[Premium-Rx] Tuning problem with Racal R1772 / R1776 receiver

[Michael O'Beirne]

Dear Nico

Your fault with frequency jumps by switching the FAST - SLOW tuning rate 
switch is quite common and is eminently fixable (so don't panic).

The problem lies in the rotary encoder producing an unbalanced mark / space 
pulsed output.  There must be a unity mark / space ratio for correct 
operation.

The setting up procedure is set out in  Part 1 of the the fat maintenance 
manuals at section 5- 5.

In essence, with the tuning switch set to FAST or SLOW (not LOCK) you must 
set the probe of your scope to ML27 pin 8 (encoder A output) and adjust R2 
on the potentiometer board (attached to the shaft encoder) for unity mark / 
space ratio.

On newer styles of encoder the VR to adjust is R4.

You may also need to check that the B output  is exactly 90 degrees out of 
phase such that A leads B by 90 degrees.  This is a bit more fiddly and 
needs a DB scope to monitor both channels, with the scope triggered from the 
A channel waveform.

If you dont have the manual contact me direct and I will email you the page.

73s
Michael
G8MOB


----- Original Message ----- 
From: "Nico Dongen" <nico.vandongen at online.nl>
To: <premium-rx at mailman.qth.net>
Sent: Friday, September 10, 2010 3:31 PM
Subject: [Premium-Rx] Tuning problem with Racal R1772 / R1776 receiver


> Hello all,
>
> I recently purchased a Racal R1776 receiver what is the maritime version 
> of the R1772 and it has a fault in the tuning. When I switch the Tuning 
> Rate switch from fast to slow or vice versa the frequency jumps up or 
> down. It is a random frequency change in the range from KHz to 100 KHz but 
> mostly in the10Khz range. It happens most of the time but sometimes the 
> effect disappears for a couple of minutes. Most of the time it will count 
> up each time I switch the Tuning Rate till it reaches the end of the scale 
> but I have seen it count down as well.
>
> I have another strange tuning effect
> When I set the MHz switch for example to 3 MHz and with the tuning knob I 
> tune down below 0 so it starts at 99999 again when it reaches 979.90  the 
> 3MHz light goes off and the 2 and 4 MHz lights come on. The tuning then is 
> locked until I switch the MHz switch to a different range.
>
> I don’t know if the two problems are related but I think it is a problem 
> in the counter chain on the display board.
>
> Any ideas on what could cause this kind of problems?
>
> Regards,
> Nico, Netherlands