18th Nov 2018, 12:54 am | #41 |
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Re: Cybernet PTBM121D4X problems
Dont know whats happened here
now no audio whatsoever checked audio ic had already done so with 10m radio earlier 10m radio this radio 1 13.61v 1 13.66v 2 12.39v 2 12.62v 3 3.96v 3 3.67v 4 8.09v 4 11.39v 5 0v 5 0.28v 6 0v 6 3.05v 7 3.35v 7 1.45v 8 1.33v 8 0.04v 9 0v 9 0v 10 6.75v 10 10.12v you can hear a clunk from speaker when you switch on thats it has whatever is wrong with it decided to give up completely |
18th Nov 2018, 12:06 pm | #42 |
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Re: Cybernet PTBM121D4X problems
And that's with the squelch turned OFF in both cases?
Pin 7 is the pin which gets pulled down towards 0V to mute the audio amplifier when the squelch circuit is turned ON. It should normally sit at about 3.3 to 3.4 volts when the squelch is OFF and there is no audio (ie, volume at minimum). The fact that you have 1.45V on pin 7 on the 'bad' radio suggests that the squelch is holding the amplifier in the muted condition, as it normally only would when the squelch is turned up past the threshold. To check whether this is the case, temporarily desolder one end of R132. That will un-mute the audio PA, if it is being held in mute. If that works then you can start tracing back to whatever is making the squelch behave that way. |
18th Nov 2018, 12:25 pm | #43 |
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Re: Cybernet PTBM121D4X problems
Hi
will do that a bit later, have to go out for awhile have taken audio chip out of scrap radio - just incase and a 2sc945 out of it aswell as that q31 is looking iffy so for now, just incase both are still available if it ever comes to that and 2sc945's are very cheap i am aware its possibily the unexpected its always the unexpected !!!! my soldering is quite good these days have an 18w to take things out and a 12w to put things in as it has a small tip and makes a tidy job and good solder braid |
18th Nov 2018, 6:27 pm | #44 |
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Re: Cybernet PTBM121D4X problems
Have now removed 1 leg of r132, back to original low volume as before, maybe a smidge better but only that
voltages on audio amp are now as follows 1 13.65 2 12.42 3 3.99 4 8.11 5 1.55 6 3.36 7 3.38 8 1.33 9 0 10 6.77 not the best solder pad on that leg of resistor still ok i take it to go back through those 3 transistors etc , i think q31 is iffy for a start, wrong voltages on it. regards |
18th Nov 2018, 8:00 pm | #45 |
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Re: Cybernet PTBM121D4X problems
Ok have tested some of the circuit in squelch, r132 is still disconnected.
with power off r132,r146,r129,r130,r147,r145,r128,d36 all check out ok r131 however tests at 1.57k supposed to be 22k switch radio on for transistors q31 is same off reading as previous post q32 is correct q33 has changed to 0v on or off, no doubt as r132 is disconnected |
19th Nov 2018, 12:17 am | #46 |
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Re: Cybernet PTBM121D4X problems
OK, you can reconnect R132.
New Rule #2: If you get a suspicious resistance reading when measuring the resistance of something in-circuit, always desolder one end of it from the circuit and re-measure just the component itself. And, while you have the component disconnected, re-measure the resistance across the pads it normally occupies. As a triple back up measure, possible only because you have a complete working chassis to compare with, measure the in-circuit resistance of R131 in the 10m radio. Note: You need to have the +/- probes of the meter the same way around that you did when you were measuring the in-circuit resistance of R131 in the 'bad' radio. If D36 is a glass bodied diode carefully desolder it and check it with the 'diode test' feature of your meter. You might find that it falls apart as soon as you desolder it, as that is a surprisingly common problem with those diodes. If it's OK, FM Receive mode, power on - Can you now check the voltage maximum and minimum on wire terminal (24) when you wind the squelch up and down, and if it is varying up and down at that point, then please measure all of the following voltages in FM receive mode: Squelch fully anti clockwise: Q31 collector Q31 base Q31 emitter Q32 collector Q33 collector And the same points again, with the squelch fully clockwise. I realise you went over this area before but the squelch was working then, at least to the point where it went 'thud' as it crossed through the threshold. At the moment we are just trying to get back to that point. I don't normally advise the 'shotgun' approach of just changing likely components to see if that fixes the fault because that usually involves spending money on ordering a part, waiting for it, and putting it in only to find that it did not fix the fault after all. In your case you seem to have at least one complete set of replacement parts so if you have a hunch about a particular component, in this case Q31, by all means just change it to either prove it or rule it out. |
19th Nov 2018, 12:54 am | #47 |
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Re: Cybernet PTBM121D4X problems
r131 definetly different
hard for me to explain but here goes on 10m radio shows a reading of 0.21 on 2m scale on cb radio it shows 0.01 on 2m scale and/or 1.57 on 20k scale havent done diode as yet, if faulty i think its an ma150, quite a few on the scrap board |
19th Nov 2018, 1:13 am | #48 |
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Re: Cybernet PTBM121D4X problems
Try not to alternate between scales on meters and scopes when you move from one set to the next. If you are expecting to see a reading of 22K then you need to use the next highest resistance scale above that value.
Remember, always use the probes the same way around when comparing in-circuit resistances in the same place on different sets. If there is a diode, transistor or chip connected across the resistor somewhere nearby, the resistance measured in one direction can be completely different to the resistance measured with the probes the other way around. This is another reason why it's advisable to isolate any component you are trying to measure the resistance of. Your abnormally low resistance across R131 in-circuit may not be the resistor at all, but something in the surrounding circuit. The easiest way to tell is to measure the resistance of the resistor on its own, and also measure the resistance across its pads while it is disconnected. It is highly unusual for low-wattage resistors to go low-resistance, their normal failure mode is to go high resistance or open circuit. |
19th Nov 2018, 1:44 am | #49 |
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Re: Cybernet PTBM121D4X problems
I've just had a look around for details about your meter - seems to have a bit of a giant leap in resistance ranges from 20K to 2M, which means that if you want to check a 22K resistor out of circuit you will have to use the 2M range.
I would have expected there to be a 200K range in between those two ranges on your meter, but that's not how it seems to be. |
19th Nov 2018, 3:23 am | #50 |
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Re: Cybernet PTBM121D4X problems
Hi
Took diode out and checked it, diode is good Pin 24 with squelch off 9.45v and squelch full on 2.83v q31 squelch off c 0.73v e 2.34v b 1.64v Squelch on full c 0v e 1.31v b 1.39v q32 sq off c 0v sq on 0v q33 sq off c 0v sq on 0v had to look at diagram for where base is etc, look at drawing see where the pins are to work out that the emitter's are nearest the chassis, diddent know that before 73 |
19th Nov 2018, 11:48 am | #51 |
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Re: Cybernet PTBM121D4X problems
forgot to say removed resistor and it checked good, forgot the pads though
as it was late when i checked again after refitting it was back to low resistance, did a brief recheck of pin 7 on audio chip and its back to 1.45v. |
19th Nov 2018, 5:37 pm | #52 |
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Re: Cybernet PTBM121D4X problems
Your readings suggest Q31 is probably working as you have either 0V or 0.73V on Q31 collector, which should be turning off / turning on Q32. However, Q32 collector is staying low which suggests:-
No supply voltage to top end of R131, or Q32 short circuit from collector to emitter, or C157 short-circuit or low resistance So, some more quick checks. Power on, FM receive mode, squelch off, what voltage do you see at the top end of R131 - not the end which goes to Q32, the other end? It should be somewhere near to the AVR voltage (9.4V, or not much less). If it is a lot less, check the AVR output voltage is still OK and if it is, check R160 (330R) near the AVR. If all of the above are OK: Power off, Desolder one end of C157, check resistance of C157 and while it is disconnected also check the resistance of Q32 from collector to emitter. If necessary desolder Q32 collector from its surrounding solder pad in order to measure only Q32 c-e, or take the transistor completely out and check it if you are happy to do that. You are looking for a possible short or low resistance from Q32 collector to Q32 emitter. |
19th Nov 2018, 7:31 pm | #53 |
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Re: Cybernet PTBM121D4X problems
By the way, it is theoretically impossible for the collectors of both Q32 and Q33 to be low, because as wired, the collector of Q33 should always be in the opposite state to the collector of Q32.
Has the connection between the collector of Q32 and the base of Q33 / R131 / C152 been lost somehow, damaged track, etc? This very high res scan of the 121 chassis diagram has a set of transistor outlines down in the bottom left hand corner if you are having problems identifying the c / b/ e leads of the various transistors. http://www.zen96216.zen.co.uk/cb/sch...ptbm121d4x.htm |
19th Nov 2018, 11:35 pm | #54 |
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Re: Cybernet PTBM121D4X problems
Just started, had to confirm to myself which end of r131 was not at q32 via contiuity, cant spell it ! so found that the end of interest has 9.02v on it.
however whilst i was on contiuity (rig off) i found that that end was going to ground, and so was all of q33. as on diagram c157 is to ground i will check it as next step,and then if need be q32 will have to take it out completely. may hear from you before then |
19th Nov 2018, 11:39 pm | #55 |
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Re: Cybernet PTBM121D4X problems
No, you're doing fine, carry on.
It'll be interesting to hear what you find. One thing to be careful of though, 'continuity' indicated by the continuity range on a meter does not always mean a short. On my Fluke, anything below 400 ohms makes the meter buzz but it does also show the resistance as well, so you can distinguish between a low resistance and an actual short by that means. Yours may be the same. |
20th Nov 2018, 12:00 am | #56 |
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Re: Cybernet PTBM121D4X problems
done the cap so far, set dmm to 2m ohms reading started around .240 rising fairly quickly
wee bit of time till i get q32 out |
20th Nov 2018, 12:24 am | #57 |
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Re: Cybernet PTBM121D4X problems
q32 Black probe on emitter and red on collector show 1 infinity ?
collector has not a very good solder pad a little missing round the hole |
20th Nov 2018, 12:58 am | #58 |
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Re: Cybernet PTBM121D4X problems
Well, as I mentioned, one possible cause of your symptom is no connection between Q32 collector and the junction where Q33 base / R131 / C152 meet. If you put it all back together you should be able to measure zero ohms between Q32 collector and Q33 base. If that connection appears open circuit, solder a neat wire link directly between Q32 collector and Q33 base / R131 / C157 positive. There should be zero ohms between any two of those four points.
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20th Nov 2018, 1:27 am | #59 |
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Re: Cybernet PTBM121D4X problems
ok so from middle pin on q32 to, would the positive pin of c157 do as its nearest
i am taking it that on 157 its the one furtherest from chassis i did make a note of which way round 157 should be in relation to chassis just incase it matters which way the pins go back here is a pic of offending area take it old age has somthing to do with it |
20th Nov 2018, 2:20 am | #60 |
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Re: Cybernet PTBM121D4X problems
The orientation of C157 and almost all electroytic capacitors does matter, so you were wise to make a note of it. If you looked at it its polarity would have been marked in some way. the PCB also usually has a + symbol on it where the + of the capacitor should go, or a shaded area showing where the -ve should go.
I don't have a 121 chassis here for inspection so the information I have marked on the attached image of part of your image is just worked out from the diagram. |