RCA TUNER/AMP M#3798, no display

[erskine]

hi friends. I am working on this RCA tuner/amp model#RV3798.
The problem it has is that the Vacuum Fluorescent display is very dim.
I found that there is a missing -24v , traced it all the way back to the transformer , replaced all active components and filter cap , but still the same . .is it possible there can be a problem with that particular winding on the transformer?
thanks in advance .
ERSKINE

[budm]

Re: RCA TUNER/AMP M#3798, no display

Is the -V for this display rectified from the winding of the main power transformer?, What AC voltage do you get from the output of this winding?

[erskine]

Re: RCA TUNER/AMP M#3798, no display

yeah the voltage comes from the main transformer , and I am getting 15v ac . and a rectified voltage of -3 volts .
the rectified voltage comes from a simple circuit of one diode ,a resistor , a 24v zenner and a filter cap .
strange thing though , i didn't replace the rectifier diode or check the output with the scope , will do this .

[budm]

Re: RCA TUNER/AMP M#3798, no display

15VAC x 1.414 = 21.21V minus about 0.6V for Diode Vf, so you should get a little less than 21V, the Zener is just to make sure it does not go over 24V if the AC line voltage goes up too high. Did you check that Zener Diode off the board or replace it?

[erskine]

Re: RCA TUNER/AMP M#3798, no display

i did replace the zener , so still have to try replacing the rectifier diode to see what happens . and for sure the voltage stays the same with no load on it .

[budm]

Re: RCA TUNER/AMP M#3798, no display

It is so simple, some time things get over look.

[erskine]

Re: RCA TUNER/AMP M#3798, no display

that is oh so right . got it fixed finally .
first thing it was the zener after all .
i did replace it and then put back in the old one.
problem was the AC was passing through 2 47mf caps that
were bad , i replaced the caps , lift the zener and the 24 v came back up.
so i just replaced the diode and the display is shining as bright as day .
thanks Budm

[budm]

Re: RCA TUNER/AMP M#3798, no display

Glad to hear, it's always the simple thing that gets you, I did that many times.