My day job is not and has never been EE. This is not a guide to repairing anything. This is a long post. End of disclaimers! I wanted a standalone analog unit. The PM had some front panel issues but generally worked ok.
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My day job is not and has never been EE. This is not a guide to repairing anything. This is a long post. End of disclaimers! I wanted a standalone analog unit. The PM had some front panel issues but generally worked ok. I lucked out on a second source for the broken panel elements and had it fully working before the arrived.
I agree that could mean anything - and indeed it did. I link some images here crammed with annotations. This is mainly to save linking extra pics of the same stuff in some cases. So after a quick inspection and a quick belching-flames-power-up test, it seemed like the tube would not light.
No whine from the flyback but some activity in low voltage rails. The 5v rail had some life at least and I think the front panel had power maybe a LED. So I removed the PSU board. Not easy - there are nasty plastic catches behind the board which need carefully prized with a long thin screwdriver. Better still, a long thin insulated stick as strong as a screwdriver good luck with that because discharging all the caps in-situ first is a bit sketchy. I discharged the main offenders at the front first, lift it out then do the rest.
Discharge that before handling. The rest are either low voltage or always tested discharged. Also that funny looking diode will test open with a normal DMM. No SMT at all. This is misleading because the back of the board has a few small islands of SMT stuff. These turned out to be opamps. Checking the schematic, these are LMs N, N which provide drive levels for the fan, graticule bulb, trace rotation and But Philips was one step ahead of that - board does not like running without a load.
The unit screeches with no output. It takes some time and headscratching to absorb whats going on. This PSU needs to be considered as an integral part of the scope and not some standalone module you can play with on its own. I used some 12V halogen bulbs in series with some high wattage, low-ohm resistors.
This did bring the board to life. Checking the rails showed good voltages for most of them This was my first really big mistake - I later noted that the filter caps on these rails are rated for v. I was intending on recapping anyway - but beware when trying to hack this unit to work out of the chassis. So rather than try to find more dummy loads to make a bigger, more precarious mess, I abandoned that and went back to the faults I already knew about.
So I replaced the missing ICs. With chips I had handy that might have been another mistake. This was made more tricky by the fact one of the pads was lifted and turned upside down. I remove the pad and made a new track from kynar wire, and raised the existing SMT resistor over it to avoid contact. Firing up the scope showed the CRT was back to life. The display was very wobbly and super, super bright. Scorching, unhappy levels of bright. Something still wrong. And sometimes the vertical axis would just collapse to a line, at random.
Also the PSU was making strange intermittent whiney, clicky, chirpy sounds from one of the transformers. Not good. I took a thermal camera to the board SeekThermal for mobile and it showed several areas getting really hot.
Not finding anything obviously wrong. This had me wondering if I had upset that feedback circuit, overdriving the tube. Some difference in opamp specs, offset or whatever. I decided to swap N for a second IC in case the first was damaged or bad, or just different tolerances. No change. I soldered wires to test points around N and noted the compared input voltages looked ok -ish 17v each but the output was very high, IIRC 27v or v. Seemed like the on-time of the EHT convertor was excessive, which might explain the tube being so bright and putting stress on the PSU via the 58v rails.
So, noting the resistor divider was a This felt like progress. But the vertical axis was still going, the PSU was still making unstable sounds and the text was wobbling and jumping randomly. Notes: the schematic shows k for PM and k for PM I found k, which drove the CRT extra bright.
Maybe this PSU was swapped in after all, but not from a PM because it is missing the 5v postregulator found in those units. I recapped the whole PSU board in the hope the chirping, ticking sounds would go away. I tested all the caps which came out. At least, it kept me busy.
For reference, this is what came out. These are original caps from the factory btw - a match for the ones I found in the other scope. Same noises from PSU. Taking thermal camera to other scope showed similar result although rechecking the repaired PSU showed temps of some areas had lowered, now matching the PM I had already checked all of the semiconductors in or out of circuit and nothing bad.
So probably an issue somewhere else in the scope So my next excellent mistake was to sacrifice the good scope for the bad one. Yes I took the PSU out of the fully working and stuck it in the I chickened out of this exercise very quickly and put the PSUs back in their respective homes. However the inductor has cooked off its insulation. Or something. Observation: These scopes were not made for DIY repairs. Now the fun part! The magic sparkles.
I spent ages prodding, poking, shaking, twisting the unit while powered up to see if i could get the trace vertical to stop jumping and collapsing at random. This sometimes felt successful, like a mechanical or contact fault. But half the time it felt like teasing. Hair dryer, hot air station, blowing on it - all failed. In frustration, I re-flowed the back of the PSU, the output stage board and neck board. At first, this seemed to fix the issue. But it came back.
I started prodding the neck board with a wooden stick and got the idea this area was involved. I found a heatsink spring-soldered against 4 ICs on the back of this board. One of the ICs was missing thermal compound almost completely and there was a gap between it and the heatsink. So I added thermal compound and tried to bend the heatsink to be sprung against the IC again.
In the process, I managed to get thermal compound on the SMT components nearby. With alcohol to the rescue, I managed to get this compound equally spread over ALL of the components, including under some of them.
And removed compound from the other 3 ICs in the process. It just got better and better. Eventually I washed the board, redid the compound and put it back. Still the trace was jumping. At this point I was I probably should have long before now.
It had got dark outside and I was seriously hungry, ready to make a late dinner. But I just sat and watched the glow from the back of the tube through a hole in the neckboard, remembering my parents first TV with valves glowing through the back. Then I notice something really weird. I saw a tiny yellow light come on.
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