[R-390] R390A- Intermittent sensitivity variation

[Roger Ruszkowski]

Eric,

I will move back over to the other computer and send you a couple of long posts.

The up shot is you have a dirty switch contact in the RF deck band width switch.
An easy fix.
All you have to do is pull the RF deck, clean the switch and readjust it a bit.
Watch for about 20 pages of detail to explain " all ".

You describe a fairly common problem. It likely has been in the receiver for some time.
It is just a mechanical adjustment of the RF deck band switch.

Do not set this adjustment as it says in the TM.
You will want to pull the RF deck and do this adjustment by eye.

The band switch has 6 segments and 6 stops. Thats 36 contact points. Moving both up and down.
Some have B+ (some power) on them. Its a matter of wanting the most surface area between the wiper and the contact on each switch segment possible.
So it becomes a subjective adjustment. What looks best for the RF band switch you are trying to get adjusted is the correct answer.

You may feel the need to clean a contact or wiper in the RF band switch. 
Take the time to get good stuff.
Go slow.
Be carfull how you touch it.
Repairing a band switch wafer after it has been over petted is hard to do.
Better to go gentle first.

Roger AI4NI





Hi Group,
                 I have mentioned in previous posts (Greetings from Sydney, 
ustralia) that I have noticed variation in calibration signal levels with my 
963 Imperial R390A.  After a longer period of investigation, I note that the 
roblem is actually confined to the 16 –32 MHz octave. On any one of the 16 –31 
Hz segments ,  a calibration signal level of say 20dB as read on the carrier 
evel meter can suddenly shift downwards to 10 dB or lower.  The problem is not 
n the calibration signal, because  if I switch back to a signal off the 
ntenna, it has also dropped in level.   This shift in sensitivity is 
ntermittent – it does not coincide with warm up after switch on , but can occur 
t any time – another observation is that sometimes sensitivity can be restored 
y moving the MCs knob a few notches – but not always- sometimes the sensitivity 
estores by itself after an unpredictable period.   I am quite new to the group, 
ut have been studying the Y2K manual , and the wealth of information on the 
390A FAQ page, as well as past posts to the group, but have not yet seen any 
pecific mention of my problem.  Since the sensitivity variation is confined to 
he 16- 32 MHz range, I suspect a problem with the RF deck 16-32MHz circuitry. 
ould it be a problem with the bandswitch contacts in the 16-32MHz section or 
ould it be a problem with the coils and/or capacitors in this section?   Any 
dvice from the group would be most appreciated.   There are no problems with 
ensitivity variation below 16 MHz.
Best Regards to all,
                                    Eric.
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