Help with an Adret 742A doing some strange things please. Now sorted :-) - Page 1
Forum section: electronics / repair

author: Aston
author_rank: Newbie
timestamp: February 06, 2025, 08:08:50 pm
content: Hi gang, used to be a member on here a while back but, what with recent events and one thing and another, I've forgotten my login details. Anyway, here I am again with a bit of a request.I have an Adret 742A RF frequency generator which for the last 20 odd years has been absolutely reliable, apart from the obvious tantalum capacitors which were replaced not long after I got it due to me noticing them being a bit off colour when I got the unit. (Glad I did as they are a nightmare when they go short)Anyway, turned it on the other day and discovered that things were not quite right.The frequency was not correct and in fact I can only get certain frequencies out of it when scrolling through the range of frequencies using the dial. These are limited to (measured with a counter with both synchronised to a GPS 10MHz source)-From 100KHz to 1.999MHz the output frequency is 5.99MHzFrom 2Mhz to 9.999MHz the output frequency is 2MHzFrom 10Mhz to 17.999MHz the output frequency is 10MHz (The level also drops too)From 18MhzHz to 25.999MHz the output frequency is 18MHz (Levels back to before)And so on and so on like this up the scale.There is also no modulation of any kind, either AM or FM (Although the display indicates fine, no modulation is applied)The output level is also limited to relay changes and is only variable on the last amplitude setting.I got a gut feeling that all the these problems are caused by one commonality, as everything was working perfectly fine the last time I turned it on. I could be wrong, but something is nagging at me that so many faults just don't pop up at the same time.I have checked for any further tantalum capacitors just in case I missed any.The CPU board has been thoroughly checked, for further acid damage (which was minimal), since it was repaired over 20 years ago, and all seems well. I have also borrowed a CPU board from a 730A, and swapped over the EPROMs, and everything was the same, and at the same time I also plugged this CPU board into the 730A, with its ROMs and all was fine. So that rules out the CPU board hopefully. Sadly all the other boards are significantly different and are not swappable.80 MHz, 10 MHz, 13.5 MHz, 2.5 khz all measure fine.All The power supply voltages are stable, +15v and -15v are set correctly and 5 V is approximately 5.2vAll the connectors have been checked for damage and as far I can see there they are fine.At the moment I'm re-writing and cleaning up the block diagram and circuit diagrams that I have printed from the available service manual which was in English and French. It's quite blurry and that, on its own, isn't exactly helping things, but I think I've got the general grasp of what they were trying to achieve.I have to say, that the people who designed these things, were either crazy genius's, or lunatics. Possibly a little bit of both. :-) Power supply is from the stone age, but functional I guess.I get the fact that all frequencies are either divided or multiplied by the source 80 MHz clock to minimise having multiple frequencies running around inside the unit. That makes sense. But why on earth did they not explain what they were trying to achieve in some coherent manner in English. Unfortunately my French is dire, so that is also not helping. (I'm slowly working out what the numbers are in the division/multiply boxes on the block diagram by tracing where the go and the use of a calculator :-P )Anyway, sorry for the ramble, if anyone out there has any hints as to where I should look next, I would be very grateful.And if George Edmonds is still on the forum, I believe he has one of these, I know it's old and crusty, and probably not worth repairing, but it's been a faithful old friend and it and I have both repaired many radios together, so I would love to get it working again.All the very best from Aston.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: February 07, 2025, 11:24:01 am
content: Hello Aston,Quote from: Aston on February 06, 2025, 08:08:50 pmHave a look in your mail inbox. If you had failed log attempts in the past, you shoud have received emails from the do_not_reply@eevblog.com adress. Your login is mentionned in the mail. If your former email adress is still valid, you may reset the EEVBlog forum password.Concerning your 742, even speaking french fluently is unfortunately not sufficient to understand how it works and particularly why it has be done in such a weird way.Because the designers even invented new french words that are never used anywhere elseAnd yes it is a complicated TE because utmost care was taken to avoid additional phase noise or spurious. An it definitely works. That's why there are 5 or 6 interwinded and nested PLLs inside...At first, you'll need some extenders to run the boards outside of their shieldings.George was involved in thisrepair thread.

author: Aston
author_rank: Newbie
timestamp: February 07, 2025, 01:01:29 pm
content: Hi there Timeandfrequency, I think I must have used a very long gone email address or something, not to worry.Thanks for replying though, I had a look at that thread, all very interesting stuff indeed.Looking at what I wrote down, it occurred to me that the steps in frequency are exactly 80 in size which probably means the 80 steps board is okay and the 20,000 steps board may have a problem, though why that would affect the other things I'm not sure.I've got a bit of a migraine going on at the moment so my brain is not really working particularly well, but will crack on with it tomorrow.I did notice how spectrally perfect these were back in the day when I bought them. Had access then to a very nice spectrum analyser at Panasonic and discovered that the signal coming out of them was very clean.An extension board would certainly be handy. I vaguely remember having a chat with George years back about the connectors and I think it might have been yourself that found out what they were. Telecommunications or something. Looked into getting some at the time but could never find any.Just out of morbid fascination, I sat down and backed up the EPROMS just in case. Won't know whether the information is corrupted or not until I find someone else's backups I guess.All the very best from Aston.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: February 07, 2025, 02:31:02 pm
content: Quote from: Aston on February 07, 2025, 01:01:29 pmEven if I suggested a replacement for the genuine connectors, I finally doubt that they will fit. Don't remember what was actually different but probably that George was right.So the most demanding work to cobble an extender is probably to 3D print these particular connectors.The fact that you could rule out the CPU board is already a great achievement.The symptoms you describe show sometimes locked then unlocked PLLs, depending on frequency.Searching for an issue in one of the digital counters/dividers assemblies might be a wise path to follow.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: February 09, 2025, 06:34:42 am
content: Hi AstonI have just come across your posting as now days I rarely logon to this repair group due to the large amount of rubbish diarrhoea posting that now occurs.I do agree that it would be nice to have a PCB extender, when I last looked at the viability of making one I gave up due to the potential costs involved, the only source of the DIN connectors appeared to be from Switzerland (not the cheapest place on earth). Taking the base cost of the two of them plus shipping, tax and duties along with the cost of a custom PCB the cost was coming out North of GBP 200, about half of the cost of a working Adret 742A on that mad auction site at the time.If you enter a frequency via the keypad does it display correctly? I did consider if you had EPROM rot but if the keypad frequency does display correctly this is unlikely, I do have a couple of 742 EPROM images, but for exactly which model I an not certain as there are at least two variants of the 742A out there.I have a full set of manuals, both in paper and PDF versions. Most importantly, but only as a paper version, an operators manual in English. This manual describes the basic operation of the synthesiser. On the KO4BB site (if it is working at present) there are PDF scans of the manuals, please note that they are incorrectly titled as 742M1 and 742M2. The M2 is actually the M1 and covers both the A and B versions of the 742, for some unknown reason the front page scan is missing. I have both of the Marconi/Adret versions as paper copies. In case you do not know this Marconi Instruments were the final owners of Adret and finally put it out of its misery.In the maintenance 1 manual is a relevant flow chart on page VI-3, it suggests, if I understand the French, that the approach board is the first suspect along with the pilote board and the various frequencies that it generates.Hope that this helps a littleG Edmonds in the UK as G6HIG

author: Aston
author_rank: Newbie
timestamp: February 09, 2025, 05:23:46 pm
content: Hi George, thanks for the reply,Yes, if I enter a frequency via the keypad it's all fine. I'd be interested in the EPROM images if you still have them. Even as an experiment to see if I can convert it to a 'B' version, as sweep would be handy.In the meantime, I think I might have had a bit of success, over the last few days I've been scoping my way around the 20,000 steps board. Rewriting my service manual as I go. Well, I say rewriting, but what I really mean is scribbling on the printouts with a pencil, correcting the pinouts and various unreadable bits.Anyway, prior to this, I finally bit the bullet, and carefully soldered 31 wires between the sub board and the mainboard, triple checking to make sure I didn't do anything stupid. And I have to say it does make life a lot easier to be able to measure things.Anyway the crux of it all is that the "charg" signal was being crushed a little by the MN23 HEF4556BP chip. I'm not sure if this is causing a problem but I didn't like the look of the signal, so I've ordered up some of those.I then set about to test all of the test points throughout the board and discovered that test point 2 seemed to be dead, which meant the comparator wasn't working properly. So after a bit of scoping round I had a look at MN5 which is a HEF4518BP BCD divider. Unsoldered it from the board, popped it into the EEPROM programmer and found it to be faulty. So I've got some of those on older as well.I'll have to see how it all goes, but fingers crossed.Many thanks to you both for the help and giving me the impetus to get the ol' girl going again. Best regards, Ast...

author: George Edmonds
author_rank: Frequent Contributor
timestamp: February 12, 2025, 10:25:26 am
content: Hi AstonI have looked at the EPROM images that I have, I can only find a 740A and a 742A image. At one time I did have access to a 742B in the Marconi archive, from memory I did take an image of the EPROM’s however I cannot find them at present. Regrettably the Marconi archive now no longer exists so I cannot get access to a 742B at present.Your comment that I find revealing is that you found a suspect HEF4556BP IC. My initial reaction to your first posting was that the instruments data or address bus had problems due to the number of failures the instrument had. Now a HEF4556BP is frequently used as an address decoder to enable a Data bus latch, incidentally I am now seeing a high failure rate with CMOS IC’s. I suspect that your "charg" signal is actually a Data latch enable signal.Now days I rarely work on Adret instruments as my interest is in repairing Marconi instruments, however despite the vast difference between the way that the output frequencies are generated the method of CPU control using Address decoders and Data latches is very similar. Most of the failures I am now seeing are related to the address decoders and data latches.Good luck with the repair and keep us informed as to what you find.G Edmonds

author: Aston
author_rank: Newbie
timestamp: February 14, 2025, 06:27:17 am
content: Thanks for looking, even at this point a 742A image would be handy to corroborate that the information I backed up is accurate. The firmware I have is 0013. Still waiting for the post to deliver the chips. I think you're right, it does sound like a data or address problem, and that is still a possibility, but for the meantime I've got to get the comparator working properly and get those vital signals back.Anyway, will update here when I get to the bottom of the mystery.Best regards, Aston.

author: Aston
author_rank: Newbie
timestamp: February 17, 2025, 01:18:35 pm
content: Okay, well, a bit of an update.Chips turned up and fitted, the comparator is now working properly, but things were still a bit intermittently peculiar, so I had a quick look round and tested some timing signals. Things didn't quite line up properly, so I whipped out the CPU board again and borrowed the one from the 730A, popped the EEPROMs into the 730's board and plugged it in.Everything worked properly except for the level outputs which were all over the place, okay I thought, at least that's a start.Had a look at the clock input to the processor on the CPU board from the 742A, apart from looking messy, it looked nothing like the one from the 730A which was lumpy but functional.On testing all the caps in the 742A's timing section, I found one of the 82pF capacitors across the tuned coil had gone down to 10pF. Replacing that made everything stable, except for the level outputs, which I'll tackle in a bit.The irony is that the 742A's CPU board worked perfectly fine in the 730A. I guess the 730A is probably just a little more tolerant of wonky signals.So that's the state of play at the moment, I've just got to find out what's causing the output amplitude, not only to be wrong, have no fine levels, just relay levels, and reducing in size over time.Pretty happy with the result so far, it's almost there, and in borrowing bits and pieces from the 730A, I also managed to spot a missed tantalum or two that I hadn't caught the first time round.

author: Aston
author_rank: Newbie
timestamp: February 25, 2025, 09:01:17 pm
content: The old girl lives!Finally got it to a state of working 'well enough' to be pressed back into use. The unstable output voltage was due to a tired 15v supply resistor in the VHF stage.The final amplification level on the output is still not switching over, however I know where the fault is coming from. The "VHF level control" board has a final stage op-amp which makes the RF output go from approximately 2 V output to unloaded 15 volts output. This involves switching over the gain stage of the operational amplifier via a HEF5053, using it as a switch. The thing is, this switch is not activating and is in the low output stage all the time, so the last stage just repeats the previous levels, The signal for this switching voltage is coming from an encased board set that is not in the manual and is mounted underneath.This board set has another HEF chip supplying the final stage activation voltage, but I'll get around to sorting that out another day, as that kind of amplitude is not necessarily important to me at the moment.So, there we are, it's given me a real 'wild goose chase', I've learnt a lot about how these things work, and at the end of the day I may have saved the other one from a fate worse than death as well. The workshop's all tidy again and I can get on with some work. Thanks again to everyone, even if you just read the posts and gave me virtual moral support.All the very best from Me, and may all your repair jobs be easy ones.