Phillips PM2523 Repair

[Ionforbes]

So bought a broken Phillips PM2523 DMM thinking it would be an easy fix. It's only 3.5 digits and is 40 years old, but I got it for the gorgeous aesthetics.

Anyway I turned itt on and the display is compeletely blank, except for a '+' symbol when in the VDC range. The caps inside all seem fine but I replaced the ones involved in power supply anyway. Voltage rails and reference all measured ok with pretty much no ripple. I caan hear relays clicking when I turn it on so I assume all the analog stuff works. Either way the dead display should be from the digital stuff.

My main suspicion is the HEF4739 IC but it shows both signs of life and death; the 'scan' outputs are all at solid 2.5V with no swittching, yet the '+' LED coming on with the DC range suggests the chip isn't completely dead. I replaced the 7404 hex inverter as the scan outputs go through it, but it didn't help. Probing the 'clock in' and 'clock out' pins shows no activity, so maybe the '+' LED only comes on since it doesn't need the clock but the 'scan' outputs do. This could mean ceramic cap C133 is shorted but I cannot for the life of me find it on the board! It's there on the layout in the service manual but not in the actual PCB! 

Anyway I'm pretty much clueless at this point so any second opinion would be greatly appreciated!

Edit: Added schematic of display bits

[PA0PBZ]

There should be a clock on pin 25 otherwise the IC will not do anything. I'm not sure why the cap seems to be missing, but the PCB is also different from the one in the manual so maybe it is another revision. There is no datasheet to be found for either HEF4739, CD4739 or GZF1200D which seem to be compatible, and also the IC itself looks like hard to get. Maybe if you are lucky it's just the clock circuit and you can apply an external clock, but the frequency of that is also a mystery without a datasheet. I'd try something like 100KHz for a start and see if it makes any difference.

[meandeev]

As somebody wrote in a german forum:

take a look in
http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Philips/pm5134.pdf

there you will find a block diagram, the pinout and a description of the function  of the HEF4739P

[Ionforbes]

Ok so I made a replacement clock with and found the 'shift out' pin wouldd output an intermittent squarewave. The display digits also light up occasionally but to be honest I don't think I'll bother continuing. This meter just isn't worth it 

[Haenk]

Note the blue PHILIPS cap - those are likely to fail, so you should check it.