[Premium-Rx] Fw: Racal 1772/78 receivers

[Michael O'Beirne]

Gents

The RAF versions of the RA1772 mentioned in several postings tonight were 
almost certainly set up for the Piccolo data transmission system much used 
by the UK military and Diplomatic Wireless Service.

One giveaway is a 400Hz filter offset into one of the sidebands so that the 
CIO is exactly 1.4MHz, derived from the synthesiser and not from the 
wretched drifting BFO.

The second giveaway is the large grey super stable 5MHz frequency standard 
(usually the Racal-Dana 9420 with a nominal temperature drift of 6 parts in 
10^10 per deg C).  I suspect it was overkill for Piccolo.

I am surprised Enrique mentioned that there was excessive bass response with 
his LSB filter.  The -6dB spec of the standard SSB filter is 250Hz to 
3,000Hz, and so I would not expect much bass response.  I wonder if the 
wrong filter has been inserted in his receiver.

For really good bass response for professional rebroadcast applications, 
Racal's RA98C ISB adaptor for the RA17 series used modified sideband filters 
giving a response down to about 150Hz.

73s
Michael
G8MOB

----- Original Message ----- 
From: "Enrique Fernandez García" <enriqueeeeee2001 at yahoo.es>
To: <premium-rx at mailman.qth.net>
Sent: Tuesday, August 10, 2010 8:36 PM
Subject: [Premium-Rx] Racal 1772/78 receivers




I purchased a 'non standard' unit,came from RAF, with these filters fitted 
0,4, 1,24, 2,7 and 8 Khz has the ISB option fitted with AFC.

It affects mainly the mode and bandwidth features. Later I did some 
modifications on the USB and mode swiches in order to have a 'standard' set. 
I removed the 0,1 Khz filter, some wire links need to be changed, and added 
a 6,8 Khz filter instead. ( Thanks Dan for your help ! ).

The difference in listening with the 6,8 and the 8Khz filters it is 
noticeable but for my taste and use I would need something more narrower.

Mainly I use it for AM BCB Dxing on tropical bands but when I need to 
identify a weak station I use the LSB filter with fair results.

My only complain it is that when I try to listen a faint signal it is quite 
difficult to understand the ID or the speech, audio it is too bassy and it 
would need a better audio recovery so I will try the ELPAF audiofilter.
Quite sure that a better headphones would mark a difference but I have still 
not found the perfect match. I tried the Japan Radio Company ST-3 but no 
significant improvement.
Another good option would be to install a 600: 8 audio transformer and 
listening with 8 Ohms headphones.

Now, I am thinking about to get a 1778 , but I do not know if I really need 
it or with the 1772 it is enough.
On latter posts somebody asked for the WJ 8718A receivers , they are again 
for sale at a fair price.

http://cgi.ebay.com/Watkins-Johnson-WJ-8718A-HF-shortwave-receiver-/180532736925?pt=LH_DefaultDomain_0



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