Superstar 360FM

[crackle]

Hi
I am looking at one of these old classic CB's now.
It stopped transmitting and then worked for a short while then stopped again.
I changed the 2SC2312 (which seemed to be functioning according to my transistor checker, albeit with a very low gain) for a 2SC1969 from Eleflow and the radio was working for a short while.
When I went back to it today it was still working, but then stopped transmitting again.
There seems to be no supply getting through the TR51 the 2SA1012 regulator transistor to the final stages..
The 2SA1012 tests OK on my tester.
Circuit attached below.

Please can someone give me some advice as to where to look next.
Thanks
Mike

[SiriusHardware]

I'm away from base at the moment - I thought Jeremy might jump in on this one as he has a lot more experience with the classic Unidens than I do.

For the time being though it sounds more like a DC problem than an RF problem. TR51 / TR50 are a Darlington pair, made from two discrete transistors rather than in one package as is sometimes seen, so if it's easy to do, check TR50 as well.

If that proves OK, with everything back in, what voltages do you see on TR51 and TR50 e,b,c with respect to 0V in

-SSB TX mode
-FM TX mode
-AM TX mode

?

[crackle]

Hi
Thanks for your reply, here are the voltages requested.

thanks
Mike

[SiriusHardware]

I know you said you checked TR51 but at first glance those results are impossible for a healthy TR51.

In SSB mode the base of TR51 is connected to 0V through a 150R resistor via switch TR52, the intention being to pull TR51 hard on and pass the full input supply voltage through to the collector of TR51 and from there to the RF output stage.

When turned on, the e-b junction of TR51 should act like a forward biased silicon diode and you would expect to see no greater than about 0.6V to 0.7V voltage difference between TR51 emitter and TR51 base.

In this case you have a huge voltage difference between TR51 emitter and TR51 base and that would suggest that TR51 is open-circuit internally between emitter and base. This wouldn't be unexpected, as it is very like the arrangement in the later Anytone / etc radios where the PNP transistor doing the same job in those is also known to fail with b-e open circuit.

[SiriusHardware]

It may also be worth checking first the 150R resistor to make sure it has not gone open-circuit and then also TR52.

If TR52 / the 150R resistor are not providing the expected pulldown, that could give odd looking results as your voltmeter on its own would not draw any significant current through TR51 e-b.

[Techman]

The very first thing I'd do after getting dodgy voltage readings like that if there was no other possibility, would be to remove the transistors and measure the junctions with an AVO on the low ohms range - I've never used a transistor tester in my life! Actually I have, but just for looking at hfe values out of interest, not for actually testing for faulty ones in a set - but that's just me, I'm 'old fashioned'.

[crackle]

Yes I have had TR51 and 52 and all the other transistors in that location out and checked on my little Chinese tester. However I did note that when testing some transistors they changed from NPN to PNP depending on which way round they were connected to the tester pins.
I wonder if my tester is playing up.
I also took out the electrolytics and they all had ESR's of over 40R but I later discovered my Peak ESR tester had a hidden break in one lead, I was surprised it worked at all. It was giving reliable capacitance readings but ridiculous ESR figures.
Anyway that tester is now repaired and I fitted the original capacitors back as they had better ESR figures than the modern replacements I had tried.
Anyway I will go and check the 150R "R295" resistor on the emitter of TR52, and do some more tests on TR51 & TR52 with an AVO.

One thing I did note was in receive the B & C voltages on TR51 and TR52 were higher, around .45v and dropped to the figures given above when the mic was keyed.

edit
I have checked the 150R resistor, it is 151R
There is no reading between C & E on TR51 and the other pins gave expected readings.

Thanks
Mike

[crackle]

Disregard the transistor checks above I am going to do them again.

I did the tests again and the results were as expected, predictable results between b and C which changed to infinite when reversed.
infinite readings between c and e both ways round.
Thanks
Mike

[SiriusHardware]

On testing conventional transistors, the 'Diode Test' feature on a DMM can be useful for showing up transistor junction problems, test b-c and b-e as though they are individual diodes. On a PNP device the black lead will need to be put on the base, red lead first to emitter and then collector. In DT mode the meter indicates the forward voltage drop across the junction.

[crackle]

I have replaced the 2SA1012 with a TIP32C and the radio is transmitting now.
I dont understand, is it likely that there is a real open circuit between C & E on the original TX
I wonder why the tester identified it as OK

Mike

[SiriusHardware]

Regarding post #8 (as edited) what about the b-e junction, which you did not mention? That's the one which your original voltage measurements suggest may be open-circuit on TR51.

Edit: The fault on the original TR51 was probably open-circuit b-e rather than c-e, as that is what the original voltage measurements suggested. Try using a DMM 'Diode test' on the original TR51 as I suggested in #9.

[crackle]

On a diode test, with black on base, b to c reads .618,
b to e is .625
both above readings are ol when leads reversed.
c to e is ol both ways round.
So that seems to indicate the transistor is OK, have you seen them fail like this before.

Thanks
Mike

[crackle]

I seem to remember I was warned about this TR51 failing before on here, https://www.vintage-radio.net/forum/...80&postcount=8
and I passed the warning on to the owner to keep the supply voltage down to around 12v rather than near 13.8 or 14v.

Thanks very much for your help guys, I was looking in the right place but did not fully understand how the regulator gets its control volts. Which was making me uncertain of what I was doing. Coupled with the tests which all SEEMED to prove the transistors were OK I was feeling a little lost.
Thanks Again
Mike

[Andrew B]

I have repaired 148/360 sets where that 2sa1012 has gone open and sometimes short, if I remember when it goes short there is no AM modulation.

[SiriusHardware]

Regarding post #12, I have to admit that sounds like an OK transistor based purely on a diode test of each junction. It's making me wonder if this is a physical-intermittent fault on the original transistor - remember when you first replaced the RF PA the set did actually work, then it mysteriously stopped working with no output from TR51.

It sounds like if in doubt, swap it out. (Which you have done).

Did you check the current capability of the TIP32C vs. that of the original 2SA device? On a quick look around the TIP32C looks to be 3A continuous, 5A peak which I would imagine is nearly on the limit in a multimode. I didn't look up the spec for the 2SA though.

[SiriusHardware]

Again based on one quick peek at a datasheet, looks like the 2SA1012 is 5A continuous / 8A peak so it has a bit more headroom than the TIP32C. If you can, try to acquire a small stock of the correct transistor.

[crackle]

Good point, I missed the current figure

looking for an equivalent, is the hfe figure critical for this application?
Farnell do a few PNP transistors. BD244CG

Thanks
Mike

[SiriusHardware]

I would say the hfe of a replacement device should not be any lower because that 150R resistor will be calculated to draw enough base current to drive the original TR51 fully on.

If you choose a replacement with a significantly lower hfe, it may not turn fully on. Best to aim for the same hfe range as the original if possible.

Apart from that, the device is only required to operate in either DC conditions or at most, audio frequencies (when being used as the AM modulator) so any similarly SPECed device in a To220 package should do.

Watch out for possible pinout differences though - Japanese devices in that package are almost invariably b-c-e but I'm not sure if that is also always the case for European To220 transistors.

[Andrew B]

I think I used TIP142 ?? as a replacement, so long ago that I don't remember

[crackle]

Originally Posted by Andrew B

I think I used TIP142 ?? as a replacement, so long ago that I don't remember

They both look like NPN Darlington types, maybe too much gain?
The one I need is a PNP type.
Would to much gain be a problem?

Mike