo four (the second photo in this post) the input frequency is again slightly adjusted such that the scope under test no longer displays a waveform (no trace).

You can see how the waveform changes.

[FlyingHacker]

So you notice in my UPDATEs above I was confused, and was scoping Q306 and Q307 when I wanted D306 and D306 (which are actually transistors used as diode in the schematic. )

So I am going to do some more probing around.

At least I am getting changing waveforms when the scope under test misbehaves. Now I should be able to track this down, I hope.

[FlyingHacker]

Well, I have been looking at this guy periodically. Still jot having any luck.

I replaced all electrolytics. This got all the LV power supplies to good values and ripples.

I was hunting a red herring on the horizontal, trigger, and vertical boards. I misunderstood why a waveform looked the way it did. I learned a lot about scopes exploring this, and am now pretty sure the problem is in the HV side, as I originally thought...

I replaced Q202, which I really thought was going to fix it, but the problem persists.

I checked the diodes, including the Zener ZD202, and they test OK. ZD202 seems to break down right at the 9.1V. However, when I put a meter on ZD202 in action I read close to 2kV on either side of it. I do not notice a difference. Should these two side of ZD202 read the same?

Of course I am reading this with an old Simpson 260 on the 5000V scale, which puts the reading way over to the left in the low accuracy area. I do not have a HV probe for any of my DMMs. So until I get one the Simpson is all I have. I also do not know how accurate it is on that scale. So it may not actually be right around 2kV. By the circuit design it should be about 1.6kV or less.

The only other thing I can think would be the three HV capacitors. These are all rated for just 1.6kV (apparently a known poor choice on Heath's part, and an issue if the circuit really is at 2kV). I get reasonable readings at the two diodes in the voltage multiplier circuit. And the display only fails at certain frequencies of input. At these frequency ranges the trigger is working, the blanking circuit on the LV side of the optocoupler IC is working, the horizontal and vertical deflection are all working.

Anyone know of a cheap source for these HV .1uF caps? Ideally 2kV. These guys seem pricey, and I hate to waste any more money of this thing, but it has become a bit of a Moby Dick obsession to get going.

I don't have any way to measure the HV side of the optocoupler. I need to get a HV probe for the scope.

I took C201 out and it measures OK.
All the resistors on the HV side read right near their ratings.

I am not sure what ZD202 is supposed to do, if someone could explain that this might help.

Any help would be very much appreciated.

Thanks.

[FlyingHacker]

I found either these:

http://www.mouser.com/ProductDetail/Cornell-Dubilier-CDE/942C20P1K-F/?qs=sGAEpiMZZMukHu%252bjC5l7YerOgAzP4CWkBluaUkSdZA8%3d

Or these:

http://www.ebay.com/itm/New-7x-0-1uF-2000V-20-Metallized-Polyester-Film-Capacitors-1uF-2KV-DC-/140590893448?hash=item20bbdec988:m:mowZQvyrA0zNS2WSDEQudJA

Open to opinions on those, and any other cheaper options.

[tautech]

ZD202 is to set a basic voltage differential between the cathode and the sucessive grids that is then further adjusted by Focus and Intensity and their various resistive dividers. Grids will be more positive than the cathode.

Many other scopes also have a PDA (Post Deflection Acceleration) source often +10kV+ to further accelerate the electron beam. Yep there are 2 +'s in that, little test for you. 

AS for the HV caps, Aliexpress or eBay will be a cheaper source, even 10kV caps are well priced.
104, 0.1 uF is a high value for an HV cap and you may find 473, 0.047 uF easier to get and parallel a pair.

[FlyingHacker]

Thanks.

Do you think the caps are the more likely culprit to fail at specific frequencies? Or should I be looking at the optocoupler? thanks.

[tautech]

Thanks.

Do you think the caps are the more likely culprit to fail at specific frequencies? Or should I be looking at the optocoupler? thanks.

Honestly, who knows  it might be a design fault. 
Or originating from somewhere else within the scope.

Have you taced the signal when it fails to be displayed throughout its path to the CRT deflection plates?
It's certainly a brain teaser type of fault. 

[FlyingHacker]

Thanks.

Do you think the caps are the more likely culprit to fail at specific frequencies? Or should I be looking at the optocoupler? thanks.

Honestly, who knows  it might be a design fault. 
Or originating from somewhere else within the scope.

Have you taced the signal when it fails to be displayed throughout its path to the CRT deflection plates?
It's certainly a brain teaser type of fault. 

I traced it to the output from the various board. In other words it is OK at the output transistors for the horizontal and vertical deflection. I traced the blanking waveform to the LV side of the optocoupler.

I guess I should follow the wires to the actual attachment point on the CRT. It would least be worth a look. Do you think a physical issue, like a bad solder joint or loose connector could fail at various frequency ranges?

I just got a Fluke 80k-6 probe, like new for $29 off eBay. For this the HV scope probe would be more useful, but that was such a good deal I couldn't pass it up.

[tautech]

Thanks.

Do you think the caps are the more likely culprit to fail at specific frequencies? Or should I be looking at the optocoupler? thanks.

Honestly, who knows  it might be a design fault. 
Or originating from somewhere else within the scope.

Have you taced the signal when it fails to be displayed throughout its path to the CRT deflection plates?
It's certainly a brain teaser type of fault. 

I traced it to the output from the various board. In other words it is OK at the output transistors for the horizontal and vertical deflection. I traced the blanking waveform to the LV side of the optocoupler.

I guess I should follow the wires to the actual attachment point on the CRT. It would least be worth a look. Do you think a physical issue, like a bad solder joint or loose connector could fail at various frequency ranges?

I just got a Fluke 80k-6 probe, like new for $29 off eBay. For this the HV scope probe would be more useful, but that was such a good deal I couldn't pass it up.

Very unlikely IMHO.

[FlyingHacker]

That was my thought.

The horizontal and vertical seem to work fine at the connection to the tube.

I think I will pull the optocoupler and try to test it at different frequencies (at a lower voltage of course). It is the cheaper part...

[FlyingHacker]

I replaced Q203 just because I had more spares. The frequency at which the issue occurred changed slightly, but that is all.

I ordered another optocoupler, just because it was only $3. 

Here is a linear frequency response (image) of the current optocoupler with a range from 20Hz to 200 kHz. Note that the problem occurs in the neighborhood of 70-140Hz depending possibly upon temperature.

Nothing is too remarkable here. This is feeding 15V into the output side of the optocoupler through a 1k resistor. The input is a sweep generator from 20Hz to 200kHz. I set the input to roughly match the voltage swing on the scope at the emitter of Q309. The input side of the optocoupler is almost linear at this frequency range through a 220ohm resistor. I suppose it is normal to expect some roll off. I have no idea what this looks like at 1.5-2kV.

I did notice some variance in the phase shift in the output wave vs. the input. Possibly this is an issue.

I guess I try the new optocoupler when it shows up, and if that doesn't work get some HV caps. If that still doesn't work then maybe defenestrate this thing.

In all honestly I will likely end up using this tube and basic control boards to create a scope clock or some other hacked display, like a game with an Arduino. A 5MHz scope is decent for simple logic stuff, but not all that useful.

I probably should have given up on this long ago. Some guys on the TekScopes mailing list said throw it away! I just couldn't bring myself to do it. I have learned a lot about how scopes work, since this circuit is so simple compared to a Tek scope.

[GreyWoolfe]

A high voltage probe really isn't that expensive- http://www.ebay.com/itm/High-Voltage-Probe-80K-6-6kV-PK-Max-Fluke-983T-bb4-/371460379690?hash=item567cc3942a:g:9G0AAOSwhcJWGIqk
For a little more, you can get a Fluke 27/FM meter with it- http://www.ebay.com/itm/TEST-EQUIPMENT-FLUKE-27-FM-MULTIMETER-WITH-HIGH-VOLTAGE-PROBE-AND-CASE-/191727208894?hash=item2ca3d509be:g:79oAAOSw9N1VpXay
You can never have enough DMMs.

[FlyingHacker]

A high voltage probe really isn't that expensive- http://www.ebay.com/itm/High-Voltage-Probe-80K-6-6kV-PK-Max-Fluke-983T-bb4-/371460379690?hash=item567cc3942a:g:9G0AAOSwhcJWGIqk
For a little more, you can get a Fluke 27/FM meter with it- http://www.ebay.com/itm/TEST-EQUIPMENT-FLUKE-27-FM-MULTIMETER-WITH-HIGH-VOLTAGE-PROBE-AND-CASE-/191727208894?hash=item2ca3d509be:g:79oAAOSw9N1VpXay
You can never have enough DMMs.

Yeah, in a posting above I mentioned I just ordered the Fluke 80k-6. Got a good deal at $29 for like new.

I have way too many meters, and keep buying more. Triplett 631 combination VOM/VTVM is my latest acquisition. None of the digital ones had a HV probe because I didn't really need one until now. But I am sure it will come in handy.

I do need to grab one of the cheaper HV 100x scope probes, though. Methinks that would be the most useful thing for debugging this issue.

[FlyingHacker]

FYI... Here is a PDF of the docs, which makes them much easier to read:

http://meanmutha.com/BellHowellScopeManual.pdf

[FlyingHacker]

Grrrr....

Well, I replaced the optocoupler... No change.

I did get the Fluke HV probe. It seems the HV is actually sitting at about -1950V. So the Simpson was correct.

It seems this is because the HV tap from the transformer (before the doubler circuit) is about 940V.

Of course the HV caps are clearly marked 1600VDC!

I know I have spent way to much time and money on this silly old student scope, but it really has become a challenge at this point,

I am trying to source some appropriate caps. I low balled some 3kV caps on eBay... Will see if that goes through.

Any other thoughts.

Thanks.

[FlyingHacker]

I have really turned into Captain Ahab here...

I ordered three new caps for the HV section. All are rated 3kV.

My only other thought is that this had a MCT2E optocoupler in it, rather than the TIL115 listed in the docs. Of course I replaced it with another MCT2E, because it was cheaper. There is also a 1K resistor bodged from Pin 1 of that optocoupler to ground (presumably a voltage divider with R202). I tried disconnecting it, but that did not help.