Cybernet PTBM121D4X problems

[SiriusHardware]

Your buzz test proves the Amp is OK from its input all the way through to the speaker. Supply volts to AN240 sound OK.

Near the input socket of the scope there will be a switch marked AC/DC/GND. Set that to AC, place the trace on the centre line of the screen with the 'Y adjust' control and try again to measure the size of the output signal on the AN240 pin 12. The trace will bounce off screen when you first touch the probe to the pin but hold the probe on there and then the trace will come back.

[John M1JWR]

did test with scope just now
set ac volts/div 5m sec/div .5
pic below

[SiriusHardware]

That seems rather tiny to me, will have to confirm by looking at the output of an AN240 on one of my radios, but in the meantime can you try looking at the size of the output on the same pin on the AN240 on your radio (10m?) which has a working receiver?

One possible approach now is to put those two radios side by side, powered up, same settings on each, and follow the output from the AN240 checking the same points on each radio until you get to where the signal at the same point on both radios is not the same.

[John M1JWR]

Got the lid off the 10m one now, as a little respite here is a pic showing
how much this radio is different
bit of an over engineered conversion that isent it
i will start with the in's and out's of this ones an240 first

[John M1JWR]

Couple of tests so far did the pin's on an240 hardly any difference between the two voltage wise
both had their speakers connected and mic's in vol up, diddent make a difference to 10m radio where the vol is same reading throughout.
two pics follow both same settings on scope had to readjust scope for 10m
radio to .2v/div as can be seen on other radio on same setting, nowt

[SiriusHardware]

The eprom mod is mainly to untangle the strange frequency jumps hard coded into the channel switch so that each upward click of the channel switch increases the frequency by 10Khz. The same principle was often used to add the UK channels in their correct sequence.

Your first pic is of beautiful (and plentiful) no-signal FM noise of the sort we have been looking for all this time on the 'bad' radio. I take it both images were taken from the AN240 pin 12 of their respective radios.

The simple conclusion is that the AN240 is probably faulty but that should only affect FM mode and not the others, so:-

Faulty radio, squelch always off, receiving aerial connected to set, RF Gain fully up or DX / LOC switch (if either fitted) to maximum / DX, modes AM / FM / USB / LSB in succession, use another set to transmit a modulated signal on the correct frequency and mode into a dummy load.

On the bad radio, in each of the modes, what do you see on the S-Meter?
-No signal?
-Weak signal?
-Full scale signal? (which is what we would expect)

Also for each of the modes, do you hear:-
-No received audio?
-Very quiet received audio?
-Received audio at normal / expected volume?

[John M1JWR]

have compleated tests and no s meter or modulation seen or heard in any mode
faulty radio was connected to a antron 99, 777 to dummy load.
ive even done this test with 777 connected to antron and faulty radio
connected to 80m di pole, bit more tx and rx oomph i think.
same results rx and s meter dead.
i always use 26.515 as when you tx on faulty radio as its better lower down.
i also tried on the next 3 channels on 777 just incase faulty radio rx's on wrong frequency, no cigar i'm afraid

[SiriusHardware]

OK - I'll have a think about where to go next tomorrow.

[John M1JWR]

Went to see an old mate of mine yesterday, he was a rig doctor back in the day
dosent bother these days, he's into cassette decks.
he said he had a similar problem with a cobra 148 back then and he thinks it was to do with a mixer, he has also seen crystal filters go down and lots of wieredness can happen if the mode switch is iffy.
all possibilities of course
i rarely see him these days and if he is not into the subject, conversation can
be difficult !!!

[SiriusHardware]

Certainly the mode switch feeds a veritable maze of diodes and a handful of transistor 'switches' intended to enable or disable various parts of the set as you go from one mode to the next.

I'll have another look at the diagram when I get home (at work just now).

[SiriusHardware]

You've mentioned a couple of times that you felt you had doubts about the mode switch. Maybe you think it doesn't feel right?

Here are the points at which to check that the mode switch is making contact correctly. The mode switch has six 'gangs' or sections altogether, so this is not a task for the faint hearted.

Power off:

With the mode switch in each of the positions stated, check there is a very low resistance between the wire terminal pins mentioned.

LSB mode:
(7) and (17) - (also in USB mode)
(5) and (19) - (also in USB mode)
(11) and (13) - (also in USB mode)
(40) and (6) - (also in USB mode)
(4) and (13) - (only in LSB mode)

USB Mode:
(7) and (17)
(5) and (19)
(11) and (13)
(40) and (6)

FM Mode:
(7) and (31)
(11) and (30) - (also in AM mode)
(6) and (39)
(1) and (28)

AM mode
(7) and (16)
(11) and (30)
(6) and (18)
(29) and (28)

If you get through all that, go back to

Power on, AM receive, squelch right off, no aerial, no microphone, and check the following receiver stage voltages:

Q20 e / b/ c
Q21 base
Q22 e / b / c
Q26 base
Q27 e / b / c
Q28 e / b / c
Q29 e / b / c

That should keep you going for a while!

[John M1JWR]

I have just compleated this monumental task !!
Lsb/usb 200ohm setting all tested with power on
7 and 17 lsb 04.2 usb 03.1
5 and 19 cant find no 5 so non starter
11 and 13 lsb 01.2 usb 01.2
40 and 6 lsb 00.3 usb 00.3
4 and 13 lsb 00.8
FM
7 and 31 00.5
11 and 30 fm 1. am 00.3
1 and 28 00.3
6 and 39 00.3
AM
7 and 16 00.0
6 and 18 00.3
29 and 28 00.3
transistors
q20 E 0.89 C 13.07 b 1.55
q21 0v
q22 E 0.83 C 12.80 B 1.50
q26 B 0v
q27 E 0.09 C 0.10 B 0.78
q28 E 0v C 0.86 B 0.11
q29 E 0.11 C 0.12 B 0.86
That should make some interesting reading !!
sick old girl isent she

[SiriusHardware]

Good work - I would be surprised if terminal (5) does not exist in some form because it carries the output of the AVR you repaired earlier. It should be somewhere on the track which joins the AVR (Q38) emitter, Q39 emitter and Q40 collector. You said all tested with power ON, I hope you meant with power OFF (always OFF for resistance measurements).

You don't appear to have any supply to the final stage of the AM/FM receiver, so could you now (with power on, AM receive mode) check the voltage on either side of R148 (47R) and if there is a large voltage difference from one side to the other, power off and check the resistance of R148 and also desolder and check the resistance of C147 and C148.

[John M1JWR]

ahh r148, is this not the one that the wire should be that goes to a leg of rv13,
that isent on this radio, which i will say is present on the 10m radio.
its not surely going to come down to somthing that somebody possibly has
removed (you never know) in the long distant past.
looking at the views we have quite an audience bye the way.
will check the area after work later, and maybe fit a wire there, see what happens, either some success or boom !
just had a quick check of r148 resistance is 47.3 ohms its got 13.65v on one end and 13.25 on the other
just found no 5 its between the two rx/tx switching transistors so 5 and 19 lsb is 0.06

[SiriusHardware]

I suggest you don't modify the radio in any way for now. Always assume that it worked exactly as it is now before it went faulty unless you find evidence to the contrary.

R148 just carries a supply voltage from 13.8V to the later stages of the AM/FM receiver, so is unlikely to be the component you are thinking of.

What you are describing (additional resistor acting on the base of the AM Darlington) sounds like a low power AM/FM TX switch mod. If you follow it on the 10m radio you will probably find it goes to a front panel switch re-designated as a high / low power switch (Tone Hi/Lo, which was otherwise pretty useless, was often used for this). At the moment your radio doesn't need this extra complication, just fix it as it is.

~13V on both sides of R148 and yet very little voltage on the collectors of Q27/Q28/Q29 indicates R148 and the capacitors are OK but there is a problem somewhere nearby if you really have such low voltages on the collectors of Q27 / Q28 / Q29. I'm beginning to wonder if this thing was hit by lightning.

Same again, power on, AM receive mode, can you check the voltages on both ends of R108 and R110 and R119?

[John M1JWR]

results are as follows
r108 0.12v/0.10v
r110 8.67v/0.86v
r119 0.12/0.12
lightning eh ! now i never thought of that
nothing looks burned anywhere

[SiriusHardware]

I was joking about the lightning. I think.

R110 reading is OK, the supply for that transistor comes from the regulated AM-receive-only supply.

Something seriously amiss with the voltage at the 'hot' ends of R108 / R119 though.

Q20, Q22, Q27, Q29 all receive their supply through R148. According to your earlier readings Q20 and Q22 do have their supply, yet it looks like the same supply is not reaching Q27 via R108 and Q29 via R119 even though it comes from the same circuit node / track.

Power off, mode switch on AM mode, lowest ohms setting on meter, keeping one meter probe on C148 positive, can you check that the resistances to all of the following other points are near-zero ohms.

One end of C147 (Not the end which goes to 0V)
One end of R148
One end of R119
One end of R108
One end of R92
One end of R89
One end of R85

[John M1JWR]

not easy this time, cant do it on solder side as i will never find them
the neg side of c147 is nearest to the case that goes round tank area
it seems, had to test sound to ground to find which leg, anyway only way i could do this
is to scrape a little off leg of cap with sharpish instrument so i could get some
form of contact it hopefully seemed to work and results as follows
r148 0.08
r108 1.
r92 0.09
r89 0.08
r85 0.07
r119 1.

[SiriusHardware]

Right, unfortunately we have two sets of incompatible results now. The thing which is interesting is that the two resistors you didn't have much voltage on are also the ones with the worst resistance readings between them and the other components - 1 ohm is quite high for a copper track running from one component to another. Even so, 1 ohm is not enough in itself to account for the huge voltage difference between R148 and R108/R119 but it may indicate that the path between them is hanging on by a thread and liable to go high resistance at any moment.

Sorry to have to ask, but can you check again the voltages on R148 (both sides) R108 (both sides) and R119 (both ends), but before doing that, work the mode switch back and forward a few times before parking it on AM RX mode again.

While you are at each resistor do the same check but with the mode switch in FM mode as well, so the results will look like:


R148
AM RX - V1 / V2
FM RX - V1 / V2

R108
AM RX - V1 / V2
FM RX - V1 / V2

R119
AM RX - V1 / V2
FM RX - V1 / V2

[John M1JWR]

results are now as follows
r148 am 13.65/13.23 fm 13.65/13.25
r108 am 0.12/0.10 fm 0.11/0.09
r119 am 0.12/0.12 fm 0.11/0.11
c148 is near by what would that do