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Old 28th Nov 2018, 10:06 am   #101
SiriusHardware
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Default Re: Cybernet PTBM121D4X problems

C148 smooths out any low - frequency bumps/variations in the supply after it comes through R148, C147 does the same but for higher frequency variations. If either of these were short-circuit or low resistance you would see a very low voltage on one end of R148 and the resistor itself would be getting very hot, so I think we can rule those out.

Although your resistance measurements suggest otherwise there seems to be a missing or high resistance connection between R148 and R108/R119.

Have you got a link to the exact diagram for that radio? I can only go on the diagrams I have, which show one end of R148 / R108 / R119 directly connected together. If they are, then the voltage differences you have between them are impossible.
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Old 28th Nov 2018, 12:23 pm   #102
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Default Re: Cybernet PTBM121D4X problems

all i have is for intek 1200fm out of export manual which is the one i think
you will be looking at, and one which is for tristar 747, which i believe it and the 320 are very similar, i cant upload it as its too big.
it was found as google tristar 747 schematic thats all
unless somebody has put something in the area that i know nowt about, for
example there is a resistor near c147 area on solder side that the scrap radio dosent have ? however when you test the two resistors 108 and 119 they are quite a distance away from r148, that kind of thing.
can take pics of that area on both when i get back from work
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Old 28th Nov 2018, 2:18 pm   #103
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Default Re: Cybernet PTBM121D4X problems

Yes, I'm working from the Intek diagram as well.

I'll try to find a specific diagram, or a selection to look at. I do have an original Concorde II diagram but that is quite a high end 121 variant, really we need a diagram for one of the simpler radios using that chassis (747, as you suggest).

There is one thing we can try, which is to link the two points which the diagram says should be linked and see if the receiver then bursts into life, and if it does, try to work out where the original connection has gone missing.

I just want to have a look around for a few specimen 121 diagrams first to see if there are any variants where those components are not directly connected together, and if so, under what circumstances that may be.
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Old 28th Nov 2018, 2:43 pm   #104
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Default Re: Cybernet PTBM121D4X problems

Managed to find a 747 schematic at Elektrotanya, very poor resolution but just good enough to see that the arrangement of that rail to the left of R148 is the same. One end of R108, R119 should have about 13V on them, not the very low voltages you are seeing.

Will get back to you on this tonight and we will try bridging what may be an open circuit or high resistance on your PCB. In the meantime any pictures you can rustle up will be helpful.
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Old 28th Nov 2018, 3:01 pm   #105
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Default Re: Cybernet PTBM121D4X problems

Something's just dawned on me looking back through some of your previous results.

When you wrote this,

Quote:
r148 0.08
r108 1.
r92 0.09
r89 0.08
r85 0.07
r119 1.
... At the time, I took it that by '1.' you meant one ohm.

Now, I think you meant that the meter was reading 'infinite', the reading it shows when there is nothing connected between the probes. Is that right? If so, all the readings do make sense and point to a missing connection between R148 and R108/R119.
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Old 28th Nov 2018, 6:13 pm   #106
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Default Re: Cybernet PTBM121D4X problems

This image (attached) shows what / where the problem appears to be, according to your measurements so far. I have rearranged the order in which the resistors depart from the rail on the actual diagram to make things a little clearer.

The 13V (approx) supply derived through R148 and across C147/C148 fans out to the receiver through five resistors, some of which obviously are getting the 13V supply, but R108/R119 are not.

This suggests that the connection outlined in red on the attached diagram is missing.
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Old 28th Nov 2018, 8:02 pm   #107
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Default Re: Cybernet PTBM121D4X problems

A bit more info. I have here two (original) paper versions of the Cybernet 'Export' service manual produced by Lou Franklin. The early version with a plain black / white cover is reproduced here in excellent resolution.

http://www.zen96216.zen.co.uk/cb/cybernet-esm.htm

The other version I have is the later edition one with the Greenish-turquoise cover and there is one really significant addition in that one, it has the PCB / track layout for the PTBM121D4X in it.

Looking at that, I can see that R148 / C147 / C148 / R85/ R89 / R92 (All the components which your measurements prove are connected together) are all grouped on a long track which runs alongside the receiver transformers T10, T11, T12.

R108 and R119 (which don't appear to be connected to the other components, but should be) are in a very different area and the connection to them first passes through a through-hole connection between the pads of R92, then, on the top (component) side of the board, travels along a winding track which goes right to the back edge of the board and travels along near the edge of most of the back of the board before turning back inwards where it goes back to the other side of the board via a through-hole between Q36 and Q28. This top side track is literally the red line in the diagram attached to my previous post.

I suspect that either that upper track is damaged somewhere along its length or one of the through-holes at the start and finish of the upper track is not making a connection.
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Old 28th Nov 2018, 11:06 pm   #108
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Default Re: Cybernet PTBM121D4X problems

yes 1. was no connection infinite
2 pics below the clear one is of this radio and the not so good one is of the
scrap radio
should i have a go at connecting a tempory between those two resistors
might take awhile as ive never been good at judging from one side to the other
i take it the thru holes are those with no component how do you deal with those if it comes to that
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Old 28th Nov 2018, 11:30 pm   #109
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Default Re: Cybernet PTBM121D4X problems

That resistor looks original, added on, yes, but I would say it was done when the radio was built, so I would leave it where it is.

The through-holes on these boards are 'blind', that is, you can't see through them. With an 'open' through-hole you can fix them by feeding a snug-fitting bit of wire through the hole and soldering it to the track on both sides of the PCB, but not with these.

For now, I suggest you bridge the gap on the track side of the PCB using some flexible insulated wire with a couple of mm bared and tinned at each end and see if that brings it a bit more to life.

If you can get me an equally decent picture of the track side with the AM Darlington's (Q36's) pins in the centre of the image I will annotate both images with where to solder each end of the bypass wire. Try to get it as sharp as your first image, the one with the resistor.

My feeling is that the top side track is most likely to be damaged where it runs very near the PCB edge at the back - there may be a crack running in from the edge of the board which has severed the track, and possibly one or two others in its path. I would look especially at the area where it runs briefly inwards in a loop, around a chassis support tab. If the PCB is cracked anywhere, next to one of those mounting points is a likely spot.
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Old 28th Nov 2018, 11:37 pm   #110
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Default Re: Cybernet PTBM121D4X problems

darlington area
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Old 28th Nov 2018, 11:41 pm   #111
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Default Re: Cybernet PTBM121D4X problems

This annotated version of your first photo shows the through-hole which leads to a track on the other side of the PCB (circled in yellow), and where to solder your bypass wire to at this end (circled in white).
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Old 29th Nov 2018, 12:01 am   #112
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Default Re: Cybernet PTBM121D4X problems

..And this is where to attach the wire to at the other end. I notice the soldering on the middle pin of Q27 (also marked on this image) is pretty poor as well, touch that up while you are in the area.
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Old 29th Nov 2018, 12:01 am   #113
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Default Re: Cybernet PTBM121D4X problems

all done on that one
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Old 29th Nov 2018, 12:10 am   #114
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Default Re: Cybernet PTBM121D4X problems

we have lots of sound i will check rx from another radio well done !
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Old 29th Nov 2018, 12:11 am   #115
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Default Re: Cybernet PTBM121D4X problems

Ok, well, FM receive mode, volume 10 o'clock, squelch off, speaker connected, microphone plugged in, PTT not pressed, turn it on.

Don't be too disheartened if it still doesn't make much noise (although it would be nice if it did). We may still have some way to go.

Edit: Cross posted. Glad it's doing something now anyway.
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Old 29th Nov 2018, 12:23 am   #116
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Default Re: Cybernet PTBM121D4X problems

Did a quick rx check with 777, 777 isent the most powerful thing on ssb as it was never finished set up wise 3 years ago.
reception on fm ok and very loud on am, nothing heard on ssb, no s meter movement
i realise that this radio may have been dormant for 25+ years and there is still a way to go, but now a foundation to work on, exellent
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Old 29th Nov 2018, 12:33 am   #117
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Default Re: Cybernet PTBM121D4X problems

Ok, We'll pick up again tomorrow.

In the meantime, can you try to find out what's happened to the top side track, the one we bypassed with the wire? Those two through-holes I indicated, turn the board over and look at them on the other side. There is an incredibly long, winding track running between one through-hole and the other through-hole on the component side. Give that a really good visual examination every millimetre of the way. There is definitely a problem with it somewhere.

It's always best to find the real cause of a problem in case it is also going to be the cause of other problems not yet found.
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Old 29th Nov 2018, 2:13 am   #118
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Default Re: Cybernet PTBM121D4X problems

have checked the track best i can, in the pic below is all i could find as suspect,
some parts are covered by components.
you will see what appears to be a hairline crack comong down from bracket through the line of the two resistors through the track and the next one below.
had to balance mobile phone under the magnifing lamp, best i could get,
no sign on opposite side.
one to sleep on !!
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Old 29th Nov 2018, 10:03 am   #119
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Default Re: Cybernet PTBM121D4X problems

Very good spot, that is 99% likely to be the cause of the original open-circuit track, which is the one nearest to the edge of the PCB in your image.

There is a danger that it has also cut across the next top side track further in, I'll work out a way for you to check that one.

I am surprised the crack is not also visible on the green side of the PCB, you need to be very sure about that, and patch any possibly affected tracks.
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Old 29th Nov 2018, 8:12 pm   #120
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Default Re: Cybernet PTBM121D4X problems

This is the relevant section of the overlay, looking down on it from the component side. The dark grey tracks are the top-side tracks, of which the uppermost one is the definitely damaged one. I have shown the approximate line of the crack - it may or may not go in as far as I have drawn it. The pale grey areas are the tracks on the underside, seen as though looking right through the PCB.

There are at least two further top side tracks which may be affected.

On the underside, if that is also cracked, the beginning of the crack goes through a wide band of earth / 0V plane which hoops around the chassis tab. That would have to be fixed as well.

We'll start by seeing if any more of the upper side tracks are cut through by the crack. With power off, meter on lowest ohms range,

-measure between R123 (end with the through-hole next to it) and the wiper / middle pin of preset RV7. Should be very low resistance.

-measure between D48 - the end with a line around it - and R117, the end furthest away from the board edge. Should be very low resistance.

-measure between C139 - the end furthest from R120, and wire terminal 21. ...should be very low resistance.

You should be able to get all of these from the component side, hopefully.
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Last edited by SiriusHardware; 29th Nov 2018 at 8:26 pm.
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