Yaesu FLdx400 help wanted

[hweng007]

Can anyone assist me with advice on repairing this transmitter please? Basic problem is no output. I have done some very basic tests which leads to think that the output valves are not being driven and that the fault is further back in the circuit. If it was just AM it would be easier to understand but with all the extra circuitry relating to SSB, etc. confuses the matter for the less experienced person like me. So if any of you have the ability to help I would be most grateful.

Many thanks

Carl

PS I know you clever chaps like to be very specific, so just ask me a question and I'll try to be a specific as I can

[Station X]

With problems like this I find it's best to start with the transmitter feeding a dummy load in CW mode.

As always though first check that all PSU voltages are present and clean relay contacts where possible.

[mickm3for]

Hi first a warning, take great care around the anode it could kill you.
Check PA screen and anode volts also check g1 for drive using an RF probe into a DVM.

Can you hear a carrier on another receiver?? Are the voltages around the driver correct?

Manual pdf attached.

Mick

[mickm3for]

Hi file too big so link to manual http://www.foxtango.org/ft-library/F...0s/FLDX400.pdf and the voltage chart

[dodgy-dxer]

Hi Carl

On the back panel there is an 11 pin round socket (like an "octal" valve base but 11 pins)

Does it have a plug in it?

This plug needs a link in it which is in series with the PA filament supply

They often "disappear" and the PA wont work without the link. Do the PA valves heaters "light up"?

73 Fred
G4BWP

[hweng007]

Hi everybody, Thank you for your advice. Just to answer your questions and add a little more detail at this point:- The anodes on both output valves read about 625 volts. Yes the heaters light up. No, sadly I cannot hear a carrier on another receiver. Yes, the 11 pin octal plug is in place. I have checked both relays and found good continuity across contacts at the time of testing. PSU voltages checked out ok so far as I can see from the circuit. I am not familiar with CW so, using a dummy load, I fed a signal of constant amplitude into the MIC and traced it as far as V1 pin 3 where the signal is attenuated significantly at the cathode follower. I have no idea how the balancing circuit works nor the mechanical filter but in this area (or rather at the cathode) is where the signal virtually vanishes, Yet a NEON lamp further on in the circuit lights up in unison with input so somehow part of the circuit sees the signal but quite how it does so across the balancing circuit is beyond my ken. V3 is the I.F amplifier but I found no frequency present on the anode of V3 only a DC level. I assume the carrier signal is modulated by the incoming audio AC at the balancing circuit area? Finally, I have checked the voltages by the chart and had to change some resistors, the levels are about right but not perfect. If somebody could please explain what I should be seeing at the cathode at pin 3 across C6 and also perhaps the same across the balancing circuit I think I would then comprehend it a little better. Well, I hope that makes some sense, Look forward to anything you could suggest
Many thanks
Carl

[hweng007]

Any further comments or advice would be appreciated if any of you have experience with this tx

[Radio Wrangler]

By 'balancing circuit' do you mean the balanced modulator?

The balanced modulator turns the audio signal into a double sideband suppressed carrier AM signal. Because of its balanced nature, you should se no, or very little audio signal at its output, nor should you see any of the carrier signal (from the carrier frequency crystal oscillator) at the output of the balanced modulator.

The mechanical filter (one made by Kokusai, if I remember) is a very selective filter which allows through only one of the sidebands of the modulated IF signal from the balanced modulator. So you now have a Single Sideband signal at the IF. Other mixers and oscillators then translate this IF signal up to the tuned RF frequency to drive the power amplifiers.

If you're not familiar with the filter method of SSB generation, there are explanations in bothe the ARRL and RSGB handbooks.

The mechanical filters in these sets are known to fail. They consist of a small box containing a group of metal discs which resonate at the IF frequency. THey are coupled together mechanically by little wires welded onto their edges. A transducer feeds vibration in at one end, filtered vibrations exit the other end into a second transducer which converts them back into electrical signals. The resonator structure is supported in plastic foam. Unfortunately by now many have had their foam turn into sticky goo.

THer is a report somewhere of someone fixing one. but mostly, peeople substitute ceramic SSB filters.

For CW transmiission, I think the balanced modulator is deliberately unbalanced to let the carriier signal through, and no audio is applied.

David