Cybernet PTBM121D4X problems

[SiriusHardware]

It's a bit late to go into this tonight, but I think I see what has happened.

What I think you have managed to do is tune the early stages of the transmitter to (2 * 10.695) Mhz = guess what, 21.390Mhz

Nothing broken but the tuning, which is wildly out.

Rule #4: Be aware that simple RF power meters only show how much power there is. They don't care what frequency it is on.

To get the tuning back into the right region, use a receiver to monitor the output on the middle channel, middle band and initially tune to increase the indicated signal on the receiver, ignoring what is happening on the power meter. That way you can be more sure that you are tuning the coils to peak on the right frequency.

[John M1JWR]

ahh yes i thought whats going on here, just to be on safe side i checked vco ch1 lo, its happy 4.19v and checked pll its happy with correct voltages and
checked c/changer side and voltages changed with channel.
was going to see if it rx'd next.
op error, part of the learning process.
now where's my vacume guages !!!

[Techman]

It sounds like the driver or output is perhaps going into oscillation. (I think I mentioned this before). I'm wondering if it's been doing this before and you have only just noticed it? If you remember I said that off peaking, or de-tuning the transmitter driver and output inductors (or coils) should be avoided, well this is a situation where slight 'off peaking' of one (or more) may be necessary. On AM/SSB, monitor the output with your frequency counter and watt meter and slightly adjust each of the inductors in turn and you may see it suddenly stop oscillating and return to transmitting on the correct frequency - some of these sets are prone to this. I'm still not convinced that there's not still an un-diagnosed fault somewhere in the drive, or perhaps it is just that the whole set was way out of adjustment in the first place.

You mentioned the 'swing' seems to be backwards on AM modulation - I think this is down to bad regulation and the audio level is making the supply rail 'dip' to this part of the circuit. Certainly they wouldn't think a lot to it on the old channel 6 AM, where carriers are wound down to a couple of watts and audio is wound up flat out and the whole lot fed into massive linear amplifiers! However, your set will probably sound quite nice to listen to on AM as it is. You can't really say that the audio on 6 is 'nice'. I'm sure you know about channel 6 AM, on what is now known as 'mid band', if you don't, then just look it up - there's nothing to be heard on there at the moment though.

As a note on those resistors with a '*', it's interesting to note that the same resistors, which are the same value but marked as R39 and R38 on the 059 chassis circuit diagram, are not marked with a '*'. The 059 board is not known for output stage biasing problems, whereas the 121 board is well known for issues with distortion and clipping of the output waveform.

As another note regarding R45 which is 4.7K, but without the point/dot shown on the drawing, I note that on the original Hi Gain and Concorde diagrams that the 'dot' is shown but is below the line compared with other similarly marked values, so it looks like the Intek circuit could be an early version with the 'dot' being mistakenly left out, but was corrected on later issues, having to be inserted lower due to spacing. Makes you wonder if there's any other corrections from this drawing onto the later ones - that is if they are indeed later.

[Techman]

Just seen post #281 - that's a good possibility I hadn't thought of.

[John M1JWR]

techman did you get the links for the alternative circuit diagrames

[Techman]

No, sorry, they're actually original paper copy diagrams that I have from the original radio handbooks. I could probably sort out posting copies if necessary, or post a quick camera snapshot.

[John M1JWR]

put them on post 275 they were done 10 years ago by an american
they look completely different to any original
they also mention the bias upgrades
ch6 am the super bowl

[Techman]

I decided to fire up the printer and print them out (post #275 links), as I can see them a lot better that way, although they have R Jackson's water mark across them on the printed version.

They're very good diagrams. I remember using one when working on a Grandstand and I found a small error on the drawing where a circuit line ended in a dead end. No problem once you'd found it, and it could just be drawn in with a pen on the paper copy.

[SiriusHardware]

Morning gents - I'm a bit more wide awake now. The rock steady 21.390Mhz -has- to be the output from 10.695 MHz oscillator (OSC 3?) being doubled.

T7 should be tuned to 10.695Mhz, but T4 / T5 / T6 should be tuned to 27Mhz in order to select the 'difference' signal created by mixing the output from the VCO with the 10.695Mhz signal. Instead, the T7 / T4 / T5 / T6 chain (and probably the output stage) have all been tuned to the first harmonic of 10.695, or 21.390Mhz.

That's why there is no Mod on FM, because the FM mod is applied to the VCO, whereas the AM mod is applied to the output finals and the SSB mod is applied to the 10.695Mhz signal before it is passed on to IC3.

John, to get back to a degree of sanity put the 10m Colt side by side with this radio and 'copy' the settings of T7, T4, T5, T6, and the output coils - set the coil slugs on this radio to as nearly the same as they are in the 10m Colt, physically. That will put them on around 28-29Mhz which is a little bit on the high side.

Then, turn T4, T5, T6 further in by another half turn. That should give you a near-27Mhz starting point to begin tuning from. T7 will need to be re-peaked as well.

What you really need for this job (and any transmitter tuning job) is a Spectrum analyser, but they are expensive. A "poor man's" version would be a tunable wavemeter tuned to 27Mhz.

[John M1JWR]

I will get after that after work, today, tomorrow and friday mornings will be tight
for any progress, evenings ok, then i am off for two weeks ho ho
i have one of those small rama frequency counters i can plug in and the 706 can do the rx work.
i will have to make up a dummy mic plug for radio to be on tx, failling that the
old elastic band around the mic trick, i have a spare 4 pin plug somewhere.
when i originally tested rx last night i could faintly hear this radio on 706 on
27.205 usb only, but basically all mod exept fm was on 21.390.

[Techman]

I'd been at cross purposes with john last night regarding what circuits he meant, but got there in the end. Also, I hadn't appreciated the bit about no modulation on FM. I found it difficult to understand with the line breaks in the text (there's another forum member that posts in broken lines of text, and assume it's down to a software incompatibility on the device they're using) and thought it was 'mode' with the 'e' missing - I also blame the late hour;-)

Now I understand, it's obvious regarding why there's also no FM modulation, as the 121 board and other later similar types do modulate the VCO for FM. I should have guessed, as I've modified the 059 chassis in the past to work this way. These earlier chassis modulate the band crystals which don't like being pulled off frequency too far. I think what happens, particularly when the crystals are already pulled several Kc down for UK FM, is that they're probably near the limit and when modulated with another 2.5 or so Kc, the negative half of the waveform gets chopped off causing that characteristic sound. I've modified these in the past to work in the same way as the 121 circuit being discussed here.

[SiriusHardware]

John, sorry if you already know this, but don't ever connect the output of a transmitter directly to the input of a frequency meter - to begin with the transmitter absolutely must always have a 50R load when being tuned up, but also the output from a fully functional transmitter is enough to damage the input circuit on some frequency meters.

It's better to use a pickup wire draped somewhere near the output stage, but not actually connected to it.

Some Ham International radios (typically) used to have a medium value resistor going from the aerial socket centre pin to a 'phono' socket nearby marked 'Frequency' - intended for safe connection of a frequency meter.

Techman: yes, transferring FM modulation from the loop mixer crystal oscillator to the synth VCO was a popular mod, a no-brainer really since the defined purpose of a crystal generally is to stay doggedly on-frequency - which makes them quite reluctant to be frequency-modulated.

[John M1JWR]

Well we are back from the nether regions
power meter is connected and 706 is in rx
interesting results all ch 20
mid band fm 15w am 4w and ssb is going to 15w without much trouble
low band fm 13w am 5.5w ssb 7 or 8w area
high band fm 10w am 2.5w ssb around 10w
this is the kind of mismatch that i get confused with
forgot to say the 10m colt has no tunable L14 just a fixed coil
also before it was using over 2 amps on fm when it was putting out
that kind of power, now its just short of 1.2amps

[John M1JWR]

forgot to say back on 27mhz tx aswell, diddent take much, approx to where those cans were on 10m radio.
followed advice and did not use frequency counter
also the power outputs are on the 5w scale my mistake so those figures are wrong
mid fm 4w am 1w ssb 4/5w
lo fm 4w am 2w ssb 3w
hi fm 3w am 2w ssb 3w

[SiriusHardware]

If you've been running directly into the frequency meter for any length of time you will have been operating into a very, bad VSWR unless the frequency meter just happens to have a 50 ohm input. Even if it does it will not be rated for much in the way of input power. We can only hope the radio's output transistor is still OK.

I may have misunderstood, maybe you just meant that you acknowledged the advice about not connecting directly to the meter and weren't saying you had ever actually done that.

At the moment, you have it peaked for best output using T7 and T4, T5, T6 and L8, L12, L14? And that's into a 50 ohm dummy load?

What about T2 / T3, did you adjust those again at any point since the first time you adjusted them?

[John M1JWR]

i have always ran radio into dummy load
when ever it was on power meter for example dummy load was on ant socket.
havent used frequency counter, just 706 on rx
havent done any other adjustments since i pulled it back onto 27mhz
bit late now, though i will have a quick slight adjustment to see if anything happens and post any result before i retire for the night

[John M1JWR]

Done a bit of improvement, have 706 on 27.205 so i know i am still at home so to speak.
mid fm 12/13w am 4w ssb 8w approx
hi fm 10w am 3w ssb 7w approx
lo fm 3.5w am 3.5w ssb 2w
poor old thing !!
can make lo band go higher with t4 and t5

[SiriusHardware]

Sorry, it wasn't clear whether you meant you copied the adjustment settings approximately from the Colt and that's the stage you were at, in which case you'd now need to start fine tuning through the T7 - T4 - T5 - T6 - L8 - L12 - L14 chain (and possibly T2 - T3) again, as physically copying the settings from the 10m Colt will only have brought it back into very roughly the right area (and frequency).

Reading your posts #293, #294, I thought you must already have tried to retune again from that point and arrived at only 2-3W out maximum.

I have to head off now, but will check back in in the morning to see how you got on.

[John M1JWR]

no probs my previous post is where i did some adjustment, one before that
is where it was after returning it back from 21mhz by using approx settings from 10m colt, by sight and age related that aint good.
so now in future i will have 706 on rx to make sure i stay where i am supposed to be.
seems to be starting to show a healthy output now, just not balanced yet.
i actually checked rv4 on 10m colt not that it may bear much relevence, just
as a guide and its at around 3pm looking from back of radio

[John M1JWR]

Good evening
this has been an evening of many small or in some cases microbe adjustments.
i could never get all 3 bands the same, t4 and t5 seemed to be key players
nearing a kind of end game, the a slight backing off of rv4 somehow levelled
it all off so now i have 12w on fm on all 3 bands, at last.
am and ssb are different though, am is just below 4w on hi band, just over 4w
on mid band and 4.5w on lo band, ssb seems to be the other way round with
approx 7w on hi band and around 5w on mid and lo bands.