Philips GM6000 VTVM repair - Page 1
Forum section: electronics / repair

author: agent_power
author_rank: Regular Contributor
timestamp: September 01, 2025, 10:00:55 pm
content: Hi allI have this Philips GM6000 that doesn't really work that well anymore (measurements all over the place)https://www.radiomuseum.org/r/philips_gm6000_gm_600.htmlhttps://www.aef.se/Matteknik/Matteknikportal/Dokument/Manualer/GM6000_Volt-Ohm_Meter_6451.pdfIt's a pain to take it apart, but what to do...I measured the resistors and some of the 1% ones are out of tolerance. Some barely above 1% and some quite a few % over. How important is the value of these precision resistors?Also, any ideas on how to get to the underside of the PCB? I cannot measure all resistors in circuit and the electrolytic cap probably needs to be replaced anyway. It does really look like I have to unsolder all those wires. I'm surprised Philips made such an unserviceable instrument, reallyThanks!

author: Swake
author_rank: Frequent Contributor
timestamp: September 02, 2025, 07:47:30 am
content: At the time 'serviceability' together with 'compact' where still difficult to achieve in a single system. Also in that time engineers faced limits in budget. Developing serviceability has a cost. I recently repaired 2 Zentro power supplies of the late 80'ies, I know that is at least 20 years younger, although I can almost assure now these where build on purpose to be unserviceable.back to your repair: I believe slightly out of spec resistors might influence the measurements but these should not be 'all over the place' as you write. Eventually the measurements are not in spec of the meter any longer but I'd expect these measurements to still be in the right ballpark. Have you tried adjustment/calibration?Are the tubes still performing as they should? What about the power supply? (careful, 120 volt...)

author: agent_power
author_rank: Regular Contributor
timestamp: September 02, 2025, 02:15:55 pm
content: Well, you're probably right, it really looks like they took shortcuts with this thing (serviceability design and assembly wise).I have tried to adjust it but it didn't really help.Tested the tubes now and they are fine (the probe was also fun to take apart, at least it only has the tube inside).I can try to power it up in this state and hope to not short something out, to see what the 120V+ looks like.If there is no easy way around it I will try to remove the circuit board.

author: agent_power
author_rank: Regular Contributor
timestamp: September 03, 2025, 08:09:08 am
content: Ok, I maanged to get to the underside of the circuit board.If you snip/unsolder the resistor next to the tube socket and four leads on the opposite side it's possible to pull the board out enough to get to the underside.The resistors and caps checked fine, but I still changed the high voltage caps (47k pF 400V and 2x 8 uF 350V) since I cannot test them at their rated voltages. Only one resistor was significantly out, it is R5 6M67 at 6.9 MOhm, I don't know how much it will influence the accuracy of the meter.I also cleaned the switches and reflowed some crummy connections on the underside of the board.Now I've got 130V B+ @ 220 VAC input, and 140V @230 VAC input.The measurements are now in the "right ballpark" and I will try to calibrate it

author: Swake
author_rank: Frequent Contributor
timestamp: September 03, 2025, 08:16:54 am
content: Its breathing, reanimation in progress

author: agent_power
author_rank: Regular Contributor
timestamp: September 04, 2025, 07:35:24 pm
content: Conclusion after calibration attempt:Since everything is drifted a bit and the line voltage is 230 VAC I don't think this will ever be accurate without swapping resistors (which I am not going to do).It's now calibrated for the 10V DC range and everything above or below it reads a bit out. AC measurements are also a bit out but I think it's good enough for what this is.Ohms measurements appear to be good.It can go back on the shelf now