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Dear all,

A happy new year to everybody on this marvelous group!

For weeks I am trying to tackle a weird problem in a recently acquired T-4XC.? In stand-by (or RX) mode, with the TRANCEIVE switch in either XMTR or RCVR position, the rig generates a low RF signal (modulated by the connected microphone!) on a freqency set by the respectve PTO.? The connected R-4C then receives this signal via its antenna input which overrides the weaker signals received via the antenna. This signal is quite strong, S9+.

When tuning the respective PTO, that RF signal moves along, hence is received anywhere on the band....... This happens in both AM and SSB mode.

So, in stand-ny (RX) mode, all oscillators and mixers seem to be opartional, hence f(RF) = f(xtal) - f(PTO) - f(carrier osc) generating? signal that is being picked up by the R-4C.

Question 1: Which of the oscillators, pre-mixer and mixer are switched off in the?stand-by (RX) mode?
Question 2: What is the voltage of the cathode (pin 7) of the Mixer V3 (6AU6) in?stand-by (RX) mode? Mine is 10 volts, rather low, suspecting that it is thertefore still operational (mixing) hence generating the unwanted f(RF) product.?

Any thoughts?
73 de Rob PA9Z

Dear all

A small addition: the unwanted signal does not appear when the MODE is switched to CW (of course again in stand-by (RX) mode) of the T-4XC

73 Rob PA9Z

Hi John

Thanks for thinking with me. Yes, I have another one, a Sure, but that is shortened when the PTT button is not pushed. Then no signal in SSB is generated, however when switching to AM of course generate an (unmodulated) AM carrier is generated.

Rob

Update:? I connected my other T-4XC to the setup and discovered that pin 7 of Mixer v3? of my other rig is 240 Volts in stand-by (RX) mode.......? I swapped the 6AU6 but to no evail.? WIll continue to look why the Mixer's cathode has such low voltage as I am getting convinced this is the reason why it is still mixing the carrier oscilator and injection frequencies while in stand-by (RX) mode.

Will keep you posted?
73 de PA9Z

No, the rig is not transmitting - no bias current shown - it is defenitely in standby/RX mode.

Rob,
To get a signal out on the band, you do need two things.? The PTO and XTAL oscillators need to be on, and the V3 mixer needs to be on.? You
have correctly suspected these.

I don't know for sure in the T-4XC but in the T-4X, when the transceive switch is in RCVR, the power to the PTO and XTAL oscillators are turned off.
If the C is the same, that would indicate to me that the transceive switch or wiring has a problem.

The cathode of V3 is grounded by the transmit relay with the same wiper that is used for the MUTE line to the R-4.? It sounds like your cathode is always
grounded, possibly a relay or wiring problem.

Has this T-4XC ever worked for you?? If not, I'd look for suspicious mods.? It would seem odd to me to have a double failure as indicated by the above.
Does your second T-4XC work correctly?? If so, the R-4 and cables is all good.
73,
Gary
WB6OGD

*** SOLVED ***

Very good work Rob!? Great to save another one.

I suspect that resistor was in the mixer cathode as you suspected.
I think that rig might have been used with an external transverter where they just wanted the signal, the TR switching was done in the transverter.? Or not...?
Does it also have the other normal transverter mods (screen voltage RCA jacks)?
73,
Gary
WB6OGD

?

Rob PA9Z writes:

After hours of wire tracing and elimination of possible defective component=
s, I finally ended up at a..... stupid mod made by a previous owner for wha=
tever reason.=C2=A0 One single resistor that screwed up the behaviour of va=
rious functions of the T-4XC.=C2=A0 @Gary: thanks for the tip!

Oh come on! At least tell us which one it was. The suspense is
killing me! :-)

--lyndon

Please tell us which resistor in case someone else has the same difficulty.

--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998

Oh boy, what an enthusiasm ?

Well, the culprit was a resistor of 270 Ohms connected to R59 (to the upper lead in the schematic) and the chassis. Hence pulling down the voltage of a.o. the cathode of the mixer V3 in stand-by (RX) mode, so activating it...
The mod also activated the Carrier Oscillator in AM and CW mode in stand-by mode.

No idea why someone put it there.

Anyway, first QSOs have been made successfully with good reports.

73 de Rob PA9Z