Cybernet PTBM121D4X problems

[SiriusHardware]

I've looked at the various flavours of 121 diagram I have here, and there is a clear difference in the supply arrangement to RV13 between variants.

On one variant as typified by the Intek diagram, the voltage feed to the 'hot' end of RV13 is from 13.8V. On other variants the voltage feed to the top end of RV13 is from the regulated output of the AVR you repaired early on in this thread, so that will be 9+ volts, which is indeed what you have on the 'hot' end of RV13.

You can verify this is definitely the case: Power off, and measure the resistance between the leftmost pin of RV13 in your image in #220 and wire terminal (5), should be near zero resistance.

If so, now power the set on and measure the voltage on the supply end of L11 in
- AM TX mode
- FM TX mode

I have marked your original image of the output stage track side with the position of the supply end of L11. Also, can you check the capacitor ringed in yellow to make sure its legs aren't touching each other or anything else apart from the pads they are soldered to?

[SiriusHardware]

- Also the supply end of L11 in SSB mode with as much modulation as you can manage.

[John M1JWR]

Have inspected cap and it is not touching leg to leg
rv13 to pin 5 is 00.9 ohms
L11 voltages, FM 13.6v, AM 7.97v, usb 12.95v as high as i can go anyway.
got the fuses today so thats a small step forward, will go onto that if you still wish at a slightly later date.

[Techman]

I don’t have a board layout diagram for the 121 and I couldn’t find one on-line. This is one of the reasons that it was a bit of a struggle working on that one the other month, with limited time, space and light. I basically used the procedure and the circuit from the 059 and transposed it onto the 121, peering down through the wiring to read the ‘T’ numbers with the aid of a torch. I found that ‘yellow’ copy on-line which I’ve now saved, but I could do with the actual one. My copy of the Cybernet manual is from more than thirty years ago and is the one that strangely has a copy of the Multimode 11 on the front cover, but doesn’t actually list it in the cover listing or the manual itself.

I did a bit of research on the MK11 Jumbo question and it’s one of those things where there was never actually a MK1 or a MK11, just that late in production of the model it seems that the 059 board was discontinued and the last of the models made were fitted with the 121 board. It’s said that even the manual supplied with the later Jumbo still featured the circuit diagram of the 059 board when the radio itself was fitted with the later 121 board. Then Ham International brought out the Jumbo 3 with I think the 133 board and actually had its model number printed on the front panel, so therefore the previous two immediately became respectively the MK1 and the MK11 by default, even though there’s no way of telling them apart externally. So, it was similar with the Ham Concorde, there was never a Concorde 1, just the Ham International Concorde with the 059 type board fitted (somewhere in the back of my mind I seem to remember a similar board known as the 058, or perhaps I’ve just imagined that). Then the Concorde 11 came out with the 121 board fitted as being discussed on this thread and the original Concorde immediately became the Concorde 1. Latterly, there was a Concorde 111, which was again fitted with the 133 board.

As regards this 121 board being discussed at the moment, I don’t have a radio with this particular board, but I had a look in the workshop and found, as the old saying goes, “here’s one I scrapped earlier” – actually they were scrapped (there’s the remains of three) more than thirty years ago and were badly ‘got-at’ beck then, hence why they were scrapped. They have been an invaluable source of hard to get parts over the years. One is definitely a Hi Gain V – possibly they both are and I should have looked to see if they both had the components added for the on-board bleep, but I forgot and they’re buried again now. I’ve included a photo of the print side of the boards below. Whether this will be helpful is debatable as they really have been messed about with, in the past and before they were broken up.

As regards to the fault on the set being discussed, there’s at least one other inductor within the output screen that can suffer burning, so might be worth a careful look. Another thing is that the output and driver look to have been changed at some time. I can’t remember whether they were swapped earlier in the thread and there’s too many pages now to go back and check. I know the voltages taken on the final outputs were within what would be expected, but I would personally be happier if I’d removed them and checked the base-emitter, base collector and collector-emitter junctions with my AVO on the low ohms range, remembering that there’s also an inbuilt diode across the collector-emitter tags on this particular RF amplifier transistor. As a last thought, is there any possibility that the output transistor is one of the notorious fakes that have been so prevalent. Some of these transistors may test roughly right on their junctions, but low RF output, and running hot is a typical indication of a bad fake – there are actually some good fakes out there, believe it or not!

It would be good to find that the fault is in the RF output stage after all, but just to go back a few pages as I can’t remember quite what was deduced, I seem to remember that there were some scope measurements taken. What did these readings indicate? Was the RF waveform measured back in the pre-driver stages and was this compared with the same circuit position in the other radio?

Other than that, keep up the good work. The above are just my thoughts, but don't let this stop you proceeding in the order that you're going with at the present time, especially as it's pretty much just been you that has been assisting the OP with this radio. I have to say that there may not be an official manual ever produced on this particular chassis/board, but there’s certainly one being written on this thread, and I think we’re all well aware that there’s a lot of interested silent viewers looking in at how it’s progressing.

Picture of boards below - I set the old camera to its highest setting of 5.0MPx for this one. Whether it'll make any difference with the forum re-sizing I don't know, as I usually take all my forum pictures at the lowest setting of 0.3MPx to save space and for a quick upload, and they usually seem adequate:-

[John M1JWR]

Also in the lucky position that i have a scrap 121 boarded radio ex tristar 747
it was killed at some point in its life in the "idiot diode area" and channel selector area.
its also a very handy source of parts of which some now reside in this radio,
i could also check its 1969/2166 as i am certain they are original, and at that brief time i got it working it had good output,
that was until the idiot diode area fell apart again, and then i made my mind up to scrap it.
its vco lives in the tristar 777.
some of these fixes seem to be from what others have done to them, the 777 was somebody trying to make it 10m
and made a mess in the process, i "luckily" fixed it but never finished it off, thats for another thread another day!

[Techman]

Make sure you look after those ITT310 and 410 varicap diodes as they're VERY desirable!

[John M1JWR]

thanks for the insight, diddent know that
as for the 1969 there is an eleflow(mrf 20 ?) one in the 777 and it seems ok
the 777's original still exists it checked ok at the time aswell though never refitted it.

[Techman]

I shouldn't have told you I should have asked you for them seeing as they're little things and of no significant use Only joking of course

Actually, there's a type that an on-line supplier has in stock that may do the same job and I really ought to get round to ordering some and trying them out as it might be a bit of the old 'snake oil' with regards to the 'ITT' ones.

[SiriusHardware]

As I mentioned, I'm open to any suggestions as to the way forward.

Alternative driver and PA have not been tried yet, although John has removed the ones in his scrapper in preparation. When we last looked in that area with a scope it appeared that both devices had larger signals on their collectors than on their bases, which I viewed as a rudimentary pass, given that no re-tuning had even been tried at the time - The idea was to establish whether it wasn't just a duff driver or PA before 'spoiling' a perfectly good tuning setup - so if John is willing to replace first the output then the driver, that's OK by me. John did also say he felt they were the original devices, doesn't mean one of them can't be dud of course.

I haven't seen any other coil than L12 go like that, so feel free to direct John towards the other one. John says adjusting L14 has little effect, so maybe look more closely at that one?

As regards checking absolute signal levels at various stages, this is a problem because although John has another Colt, it was modified for 10m (not a problem in itself, the levels would be the same, the frequencies would be a bit different) but unfortunately somebody carefully tuned the output down so that the output level matched the input level of a linear which was used with it - so we can't do direct level comparisons.

We need, I guess, someone who has a 'normal' 121 chassis which we can ask them to make level measurements from, but even then different combinations of scope and probe may load the various test points differently and have different sensitivity at the frequency of interest - the best option is another working and optimally adjusted 121 chassis radio alongside the faulty one, with test points in both being measured with the same scope and probe so that a direct comparison can be made.

[SiriusHardware]

John, Ref: The bias current setup, yes, I'd still like that to be done as it does need to be set even though it will not directly fix your fault.

Your latest measurements on the supply voltages are again OK, by the way, and seem to confirm that this is a high-power FM variant as the supply to the output stage is 13V in FM mode, rather than 7V as it is on AM.

[SiriusHardware]

About this:

Originally Posted by John M1JWR

ok checked those to 0v ground tested infinity and to centre of antenna socket
it measured 00.3 ohms

The measurement to 0V should have been 47K rather than infinite - my guess is that you used one of the lower ohms ranges on the meter for the measurement.

[John M1JWR]

Yes i think i had meter set to 20k
will re do that a little later
i have the 1969 and 2166 out of scrapper on the table just now
and going over them with meter
a few months ago there was an ad on ebay selling 1969's from a well known
ham radio retailer for £4.50 each new old stock, because it was a respected
retailer i had a punt at the time and bought two
the company is a state in the usa
will leave them alone unless desperate !!

[John M1JWR]

Have re checked that connection again on 20k range and it says 4.68 k ohms
bit of a strange result compaired to 47k or does it mean 46.8k ?
results from scrapper 1969 and 2166
1969, ofcourse red means red lead etc
base to emitter red/black 629
emitter to base red/black 1.
emitter to collector red/black 614
collector to emitter red/black 1.
base to collector red/black 629
collector to base red/black 1.
and now 2166
base to emitter red/black 621
emitter to base red/black 1.
collector to emitter red/black 1.
emitter to collector red/black 622
base to collector red/black 615
collector to base red/black 1.
to me they look good, i tested them on continuity
probably weekend now when i may have more time if i swap say 1969 first
i can also do the milli volt check at the same time, and if there is a large
improvement in output i may not need to change 2166, that would be fingers crossed !!
its a bigger job than a 125 board, none of those nice ali heatsinks in one of those.
one day i may put the 777's original back in.

[SiriusHardware]

That 4K7 would be interesting if true. What you should be seeing is the resistance of R45 which on the diagram looks like 47K - might possibly be that my diagram has just lost its 'dot' between the 4 and the 7. Have a look at R45, it's near L13 / L14. Is it marked as 4K7 (Yellow/Violet/Red) or 47K (Yellow/Violet/Orange)? It would be a staggering coincidence for a 'bad' resistance to be precisely one tenth of the resistance expected, so I have a feeling R45 is probably a 4K7.

I'll defer to Techman to review your transistor measurement results because he has more recent experience at diagnosing these with a meter than I do. When I last worked on these the RF transistors were widely available and cheap, so it was just as easy to substitute a suspect transistor. Nowadays with genuine transistors being expensive and hard to find, we don't have that luxury.

[SiriusHardware]

Afterthought:

If R45 is marked 47K but you really do see 4K7 down to 0V, check C63 in-circuit to see if it is short-circuit / very low ohms. If the results are ambiguous desolder one end of C63 from the surrounding pad and check the capacitor by itself, should be infinite / high resistance.

If C63 was short-circuit that would be one way you could get a resistance of 4K7 (the resistance of RV2) down to ground instead of the expected 47K.

[John M1JWR]

yes its yellow violet red and gold
its one of those upright ones that i had to pull back to see it

[SiriusHardware]

OK - it was a long shot. Let's see what Techman has to say about the transistors.

I think I remember you saying you'd already taken the transistors out of the scrapper, so in the meantime try comparative testing with the two devices side by side. Mark the ones from this set somehow before you bring them together, then compare them by methodically testing the same junction on each transistor in turn the way you did above.

If you see any major difference between two devices which are supposed to be the same then one is probably faulty. If it does come to that, make a further comparison with one of the never used devices you got from the American state.

[Techman]

Yes, R45 is 4.7K.

Those transistor readings would seem to be spot on and read close to what would be expected with a DVM on the diode test/continuity etc. setting. Using an AVO those same readings would come out at around the 50 ohm mark and I much prefer using an analogue meter such as an AVO for tests like this, in particular the reverse junction tests. All the readings, including the inbuilt diodes which conduct in the direction emitter to collector look as you would expect. I think that rather than just fitting those in place of the others, it would be worth first doing the same tests with the ones in the set, but it does at least look like those spares are good.

The other inductor that's worth looking at for any signs of burning is L15. It's in series with L7, so probably worth looking at that too. They're in the line from the collector of the driver via the mode switch to the emitter of the Darlington.

[Techman]

I should probably add that there's obviously a bit more to that circuit with the inductor in than the simplistic description that I've given above as there's also the connection with L11 and L10. I don't think there's usually any problem with these and it's just L15 that sometimes suffers. The description was basically just to help locate it in the circuit.

[John M1JWR]

have had a quick look at l15, it "looks" ok, is there a way of checking it to see if
its ok, l11 and l9 look like they have some form of black tape around them
must be standard as scrapper ones are the same.
after that either after work or tomorrow i will remove 1969 and check it and
if need be then i will move onto 2166