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Old 16th May 2021, 3:31 pm   #1
crackle
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Default Husun 55

I couldn't resist buying this today for a miserly 8.
The radio was dead on arrival, which turned out to be the on/off switch not making contact.
It now seems to be working OK into a dummy load.
But one think I would like advice on is the 78005AP regulator. It is permanently live when set is switched off but with a 12v supply connected to radio. the output is about 6.3v. I suspect it is something to do with the circuit that enables the retention of the last channel selected. If the supply is disconnected the radio reverts back to ch16 when it is powered on again.
I burnt my finger on it, it was really hot, not quite so hot as to make a sizzling noise but enough to make me pull away rapidly.
It is not clamped to any heat sink, it just appears to have been stuck to the side chassis with a sticky pad which has lost its stick on one side.
When the radio is switched off it cools down, it only gets very hot when the radio is switched on.
Does anyone have a circuit for this radio, I would like to see what the most likely cause for the excessive heat being generated is.
thanks
Mike
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Old 16th May 2021, 4:11 pm   #2
ex seismic
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Default Re: Husun 55

Well a very quick google shows that to be a 5 volt regulator, so if the output is 6.3v there is something wrong.
Gordon
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Old 16th May 2021, 7:26 pm   #3
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Default Re: Husun 55

The 78005 looks to be identical to the popular 7805, the extra zero appears to be particular to Toshiba versions. I wonder if this transceiver is actually made by Kelvin and Hughes in the UK or whether it is a re-badged set made by another manufacturer. The regulator should be properly heatsinked to the case for it to operate correctly, not just resting on a foam pad. I would be surprised if a company such as K&H would let this construction method get past its QA systems without modification.
Neil
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Old 16th May 2021, 9:46 pm   #4
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Default Re: Husun 55

"Well a very quick google shows that to be a 5 volt regulator, so if the output is 6.3v there is something wrong.
Gordon."
Not necessarily so, a good many manufactures put one, two, or three diodes in the "Earth" leg to raise the output voltage to, say 6.3V. for valve heaters. I know that this is an S/S unit but even so look at the schematic just in case. Definitely needs a heatsink and I suspect isolating pad and washers
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Old 16th May 2021, 9:55 pm   #5
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Default Re: Husun 55

Fee fo fi fum
I spot a little blue tantalum...

Right by the legs of that reg. It might be decoupling input ot output, but sometimes people playing games with the ref pin to adjust the voltage may add a decoupler there to reduce noise. If it goes leaky, it can muck up the voltage setting. Worth checking, just in case.

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Old 16th May 2021, 9:56 pm   #6
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Default Re: Husun 55

It looks like there is a tiny orange and black diode D09 in the connection to earth.
So that could account for the higher voltage.
I will have to work out a way of fitting the regulator to the chassis.
Yes it does look like a tant in there.
thanks
Mike
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Old 17th May 2021, 9:27 am   #7
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Default Re: Husun 55

I have replaced the blue tantalum cap. it was .1uf 35v with a .1uf 50v ceramic type.
The regulator is now bolted onto the side of the chassis, and there is very minimal heat gain now.
A service manual would still be useful to complete my recording of the details of this radio.
thanks
Mike
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Old 17th May 2021, 5:05 pm   #8
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Default Re: Husun 55

I had second go at cleaning the radio up and it looks a lot better now.

Mike
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Old 17th May 2021, 7:12 pm   #9
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Default Re: Husun 55

I don't think the black foam was intended for heatsinking purposes but simply to stop the regulator from flapping back and forwards under its own weight and snapping its legs off. (Remember this is a mobile radio, subject to vibration).

The metal chassis on radios like this are often not actually connected to supply 0V directly, rather by a number of parallel RF capacitors - I think probably so that the radio could be used in either negative or positive earth applications.

By bolting the regulator directly to the chassis you may have created an unintended connection between chassis and 0V - whether that will do any harm, other than make the unit unusable with a positive-earth host, is a moot point.

Incidentally PLL circuits in radios like this often run on around 6V rather than 5V so it is quite possible that the fixed output of the regulator has been intentionally jacked up, as John Caswell suggested.

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Old 17th May 2021, 8:01 pm   #10
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Default Re: Husun 55

I agree about the foam fixer being used to stabilise the regulator against vibration. But it was getting excessively hot, hotter than any other semiconductor I have come across.
I have now fixed the regulator to the chassis and it is isolated from the chassis.
See photo post #7
Mike
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Old 17th May 2021, 8:13 pm   #11
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Default Re: Husun 55

Yes, sorry, I missed your image in #7 in which the reg is clearly properly insulated from chassis.
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Old 18th May 2021, 5:34 am   #12
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Default Re: Husun 55

I forgot to mention, the ground leg of the regulator was 1.26v above 0v accounting for the 6.3v output of the regulator. No doubt because of the series diode in its ground connection. This is also decoupled by a disc capacitor. The small tantalum capacitor was decoupling the OP.
Mike
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