Cybernet PTBM121D4X problems

[SiriusHardware]

Yes, always worth hoarding stuff like that, especially for things like the VCO modules which have been unobtainable for many years now. Away from base as usual at this time of the week, will have a think about where to look next. Don' t bury your scope yet... In the meantime you could run through the b-c-e voltages on the last 4 transistors in the transmitter, USB TX mode but with zero modulation. That will allow you to measure the DC voltages without too much RF getting in the way.

[John M1JWR]

Yes the vco block came in very handy, it went into the 777 when i got it as the vco was u/s, and a spare pll 02a can always be handy.
scope's still on the table !
bit late now, i will look at those transistors tomorrow.
regards

[SiriusHardware]

OK - as you have now aligned everything earlier in the TX you could try tuning the three or so large coils in the output stage, at a guess L8 / L12 / L14.

[John M1JWR]

Have got some response out of it now, firstly adjusted L8/12/14, got a bit extra out of L8, and most out of L12, very little out of L14.
On FM CH20 mid 8.5w.
as a guide checked CH1 lo band is also 8.5w and CH1 hi band is 7w.
will check voltages on those 4 transistors, i think they are Q8,Q7,Q1,Q4

[SiriusHardware]

Question: Is the output on AM similar to the output on FM? If so then I may have been mistaken about this being a high power FM variant, and if so then that's actually the most you should expect out on those modes. ~7W on AM/FM would be normal on the 'normal' variant of the chassis, and normal on AM on any variant of the chassis.

Easy enough to find out: A strongly modulated SSB signal (sustained loud whistle) should now peak at about 12-13W.

You can try balancing up the output across the range (FM TX mode) by slightly offsetting the tuning of T4-T5, peak one for the middle of the high band and one for the middle of the low band to begin with and then make the fine adjustments needed to level out the output across the whole output range.

[John M1JWR]

only 2w on am and about 2.5 to 3w 0n usb cant whistle loud
have got it to 7w output on 20 lo and 20 hi, 20 mid remains at 8.5w on FM

[John M1JWR]

Give or take a smidge, all levelled off to 7w on each band ch20 fm
AM is 2w on 20 mid and ssb tops as far as i can whistle which isent much
at around 3.5w.

[SiriusHardware]

Can you see what voltage measurements you now have on the transmitter transistors c-b-e, the driver, output and the two further back? USB TX mode, but zero modulation.

[John M1JWR]

Lets see, ch20 mid usb tx
Q10 B 0.56v C 13.42v E 0v
Q9 B 0.74v C 13.37v E 0v
Q8 B 1.45v C 13.36v E 0.72v
Q7 B 2.15v C 7.26v E 1.44v
on a side note did ic's 1,2 and 3 all voltages are in spec

[SiriusHardware]

One thing which stands out is the 0.56V on the base of the O/P transistor. This is a linear RF amplifier, so the output transistor needs to be partly turned on any time the radio is in TX mode. The degree to which the O/P transistor is partly turned on (biased) is set by RV2. If set too low, the O/P stage is not biased on at all because the voltage on the base is not high enough to turn on the O/P transistor.

As you turn RV2 to increase the bias the voltage on the base of Q10 will get to about 0.7V and then will stop rising, but as you continue to turn RV2 the standing current through Q10 will continue to increase. Unfortunately the 'Export Manual' only gives detailed information for setting the bias current for the PCMA001S (Nato 2000 etc, chassis) and not for the PTBM121D4X.

If we assume that it should be broadly similar for the PTBM121D4X then the way to set the bias current is to:

-Temporarily desolder Q10 emitter from 0V

-Set your meter to a milliamps range, plug the +ve lead into the A/ma input socket, connect the meter red lead to Q10 emitter and the meter black lead to the 0V point you desoldered the emitter from. Make sure these connections are secure, you don't want the meter to become disconnected and you need to be able to make this measurement / adjustment hands-free without trying to use your hands to hold the meter probes.

-Power the radio on, go into USB TX, zero modulation, adjust RV2 until the emitter current of Q10 is ~35mA. Power down, remove the meter, resolder Q10 emitter.

-----

Alternative method, which assumes that the 10m Colt is perfectly set up. (This method assumes that all other parts of the radios are drawing roughly the same amount from the supply when in USB TX mode, and that any significant difference between them is accounted for by the difference in the setting of the output stage bias current). Make sure both radios are set to the same indicated channel, because even having different channels displayed on the channel indicator will cause a difference in the current drawn. Ensure all switches and controls are set to the same positions on both sets.

-Power up the 10m Colt with your meter inserted as an Ammeter / milliameter in the main +ve supply lead. Go into USB TX with zero modulation and note the exact current consumption of the radio in that mode / at that point.

-Replace the 10m Colt with the faulty Colt and note the supply current on USB TX, zero modulation. Adjust RV2 until the faulty Colt draws the same current from the supply as the 10m Colt did.

[John M1JWR]

I had a quick check on the 10m radio took note that Q10 has 0.67v on base
and Q9 has 0.71 0n base, all i know about the 10m one is that the G4 that had
it before me had it professionally done in the early 80's as he bought it new.
that radio has an output of 4w on AM 3W on FM and about 3w on SSB,
it came to me with an 80w amp that takes upto 5w input, ive owned it for
about 18 years, i wonder how compatable it is in the regard of that it has
a prom in it.
so could either gradually adjust RV2 to get to 0.67v area or do plan A temp
de solder emitter, thing is i have never seen that meter read anything on
ama setting. dont have crocodile clips small enough to clip onto those two
points at the same time.
possibly try plan B first as its the easiest option, if that dosent work the only
way i can think of the other way is to solder wires onto the two points then croc clips onto the two wires, or something like that

[SiriusHardware]

The output on the 10m Colt is surprisingly low, as you say it seems to have been set / modified so it provides the correct drive for the companion amp - in that case it is not as good for use as a direct comparison as I would have hoped, and you're right, the presence of the EPROM mod in one radio and not the other will also make a difference so really the better method - and the best method anyway - is the first one, direct measurement of the emitter current with a milliammeter in series with the emitter. If you have a couple of 4mm / banana plugs spare, make up test leads with bare / tinned ends at the other ends which you can solder temporarily to Q10 emitter / 0V.

The Amps ranges on multimeters are always protected by an internal fuse which may have been blown at some point in the life of the meter and not replaced, so if you don't remember it ever having worked I suggest you have a quick look in the meter to see if the amps fuse(s) have blown. This may seem to present a surreal repair paradox - how do you check a fuse in your meter if a fuse in your meter is blown? - but a blown amps fuse generally only stops the amps / milliamps ranges from working, the ohms / continuity ranges will still work even with the fuse removed so you can check it.

[Techman]

It's quite common to find that sort of voltage on the base of the output transistors on these sets, hence one of the reasons that they sound so rough and clipped on SSB. As a personal opinion, I don't get on well with the method of adding a meter in series with the emitter to measure current, due to the presence of the meter itself upsetting the circuit. I always use a DVM to set the base bias to 0.7 volts, but that's just me. In reality, there sometimes isn't enough range on that pot to actually be able to set the bias correctly, so you then get into the realms of circuit modification.

As for that output stage coil in the output of the final being burnt, I agree that if it doesn't look burnt then it's likely to be alright. I remember back in the old days, probably when FM was first legalised and everyone began using these sets on the 'new' FM mode, they started failing with low output and local rig repairers couldn't find the fault. I remember thinking I'd struck gold when I first discovered the infamous burnt out coil with shorted turns. I thought I must be the first to find this fault in the UK. The reality of course is that in a world without the internet, you didn't know what was going on in any other part of the country unless you actually went there, so no doubt rig doctors all around the country had probably already discovered this particular fault long before I did. I'll have to admit to bodgery now, as the easiest way to fix this fault so long as the former isn't too badly damaged, is to carefully go round all the turns with a small jewelers screwdriver and just separate them all. I find that this actually produces a permanent repair without the need for rewinding. I think this circuitry is part of a band pass filter which would originally be factory set, so I now try to avoid touching the adjustment of it unless it's obvious that it's all been twiddled in the past, as in this set.

Reading through this thread has been just like a 'ground hog day', as earlier this year, during the summer, I went to a chaps house to have a look at a set which turned out to be a 121 board with exactly the same fault as the one being discussed in this thread. I'm not referring to the original problems with the broken circuit board, but the latest issue with the low output. The set was a Ham International Jumbo home base, so armed with all the service information for the expected 059 board, I had a bit of a shock when I took the cover off and was confronted with a 121 board - I wasn't expecting that and neither was it's owner who had recently done a trade with another set and wanted a proper booming out 'muffle mode' type set!

I can confirm that several months ago working on this chaps kitchen table, that I went through all the alignment procedure exactly as you've done in this thread with exactly the same result. I improved the operation of the set and got the output better than it was, but still a lot lower than it should be and still lower on SSB than FM, exactly as you have on the set being worked on, on this thread. I only agreed to look at this set as a favour, hoping it would be a quick fix for the chap and I wasn't prepared to spend much time on it. So that was how it was left and if I'd had time and I was looking for a paying job, then I'd have taken it away and spent some time on it on the bench. I advised the chap to either run a small amount of power on the end of it to give it some help or advertise it for sale as working, but with low output and get rid of it.

Several hours ago earlier this evening the phone rang and it was the new owner of the Jumbo. I was on the phone with him for about an hour discussing the radio. All the Jumbo home bases I've ever seen have had the 059 board with the separate front board. I've never seen a Jumbo with a 121 all in one board fitted. I think that this must be what they call the MK11 Jumbo and the ones with the 059 separate boards are the MK1 Jumbo models. The chap that's bought it knew what he was buying and really only bought it out of curiosity, as he thought the difference between the MK1 and MK11 Jumbos were down to differences in having a high band or a low band, but as far as I know, all old type Jumbos had just the high band going up into 28 MHz. I've noticed recently that Ham Jumbo sets offered for sale have had it stressed in the advert that they're a MK1 model, as the 121 board isn't the best, unfortunately, so if it is a fact that the MK11 has the 121 board, then this is the less desirable type.

So were're waiting to see the outcome of this fault on the Colt, and I'll be cross if it turns out to be something really simple that I should have found while looking at the set on that kitchen table a few months ago! I was so surprised to see this 121 in a standard Ham Jumbo home base that I took some photos of the innards for interest sake which I can post if you're interested - you'll probably say that you've already seen loads with this board in and it's just a coincidence that I never have. Also, I don't ever remember hearing any on the air, as there's a considerable difference in the audio on FM and SSB from the two boards and I would have noticed it when someone said they were using a Jumbo. I've been told that there were some Ham International Multimode 11 sets that had the 121 board fitted rather than the 059, although I've never come across one. Anyway, good luck with the Colt

[John M1JWR]

Firstly did an experiment on RV2 on minimum and maximum settings and what its
capable of and checked voltage on base in the usual manner in usb tx
on minimum setting its 0.32v and maximum is 0.66v, if and i mean if i am
thinking right it wont get to 0.70v whatever happens and the 10m one is set flat out.
just thinking more kind of mechanical because i am more that than electrical !!
so at the mo set it to flat out and backed it off a little to read 0.64v
techman, thanks for joining in, i think many moons ago this radio might have
been banged or dropped caused the cracks and then mr twiddler got in
and had a play, me, factory spec is the order of the day, however ive seen
one or two small mods to improve the mod on the 121 swissradios on you tube.

[SiriusHardware]

Anyway, good luck with the Colt!

Uhhnhh, and there I was, hoping that you were going to tell us what it turned out to be.

The 'set base to 0.7V' advice (as also given in the 'Export Manual') isn't great when you think about it, because the base-emitter junction will act like a 0.6-0.7V zener diode once the voltage supplied by the bias network has risen to that level. Once the junction starts conducting, the voltage will stay stuck at 0.7V no matter how much you increase the bias drive, therefore it is essential, really, to measure the current rather than the voltage. All the 0.7V tells you is that the transistor is turned on: But not how much.

I agree that if the meter being used to measure the current has significant resistance of its own (and they all do, because they work by measuring the voltage drop across a low value internal resistor) then it will have some small influence over the value being measured - but that can't really be helped, and may actually have been included in the reckoning for the original 35mA figure.

[SiriusHardware]

Originally Posted by John M1JWR

if and i mean if i am thinking right it wont get to 0.70v whatever happens and the 10m one is set flat out.

You are thinking right.

The 0.7V figure I have been quoting comes from the circuit diagram, but if the voltage on your actual set only rises up to a point (seemingly 0.66V) then that is the forward voltage drop for the base-emitter junction on your output transistor.

You should find that as you adjust the voltage from the lowest to the highest, it rises to 0.66V and sticks at about that voltage no matter how much further up you turn RV2, but, the further beyond that point you turn it the greater the bias current will be. If you leave it set too high the output transistor will draw more current than necessary and may get very hot.

I'd still suggest you set it with a milliammeter in series, for now.

[John M1JWR]

the fuse is blown in the meter, you will get a laugh
i needed 2 pairs of specs magnifying lamp and a magnifying glass to see it,
this age releated blindness problem is a bad thing.
all i can do is leave as is for now until i get a fuse, its set to 0.64v

[Techman]

Originally Posted by SiriusHardware

Uhhnhh, and there I was, hoping that you were going to tell us what it turned out to be.

I know, sorry about that - I thought you would say something like that

I think the trick with the bias is to set it until it just reaches 0.7 volts and then leave it at that, or things can start getting hot.

I'm sure that Ham international themselves actually recommend the 'voltage' method, and I don't mean the manual that's being referenced here, but I've looked and I can't actually find where I read it now.

It sounds like it's one of those with not enough adjustment on the pot from what's just been said, but getting it as close as you can should be OK for the time being.

[Techman]

Originally Posted by John M1JWR

its set to 0.64v

It's not ideal, but should be OK for now - most of them have run like that for years anyway.

[Techman]

I might have to try the 'current' method again myself sometime, it's just that I remember I had problems with that method, but it was a long time ago.