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Old 10th Dec 2018, 2:47 am   #181
John M1JWR
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Default Re: Cybernet PTBM121D4X problems

Gentlemen, interesting one of you prefers the 059 the other the 121,
i will throw a spanner in the works based on the little in relation to how much
i know compaired to both of you.
i have worked on the 125 board i think its better, well better made anyway !!
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Old 10th Dec 2018, 2:55 am   #182
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Default Re: Cybernet PTBM121D4X problems

Not a spanner, as I actually really like the 125 board. I know Sirius has mentioned on more than one occasion that the receiver isn't very selective when other strong stations are close by, which I think is true. I like the 059 and the 125 board, but not a big lover of the 121, probably due to having to try to decipher what was being said by someone (on many occasions - Tristar 747) operating one on USB back in the day!
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Old 10th Dec 2018, 3:00 am   #183
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Default Re: Cybernet PTBM121D4X problems

and this radio is a tristar 747 in drag !!
far better looking though
but yes better rx than the 777
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Old 10th Dec 2018, 3:03 am   #184
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Default Re: Cybernet PTBM121D4X problems

Traditionally the 'turn on' voltage quoted for a silicon transistor is about ~0.6V, rather than 0.7V, but in reality probably somewhere in between.

The only way to find out if the transistor really is / is not turned on by 0.64V in this case is, unfortunately, to measure the emitter or collector current. We'll have to wait for John to get another fuse. What size (Physical length) / value (Milliamps or Amps) is it? We have a wide range of them here at work.
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Old 10th Dec 2018, 3:09 am   #185
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Default Re: Cybernet PTBM121D4X problems

i think its 500ma not sure
there are some fuses like that in the box of bits that came with the 706
when it was new, i will look tomorrow, fingers crossed
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Old 10th Dec 2018, 3:10 am   #186
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Default Re: Cybernet PTBM121D4X problems

I suspect it may be a short 200mA type. Better not tell him to bodge the fuse and just be careful

Edit: You'll get no second chances if you do bodge the fuse!
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Old 10th Dec 2018, 3:12 am   #187
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Default Re: Cybernet PTBM121D4X problems

OK, could send you one if you have difficulty finding one. Would need to know the length in mm, the amp or milliamp rating and whether it has a letter 'F' or 'T' in front of the current value.
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Old 10th Dec 2018, 3:22 am   #188
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Default Re: Cybernet PTBM121D4X problems

Shame about blowing the protection fuse in the meter.

I've found postage is quite slow at the moment, probably due to Christmas.

It's worth feeling the temperature of the transistors or their heatsinks on key up, just in case - even with this low output.

I seem to think that mention was made some pages back about the heatsinks feeling too hot - I suppose it's not oscillating at some frequency that the watt meter isn't registering?
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Old 10th Dec 2018, 3:26 am   #189
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Default Re: Cybernet PTBM121D4X problems

f500ma al250v 20mm
just scored a pack of five on fleabay 2 quid inc 1st class post
meanwhile i will still look in icom box tomorrow
thanks for the offer
bed time, dream about fuses tonight !!
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Old 10th Dec 2018, 3:30 am   #190
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Default Re: Cybernet PTBM121D4X problems

I've been buying little bits and pieces from the above mentioned in the last week or so and even first class postage is taking several days, so hopefully you'll find one among your bits.
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Old 10th Dec 2018, 3:34 am   #191
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Default Re: Cybernet PTBM121D4X problems

What are the thoughts on this? Somehow, and with the evidence so far, I don't think that this is going to be an RF output stage fault.
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Old 10th Dec 2018, 10:25 am   #192
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Default Re: Cybernet PTBM121D4X problems

It's a shame the 10m Colt isn't set up for maximum output because then the approach could have been to do a stage by stage comparison of the RF levels through the transmitter.

I can remember in one single instance - decades ago - one of the ceramic capacitors near the output going low resistance - possibly C57 / C58, I don't think it was C59, but we may as well check them all, so, John, can you...

...Desolder one end of C56, C57, C58, C59 and check the resistance of each of those capacitors please. You may see a brief flash of low resistance as you connect the probes but then the reading on each one should be very high / infinite. Suggest you check on a 'K' range rather than low ohms or continuity, as any measurable leakage at all on this type of capacitor would be a fault.

(C59 may be soldered on the back of the SO239 socket).
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Old 10th Dec 2018, 12:13 pm   #193
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Default Re: Cybernet PTBM121D4X problems

Unfortunetly icom box was not forthcoming, yes glass type fuses, wrong size.
age related memory problems now !
not to worry other areas can be explored meantime.
job on this morning, will tackle those after work.
c56 and 57 is beside L12, c58 and 59 are beside L14, all inside the
screened tank area's, i seem to remember L14 did not adjust anything
at the time.
techmans thought of " I don't think that this is going to be an RF output stage fault."
could end up in a hot debate, over my head though !
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Old 10th Dec 2018, 12:34 pm   #194
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Default Re: Cybernet PTBM121D4X problems

No, we're both just interested in getting to the real cause of the problem, especially as it may fix his rogue set as well. I welcome alternative suggestions for checks on other areas as well, it may get us there quicker.

Rule #3: Any component which is excessively difficult to get to is exponentially more likely to be the cause of the fault. (Okay, I did just make that one up).

Access to those capacitors will obviously be easier from the underside if you can work out where the pads are and just desolder one pin from its surrounding pad. I definitely had one instance of failure of one of those capacitors in the distant past.

If you have difficulty identifying the capacitor positions from below, take a shot of the green side of that area wide enough to include the whole of the TX output strip and I will mark the positions of the relevant capacitor pads. We could do this faster if I had a radio with a 121 chassis myself but unfortunately I don't.
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Old 10th Dec 2018, 1:16 pm   #195
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Default Re: Cybernet PTBM121D4X problems

One thing I have overlooked previously is the role of transistors Q14 / 15 and their associated coil / transformer T7. Try peaking T7 on FM TX on the middle channel, middle band. As well as going to the SSB I.F section, an output from that coil also goes from the secondary (via C94 / R72) to one of the inputs on the second TA7310 (IC3).

In AM / FM mode, part of the output from OSC 3 comes through C76, through D17 (which is forward biased via R170 from BTA (AM/FM TX only regulated supply)) and then through Q15 to T7. In SSB (RX and TX) Q18 is turned on to kill this feed to Q15.
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Old 10th Dec 2018, 2:17 pm   #196
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Default Re: Cybernet PTBM121D4X problems

Yes, the ALC circuitry can certainly be responsible for lower than normal output. I'm also wondering whether RV12 may have been wound down in a previous attempt to cure 'gruffness' on SSB. It's the fact that it's not just general low RF output, but that SSB is even lower than the other modes, although John did say that his 'whistle' wasn't good.
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Old 10th Dec 2018, 3:10 pm   #197
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Default Re: Cybernet PTBM121D4X problems

I think the output from Q15 via T7 is the AM/FM TX 10.695Mhz feed to IC3. The other frequency coming in to IC3 is from the VCO. IC3 mixes those two together, subtracting 10.695Mhz from the VCO frequency to produce 27Mhz, which then heads out through the TX stages.

If either of those feeds to IC3 is lower than it should be, then the 27Mhz output to the TX stages may be correspondingly low as well. That's what I am hoping anyway. John has already retuned the VCO output coils, but not T7 as far as I know. T7 is also involved in both SSB TX and RX, via Q14 rather than Q15.

IC3 does also handle the RF ALC and one future line of attack is to try disabling the ALC to see if that is what is throttling the output, but, one thing at a time I suppose.

Last edited by SiriusHardware; 10th Dec 2018 at 3:16 pm.
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Old 10th Dec 2018, 8:43 pm   #198
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Default Re: Cybernet PTBM121D4X problems

While we wait to see if John has any luck with T7 and then those capacitors in the end of the output stage, let's take a look at the bones of the RF ALC circuit (sketch attached).

C63 siphons off some of the RF output and a portion of that, determined by the setting of RV4, is rectified to form a negative DC voltage at point 'X'. The more RF, the larger (more negative) the DC voltage generated there.

D11, which is a 5V1 zener, doesn't begin to conduct until the RF signal is so large / voltage at 'X' is so far negative that there is a voltage difference of 5V1 between the voltage at 'X' and the junction of R33/C44. Whenever that does happen, D11 turns on and causes a dip in the voltage on C44 and, through R33, the ALC control input. In SSB TX mode, RF peaks cause a dip in the voltage on the ALC control input of IC3. (Pin 7).

In AM/FM TX mode, although the combination of RF / C63 / RV4 and D10 is still generating minus voltage it is partly cancelled out by the positive DC bias being applied to point 'X' through R47 in those modes, so the voltage across D11 probably never reaches the 5V1 threshold necessary to turn it on, and it looks like there is essentially no RF ALC action in AM or FM TX mode.

We can turn off the ALC in three different ways:

- Select FM TX mode
- Select AM TX mode
- Turn RV4 wiper fully towards 0V, and then the RF ALC is disabled in SSB TX as well.

John already measured the static voltages on IC3 including pin 7, and found them to be OK but they may bear closer investigation in a wider range of modes.
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Last edited by SiriusHardware; 10th Dec 2018 at 8:57 pm.
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Old 11th Dec 2018, 12:13 am   #199
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Default Re: Cybernet PTBM121D4X problems

Firstly had a little adjustment of T7, only a very slight peak from that
have de soldered the pads on one end's of those 4 caps, if i have it right
each one tested infinity, on the pic, A to make sure i have the right ones and
B you will see a red mark that i drew on pic to show that those two pads are
joined together
i did all the voltages on ic3 and they look to be in spec will post results if needed
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Old 11th Dec 2018, 2:20 am   #200
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Default Re: Cybernet PTBM121D4X problems

Cap i de soldered bottom left of pic was c210, i then desoldered leg of correct
cap just to left of it c58
the moral of the story is they all checked infinity including c210
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