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Old 12th Apr 2021, 8:17 pm   #1
crackle
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Default Superstar 360

Hi
I wonder if somebody can please tell me what the function of the power transistor 2SA473 is in the circuit below.

Thanks
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Old 12th Apr 2021, 8:27 pm   #2
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Default Re: Superstar 360

It is a switch in the Vcc line to the little power amplifier TA7222ap.

I think the circuitry before it only turns on once the power supply has reached a chosen voltage, and I think that once on, the supply to the TA7222ap is at a reduced voltage compared to the incoming supply. As for why? the answer must be on the rest of the diagram.

Weird!

David
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Old 12th Apr 2021, 8:45 pm   #3
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Default Re: Superstar 360

I've never paid much attention to that part of the PB010 circuit but that looks to me like the voltage regulator circuit which usually supplies around 8V regulated - often called 'AVR' on this sort of circuit diagram. It takes the raw switched 13.8V supply (coming up from below on your excerpt) into TR41 emitter and on the collector of TR41 is a steady regulated voltage which is later routed through the T/R switching transistors so those supply rails which are RX or TX only. Note that TR40 is also part of this regulator circuit and should be checked as well, as should the 7.5V zener diode.
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Old 12th Apr 2021, 8:50 pm   #4
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Default Re: Superstar 360

Actually, the connection to pin 1 of the TA7222 is its main power feed from 13.8V. The incoming 13.8V turns right to go to the audio PA IC and left to go to the input of the general voltage regulator comprised of TR40 / TR41 / the zener etc. There is no relationship between the TR41 circuit and the audio PA IC, except that they happen to share the same incoming supply.
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Old 12th Apr 2021, 9:41 pm   #5
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Default Re: Superstar 360

I've extracted the circuit from the surrounding mess and rearranged it a bit. See if this makes any more sense.
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Old 12th Apr 2021, 11:59 pm   #6
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Default Re: Superstar 360

Yes, it seems the Japanese etc. designers were rather fond of arranging everything on these CB circuits, to cram as much as possible on one sheet with little regard to more usual conventions on drawing circuits with logical-flow to aid understanding
And it does look like their own Low-dropout PNP-based regulator, from the days before LDO voltage reg IC's were common.

And the SAMS Photofacts service manuals would redraw the circuits across many sheets(Although I didn't always find these necessarily much easier to work with, like the original R&TV servicing books re-draws to fit across several pages).
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Old 13th Apr 2021, 12:17 am   #7
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Default Re: Superstar 360

Wow, I haven't seen that circuit in a few years! It's a low dropout 8V regulator circuit. This dates back over 40 years and the earlier Uniden radios that had this circuit could suffer temperature related instability in this circuit.

The 2SA473 is quite a fast device and Uniden had to add some decoupling caps to improve the stability on the early versions of this regulator circuit. From memory, the early version of this circuit (used on the mark 1 radios) can go unstable up in the MHz region without the extra caps.

The circuit shown looks to be the later version. It serves as a low dropout regulator plus it has a fairly wide loop bandwidth so it can filter out any input side noise and spurs at AF frequencies and higher.
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Old 13th Apr 2021, 12:23 am   #8
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Default Re: Superstar 360

I think the early version had a small startup resistor across the collector and emitter of the 2SA473 and I think the 47k resistor in the base of the 2SA473 now helps with the startup of the later circuit. It's so long ago I can't remember now.
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Old 13th Apr 2021, 12:35 am   #9
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Default Re: Superstar 360

From memory I think it outputs just over 8V and this is also current limited. I think the circuit can cope with a short circuit at the output and the current limit is defined by a resistance value. I can't remember what the current limit is but I do recall that this circuit is quite robust. However, failures did occur and a lot of repairers put a slower PNP device in place of the 2SA473. Ideally, this circuit needs to cope with input voltage fluctuations at audio frequencies and also it will be fast enough to cope with fast impulse changes as long as the original 2SA473 is fitted (not something slower and cheaper).

However, I'm remembering this stuff from a long time ago so I may have got a few details wrong.
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Old 13th Apr 2021, 7:25 am   #10
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Default Re: Superstar 360

Thanks every one for your useful and detailed information.
A friend has been messing with the radio that was not working and thought TR41 was the TX PA device, he swapped it for a NPN output transistor.
He says he has put the original TX41 2SA473 back and the radio is now transmitting but it has some other issues.
I think this may be another radio to come my way to take a look at.

thanks again
Mike
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Old 13th Apr 2021, 8:10 am   #11
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Default Re: Superstar 360

I think you need to kidnap that radio for its own safety, by the sound of it!
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Old 13th Apr 2021, 10:13 am   #12
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Default Re: Superstar 360

I'm a bit rusty on these old radios but I can help with troubleshooting this circuit if it turns out that it has been zapped by your friend.

I think the little diode D89 is a 'stabistor' type diode and back in the day this odd looking diode was difficult to buy. From memory it is an exotic looking device, it doesn't look like a conventional diode. I recall it looks like a little 2 legged blob about 1mm diameter. I think it is painted with two colours to indicate the polarity. This is the part you really don't want to damage.
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Old 14th Apr 2021, 6:06 pm   #13
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Default Re: Superstar 360

I don't know why he didn't contact me first and I could have told him which was the OP transistor.
Unfortunately I no longer have a dedicated workshop with tools and instruments to hand, it is now a nursery for my grandson, so I am not volunteering to look at it.
I have suggested it goes to a proper rig doctor, but it has been put back in a cupboard for now.
thanks
Mike
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Old 29th Apr 2021, 3:20 pm   #14
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Default Re: Superstar 360

This radio finally made its way back to me to repair.
The cause of the problems with the set was it suffered an accidental reverse polarity.
Unfortunately the 1N4003 protection diode was not able to blow the 5A (rated) fuse and the audio amp suffered as a result.
The reason the inline fuse didn't blow is because I believe that despite being marked as 5A it looks more like a 35A fuse. Take a look at the photo of the fuse compared to the 4A fuse next to it.
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The diode surprisingly didn't explode but it certainly does not function as a diode any more, it is more like a 4 ohm resistor one way and a 13 ohm resistor the other way.
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Anyway a new old stock TA7222AP was fitted and the diode replaced and a 4 amp fuse fitted and the radio was working again.

The original audio IC did not fail completely as a result of its reverse polarity experience. But it was behaving in a strange way. At 13.8v it would squeal
and crackle and distort badly. If the supply voltage was turned down to 12 volts it would stop the squealing and crackling and just sound slightly distorted.

Mike
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