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Old 19th Apr 2017, 9:12 am   #1
vosperd
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Default RA1772 again. Frequency 100kc/s out

As title says.
I have to tune to an indicated 3.7 megs to receive a signal on 3.6.
On switch-on the display reads zeros and goes up to zeros. So I don't think it's a display fault. The only thing I've checked so far is the output of the upper loop board and the 4.6 to 3.6 output reads ok. Making further tests today.
Any ideas most welcome as I am not fully familiar with the intricacies of these sets.
Don m5aky
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Old 19th Apr 2017, 11:08 am   #2
Sparky67
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Default Re: RA1772 again. Frequency 100kc/s out

Don - there is a 100kHz BCD line on the display board which feeds the 100kHz digit display and the upper loop board's programmable divider N2. Is the 100kHz offset at every 100k step, if not it might be worth looking at the '1' BCD around ML2 and ML41 on the display board and the wiring to the upper loop board?
Martin
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Old 19th Apr 2017, 4:17 pm   #3
vosperd
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Default Re: RA1772 again. Frequency 100kc/s out

Hi Martin, Thanks for the reply.
I'm just checking those areas now.
As the displays are behaving normally I was assuming that ML2 was doing its job.
I've checked the output of the upper loop board and it goes from 4.6 down to 3.6 at either end of the scale.
Cheers
Don
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Old 19th Apr 2017, 6:49 pm   #4
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Default Re: RA1772 again. Frequency 100kc/s out

Another thought Don... Is the output of the HF loop board tracking the reception or display freq (+ 35.4MHz)?
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Old 19th Apr 2017, 6:55 pm   #5
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Default Re: RA1772 again. Frequency 100kc/s out

I'm heading in that direction but my frequency counter might run out of steam at +35.4.
Don
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Old 19th Apr 2017, 7:38 pm   #6
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Default Re: RA1772 again. Frequency 100kc/s out

Would this be the signal that could be read on the BNC marked LO in/out on the rear panel?
Don

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Old 19th Apr 2017, 7:50 pm   #7
G6Tanuki
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Default Re: RA1772 again. Frequency 100kc/s out

I'd suggest:

Tune upwards in decades from a known frequency, and see how the "supposed" tuning follows the actual.

Map this out into the bit-patterns that are being fed to the synth.

0000
0001
0010
0011
0100
0101
0111

and such.

Part of me thinks that you'll find one of the lines is 'stuck' high or low.
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Old 19th Apr 2017, 7:52 pm   #8
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Default Re: RA1772 again. Frequency 100kc/s out

Because if it is this is what I get:-
Kc/s all zero
Mc/s=0 ; 35.400
1 ; 36.285
2 ; 37.400
3 ; 38290
4 ; 39400.
Don
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Old 19th Apr 2017, 9:41 pm   #9
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Default Re: RA1772 again. Frequency 100kc/s out

Yep, that sounds like it's the synth that's misbehaving, not the display.

I don't have the RA1772 synth block diagram in my head, but it's a multi-loop synthesiser. There should be a decent fault-finding section in the manual.

It has a few phase locked loops which each synthesise a signal in frequency steps. The steps are quite large so that it settles quickly and its noise performance is good. The signals from these loops are then added together.

Don't start at the end of the combining chain and try to logic your way backwards, that can get you in a right muddle.

Find the test points for each loop and check it covers the range the manual says, and hat the relationship between the tuning display number and the measured frequency is right. Only if these are right, move on into checking the summing arrangements. You are going with the flow of the signals in the synthesiser, not trying to go backwards, guessing which tributary must be wrong.

I had the job of designing a five-loop synth many years ago and had to debug prototypes which had never worked before, so I didn't have the help of knowing it once worked. It's manageable.

Each loop has a tuning voltage. It is very likely that a bad loop can be spotted by its tuning voltage being out of range. You can get fair bit of value from a basic scope in these tests.

It looks complicated, overall, and it is. But it is really an array of many fairly basic circuits flying in close formation. Debug at system level without worrying about details until you find a bad block of circuitry, then you only have to delve into that block in detail.

David
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Old 20th Apr 2017, 3:40 pm   #10
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Default Re: RA1772 again. Frequency 100kc/s out

Hi,
Thanks for all the comments received
Thanks for the advice regarding the multi loop synthesiser. I am shall be reading through the relevant parts of the manual.
It would appear that all of the even Mc/s are correct and all of the odd ones are 110kc/s out, not 100 as I first thought. The outputs from the logic board appear ok so far.
Don
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Old 20th Apr 2017, 9:39 pm   #11
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Default Re: RA1772 again. Frequency 100kc/s out

With a synthesiser, you have to be quite careful in talking about frequency.

If something is exactly 100kHz out, then it sound like the loops are locked and the control data is wrong.

If loops are unlocked, then there could be any amount of error. It is most unlikely that it will be an exact round number and it will drift about a bit.

So looking for exactness or digit is a quite helpful indication of the sorts of things it could or could not be.

I've designed multiloop synthesisers (and DDS assisted ones, and Fractional-N ones) but I was a bit spoiled by having fast scopes and spectrum analysers to hand. It's harder to see what's happening without such things, but looking at the tuning voltage on each loop is a very good approach.

I wrote some description of how multiloop synths work for the ARRL handbook. It's in any edition from 1995 onwards in the oscillators and synthesisers chapter.

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Old 21st Apr 2017, 8:13 am   #12
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Default Re: RA1772 again. Frequency 100kc/s out

Last night I clipped my new logic probe onto the Upper Loop board, moved the wiring about in order to identify the pin numbers and the fault cleared. No obvious bad connections and I can't get the fault to come back for now.
So now the 1772 I bought as spares or repair seems to be better than my original!
I would like to read about the multiloop synthesisers but I see my latest ARRL handbook is 1991. However, I see there is lots of info on the internet and in the manual.
Thanks again for all the interest and advice.
Don m5aky
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Old 21st Apr 2017, 10:14 am   #13
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Default Re: RA1772 again. Frequency 100kc/s out

Oh, that can be frustrating when that happens!- whilst it's pleasing to have something working again, it's always good to be able to say, "It was x that needed changing/reflowing/whatever". I'm not familiar with this set, but I'm sure I recall something about Racal biting the bullet and going for swing-out but hard-wired boards. From what I recall of working with edge-type connectors of that sort of era, it might have been wise!

With the complex but hemmed-in replacement transformer requirements, perhaps an outboard PSU with a number of easier small transformers and an umbilical will keep the original going, HRO-style?
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Old 23rd Apr 2017, 9:42 pm   #14
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Default Re: RA1772 again. Frequency 100kc/s out

Quote:
Originally Posted by vosperd View Post
Because if it is this is what I get:-
Kc/s all zero
Mc/s=0 ; 35.400
1 ; 36.285
2 ; 37.400
3 ; 38290
4 ; 39400.
Don
Well, I was going to ask you for some more readings higher up from 4MHz as the odd MHz freq error was changing Don... No need now! Enjoy the Rx. I have used the 17, 217, 1772, 1778 and the 1792 and I know which I prefer.
Martin
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Old 24th Apr 2017, 8:34 am   #15
vosperd
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Default Re: RA1772 again. Frequency 100kc/s out

It was the same all the way up, my Cumbria Designs frequency counter worked fine in the end.
I will spend some time cleaning out the melted varnish and wax from the old set and consider getting a transformer made up at a later date. I see there is a transformer in Greece but the carriage cost is high and I've spent my pocket money now.
Faults have a habit of returning, at least I know a bit more about it now and another subscriber to the forum has kindly sent me the relevent chapter from the ARRL handbook regarding the synthesiser.
Cheers
Don m5aky
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