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Old 11th Dec 2018, 12:54 pm   #201
SiriusHardware
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Default Re: Cybernet PTBM121D4X problems

OK, thanks for testing so patiently as usual, that's a little bit more ruled out.

We are approaching the point of needing to know what the levels at each stage should be in order to know where things are going wrong. The obvious way to get them, in the absence of a 'proper' service manual, is to measure them from a working 'normal' 121 chassis, ideally the high-power FM version although the normal variant would do (in which case we would have to use AM rather than FM mode for the comparison). Does anyone have one?

In the meantime a few more random checks you can do:

Check the resistance from the junction of C56 / C57 / C210 / L13 to the centre pin of the aerial socket. (Should be close to zero ohms).

Check the resistance from the same junction to 0V. (Should be ~47K)

Check the resistance of L11 (should be near zero ohms)

Check the resistance of R39 (should be 10 ohms)

Check the resistance of R37 (should be 68 ohms)

Check the resistance of R32 (should be 100 ohms).

Mark or otherwise record the current setting of the TX ALC preset (RV4) and then set it to its half way point. Power on, what voltages do you see on pin 7 of IC3 in:

- AM TX mode
- FM TX mode
- SSB TX mode

Last edited by SiriusHardware; 11th Dec 2018 at 1:07 pm.
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Old 11th Dec 2018, 1:05 pm   #202
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Default Re: Cybernet PTBM121D4X problems

can i do these mesurements with the components in situe or do
i have to desolder them again.
i dont know anybody else with a 121 boarded radio.
the current marketed cybernet export service manual isent upto much
by the looks of it, and as we go through this i have discovered its very
basic and favours 125 boarded radios.
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Old 11th Dec 2018, 2:14 pm   #203
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Default Re: Cybernet PTBM121D4X problems

The very low value components (10R or less) will be OK to test in-situ. You might see a value which is slightly lower than the expected value when in-situ. Anything higher, it is advisable to desolder one end - no need to take it out, just make sure one end is completely disconnected from the surrounding pad and measure the resistance from the track / green side of the PCB.

Yes, the 'Export manual' is a pretty strange hybrid as it mostly focuses on the 125 /131 / 001 boards which must all be quite similar - but oddly, the only PCB track layout in the second edition of the Export Manual is for the PTMB121D4X, luckily for us I suppose, as it made detection of that crack much easier. Speaking of which, I'm wondering if we should try a thick wire bridge across the line of the crack on the green side of the PCB where that earth / 0V track runs around the metal tag, even though the crack does not seem to be visible on that side.

The 121 chassis in its original form was a 40 channel USA AM/SSB radio for which it would have been possible to buy one of the famous SAMs Fotofacts series of service manuals, at least in the USA. I may try to look out for one of those.

I will put feelers out and see if anyone I know locally has an original or 10m converted 121 chassis set that I can borrow to take some measurements from.
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Old 11th Dec 2018, 5:40 pm   #204
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Default Re: Cybernet PTBM121D4X problems

In case I didn't make it clear, when I said

Quote:
Check the resistance from the junction of C56 / C57 / C210 / L13 to the centre pin of the aerial socket. (Should be close to zero ohms).

Check the resistance from the same junction to 0V. (Should be ~47K)
...I didn't mean you needed to desolder those components - just make a resistance measurement from the track which one end of all of those components are connected to, to the aerial socket centre pin, and also from the same track to 0V as well.
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Old 11th Dec 2018, 11:38 pm   #205
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Default Re: Cybernet PTBM121D4X problems

Have checked the following
Resistence L11 00.4 ohm,/ R39 10.1 ohms/ R37 64.5 ohms/ R32 102 ohms
then put RV4 in middle, wasent far away anyway as pin 7 on tx mixer was
reading 2.11v on AM TX to begin with, so after adjustment to centre,
AM 2.16v/ FM 2.10v/ USB 1.93v.
i am afraid i havent a clue where the junction for c56/57 etc is
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Old 11th Dec 2018, 11:57 pm   #206
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Default Re: Cybernet PTBM121D4X problems

Reading the voltage on pin 7 before adjusting RV4 was a good way to 'remember' where to set it back to if necessary. Everything you did measure sounds OK.

Can you give me a photo of the area around the output transistor? (Green side, output transistor in the middle of the image and maybe up to 5cm in every direction from that - as sharp as possible please). I'll mark a point from which to measure the resistance to the aerial socket centre pin and to 0V.

Basically, if you start at the centre pin of the output transistor, go from there through L12 to C56 and then go to the other side of C56... you're there. C56 is one of the capacitors you checked earlier, the point you want is not the side of C56 which is connected to L12, but the other side of C56.
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Old 12th Dec 2018, 12:13 am   #207
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Default Re: Cybernet PTBM121D4X problems

here we go
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Old 12th Dec 2018, 12:53 am   #208
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Default Re: Cybernet PTBM121D4X problems

That image looks the way I feel on a dark winter's morning.

I've marked the point from which to measure the resistance to the aerial socket centre and to 0V.

I've also marked a joint which needs tidying up a little as it seems to be standing up in a 'bead' - hard to tell for sure as the image is not too clear. Needs to be more like the 'volcano' shape of the other joints nearby.
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Old 12th Dec 2018, 1:34 am   #209
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Default Re: Cybernet PTBM121D4X problems

ok checked those to 0v ground tested infinity and to centre of antenna socket
it measured 00.3 ohms
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Old 12th Dec 2018, 1:36 am   #210
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Default Re: Cybernet PTBM121D4X problems

OK, that's another thing it is not.
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Old 12th Dec 2018, 1:57 am   #211
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Default Re: Cybernet PTBM121D4X problems

Possibly getting to the experimental stage now
i know you dont really agree with it and i think its a factory thing
how about the green wire connection, nothing ventured etc
as seen in the pic of another one
or this hygain v
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Old 12th Dec 2018, 2:07 am   #212
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Default Re: Cybernet PTBM121D4X problems

Sure, if you can reproduce it from those photos. My approach is always to assume that the radio worked exactly as configured before it went duff. Mods should only ever really be applied to an already working radio, but, I see no harm in trying it as long as you get it exactly the same as it is in the example and make it reversible in case it makes things worse.

When I clicked on the image just now it, along with all the smaller images in it, were rather small. The forum does that unfortunately, reduces the size of most uploaded images so they don't consume too much storage / bandwidth. Do you have a link to the original images? If I can see it properly I might be able to work out what it is supposed to do.
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Old 12th Dec 2018, 2:08 am   #213
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Default Re: Cybernet PTBM121D4X problems

i know where it connects to as there was one in the scrapper
there is another pic on that post now of a hygain v
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Old 12th Dec 2018, 2:31 am   #214
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Default Re: Cybernet PTBM121D4X problems

We're talking about the green wire in the Hy Gain V image? I'll look at the layout and correlate it to the components it connects to.

It could take me a while, probably won't get back about this now until tomorrow.
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Old 12th Dec 2018, 2:40 am   #215
John M1JWR
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Default Re: Cybernet PTBM121D4X problems

As you were
it did increase am carrier to 4w output up from the 2w before
though there was no FM output
ah well back to the drawing board
but at least it did show me that more AM output was there
i think the colt pics were from google images
radio is now back to as it was before

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Old 12th Dec 2018, 2:53 am   #216
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Default Re: Cybernet PTBM121D4X problems

You remember we talked about how the emitter voltage of the AM Darlington needs to be at about half supply when there is no modulation? Whatever you did there just made the Darlington emitter voltage rise, giving the output stage more voltage to run on in AM mode.

People have a bad habit of adjusting RV13 in order to maximise the output on AM (and on FM, if the set is not already a high power FM variant), but with the same result: Broken AM modulation.

Your AM output is lower than the FM output because on that variant of the chassis, it always should be: In FM mode the transmitter finals run on 13V, in AM mode they run on ~7V. When you eventually get the FM output up to what it should be, probably about 12-13W, the AM output will be about 7W. That would be normal.
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Old 12th Dec 2018, 3:19 am   #217
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Default Re: Cybernet PTBM121D4X problems

Right, your green wire connects the 'hot' end of RV13 to wire terminal 13, which is the unregulated supply voltage. (13V). On the Intek diagram at least, that is actually how it is supposed to be. At the moment I can't think why that wire would not be present on your particular chassis.

Still with the green wire removed:

With power on, AM -TX- mode, what voltages do you have on the three pins of RV13? What is the voltage on the emitter of the AM Darlington in AM -TX- mode now?
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Old 12th Dec 2018, 12:19 pm   #218
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Default Re: Cybernet PTBM121D4X problems

Have done voltages no's 1/2/3 are i think rv13, no 1 is where the green wire
would go and are as follows
no1 9.38v,no2 4.51v,no3 8.09v
and the darlington
no4 base 7.58v, no5 emitter 6.64v rising slowly
pic of area below.
did you ever watch the cobramans vids on youtube, he was as he said just
a hobbyist, though he knew his limitations and was quite entertaining, he died a few years ago.
there is a ten part saga where he tries to fix a concorde 2 with a low output problem.
he even replaced all of the tank area and it wasent the cure, unfortunetly he
must've died before he cured it. it will be under tribute to the cobraman
concorde 2 title.
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Old 12th Dec 2018, 1:28 pm   #219
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Default Re: Cybernet PTBM121D4X problems

I fouled up a little - I should have asked you to see if there were any signs that the green wire had once been fitted, or whether it looked as though it never had. Maybe you can remember?

If the circuit diagram is to be believed you should have +13V on one end of RV13. What position is RV13 set to (if fully left = 0 and fully right = 100?)

I need to look again at the diagrams of radios which do / do not have high power FM to see if there is a difference in that area.

I didn't see those videos you mention: Traditionally you were either in the Cybernet camp or the Uniden camp, so I admire the fact that he persevered for so long with a Cybernet despite - as 'Cobraman' - having an obvious affinity for Uniden radios.
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Old 12th Dec 2018, 11:35 pm   #220
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Default Re: Cybernet PTBM121D4X problems

Best wat to demonstrate position of rv13 is a pic
cant remember if there was any sign of a wire on those two points
possibly not, although it could have been de soldered tidily.
me, back in the day definately cybernet, cobra's were deaf !!
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