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hi all, i have ham international jumbo it works fine when switched on from standing for a few hours works for about 5 minutes then the sound goes very low and thers no rx or tx,then if i switch off and let go cold it does the same and the longer it is left off the longer it will work ok.seems like something is over heating to me.
any help or advice would be much appreiciated

btw all this looks nice forum very interesting with great help and
some great down to earth users

It sounds like the PSU. check the volts at the output of the PSU, it should be about 13 to 13.8 volts.

hi rick,
checked power supply its putting out 13.8v,
after 1st switching on ,everything on radio works fine for about 10 mins then the sound volume drops with no tx or rx,but everthing still lights up and tx light comes on if mike is keyed but thers no power going out.and reception goes as if the aerial has been unpluged,you can still ear the quelch noise if you turn the volume right up,

I assume you have check the main PCB for cold solder joints?

You do not say which model of Jumbo you have but assuming you have a MK1 with the PTBM059COX PCB, if it is then check the AVR, which is Q44 the collector should be at 13.8 volts and the emitter should be about 9 to 9.5 volts.

Also check cap C166 they are known to dry-up due to age, it maybe best to just replace this cap.

Also check R157 for cold solder joints.

If the fault is still there after check the above, then check the RX/TX switching transistors Q40, 41, 42 and 43 and the diodes D44, 45, 46, 47, 48, 49 and 52.
Let us know how you get on.

ok rick will do,yes thats the mother board it has,i also noticed last night if the radio is placed upside down on the table switched on it seems to run ok for a lot longer time does this suggest anything.i have checked for dry joints and im pritty sure thats not the problem.

i checked q44 volts and they are ok,r157 is ok but c166 is a jumper wire no cap present,

thanx angus

This must be a power distribution fault, are you sure that the voltage at the emitter of Q40 stays constant under the fault condition?

If voltage stays constant at Q44, check the voltage at pin 8 of the PLL chip (IC1) under the fault condition, and let me know the voltage?

Also check for components which maybe getting hot, a can of Servisol Freezer would be useful.

hi rick,i did try some freezer yesterday the radio kicked back in 2 or 3 times when i sprayed around the x1 ,10.2400 cristal but now has no effect.

??
rick i just checked pin 8 of pll under the fault and it is 5.35 v,also checked
q40 under fault emiter is 0.07 v collector is 0 v.

What are the voltages around Q41 and Q43 in RX mode under the fault condition? try switching between SSB and AM/FM.

I know it will TX but what are the voltages around Q40 and Q42 in TX mode under the fault condition? try switching between SSB and AM/FM.

Also let me have the voltages under the fault condition of Pins 1 to 9 of IC2 (TA7310).

You could try replacing the 10.240 MHz crystal in case the crystal as gone dead.

volts are
with and without fault on fm in rx
q40=0.15
q41=0.00
q43=8.59
with and without fault on usb in rx
q40=9.18
q41=7.8
q43=0.01

there is not any change in rx with or without the fault on volts on q40,q41,q43
---------
ic2 pin volts under fault condition
1=2.6
2=1.94
3=1.24
4=2.34
5=0.0
6=8.97
7=2.07
8=4.30
9=5.34
---------------
volts are
without fault on fm in tx
q40=0.13
q41=0.52
q43=0.22

volts are
with fault on fm in tx
q40=0.13
q41=0.64
q43=0.22

All the voltage look ok, but I see you did not test Q42?

Do you have a standalone frequency counter?

yes i have frequency counter ,will check q42 today,fault seems to be changing ,i have to leave radio off for a lot longer now before it will work ok.
and the longer it is left off the longer it will run ok for

All check under the fault condition.
With you counter check the frequency at the following points TP1, the base of Q22 and TP5 in RX and check the frequency at base of Q7 and TP5 in TX.

hi rick,my frequency counter doesent seem to be working right, cant get any readings with crocodile clip on tp1 or tp5,so under the fault i put it in line still no readings so started turning the channel selector round then on 73 high band keying the mike about 25 times i seen the power metre go up and the channel frequency showed so turned the selector up and down it keyed for about 5 channels then droped out i also noticed it was 15 kc between each of the 5 channels,
so now ive found if under the fault if i key the mike several times it will tx
at normal watts and off frequency but still no rx.
what u reckon rick

It would be a good idea to do a full alignment before you do any more work on the radio.

hi rick,my frequency counter is ok,
started alighnment
freq counter on tp1 set ct1 to 10.240
freq counter on tp4 tried to set ct2,ct3,ct4 but frequency does not alter
thers is frequency readings when fault not showing,
but when fault shows thers no fc readings

So let see if I have this right, under the fault condition you can not get a reading on your counter at TP4, Yes or No.

fc on tp4 without fault reads 19.??? fluctuating
fc on tp4 with fault reads 8.??? fluctuating

Ok then, check the voltages at Q4 Emitter, Collector and Base with out the fault and with the fault.

Also check the voltage at TP2 on Low band channel 1 again with and without the fault.

q4 seems ok rick
q4 without fault
e=1.56
c=5.35
b=1.18
q4 with fault
e=1.56
c=5.34
b=1.22

now volts on tp2 low band channel 1
without fault=3.36
with fault=5.26

It looks like your PLL chip as failed, but I would change IC2 first just in case it giving the wrong signal back to the PLL. The PLL chip is not very easy to get in the UK any more, but if you get stuck trying to find one let me know as I think I have a few left in stock.

ok rick will see if i have any of these chips and get back to you,
also i just bought a cb power supplyand its putting out 14.14 volts will this be ok for running a mobile cb.
thanx for your help rick.

angusuk wrote:ok rick will see if i have any of these chips and get back to you,
also i just bought a cb power supplyand its putting out 14.14 volts will this be ok for running a mobile cb.
thanx for your help rick.

That a bit on the high side, 13.8 should be the max. I run my radios at 13.5 volts.

what you think rick to get it down from 14.14 to 13.8 just put a variable resistor on the + volts,i cant see any adjuster inside with the top off.

When you need to talk about a new issue. It is best to start a new Topic, as this is how you help others with the same problem.


It sound like a bad PSU you have there.
Any good PSU has an adjustment control.

You cannot just put a pot in the + line, this will not work.
You need to be able control the Base of the power transistor and in the case of a switch mode PSU the PWM.

There are lot of circuits for building a PSU on the net, just be careful when playing around with the Mains side it can KILL.

I do not use switch mode PSU as they can cause lot of weird noises on RX and TX.

hi rick,
the power supply is a 12 amp moonraker,would like to get it working at 13.8v
and for the jumbo the guy who owns it came and took it back yesterday before i had time to try those 2 chips, says he can get it sorted,i was going to buy it if i could of got it working.