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Old 8th Nov 2018, 11:59 am   #1
John M1JWR
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Default Cybernet PTBM121D4X problems

Hi
I have a CB radio that has voltage too high at base, and therefore emitter of voltage regulator transistor (2SD325). The spec is base 10.05v collector 13.60v emitter 9.45v. In this radio it is as follows, base 13.04v collector 13.61v emitter 12.38v, there is a mismatch of around 3v.

There are some other components on the base circuit of the transistor, a 390ohm resistor, 68uh coil, MZ310 diode, .047 ceramic capacitor and a 100/16 electrolytic capacitor. I have tested the resistor, and it only drops voltage from 13.61v to 13.04v. The circuit diagram is for a Cybernet PTBM121D4X board and is easily viewable on the net. The resistor is a replacement as the original did the same.

The question is, Is the resistor faulty as well, or is another component faulty. Any ideas appreciated.
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Old 8th Nov 2018, 1:13 pm   #2
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Default Re: Cybernet voltage regulator issue

The MZ310 is a 10V 0.5Watt-rated Zener Diode; it is this that sets the base voltage.

If it's gone open-circuit it will give the symptom of excess base-voltage.

I'd replace the MZ310 as the first step.
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Old 8th Nov 2018, 5:07 pm   #3
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Default Re: Cybernet voltage regulator issue

Silly question possibly, but did you check the 2SD325 itself?

The most likely failure mode in this type of transistor regulator circuit is the 2SD325 failing short-circuit / low resistance from collector-emitter and passing on the input (collector) voltage straight to the output (emitter). If the device is faulty to that extent, anything can turn up on the base. Try putting your finger on the MZ310 zener, is it getting warm? If so, it's not open-circuit.
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Old 9th Nov 2018, 12:31 am   #4
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Default Re: Cybernet voltage regulator issue

Hi. Thanks for the replies and advice. I have replaced the zener with a new one and problem is solved B 10.07 C 13.62 E 9.43 ch1 lo band AM RX. They are 10V ones with a slightly higher wattage that I got for another job a few years ago. I am not great in knowledge in this subject and a faulty old rig is the best way to learn I think. I had already checked the 2SD325 and was good.

This radio has some other issues (haven't they all)? No RX audio though it does TX all mode albeit it with very low power. Wet finger test says audio amp is good and is fine through the PA. I will check voltages around the radio places like the AM Darlington and the pins eg br bas etc. Somebody suggested through holes, which could be most likely, and an
RF sig gen is required, I dont have one of those.

Would this warrant another thread as slightly different subject or just carry on with this one? The radio is a Colt 320 FM black shadow SSB.

Regards, John.
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Old 9th Nov 2018, 11:27 am   #5
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Default Re: Cybernet voltage regulator issue

One down, more to go. For future reference, please always state which component numbers you have already checked when chasing down a particular fault.

I would suggest you just continue this thread, perhaps the mods could change the title to 'Cybernet PTBM121D4X problems'.

What do you want to have a go at next?
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Old 10th Nov 2018, 2:21 am   #6
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Default Re: Cybernet PTBM121D4X problems

Hi. Yes this old girl has a few issues.

For example I know there is nothing wrong with the Q36 Darlington itself but it has low voltage on its base and emitter, 7.10V on base and 6.60V on the emitter as it stands now. Before when the AVR was high the Darlington showed 10.02V on the base and 9.39V on the emitter. I do notice on various pics on the net that on the solder side of the 121 mainboard there is a green wire that goes from a pin on R148 and to to a leg of RV13 which will feed full 13.6 volts as that is the voltage on that leg of R148, this radio does not have that wire, on this radio the voltage is around 9.5V on that leg of RV13, there's a one to ponder, attaching a wire to those two points may cause other problems, I dont know. I have done lots of voltage checks around that radio, some are in spec some are low, but one thing at a time.

I am lucky that I have a scrap board that at one time was blown away in its "idiot diode" area and somebody messed up the pins and board area for the channel selector, think it was a Tristar 747, so some parts are in stock.

Regards.
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Old 10th Nov 2018, 11:47 am   #7
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Default Re: Cybernet PTBM121D4X problems

With regard to the wiring differences around the AM Darlington, there is a specific difference in the way that transistor is used to supply power to the TX final in some models compared to some others.

In AM mode on all models, the emitter supplies the power for the TX finals and is set so that the TX supply when unmodulated is something like a bit over half the supply voltage, maybe around 7-8V. When modulation is applied the emitter voltage then varies plus and minus from that set voltage, amplitude modulating the transmitter.

In FM mode on some models, the power for the transmit finals is taken from the same point (the emitter of the AM darlington) but in that mode the transistor is not modulated so the finals run on steady 7V-8V, which means that the FM output power is the same as the unmodulated AM power. Example: Concorde II.

In FM mode on other models using the same chassis the supply for the TX finals is switched away from the emitter of the AM darlington and taken directly to the main 13.8V supply instead, which means that the FM output power is much higher. Example: Hy Gain V.

This may account for the differences you see in that area between the two chassis.

On chassis using the first arrangement, a common mistake is to adjust the AM darlington emitter voltage (Via RV13, possibly?) for maximum output on FM mode, but then the modulation in AM mode no longer functions correctly.
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Old 10th Nov 2018, 4:20 pm   #8
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Default Re: Cybernet PTBM121D4X problems

Hi have checked that area just now using the manual for the 125 board as there is no spec for the 121 board.

On circuit diagram voltages on Darlington are as follows (spec) RX B 9.06V C 13.67V E 8.68V TX B 8.06V C 12.80V E 6.83V. I have adjusted RV13 to get as close as possible to 9.06V on the base. Looks like you adjust it by microns though, I got it as close as possible then let the radio warm up for a few minutes. As a result I get the following:-

AM RX B 9.07V C 13.66V E 8.63V and TX B 8.73V C 13.49V E 7.35V
FM RX B 9.07V C 13.66V E 8.63V and TX B 9.01V C 13.46V E 8.58V

I have noticed with the meter that when I first touch the emitter it initially starts much higher then drops down to voltages stated on both modes and settles there. Doesn't do that in TX. When you go back to RX it does that all over again, I don't know if that is normal.

I have a decent power supply, a PL330, it's set to 13.75V, the current draw on AM RX is 184mA and on TX is 562mA which rises slowly the longer the PTT is keyed. On FM RX is 194mA and TX is 625mA again rising slowly. When the AVR was in the failed state the draw was much higher something like 350ma in RX and 1.3 amps in TX can't remember now which mode.
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Old 10th Nov 2018, 11:00 pm   #9
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Default Re: Cybernet PTBM121D4X problems

The voltage on the Darlington emitter is only significant in AM TX mode. Broadly, the idea is that the emitter voltage, when the transistor is being modulated, needs to be able to sweep up and down from the set voltage by roughly the same amount plus and minus, so it needs to be about half way between 0V and the supply voltage, hence 6.83V. (My guestimate of 7-8V was a little bit high). It doesn't matter if the voltage is not precisely the same in FM TX mode because in that mode the voltage on the darlington emitter stays fixed and does not vary up and down.

As long as you can use RV13 to set the emitter voltage to 6.83V in AM TX mode, that part of the circuit is working as it should, so don't worry about what it does in any other mode, and don't worry about trying to set it to within a hundredth of a volt either.

I suggest you assume for the moment that the alignment of the RX coils is still factory set, in which case you will not need to do any re-alignment yourself. If they have been twiddled then the only really satisfactory way to align them properly is with a synthesised RF signal generator, so do not adjust them yourself without good reason.

Maybe you can have a go at the lack of receive audio now ? Is this on all modes? Are you getting indication of received signals on all modes, do you get the expected no-signal roar if you go to FM mode and turn the squelch down? I see you've already verified operation of the audio power amp IC by trying the radio in PA mode, good test.

Do you have an oscilloscope, do you have a frequency meter with a reasonably high resolution at 27MHz - ideally down to 10Hz or 1Hz?
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Old 10th Nov 2018, 11:26 pm   #10
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Default Re: Cybernet PTBM121D4X problems

Hi
i will readjust rv13 back to have am tx at as near 6.83v as possible
i have never touched rx coils, cant speak for the past of course.
an rf sig gen is on the shopping list.
rx audio is very low i have never heard an rx signal on the radio even when using another radio close by, i can hear low volume hiss of fm
and very feint on am, so i can just hear change from one to the other. nowt on ssb but i suppose thats to be expected.
i do have a scope my working knowledge is rather limited with it, its a tektronix 2225, and i have a black star meteor 600 which takes a while to warm up, i also have a pocket thandar one somewhere.
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Old 11th Nov 2018, 4:18 pm   #11
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Default Re: Cybernet PTBM121D4X problems

Just a thought about the reception noise, i have another radio a 777,
which works ok ,could some of this be to do with the squelch ?
if i turn the 777 on and put squelch all the way round to right on fm i can
still hear hiss all be it very low with volume turned up all the way, just like this radio.
On this radio when you operate squelch it has the very low volume then so
far round you hear the cybernet thump on squelch after that no hiss, volume
or anything, just blank/dead rx all mode
its like the squelch is on all the time then after thump it becomes super squelch
and blocks everything out
dont want to go o/t but just an observation
posts from me at the mo have a two or three hour delay before being uploaded
regards
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Old 11th Nov 2018, 9:09 pm   #12
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Default Re: Cybernet PTBM121D4X problems

Yes, that's an angle worth considering.

The fact that you aren't even getting loud no-signal 'roar' from the AN240 in FM mode points to something in that direction, possibly.

The squelch audio muting in virtually all Cybernet radios from that period is done by the very crude method of yanking one of the two differential inputs of the audio amp IC down to 0V, it causes a very large change in the voltage on the output of the IC and that is what causes the characteristic 'bok' through the speaker when a Cybernet squelch goes in and out.

So the question is, are you hearing that 'bok' (thump) at any point as you rotate the squelch up and down on the faulty radio?

You seem to be a meticulous voltage checker, so have you measured all of the voltages such as the RX-only supply voltage to the receiver section, the TX-only voltage to the early stages of the transmitter section, and so on?

I'm away from base at the moment so can't really consider this in detail just now. I may (possibly) have the exact diagram for the 320, as found in the original user manual - Cybernet were always very good like that. I'll have a delve through my box when I get home later.

Your posts (as a new forum member) will only be moderated for a while longer and then you will be able to post in 'real time'.
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Old 11th Nov 2018, 10:06 pm   #13
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Default Re: Cybernet PTBM121D4X problems

Hi
looking at the schematic would the rx only supply be around q20 or am i wrong or is it q27 to 29 for rx if thats the case not yet.
havent done tx either in that respect only base of 2166 and 1969 on tx and they are 0.76v and 0.36v respectively and look to be original transistors.
the radio gives the "thump" on squelch and can be heard no problem its after that it all goes totally silent as previous post.
i do have the 320 manual too but i cant see the diagram anymore lol
so i use the one from the cybernet workshop manual that covers all 121's
thats easily got over the net.
i did get a sig gen its on its way, its one of those digimess 100 things, i got it for a good price, i know they are not perfect but for a cb radio they should suffice.
diddent fancy a valve jobbie and us cumbrians are tight so we wouldent consider 500 quid things. and those lsg 16/17 ones are thin on the ground.
having a look at transistors in the squelch/noise blanker circuits at the minute.
the other radio i was on about the 777 was another rescue from death job
from a few years ago, its mint on the outside and somebody tried to make it
into 10m and made a mess,it needed an avr vco rx tx switching transistor and a feint cut trace between pll and ch selector that i bridged with a wire link,
the mode switch was broken too, had to replace that with a 6 position one
as no 3 wafer 5 position ones could be found had to draw it all out so as to leave pos 6 blank what a sod of a job, never finished it off as its low power on ssb, works great otherwise thought i would leave it for another day.
but thats for another time.
not a collector or anything just fancy doing this kind of thing from time to time and an old cb is the best way to learn because most of them are busted.
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Old 12th Nov 2018, 1:12 am   #14
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Default Re: Cybernet PTBM121D4X problems

I know what you mean about the diagrams. Time was, I could read the diagrams in the back of the user manuals (A5 size) by the light of a firefly. Nowadays I have to blow up the original A4 copies I made of those to A3 so I can see them.

If you can work through the various TX and RX supply voltages to verify that they are correct, these would appear to be:

"BT" / Q39 collector dropped via D44 + D52 = TX-only supply for all modes = 7.9V on the cathode of D52, can be measured on wire wrap terminal (6).

The mode-specific TX-only supply rails below are basically the above TX-only supply after it has been routed through the mode switch.

"BTA" - TX only supply in AM mode - Wire terminal (18? diagram not clear)
"BTS" - TX only supply in SSB modes - Wire terminal (40? diagram not clear)

Receive-only supply - not tagged with a label like the rest = 8.6V - comes from Q40 emitter and can be measured on wire wrap terminal (7).

The mode-specific RX-only supplies below are just the above RX-only supply after it has been routed through the mode switch.

"BRA" - RX only supply in AM mode - measure on terminal (16)
"BRS" - RX only supply in SSB modes - measure on terminal (17)

I'm suggesting you check these because you say you don't get any obvious reception of signals even on the S-Meter, so there is probably more wrong than just the squelch circuit, which may actually be OK.

The fact that you get the usual 'thud' from the squelch suggests that the DC part of the squelch circuit (VR2, R129, R10, D36, Q31, Q32, Q33, C157) is probably working.

You can verify this by (in FM receive mode) monitoring the voltage on the audio amp IC output, pin (10). With the squelch fully up that should be at about half supply volts, but if you then turn the squelch smoothly downwards you should see a dramatic change in the pin (10) voltage at the point where you hear the 'thud'. If you do, the DC / switching part of the squelch control is working.

You can also temporarily manually disable the squelch audio mute to see if that allows the RX audio to come up: Remove either Q33 or R132 if you want to try that.
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Old 12th Nov 2018, 1:53 am   #15
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Default Re: Cybernet PTBM121D4X problems

Hi
interesting stuff, diddent know about a lot of this, will go over it over the next few days, will be back in touch when i am done.
work tomorrow i do shifts 1pm to 9pm, fork lift, haulage company, 50ft double deckers.
regards john
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Old 12th Nov 2018, 2:25 am   #16
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Default Re: Cybernet PTBM121D4X problems

Quote:
Originally Posted by SiriusHardware View Post
With the squelch fully up that should be at about half supply volts, but if you then turn the squelch smoothly downwards you should see a dramatic change in the pin (10) voltage at the point where you hear the 'thud'. If you do, the DC / switching part of the squelch control is working.
I knew I shouldn't try to type this stuff so late at night: The above statement is backwards.

Squelch at minimum, output of audio amp should be at half supply volts.

Winding squelch up, the amp output voltage should change state as the squelch passes through the threshold - I honestly can't remember whether the output voltage rockets up (to nearly the supply voltage) or down (to nearly 0V) when the squelch is 'on'.
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Old 13th Nov 2018, 12:53 pm   #17
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Default Re: Cybernet PTBM121D4X problems

Hi
Did a wee bit last night and found the following
Rx
Pin 7 8.67v am,fm 8.50v,usb 8.12v
BRA 8.67v
BRS 8.15v
TX
BT Pin 7 42v am,fm 7.44v,usb 7.37v
BTA 7.49V
BTS 7.41V
Squelch
6.78v in low noise area until thud at that point it changes
to 10.15v thereafter, i took it on pin nearest to chassis on chip.
also checked the squelch pin itself 9.48v on minium and decreases
as you turn squelch to max then voltage is 2.83v
may remove resistor 132 tonight after work
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Old 13th Nov 2018, 2:01 pm   #18
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Default Re: Cybernet PTBM121D4X problems

I have a feeling that won't be necessary because you seem to have proved that the squelch action is working. No harm in trying though.

The various +Ve supply rails seem close enough don't they? (the voltages are not critical, it's more a case of checking whether they are there when they should be, and not there when they should not be).

How confident are you about using your oscilloscope to follow audio signals? If not very, we can try using the audio PA as a signal tracer by linking its input back to places where we expect to be able to find sound, especially the audio output from the AN240 FM discriminator.

Last edited by SiriusHardware; 13th Nov 2018 at 2:09 pm.
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Old 13th Nov 2018, 11:11 pm   #19
John M1JWR
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Default Re: Cybernet PTBM121D4X problems

Hi
got to learn sometime !!
i take it that the scope earth crocodile clip will be grounded to earth pin in radio and using the probe touching various points in radio and looking at screen for a signal pattern. (signwave ?)
then when you get to a component that is not showing on screen and it should be that could be the fault.
could be at weekend when i get to that, we will see !
it looks a stright forward scope as i have had an audio generator into it before
the screen showed the curved up and down pattern that i would expect.
it showed me that the scope and the audio gen were both working.
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Old 13th Nov 2018, 11:57 pm   #20
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Default Re: Cybernet PTBM121D4X problems

Yes, that's more or less it.

Be aware though that on radios like this the chassis metalwork is often DC-isolated from the 0V / ground plane of the PCB itself, so to get a satisfactory earth for the scope you need to connect the earth side of the scope probe or the scope's front panel earth terminal to true circuit 0V, which can usually be found on the metal can of any of the shielded coils on the PCB.

Before we go circuit tracing though, can you just try:
-In FM receive mode
-Squelch fully off
-Volume down to 30% to start with
-Make a temporary link (with a crocodile lead or similar) between wire terminal pin (42) on the PCB and any substantial 0V point (not the metalwork / frame).

If you do that, do you get audio out of the speaker?
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