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Experienced CRT people - looking for some help/advice on a Commodore 1702

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First post, by hpxca

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Hello,
I'm looking for some help/advice on a Commodore 1702 monitor. I'm an experienced electronics tech having worked in the industry for many years. I've successfully recapped/repaired many a motherboard and peripheral, particularly vintage stuff but what I haven't done much of is CRTs. I knew a guy in an old job who worked on them and watched him discharge and service one once, but I didn't perform the process myself. Still, I'm aware of the safety considerations of them.

With that out of the way I have two Commodore 1702 monitors, one was from a family friend, which was a late 1984 built one with some "sunburn" and a missing door (but of course!). The other was mine, which is a 1983 model. Both have JVC innards (maybe they all do?) and Hitachi tubes.

I wanted to refurbish/recap them, but I was hesitant to make the investment the required HV probe and/or obtain or build a "proper" discharge tool just for this. It's a shame commercial tools for discharging them aren't available any more - I've been rather leery around HV stuff since I zapped myself on 1kV calibrating a multimeter many years ago. However, KNOWING that both of these monitors haven't been connected to a power source for more then a decade I was confident that so long as I didn't plug them in prior to disassembly it would be pretty safe to build a relatively rudimentary tool with a screwdriver and alligator clipped cable to ground to ensure there was no charge on the tube and then disassemble it.

So I grabbed one of them - the 1984 one which had a busted/unsafe power cable anyway (so I couldn't really plug it in even if I wanted to) and cracked it open. Quite certain there was very minimal risk involved I built the rudimentary discharge tool and ensured there was no charge, then removed the anode cap and disassembled the monitor. I carefully inventoried the caps on the board (I know there's also lists available and even kits for these things) and ordered some caps for the monitor. I recapped the board doing 1 cap at a time and taking my time doing it (the whole thing took me about 4 hours even with my FR-301). I am aware that:

-There are 2-3 bipolar caps on the board depending on board variant and they were replaced with bi-polar caps. In my case the board in the 1984 monitor had two bi-polars.
-C101 is reversed as compared to the silkscreen and most other caps. I ensured that C101 was installed in the correct (reversed) polarity - the same polarity as it was when it was removed.
-I double checked all polarities prior to reassembling the monitor.
-For a few of the caps I installed higher voltage versions to reduce the variants I had to order - I replaced some smaller 25 and 35V caps with 50V caps and some 25V caps with 35V caps. The capacitance values weren't changed.
-I did apply some dielectric grease to the anode cap and top of the tube prior to reinstalling the cap. I didn't put grease on the clip of course or in the hole - we want that part to conduct.

After the recap and a good cleaning of all the plastics I replaced the faulty power cord and reassembled the monitor. As I didn't want to power it to ensure it remained discharged I didn't test this monitor prior to performing this work. As far as I knew this monitor worked when it was last plugged in, but that was a very long time ago and with the busted power cord and desire to keep it non-energized to make the work safer I figured I'd just proceed and cross my fingers that it would work when I was done.

Well, it didn't work when I was done - hence this thread - here's the symptoms.

-When power is applied you get a steady power light. With the brightness set in the 'notch' the screen shows no picture. I had it connected to my C64 via the rear inputs, and the switch was set correctly on the back. There is audible arcing going on so there's an issue with the high voltage. Note I had the cover on when I did this, so the arcing wasn't visible to me.

-If you raise the brightness the frequency of the arcing increases. The arcing doesn't have a constant frequency or pattern however, but does happen more often when the brightness is higher. The screen does brighten as expected but there's visible "noise" in areas of the screen which are related to the arcing I think. There's also a few bright "streaks" that can appear near the bottom of the if the brightness is set quite high (they almost look like lightning bolts). I didn't have it on for more then a few minutes, with the arcing I didn't want to leave it powered.

-Even with the screen brighter there's nothing that resembles a picture, not even distorted or a shadow of one. I didn't try the composite input - again I didn't really want to mess with it too much with the arcing going on.

Finally the questions:
I know if I want to try repairing this or looking at it again I'll now need a HV probe and a safer means of properly discharging the monitor. This is probably a $300 or so tool investment for me that I'm not sure I'd ever use again.

Is it worth it for this monitor?
Is it likely this monitor is now permanently damaged and I should just throw a "for parts" sticker on it?
Or is it likely I just made a simple mistake here like not seating the anode cap correctly? Other?

I'm worried that the no picture and the high voltage arcing could be two separate problems and I have no idea if it worked to begin with. Like most humans I make mistakes and I could have made a mistake in the recap, but I don't think that explains the arcing. If I had to guess based on the sound it seemed like it was coming more from the area of the anode cap.

Reply 1 of 2, by momaka

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I'm not a professional CRT serviceman, nor a professional electronics serviceman even... but I've done a fair share of CRT repairs and all of them with nothing more than basic tools. In fact, one of my first and more serious electronics repair was my family's 17" CRT monitor. I only had a cheap $5 multimeter back then and a 30W soldering iron. Managed to fix it just fine after finding an open resistor in the power supply section. More than anything else, the multimeter saved me from getting zapped on the bulk 400V input capacitor, which was charged to 160V DC (120V US AC line rectified is about 160V DC... for 220/230/240V countries, it's approximately twice that). Not that this cap would have killed me if I did get zapped (and eventually, I experienced shocks from much bigger caps - some of them by choice too / for "shits and giggles"). It's not pleasant, that's all I will say. But not deadly either.

That said, I don't think you need to invest in an HV probe - not at this stage, at least.

For starters, just open the monitor and try to see where the sparking/arcing is coming from when you plug it in. Since you removed the anode cap, I suggest to check around there first. It's possibly the dielectric grease is not suitable for HV applications. I found out something similar when I had to replace the HOT on a 19" CRT monitor and decided to put MX-2 thermal compound between the HOT an its silicon thermal pad. The thermal compound allowed HV to leak through from C to E and arc-over. HOT didn't get damaged, or anything else for that matter... aside from the silicon thermal pad, of course... which is what stopped the monitor from working. So do check if your dielectric grease is OK for HV applications. If it doesn't say, perhaps wipe it off and try again.

Also have a look around and under the flyback transformer area too. Sometimes the plastic on certain flybacks can get old and brittle and develop cracks... allowing HV to leak through. It help to do this in a dim room (but not completely dark, as you don't want to accidentally trip on something, especially live-powered equipment.)

As for discharging tool - I don't think you need to go with anything that fancy either. A screwdriver with just an alligator clip (or actually, TWO, just for redundancy) going straight to ground should be OK. Obviously with no series resistor, expect a loud pop if the tube is still charged. But it should be harmless to you or the monitor (unless you have a heart pace maker that can get affected by stronger EMP... in which case, you probably shouldn't be working on electronics, so probably moot to mention this... but eh, why not for the completion 😉 )
That said, if you do want to avoid the loud pop, you won't need such large resistance specialized resistors. Even 4.7 to 10 kOhm should be fine - just use something with a higher power rating so that it can handle the energy from the tube without bursting. 3 or 5 Watt resistor should be OK. 2 or 3 in series: better. And probably better to put them on the end close to the anode cap with their tail grounded so as to limit both EMP and danger of HV leaking nearby in case the grounding gets lost for whatever reason (or if the resistors open.)

I personally don't discharge the anode when working on CRTs unless I really do have to remove the HV anode cap. Of course, it's possible that some older monitors (90's and earlier, I presume) might be safer to work on if you do, depending on how they are designed. For mid and late 90's (or older) CRTs, the anode usually self-discharges to relatively or completely safe levels within a few seconds of the monitor going off. So after 10-15 minutes of sitting, it should be OK to work on.

Reply 2 of 2, by hpxca

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Thanks for the reply. I did order a few things to make a better discharge "tool". I didn't order any resistors, it does seem that most people just do it without. I used some permatex dielectric grease because it was easy to get here - it's mostly for automotive applications, I'm not 100% sure if it's perhaps not good enough here. I'll try powering it up from a safe distance in a dark-ish area and see if the arcing makes itself visible.

Edit: I powered it up in a darkish room with the cover off. It doesn't arc much unless I increase the brightness and I can see the glow of the heaters in the CRT and I could hear the arcing but it wasn't visible. If i listen closely it seems to be coming from under the anode cap.

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