Oscilloscope repair - Bell & Howell / Heathkit

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absrec

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Aug 11, 2017
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I hoping someone here might have messed with one of these before. I just bought an old oscilloscope on eBay. It's a Bell & Howell 9560. It looks like it was sold as a kit that electronics students put together. It was listed as tested and working so I took a chance and it did in fact work when it showed up. I don't have any experience using one but after messing with a bit, I started to get the hang of it and decided to do the calibration procedure outlined in the original build manual. I figured... couldn't hurt, right? It tells you where to set all controls and switches and then has you adjust trim pots as well as certain controls on the face and eventually a little square appears on the display, which it did. I then realized that I hadn't set ALL of the controls correctly and some of the adjustments seemed more extreme than the manual was alluding to so I figured that was why. I zeroed everything out and went back to square one to begin the calibration procedure once more. At a point, I should have been seeing the little square dot on the display but I wasn't. A bit later, I saw it but it started fading and acting weird and then disappeared completely and I haven't seen it since.

I'm pretty careful with things like this and I don't remember shorting anything out with my probes but it certainly is possible. I asked a knowledgeable friend about it and he said there was probably a flyback transformer somewhere and that is what normally goes bad with a CRT based system. I don't see anything on the schematic aside from the main power transformer which is a multitap much like you would find in a tube amp. One of the windings goes straight to what looks like the filaments of the CRT. As I'm carrying out these measurements and adjustments, the voltages are all there so I'm thinking the driver transistors are probably fine. Just can't seem to get anything on the display.

Does it sound like the CRT is toast? It's ironic that I got an oscilloscope to help me troubleshoot electronics builds and repairs and now I'm stuck trying to repair the scope. I really don't know where to start with something like this. All suggestions welcome and appreciated.
 

absrec

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If it's old, electrolytic capacitors could be challenged.

JR
I considered that. Get this.... there are (3) .1uF 1600V caps. Such a wierd value for an electrolytic, right? I looked around and they don't seem to exist. Probably have to do a series combo if they need to be replaced. Just wish I could figure out if that's the issue.
 

absrec

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Could it hurt to replace those with film capacitors? Better odds of finding such high voltage ones, than electrolytics, i'd reckon.
Probably not. I removed them and tested today and they tested over spec on my meter.
I doubt 1600V 'lytics ever existed. They're probably film or paper-in-oil.
I also doubt there is a flyback xfmr in a scope.
There's not a flyback. The PT has an 800V AC secondary that is rectified to provide the 1000V DC going to the.... screen? grid? Not sure what it's referred to on a CRT.

Anyway, I fixed up some bad solder joints and a source resistor that was testing way over spec. Turned it on and after twiddling the controls a bit, I saw the little dot come back so the CRT isn't toast. However, the calibration sequence outlined in the manual went fine until I got up to the last step which involved the vertical driver. The "vertical calib" trimmer inside the unit seemed to do nothing. I just left it centered and moved on. I then replaced the IC the manual instructed me to remove for calibration and switched the unit back on. At first everything was working and then I heard a click that sounded kind of like a relay and the display went blank. I then began to smell a faint burning smell. Ya'll know the one. I promptly shut it down and did something else.

Maybe this was more than I bargained for. Again, I can't help but recognize the irony here.
 

murrayatuptown

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Bell & Howell 9560 also had a Heathkit model number. I think IOD-203-3.

This bunkerofdoom.com electronics website (it is safe, despite the name!) has a partial manual as individual JPG pages. I think they got the Heathkit model by scan-to-text, so it's corrupted into 103-203-31...but here it is:


When I hear 0.1 uF 1600V, I immediately think of a pair of paper (non-polar) capacitors used in the CRT high voltage voltage doubler. Many Heathkit oscilloscopes had one 50/60 Hz power transformer with a winding in the range of 600-1000 vac at somewhere between 0.5 and 3 mA DC. A smaller number of models had a flyback inverter power supply. Some had two line/mains transformers, one for high voltage and CRT filament, and a second for low and medium voltages.

This scope looks like it is solid-state, other than the CRT. Here is one of the schematic pages.
https://www.bunkerofdoom.com/lit/bellandhowell/4bellandhowellscope90.jpg I see the 0.1 uF pair at the very bottom, a pair of 5D20 rectifiers, a 470R resistor and another 0.1/1600.

This model has a single power transformer. I don't think this manual is complete. I couldn't find a parts list. I wanted to see if there was a 54-xxxx part number for the power transformer. I have been selling NOS Heathkit power transformers halfheartedly on eBay since the 90's. I had a hard drive crash and really have no idea what I have or where, but I recently pulled out a box of I think 54-953 and one 54-1032. Also 54-282, which were HV & filament only, but I buried that box somewhere again (LOT of stuff in the basement).

Sometimes those capacitors go, sometimes the HV winding opens. I've only heard from people who discovered an open winding. Less frequently bad doubler capacitors, but if I had an open hv winding, I would replace those for various reasons. I have never heard from anyone who actually witnessed a transformer failure...just realized it after the fact. So I do not know how audible such a failure would be.

I have disassembled two other Heathkit scope transformers, one with an open HV winding (measurable at the wiring). The other blew fuses. The confirmed-open winding specimen had a 'mild case' of paper insulation darkening...almost charred, but not black. 'Ground Zero' was found, but I did not know what the chain of events was. If the winding simply fused open, I wouldn't have expected charring. It might have been a turn-to-turn insulation failure that caused gradual insulation failure, or some other insulation system failure like corona. Other than a simple open winding (I assume due to an external cause for over-current such like a bad diode or capacitor), most other transformer failures have to be insulation system failures. Not the iron.

Maybe the linked manual will give you a visual reference, and maybe you can see something else on a circuit board that looks unhealthy.

I don't have any idea what else can fail with enough energy to be audible other than AC primary and high voltage circuits. After you are certain any charge is dissipated, the diodes can be checked with a DMM. If a diode in a doubler shorts it's hard to measure the rest of it if there is a short across the winding. I am currently working on a very old machine with a hipot and IR tester and both diodes were shorted in the voltage doubler, making the capacitors appear shorted (but were not).

You may also smell, as well as see, what failed, if you're lucky.

I don't have any opinion about how common a HV winding failure is, but if you can tell me the 54-XXXX p/n, I can look.

There is a two-page post from 2009 with someone trying to trouble-shoot one of these that suffered a failure induced by a tool inside the scope while the power was on. Not sure how it ended, as the guy was hinting at converting it to a CRT clock. It may not end well.


Murray
 

murrayatuptown

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I also agree with a previous post that if you need to replace a 0.1 uF/1600 VDC capacitor, film is the way to go, particularly in a power supply.
 
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