Electronics > Repair
Understanding and Repairing Clock Generator on Adret 742A UHF Generator
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timeandfrequency:
Hi George,

Two rows IDC connectors always have an even number of pins (usually between 6 and 50).

On both CPU boards pictured in this thread (here and here), I guess that the 'CPU card to motherboard connector' you mention is the light grey one at the bottom of the board, a bit on the right side, which has 31 pins, as printed on it (1-3-5-7-9-11-13-15-17-19-21-23-25-27-29-31).
Concerning the 'TRELEC TM31' model part # written in the SM, '31' stands for...31 pins.
The extender and the connectors I pictured also have exactly 31 pins.
All of the replacement parts I suggested have 31 pins (see page 206 & 207 here).

Well, I might be wrong, but is there another uncommon connector on the CPU board ? In the affirmative, can you provide a picture ?
George Edmonds:

Again someone who does not read diode specifications.

Part of the specification for the 1N 540X series says “For capacitive loads derate maximum current by 20%”  This had not been done hence the high diode failure rate.

I said I have never seen that connector, it was never used in UK and US electronics, even Adret stopped using it in the 7000 series, they are also prone to bent pins.

G Edmonds
PerArdua:
Evening all.

With the successful test of the power supply (unloaded), the instrument checked for visible signs of damage, etc I decided to attempt to boot the instrument. Upon apply AC mains, the standby LED came on and everything appeared stable. I the flicked the standby switch to ON and the instrument appeared to try and start (a few numbers flashed on amplitude display) and a flashing LED above the execute button. However, at that point I began to smell some acrid scent, so I turned the supply at mains. Opening the VHF module I discovered a tant. capacitor for +15V in the output amplifier section (C716 on schematic, C16 on the PCB view) was charred. Furthermore, opening the UHF section (the section containing the two frequency doublers, with no schematic) revealed a charred capacitor in the section containing the attenuator EPROM for the -15V supply. There is a possibility I missed the first capacitor (though I believe small), but the second capacitor was definitely damaged on this attempt (photographic evidence from the first repair to the +15V capacitor haha).

I am running out of time for further tests this evening, but do we think these caps are failing due to a past catastrophic event, and it is worth chasing down and repairing each cap now, or is it more likely there's a PSU issue that needs to be addressed first? I will replace the two caps above, and will then measure the startup behaviour of the PSU outputs under no load (disconnected). Depending on those results, I may reattach the PSU to the instrument and again measure the rails.

Would it be beneficial to measure the resistance across each supply rail on the instrument motherboard with the PSU disconnected? Whilst I get this might change during operation, would it at least be a good idea to verify the 'cold' state?

I am both a bit bit encouraged and disheartened by these results - on one hand the flashing LED gives me hope that the CPU is doing _something_, though chasing this PSU/blown caps is a bit repetitive. ah well.  |O :-/O

Attached are combined schematics for the VHF PCB. I've combined the four pages onto a single sheet - the manual shows this as two different schematics.
George Edmonds:
Hi Time and Frequency

Thank you for the information on the PCB connectors, I now understand what they are and why I have never seen them, nor had anyone else in the UK that I asked.  They appear to have been used solely in the telecommunications industry, until recently the telecommunications equipment industry in the UK has been dominated by two UK companies and an ex nationalised industry.  Shall we say that it was not an open market place.

The shrouded DIN multi pole connectors I am very familiar with as I have a number of Rohde & Schwartz instruments which make extensive use of them, I even have PCB extenders for them.  A quick search in the UK for the type used by Adret reveals no stock held with the only stockist being in Germany at high prices and expensive delivery costs.

When I used to get a 700 series instrument in for repair I automatically shotgun replaced all of the power supply diodes with BY550-400 and never had a diode fail again, these are 5A 400V PIV and dimensionally almost identical to the 1N5404.

Hi Per Ardua

Please do not give up, you have tackled a complex instrument and are making real progress.

Before you power the instrument up again please replace ALL the bead tantalum capacitors before you have one go on fire and cause considerable damage.  When they were first used they were sold as a magic solution having low ESR and being able to work very close to the specified working voltage.  Over time and as they age this has certainly not proved to be the case, they need to be specified to be some 50% over their operating voltage, for example on a 15V rail use 25V ones not the 16V ones used.

This is NOT an Adret only problem, it is industry wide.  Racal had massive problems with failing bead tantalums which sometimes caused extensive damage.

It would also be a wise move to  replace all of the aluminium electrolytics, over time they go high ESR and lose capacitance, after all they must by be thirty years old.  They can also leak electrolyte which is also corrosive causing PCB damage and is conductive.

Please keep us informed of your progress.

G Edmonds
timeandfrequency:
Hi everybody,

@George
About the PCB connectors : you are right when saying that those open models were never used in UK and USA TE. But that's only half of the world, and there might have been some regional preferences/choices. I could also spot these connectors on Siemens automation systems designed in the mid/late 70' (see attached pictures), which is about the time the 74x was designed.
Furthermore, Adret management teams first target was the telecommunications industry.
You may buy these connectors here and also here, for immediate delivery.

The more common shrouded version (which only exists with even number of contacts : 16, 32, 48, 64 & 96) was also much used in VME racks/boards.


@PerArdua
George's advice to replace all of the tantalium capacitors is wise. It's a nightmare for every vintage TE in which they were used. That some of them prevent your Adret from working properly is not so surprising.
You may replace them by low ESR 105°C Electrolytic capacitors from first tier manufacturers like Panasonic, Cornell-Dubillier, Nichicon or Rubycon. Sometimes it is wise to also add an 100 nF MKT capacitor in //, in case not other plastic capacitor prowls in the surrounding.

For low voltage rails, a direct replacement for tantalium capacitor are also those ones. With no liquid electrolyte, they last forever, but are rather expensive, exist only in SMT form factor and are 10VDC maximum. The fact that they are SMT should not refrain you from trying : sometimes, you can solder them on the...solder side of the PCB.

Thanks to his experience on this stuff, George could spot the usual failures of the PS inside the Adret, so replacing the diodes and put new electrolytic capacitors is also desirable.


--- Quote from: PerArdua on February 05, 2023, 10:39:12 pm ---Would it be beneficial to measure the resistance across each supply rail on the instrument motherboard with the PSU disconnected? Whilst I get this might change during operation, would it at least be a good idea to verify the 'cold' state?
--- End quote ---
Yes you can measure the resistance of each supply rail but the test is not always valid (you will get some false negative). You may detect dead shorts, but tantalum capacitors sometimes have a voltage threshold where they begin to become a short... And the voltage provided by your Ohmmeter (usually <= 2VDC) might not be sufficient to trigger the short.
Do not attempt to use your lab PS to make this test : only supplying one of the ±15VDC rails could kill all of the opamps. To be on the safe side, you would need a dual lab PS with tracking mode and common overcurrent trip mode, which is a rather rare piece of equipement.

And thanks for the rebuilt of the VHF board schematic, which is now much easier to read. A new item just appeard on my wish list : an 8K PC monitor.

You have already made good progress in the repair of your TE and going further would be really interesting. It is true that reparing the RF and analog boards is more demanding than the PS & CPU assemblies. It usually requires specific TE for probing inside RF sections. But maybe you get lucky and the sole issues you'll find are charred/shorted capacitors.
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