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Philips GM5603 oscilloscope

The GM5603 is one of Philips' high-end scopes of the early 1960-ies. It contains a large number of valves, consuming a lot of power. I got this scope in 2002 and made a number of repairs so that it actually works again. Only the time-base has some problems remaining.

This scope dates from 1961, if I'm correct. It contains 44 valves, including the DN10-78 CRT. This is a high-end scope for its time. It has a balanced Y amplifier with a 14 MHz bandwidth, a CRT with acceleration spiral and a total acceleration voltage of 10 kV. It weighs 35 kg and uses 450 W. It needs a fan to get the heat out, which is not too noisy, by the way.

I got this scope in spring 2002 from a vintage radio enthousiast who works at Philips. The scope was removed from the company's museum. Unfortunately, there were no manuals or probes with it. But he knew somebody who once had the same type of scope and had kept manuals and probes. So I was lucky as this scope is really too complex to fix without service manuals. And it sports some peculiar input connectors, impossible to find, I fear. These connectors are a marriage between an 'N' connector and a 7 pin miniature valve base. The 7 pins provide power for active probes.

While cleaning the inside and studying the schematics, I could not help comparing this scope with the ultimate high end scopes from Tektronix. It you find this offensive, please stop reading :-).

I think Philips did a very clever engineering job.

row of PL36 power regulator valves

The power supply contains 5 PL36 penthodes.

They preferred easily available and cheap valves, except where SQ (Special Quality) types were really needed. It's nice to see a row of 5 PL36's, normally used as line deflection pentodes, sitting on the power supply board. And a lot of PCF80's and PCL84's are used, these are HF and video amplifier tubes Philips used a lot in their TV sets of the time. And Philips has applied printed circuit boards, while Tek kept on using mounting strips for many years. So this scope is better suited for industrial serial production than the Tek designs. But its serviceability is not yet optimal, especially in the HV part.

The scope has been sitting on my work bench for over a year before I started its rehabilitation in May 2003.

When I first switched the scope on, the screen remained dark. I fumbled with the controls for a while and finally got a time base line. Feeding it some input signals, I concluded that the image was too large and that the scope had some focus problems. The lines were reasonably sharp in the middle of the screen but blurred near the sides. I suspected the HV stage and measured the -1450 on the cathode. This gave some strange effects because even the 10 MΩ of my DVM was too much extra load. I invested some extra time to construct a HV probe with a total resistance of 400 MΩ. With this, I could measure the -1450 V and +8400  voltages safely and reliably. I found out that all the voltages were too low.

HV unit outside the scope

HV unit being tested outside the scope.

Then I took the HV unit out of the scope. This is a part Philips did not build very service-friendly. It is located in the centre of the scope and can only be taken out through the bottom, after disconnecting all the wires. After taking it out and removing the side plates of the HV unit, the components inside become accessible. I decided to attach it to the side of the chassis to be able to test it in circuit.

After some testing, it became clear that the control circuit of the HV unit did not work right. It kept the voltage too low. The cause turned out to be 2 capacitors in the voltage divider in the feedback loop. THese were paper capacitors that had become leaky. After I replaced them, HV was OK. I checked the HV rectifiers, just to be sure, but these were in excellent condition.

But now, though the HV unit worked fine, I did not get an image any more. I realised that the CRT was probably cut off now because the total HV had increased. As the voltage between cathode and Wehnelt that determines the brightness is derived from the -8400 V supply, it had become more negative and the CRT was contantly cut off. I corrected the settingd for minimum and maximum brightness, and had a good picture again.

After this, I followed the manual to set DC balance and amplification for the vertical amplifier. The scope became better. After this, some errors in the time base remained.

In August 2003 I decided to stop working on it for a while, and haven't yet restarted. To be continued.

Copyright © 2003 by Onno's E-page         published 2003-05-18, last updated 2008-08-16