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Old 25th Apr 2017, 7:02 pm   #1
chriswood1900
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Default Marconi 2019A frequency fault.

I need some help!
I’m having some problems with my 2019A. It is later model 52019-910E SW V3 .
On start-up it sometimes shows Error ---15---, (the oscillator is unlocked) but can be overcome with an external source.

My mine concern is the frequency is out by a consistent amount at any frequency as follow:-
1040MHz is 1039.970MHz
1000MHz is 999.971
100MHz is 99.996
10MHz is 9.9995
The above is measured with the external frequency input and the frequency counter locked to the same accurate oscillator.
I have opened it up and check the power supply AE1 and all rails are clean and on spec. I suspected that the 10MHz master oscillator was out but on opening up the top cover the OCXO can be set bang on frequency measured on its output pin 1.
Following Table 6 Output frequency error in the manual. I moved on the lower boards they tested OK and I have ended up on the top with a Fault on Board AA1 LSD Loop.
On AA1
TP1 is approx. right 4 to 5v @8.5mhz
TP2 Varactor voltage is 1.1V
Removed jumper TP3,4 and connected a DC supply this allowed me to vary the frequency @ TP1 from 8mhz to 17mhz, and this also allows you to correct the main output manually.
I’m now a bit stuck, has anyone had experience troubleshooting AA1?
Thanks in anticipation
Chris
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Old 26th Apr 2017, 9:41 am   #2
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Default Re: Marconi 2019A frequency fault.

I recently had to fix numerous faults on a 2019, which is not that dissimilar to the 2019A.

As I've no doubt you know, frequencies in the range 520 to 1040 MHz are generated by doubling frequencies in the range 260 to 520 MHz. Have you checked that when doubling is used the error is exactly twice that for when half the frequency is generated? ie the error at 600 MHz should be exactly double that at 300 Mhz.

The manual gives an example of 512.34567 MHz and states how this is generated, giving the frequencies to be expected at various places in the SG when 512.34567 MHz is selected. Try checking these frequencies to find which one is in error, which should localise the problem to a small section of the circuit.

My 2019 was totally non-functional until I changed the microprocessor and one of the IO chips. After that I had no output and when I got output I found that frequencies were wrong. All these problems were down to faulty BFR 19X transistors and could have been found by taking voltage readings on transistors.

Let me know if you want any readings checked for comparison purposes.
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Old 26th Apr 2017, 10:21 am   #3
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Default Re: Marconi 2019A frequency fault.

I assume you have checked all the OM-345s on the RF deck? if any of these are playing up they immediately impact loop locking. Check input and output levels. If the OM-345 looks more like an attenuator than an amplifier, swap it out immediately! An even quicker check is the OM-345 output pins (Pin5 from memory). If it's sitting at rail (+12V) it's blown.
And you can borrow a spare grey plastic 10MHz module from me if you need one.
Call me if you want to discuss.
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Old 26th Apr 2017, 10:29 am   #4
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Default Re: Marconi 2019A frequency fault.

I think these are definitely heading towards EOL now. Too complicated to effectively repair going forwards. My 2019A blew out a regulator and took most of it out. This is after the entire attenuator assembly died and it ate about £100 worth of parts over a couple of years before that. Gave up and got rid of it in the end.

As for the precise problem, the AA1 board is basically a big programmable PLL. It has registers on it (IC8-11). It's possible it can't latch data on those registers or a low significant bit is not latching causing the PLL to generate an erroneous frequency. Check the cables and signals around those to start with, then check the output frequency across the range of 2KHz - 8KHz if I remember correctly. You really need a reciprocal counter to get low frequency resolution though.

SMM is here: http://dedisol.unsads.com/~squalyl/k...%20Service.pdf

See note 62-68 WRT AA1 board.

I had to replace one of the 74LS273's in mine on a similar fault.

I had a logic analyzer though - may be quite hard to diagnose if you don't have one.

Edit: just to add that swearing at it and hitting it doesn't work - I tried
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Old 26th Apr 2017, 11:20 am   #5
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Default Re: Marconi 2019A frequency fault.

Thanks for the replies, I don't think it has anything as modern as an OM-345 certainly not on board AA1, looks like a search in the loft for my logic probe, it amazing how these things hide them selves when you are looking for them!
I think Mr Bungle is in the right area as my searches and tests have got me to board AA1 and an unlocked PLL. Its just been such a long time since I did this type of troubleshooting I'm a bit rusty.
Thanks
Chris
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Old 26th Apr 2017, 11:21 am   #6
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Default Re: Marconi 2019A frequency fault.

I recollect that I too changed a 74LS273, but can't remember which board it was on. I would have found it to be faulty by using nothing more than a logic probe to check for "stuck" pins.
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Old 26th Apr 2017, 11:23 am   #7
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Default Re: Marconi 2019A frequency fault.

Logic probe is probably good enough if it has a pulse stretcher on it. You can shift the frequency up and down and see if the outputs change on the '273 ICs.

Mine was blowing up TTL ICs because the 5v regulator on the unit was on its way out. Voltage was floating around 5.8v which is a little high and then it went short and blew the whole thing up. May be worth checking the power supply for this fault as well. I'd hate to see another one have the same fate.
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Old 26th Apr 2017, 8:47 pm   #8
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Default Re: Marconi 2019A frequency fault.

Looks like we have lost the PLL frequency feedback to the phase detector with negligable output from IC7 a 74LS160 so I will get some chips on order and report back in a couple of weeks when I get back from holiday.
Chris
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Old 26th Apr 2017, 9:56 pm   #9
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Default Re: Marconi 2019A frequency fault.

Interesting. Please do post back - lots to learn from these failure modes.
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Old 19th May 2017, 7:59 pm   #10
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Default Re: Marconi 2019A frequency fault.

I returned from my break and the ICs had arrived so once I got the NVCF and family visits done I got back onto the repair.
During my break I purchased another 2018A for spare parts and swapped the AA1 board from that into my 2019A and things worked properly and frequencies were now correct. This at least confirmed that I had correctly identified the right board, it also gave me chance to check what the pulses and voltages should look like on a working unit.
I put back in the original board and did a bit more probing and confirmed with my scope that there were no pulses from IC7 to IC1 and consequently no change to the voltages at TP2 the phase detector output the voltage was low and stayed low around 1v. As per my last post I had suspected IC7 74LS160, so out with my de-soldering tools and the chip came out cleanly and a new one went in. On powering up the frequency was still out but on scoping the PLL line from IC7 to IC1 we now had pulses but still no voltage output from IC1, In anticipation of other problems and because they were so cheap I had ordered a couple of each IC on the board, so out with the tools and IC1 74LS74 was replaced. This board was refitted to the signal generator and it was now behaving as expected.
I then did checks across the frequency range particularly at the points where different oscillators take over to confirm they were all working which they were I stepped the attenuators up and down the range whilst looking at the output to ensure I had a clean sign wave and that the AM and FM modulators were working all seemed good so I screwed all the lids down and set it aside and looked for my next item in the pile. I decided to have a look at the 2018 I had just acquired as I was in the mood, the story of that will have to form another post.
Whilst I was doing the 2019 my 1992 Frequency counter packed up with no response to key presses so I had to fix a dry joint on SKt 2 which connects the keyboard to the motherboard. This was needed so I could select the high range on input C to check the higher frequencies on the 2019.
Regards all
Chris
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Old 20th May 2017, 10:09 am   #11
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Default Re: Marconi 2019A frequency fault.

Chris,
Well Done. Nice to complete all that fault finding and get a result.
And that 2018A may well live again as well .....
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Old 20th May 2017, 6:17 pm   #12
chriswood1900
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Default Re: Marconi 2019A frequency fault.

Jeremy
Thanks for the kind words, it was an interesting challenge as I had not done anything with digital ICs for 30 years!!
Regards
Chris
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