Rick -
The '40M' peak in the 80M region of the RF TUNE usually indicates that there is no PreMixer signal
coming to the 1st Mixer. The PreMixer is derived from the BAND oscillator and the PTO.
Example.
BAND Oscillator (18.1 MHz) <minus> PTO set to 0.000 (5.455 MHz) <equals>
PreMixer (12.645 MHz).
PreMixer (12.645 MHz) <minus> Carrier Oscillator (5.645 MHz) <equals> Mixer
Output (7.000 MHz).
Absent a PreMixer signal, the RF TUNE can tune the RF chain to the 5.645 MHz CO frequency which is
then amplified by the rest of the RF chain. Easiest test, if you have the pair cabled together for
transceive is to switch to RCVR control, with the PTO set to 0.000. This routes the identical
PreMixer signal from the receiver in place of the one in the transmitter, and should result in an
output signal on 7.000 MHz.
The 6CA5 is a 'fair' replacement for the 6EH5. It's only a little over one half the
transconductance of the 6EH5, but will probably work OK. I'd be interested to hear what difference
(if any) you note when you get a 6EH5 in there.
The 15M problem is obviously in the BAND oscillator, and could be caused by dirty bandswitch
contacts (DeoxIT), or a bad crystal. The corresponding crystal in the receiver is identical and can
be used to swap either way to determine if it's the crystal. You have a scope, just bring the probe
near the BAND oscillator (left front corner with transmitter upside down, panel toward you) and KEY
(PTT) the transmitter. You should see a strong sine wave for each crystal.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Rick Gunderman wrote:
Ok, connected up the AC-4, fired up the transmitter (sorry, poor choice of words) and tried to
tune it up on 40 after adjusting AC-4 bias. It seems my peak when adjusting RF-Tune was down in
the 80M range...I'm not sure if that is normal. Anyway, I need to clean the switches before I
proceed on the transmitter.
On the R-4B, I received the 6CA5 to replace the tube that was broken when I received the
equipment. The receiver seems to work like a charm on all bands but 15, absolutely nothing on 15.
Without an antenna I can adjust the pre-selector for a noise rush on all bands but 15. Where do I
begin?
The audio sure sounds sweet from the MS-4 on the other bands, great audio!
Tnx,
Rick - WA4RG
Test equipment on-hand for troubleshooting: DMM & 100Mhz Scope
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