Help me with the Philips PM3295 350MHz - Page 1
Forum section: electronics / repair

author: ffliere
author_rank: Newbie
timestamp: October 10, 2021, 12:07:09 am
content: I tried searching for this issue alot but couldnt find the solution yet.Hi, I recently acquired a pm3295 crt based scope. It came in as "not working". Although I already knew this was a common rifa job. Fast forward a bit and I have this problem sorted. Scope came on, everything worked. No weird smell nothing. Well this scope has one nasty problem with it and that is that you have to remount the outside cover, which is super hard. I followed the instructions in the manual 100% and overdid the instructions (read extra securely). (Add cover and lay the scope crt facing down) Now comes the fun fact that after reseating the cover, the crt wont come on again (everything else is working , auto set, triggering). No whining noises anymore, no trace or anything on the crt. Im thinking that the HT is somehow off since I cant hear a whining noise.Thats where I need some advise. I already checked the focus and that part seems to be functioning as it should. The fillaments are also on. I dont think I killed the crt by following the instructions, but yeah, where should I go now? I have the full, printed service manual with it.

author: bob91343
author_rank: Super Contributor
timestamp: October 10, 2021, 02:10:05 am
content: Measure all the crt voltages and compare to the manual chart. It helps to have another scope to see if a waveform is being sent to the deflection plates. Measure the high voltage. If everything checks normally, it's a bad crt.

author: ffliere
author_rank: Newbie
timestamp: October 10, 2021, 08:36:58 am
content: Thats what I thought as well. But the first sign Im focussed on is that the whining noise disappeared. This is caused by the scanning right? Or could this also be caused due to lacking HT?

author: ffliere
author_rank: Newbie
timestamp: October 10, 2021, 01:09:11 pm
content: I measured the XZ and Y amplifiers, they seem to work fine.The high-pitched noise is still missing and so is the trace, I've looked in the manual and there is a protection circuit for the oscillator for the HT supply.Atm, I can only measure 1kV top, any tips to do this safely?I can't probe the HT while it's on. The whole PSU has to be removed before I can even think about putting a probe near it (that's how compact this scope is)

author: ffliere
author_rank: Newbie
timestamp: October 10, 2021, 09:01:30 pm
content: I've not yet opened up the HT supply, but I strongly feel like it is the HT supply. Maybe a failed transformer or voltage triple, I'll have to open it and check. I know there is a issue with the voltage triple that makes it short internally due to age (or earlier with some extra help aka vibrations).Does a healthy HT supply normally have steady whining, just like it did when it worked?

author: bob91343
author_rank: Super Contributor
timestamp: October 11, 2021, 02:43:07 am
content: The whine from the scope is not a proper measurement procedure. It could be absent due to lack of load current or other things beside failure of the supply.With another scope, move a probe around the face of the crt. You should pick up high frequency noise from the power supply; they radiate a lot. If you don't, then you can surmise a failed HV supply.

author: ffliere
author_rank: Newbie
timestamp: October 16, 2021, 10:33:40 pm
content: That's true, whine is not a proper measurement procedure, just an indication. I checked and saw zero noise radiated from the face of the crt, so I'll check the HT supply tomorrow.

author: ffliere
author_rank: Newbie
timestamp: October 19, 2021, 11:56:53 pm
content: So I had a good look through the HT supply. Fixed heatsink (the old compound dried up) of switching transistor, fixed some suspicious connections and checked all transistors.Nothing yet. Glow is good, no sign of a trace yet. Gonna give it some more tries tomorrow.If I cant fix it, ill replace the crt with a lcd. Cant throw away a 350MHz dual channel scope with all functions you need for having a bad crt.Any input is welcome

author: ffliere
author_rank: Newbie
timestamp: February 03, 2022, 08:47:35 pm
content: Since the last post I've been quite busy with other repairs such as a pm3260. As of now, I think I identified at least 1 problem: zener V6612 (BZV11) in HT supply is open in both directions. Without this zener, the negative input of OpAmp N6601 (ua741) will be biased too much. This OpAmp is used for the voltage feedback and will otherwise turn off the oscillator. No wonder then that I didn't hear the oscillator going and thus no crt trace. Will try replacing it with a 6.2v zener shortly and probing some more around. I'll keep y'all updated.

author: macboy
author_rank: Super Contributor
timestamp: February 04, 2022, 05:09:24 pm
content: You certainly can't replace this CRT with an LCD because this is an analog scope. The CRT is a fundamental part of the machine.

author: R.J.
author_rank: Contributor
timestamp: February 10, 2022, 10:07:48 pm
content: Nice work, repairing old Philips Scopes!

author: ffliere
author_rank: Newbie
timestamp: November 11, 2022, 02:28:05 pm
content: The last time I posted, I promised to follow up as soon as I have more information.I'm glad to announce the fact that this scope is FIXED. This scope is riddled with old insulated crimp spade connectors. I found 1 connector whose wire was loosely connected, it operated under constant stress without a strain relief. I remember me checking this connection 6 months ago, I guess I did a poor job. Well, it might be a surprise that this connector connects to the filament wires of the CRT. It now makes a lot of sense why the scope stopped working after mechanical movement due to the reseating of the (not user friendly) casing. Trust me, it is not fun to reassemble the case.As far as I can see, this scope is fine. The only minor thing wrong with it is the connector for channel B. The connector works but is a little bit loose. I can live with that, it's not worth the pain to remove the front panel.This is maybe the best 100$ I've spent so far on equipment, a nice 350MHz analog scope with auto-set, 50 ohm input impedance and some other nice features. I only needed to replace 2 Rifa caps and some bad connectors, can't complain.