Tristar 747

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M1JWR
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Tristar 747

Post by M1JWR » 14 Oct 2018, 21:57

I have a tristar that has a fault that i need confermation on if i am right.
it has hardly any audio and no rx, it may have if you are transmitting to it with a burner
whilst standing next to it, ive never tried so thats a guess.
it does tx, though very low less than quarter of a watt and thats with it turned up flat out.
it does work on pa and you can hear the thump when you switch on etc so audio ic is ok.
the vco is locked and all near enough on frequency when on tx all mode.
the radio has no mods whatsoever.
i have done some voltage checks on various parts eg the voltage regulator in this case the
collector is correct and emitter and base are nearly half a volt low which seems to be a recurring
thing around the radio
my guess is that the voltage regulator is a bit duff or somthing is dragging it down so therefore
correct voltages are not given around the radio so it is in its how should i say very QUIET state
any other ideas appreciated

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1968neil
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Re: Tristar 747

Post by 1968neil » 15 Oct 2018, 07:33

Hi M1JWR,

Most likely the AVR transistor, from memory its a darlington pair.

However one thing worth checking are the thru board links (thru holes), these are becoming very fragile in radios of this vintage, I had an excalibur a while back that had never been out of the box but it was faulty on test and it turned out to be a thru board link that had corrosion due to age.

Regards
Neil

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Re: Tristar 747

Post by M1JWR » 15 Oct 2018, 10:42

Hi thanks for the reply
The avr is q38 thats the one on the little heatsink that sticks up on its own
is the darlington the one that is on the same heatsink opposite side to audio amp i think
q36, through holes could be an exploration and a half, if it comes to that, luckily there
wont be many leading up to the avr and then there is its own circuit.

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Re: Tristar 747

Post by 1968neil » 15 Oct 2018, 13:09

The Darlington transistor DC Switch buffer Q36 2SD837 can go open circuit causing no power on AM or short circuit resulting in no modulation on AM but
FM and SSB unaffected.

A very common fault includes the AVR (Q38) 2SD325 going open circuit (Rig lights but theres no tx or rx) or goes short circuit causing
strange effects like no tx on FM but needle hard over on SSB with a loud whistling on the carrier.

If these are ok then my money would be on a thru hole being high resistance or completely open circuit, i usually use a pcb drill and drill them out and then put a vero pin or a header pin in the hole and solder both sides. This also gives you a useful test point for future use :)

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Re: Tristar 747

Post by M1JWR » 15 Oct 2018, 21:18

Hi
i diddent suspect the darlington as am is working on tx
its just that everything including the pll is getting on for half a volt low
like its in hibernation, alive but not as we know it
pa works
you can hear fm noise with volume on flat out but very quiet ext socket headphones
it can tx on all modes i can hear it on another radio it sounds good for a 747
but output is very low
s meter lights up no movement
never heard any rx through headphones dont have a sig gen and a very large
signal will be required
its like its being run by a 50cc engine when it needs 100cc to work properly
thats why i suspect the q38 voltage regulator just not 100%

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1968neil
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Re: Tristar 747

Post by 1968neil » 16 Oct 2018, 08:40

I would seriously start checking the lead thrus,
The easiest way i found was to set the radio up on a known frequency in RX mode. squelch open or with a sig generator on with a 1k tone.

Then using a very fine sewing needle just gently prod the suspect leadthrus, you will soon find the culprit, i found three in my scenario, all has been good since.

Good luck :)
Regards
Neil

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Re: Tristar 747

Post by M1JWR » 17 Oct 2018, 10:17

Hi
as i dont have a sig gen it will have to be trial and error
so replacing the voltage regulator and possibly one or two parts associated with it
cant do any harm
i do have 2sd325's in stock.
i might get lucky

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Re: Tristar 747

Post by M1JWR » 01 Nov 2018, 01:31

Have replaced avr it is now putting out correct voltages, it wasent the cure though.
have checked voltages on some of the transistors in radio
as i dont really know which pin is which only the middle reading will be accurate
q20 0.72v/13.5v/1.38v
q22 1.33v/12.97v/0.66v
q27 0.76v/0.8v/0.8v
q28 0.7v/0.84v/ground 0v
q29 0.1v/0.11v/0.84v
something seriously wrong in this area
q14,16,17 and 19 all read near enough correct
i do have a cybernet manual but it dosent relate to the 121circuit board very well
the radio was tested without the speaker or mic connected, as there is no shall we say
output to them i did not see the point
the voltage on pin bra is 7.9v
needle in a haystack indeed

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1968neil
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Re: Tristar 747

Post by 1968neil » 01 Nov 2018, 09:41

Have you replaced Q36 ? 2SD837
This is the DC switch buffer, without this functioning correctly not a lot works.........

It supplies IC5 the TA7205 which is the audio amplifier, may explain no rx audio ?
It also feeds some of the stages you mentioned in your last post.
This is a common failure point and always the first device i check along side the AVR.
If this is ok then as i've said before its probably a bad lead thru.

Regards
Neil

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Re: Tristar 747

Post by M1JWR » 01 Nov 2018, 10:54

Hi
have checked this one now
looking from back of radio solder side left to right
6.77v/13.66v/7.18v
the left joint started around 9.5v on meter then dropped down to 6.77v
it remained steady on 6.77v after that.
no spec in cybernet manual for this, so i wouldent know what is correct
ive seen cobramans vids on how bad this area can be, not enough track apparently !
i have a spare on a scrap board that wasent at fault if it comes to that.
regards

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1968neil
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Re: Tristar 747

Post by 1968neil » 01 Nov 2018, 14:36

Q36
In Rx Mode (channel 1 Band C)
Emitter = 8.68v
Base = 9.06v
Collector = 13.67v

In Tx Mode
Emitter = 6.83v
Base = 8.06v
Collector = 12.8v

Q36 2SD837 from the front of the device (text facing you) = B C E
Don't forget this is a Darlington transistor......

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Re: Tristar 747

Post by M1JWR » 01 Nov 2018, 22:18

AM mode rx
ch1 hi band
B 7.18v
C 13.67v
E 6.79v
and tx
B 6.98v
C 13.55v
E 5.75v going up slowly
Ch1 mid band rx
B 7,21v
C 13.67v
E 6.80v started at around 9.3v and went down to 6.8v
TX
B 7,00v
C 13.55v
E 5.75v rising slowly
hope this helps

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1968neil
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Re: Tristar 747

Post by 1968neil » 02 Nov 2018, 17:34

I would replace "2SD837" sounds like it could be leaky ?
also replace the 10uF electrolytic in its base, this may well have dried up in the past 30 years !

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Re: Tristar 747

Post by M1JWR » 03 Nov 2018, 01:51

Hi
replaced both parts, checked voltages,no change
still rising and dropping etc, i know the 2sd837 i put in is ok
audio through speaker is a smidge better
next part in chain looks like mode switch ?
a real puzzler thanks for your help so far

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Re: Tristar 747

Post by 1968neil » 04 Nov 2018, 11:21

seriously .....

Check the lead thrus,
They are more than likely the problem in that case...

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