DC-DC output voltage

[smithy959]

after replacing the caps in the DC-DC unit when I check the 24 volt rail it is reading -11.5 volts
what have I missed or got wrong,the receiver still powers up ok and receives aerotraffic both Civil and military ok and is ok on the 2 meter band but the display will go dim and go off then come back on,
any suggestions welcome.

Thanks :-Ian

 

[prcguy]

Does the 24v rail then feed a sub regulator for 12v or similar? If so it sounds like the DC-DC converter is broken and the regulator is starved for input voltage leaving the receiver running on a very low voltage. Or the 24v is only for powering the display like a florescent back light?

Can you give more info on what the diagram shows for input voltage to the radio and what the receiver runs on vs the display, etc?
prcguy

after replacing the caps in the DC-DC unit when I check the 24 volt rail it is reading -11.5 volts
what have I missed or got wrong,the receiver still powers up ok and receives aerotraffic both Civil and military ok and is ok on the 2 meter band but the display will go dim and go off then come back on,
any suggestions welcome.

Thanks :-Ian

 

[smithy959]

Icom IC-R7000 DC - DC unit

There are 4 wires that come out of the DC - DC unit that should be as follows and they all go to the RF unit

24v
9 v
-7 v
-12 v
as I said before the Receiver works OK and is sensitive enough but I don't understand why i'm
getting negative voltage where it should be positive,a bit of a mystery,could it be i've got a capacitor located wrong?

Regards:- Ian

 

[majoco]

I can't see how it can be working correctly - the +24v out from the DC-DC unit feeds an electronic attenuator and a few IC's in the RF front end. Unless you have removed the diode D1 on the DC-DC board and reversed it, then there no way that it can give out -12v.

I'm suspecting finger trouble - where are you connecting the negative lead of your voltmeter? Digital or analogue voltmeter?

 

[WA0CBW]

Ian,
If the minus 7 and minus 12 volts are correct then it is probably Q3 the 24 volt pass transistor regulator that is bad.
BB

 

[smithy959]

Icom IC-R7000 DC - DC unit

Hi
The negative lead was connected to the Earth socket on the rear of the receiver
and it's a digital multimeter,maybe I have this connected wrong or something.

Thanks :-Ian

 

[majoco]

Sorry, I put you wrong with what IC5 was in the RF unit - it's an LED driver.

Forgive me for being stupid, but are we talking about the same DC-DC converter?

You said:

but the display will go dim and go off then come back on,

Now the single board converter has nothing to do with the display BUT there is another DC-DC converter that supplies the flouro display and all its outputs are negative apart from the heaters, This board commonly suffers from bad capacitors (which leads to fading of the display) and poor soldered joints on the flexible PC board, which I fixed by adding an external pair of wires for the heaters.

 

[majoco]

The only way I can see that you get a negative voltage on the +24v line is if the diode D1 has popped and the series pass transistor is acting like a rectifier - it will be struggling of course - all it's polarities will be wrong. Of course you haven't replaced the diode D1, have you......and got it round the wrong way....

No blobs of solder across any tracks when you replaced the capacitors.... we've all been there :wink: