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Understanding and Repairing Clock Generator on Adret 742A UHF Generator
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PerArdua:
Interestingly I think I have found that there are at least two quite different revisions of the analogue PCB in the 742A - the right hand side of the PCB in the manual provided by K4OBB (first attachment) and the position of potentiometers does not correspond with what I have in my unit (see second picture).
timeandfrequency:

--- Quote from: PerArdua on February 09, 2023, 10:43:35 pm ---Interestingly I think I have found that there are at least two quite different revisions of the analogue PCB in the 742A - the right hand side of the PCB in the manual provided by K4OBB (first attachment) and the position of potentiometers does not correspond with what I have in my unit (see second picture).

--- End quote ---
Hi Per Ardua,

Indeed interesting : the PCB version of the analog board inside your Adret seems not easy to find in the currently available documentation. Any serial numer on the PCB beginning with D97027' ? Did you spot an issue on this board ? Does it need recalibration ?

About the attenuator, as @George underscored, the 730A SM includes its schematics, completed with the reverse power protection circuit.
C8 (4.7µF/35V) might be a tantalium cap on the +15VDC rail.

However, I don't know if 730A and 742A attenuators are identical.
PerArdua:
Hi timeandfrequency,

the only part number I can find for the PCB seems to call it H12774902 (unsure whether the first character is H). I've attached a high resolution image of the reverse of the PCB which shows this in the lower right quadrant.

Regards,
George Edmonds:
Hi Per Ardua

Looking through my Adret archive I have found a zip file of the Adret service manual for the 740A.

This zip file also contains images of a version of the firmware and EEPROM along with some photographs.

Most importantly it does contain full details of the attenuator assembly, including a schematic.  Regrettably it does not contain any information on the final frequency doubler stage, you need a 742A manual for that if it was ever provided.  I suspect that the final frequency doubler stage uses devices containing Beryllium, in which case it would have been a factory serviced only module with no schematic provided.  The original UK sales and service representatives for Adret were Racal Dana and they operated a policy of factory service only for anything containing Beryllia.

There appears to be details of yet another different Analogue board in this zip file.

I was always under the . probably mistaken, impression that the only major difference between the A and B models was the case, the A case has four plastic rear corner moulding whereas the B case does not.

This zip file is some 24mb so emailing it to you may not be possible.  I possibly can upload it to the Marconi Test Instruments group on groups.io for a few days only as I am a joint owner of the group, but it cannot remain there as the group has a very limited file storage allocation.

Let me know what you want to do.

G Edmonds
PerArdua:
Hi George, if you could send me that via email I would be very grateful thank you. I will send you a PM with email address.

I am currently putting together a digikey order for replacement capacitors, and a couple of values of resistors I did not have to hand when repairing the CPU PCB (ie completely fresh, completely corrosion free etc). When that arrives I will add them to the instrument. In the mean time I have a couple of EPROMs I am using to build and test my reader with, before using the devices in the 742A. It is unfortunate that it does not use a Z80 CPU - I have already about half a dozen of those to hand.  ^-^
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