756Pro meter LED issue

[K9FV]

Hello all, I've had a 756Pro for several years now with good service. Lately the back-light in the S-meter has an intermittent flicker. Looking at the PCB board in the service manual shows 4 LEDs - DS901-DS904. Since it looks like these LEDs are soldered direct to PCB, how can it flicker? Has anyone had this issue with a 756Pro? Any ideas or suggestions?

Thanks for any help,

73 de Ken H>

[G4COE]

Very simple, how about bad soldered joints... or their series resistor going faulty? A test meter will reveal all. Yes, LEDs themslves can go like that but you are unlucky if it's more than one.

Dave

[G4COE]

I forgot to add, check any plugs and sockets including any crimped terminals.

Dave

[K9FV]

.......Yes, LEDs themslves can go like that but you are unlucky if it's more than one.Dave

Thank you for info - I did not realize LEDs could go intermittent - I always thought LEDs were like fuses - good or bad?

Thank you again,

73 de Ken H>

[G4COE]

Just a bit of silicon or what have on a pedestal... and a connection to form the circuit onto the silicon slab.

Some LED info here

Dave

[KA1MDA]

Do the LED's have adjustable brightness? If so, they may be driven by an PWM circuit, which could be causing the flicker.

[G4COE]

Oooops silly me, sorry I sent you down the wrong track.

The Back lights are CCFLs that are driven from an inverter generating about 1.4KV, flicker is usually due to the tubes aging, but the inverter can also produce this effect. CCFL's stand for Cold Cathode Florescent Lamps.

See if this link is any good

Thanks for the wake up call , sorry about that Ken, why LEDs came to my mind I don't know!

Dave

[K9FV]

Dave, thanks for the effort of looking for info - but I do think you have the main display in mind. Note my original post says "back-light in the S-meter". The service manual has the part numbers as shown - and I'm pretty sure I remember the parts list calls those "LED".... I think anyway, I don't have the service manual handy at this time.

Thomas, Nope, the brightness is not adjustable...... don't think anyway. Not seen anything on it.

At this point it's not a "big" problem - but expect one of these days there will be an issue. Maybe at that time the problem will be easier to trouble shoot.

Thank ya'll for the suggestions and links.

73 de Ken H>

[G4COE]

I don't have the schematic on hand right now but see this note and look for this heading Outside Observations /Meter Lighting / Spectrum Scope / No Rubber Feet. The Lamps can be dimmed and from memory 'I think' this is done from by a transistor, if so the transistor is worthy of checking. A volt meter should show the dropout or flicker, my guess is the transistor runs warm and just wants a re-solder, just a guess mind.

Dave