Re: Greetings and RA.1217
Ian Ropper has worked on these radios, along with many other items from Racal. Go to his website: ?chavfreezone.me.uk You will find that he last restored one in 2010, and he discusses his experiences on his website, in restoring this radio. Best regards, Julian Barnard.
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On 1 Mar 2025, at 20:44, Gianni Becattini via groups.io <Giovanni.becattini@...> wrote:
? Hi Friends,
I am new (only here, unfortunately I am all but new...)
I am trying to restore initially at least one of three RA.1217 I have here.
They are probably the most difficult equipment I worked on.
Is there someone willing to give me some suggestion??
Thanks,
Gianni from Florence, Italy
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Hi Friends,
I am new (only here, unfortunately I am all but new...)
I am trying to restore initially at least one of three RA.1217 I have here.
They are probably the most difficult equipment I worked on.
Is there someone willing to give me some suggestion??
Thanks,
Gianni from Florence, Italy
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Re: Synchronous AM on the RA6790
Hi guys:
Not a technical guy, however the 6790GM here is not improved by the addition of the Sherwood SE-3.
That says something
I do use it anyways however, because the audio out is strong enough to drive bookshelf speakers.
Incidentally, the SE-3 with the Racal ra6217E is a revelation.
Dan from Saskatchewan.
Hi everyone,
?
I have a question for the technical guru's.
?
When I tune in AM stations using SAM on my WJ8711, its quite obvious that is generating and reinserting its own carrier. There is a momentary heterodyne if the AM signal changes in frequency suddenly while it adjusts to the correct frequency.
?
The RA6790, while it performs fabulously well as an AM receiver, does not show any evidence of this happening at all. It also doesn't perform as well as the WJ on selective fading. Why then is it widely believed that it has full time synchronous AM?
?
Cheers,
Peter - VK2AN
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Re: Racal RA6790 Hang AGC problems
Hi Graham,
?
There is a guy in Germany selling Plessey SL621C's for about 28 Euro. Is that the one you are referring to?
?
Cheers,
Peter - VK2AN
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Synchronous AM on the RA6790
Hi everyone,
?
I have a question for the technical guru's.
?
When I tune in AM stations using SAM on my WJ8711, its quite obvious that is generating and reinserting its own carrier. There is a momentary heterodyne if the AM signal changes in frequency suddenly while it adjusts to the correct frequency.
?
The RA6790, while it performs fabulously well as an AM receiver, does not show any evidence of this happening at all. It also doesn't perform as well as the WJ on selective fading. Why then is it widely believed that it has full time synchronous AM?
?
Cheers,
Peter - VK2AN
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Re: Racal RA6790 Hang AGC problems
On Fri, Feb 21, 2025 at 11:54 PM, Peter E wrote:
definitely engages hang AGC in medium
Hi Peter?
?
That looks to be,? I checked again to day, using spec-lab and am carrier, there is a profile attached to the agc recovery, when the? carrier is removed, using the rx off tune noise, its in the? 10's of mS range, but the noise level remains constant, before the rx gain recovers in short time .
not a detailed check, but, enough to 'observe' the? action,? long gives something like 4 second hang , with a [fast] ramped gain recovery.
?
Without knowing the? designe aims, hard to tell if its good, bad or indifferent, the large jump from medium to long is 'novel?
?
?mods to what is a classic receive
?
Yes having ''improved'' radios in the? 1970's only to see 'Non improved' one's change hands for silly money much later on , thats a good code to live by -?
?
The? Plessey AGC generator IC, did? back in the? 1980's provide a? complete? Audio derived agc system , with fast attack , hang, qsb tracking,? noise pulse management and tailored release.
?
I used one in a ''add on box'' , for the? RA17,? Using the? audio from one of the? 600 ohm linefeeds, and the? diversity AGC connection, the add on provided hang Agc , with selectable time constants . NO mod to the? RX !?
?
The? Plessey IC required? low level? audio to provided a? AGC output of? 0<>5 V , I used a? high voltage? Op-amp , [inverting, gain ~6] with 50-0- -50 Vdc rail to translate the +5V Dc to? -30 v Dc , which in my RA17 fed the? last valve in the IF, the? RF stage used E180F?
?
That totally transformed the RX [with side band adaptor] not only did the AGC provide? hang , it also provided? qsb tracking, which, was quite odd to watch the? s-meter swinging up and down - when the? Tx stopped, after the? hold period, rapid re-set reduced the? gain to the? point where the Rx noise triggered the threshold level , no loud noise blast ! , if there was a carrier, then the? normal agc took hold , for Am simply switched it out.
?
To function the system needed to over ride the RX agc, which was still working , ie, reduce the? gain slightly, but , in practice with was not intrusive.?
?
I believe at the? time the IC stopped production,? some one did programme a? PIC to do the? same thing, but, to day, with the? min pc's , it should possible to code something that replicates the? action[s] - series of loops monitoring the AF level and the? history , making decisions based on rates of change.
?
With no Tx/Rx mute , it is possible to mute the? 6790 Rx by using the? diversity Agc function -again an? external? circuit?
?
73-Graham
g0nbd?
?
?
?
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Re: Racal RA6790 Hang AGC problems
I can write Arduino code. Might be an interesting project...
However, I am always a bit loathe to do too many mods to what is a classic receiver after all.
Mine definitely engages hang AGC in medium as well as long. Maybe something they changed in various firmwares?
Cheers,
Peter
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Yes just checked ..hang only engages in long?
?
The agc is a little clunky compared to the 1778 ..but that radio was profiled for operator use?
?
Theres opportunity to provide external agc generation accessed via the diversity agc port ..tx mute etc , unfortunately the plessey agc ic is now virtually unobtainable..with no replacement..there was? pic replacement..I'd suspect a few lines of code with
one of the mini Micros would provide a solution..first find someone who can write code ..
?
73 Graham?
?
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Re: Racal RA6790 Hang AGC problems
Yes just checked ..hang only engages in long?
?
The agc is a little clunky compared to the 1778 ..but that radio was profiled for operator use?
?
Theres opportunity to provide external agc generation accessed via the diversity agc port ..tx mute etc , unfortunately the plessey agc ic is now virtually unobtainable..with no replacement..there was? pic replacement..I'd suspect a few lines of code with one of the mini Micros would provide a solution..first find someone who can write code ..
?
73 Graham?
?
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Re: RA1778 Tricks of the Trade
Hi Pete?
?
It was a option fit on the 1771 and 1772 as well..that fit was analog...fun to set up too ....
?
Who else worked in the annext at western road back then ?
?
You can do the same with sdr sharp and a rtl dongle in direct sample ... too?
?
73 Graham?
G0nbd?
?
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Re: RA1778 Tricks of the Trade
Unfortunately my 1779 wasn't fitted with the AFC lock option, however I often use SSB on the appropriate sideband to listen to difficult AM stations such as Radio Pyongyang (for a bit of amusement!).
?
The 5Khz sideband filters make this particularly good, and often one sideband is unintelligible while the the other is perfectly clear.
?
Cheers,
Peter - VK2AN
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I saw a post about the hex files for the 6790 with LF and 1-10-100kc tuning but cannot find it now for some reason, I would like to find those files.
Thanks
W5DTW
--
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MOGMAN
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RA1778 Tricks of the Trade
RA1778 Tricks of the Trade -
?
VMARS AM net resolved as SSB -
using AFC carrier phase lock, full carrier AGC and selectable USB/LSB side band -
eliminating co-channel QRM.
?
Not quite its intended use, but handy as the Vintage net's 'drift the night away'
?
73-Graham
?
https://youtu.be/vdIwBHLOoes?si=5ibtYZyzBVBw22q7
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Re: Racal RA6790 Hang AGC problems
Change all the? 1uF tants in the AGC and meter D/A cts round the? diversity cct's?
the old ones now have voltage/leak problems , which clamps the?
agc pk voltage and makes the? meter read low, as well as preventing?
agc from reacting as intended.?
?
73-Graham
g0nbd?
?
?
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For anyone who is interested, I thought I would do a quick write up of how to change the AGC time constants on the RA6790/GM
There are two separate stages that control AGC on this receiver, a "hang" AGC section and a normal AGC decay section on the main IF/AF board, A4. The hang AGC holds the AGC constant for a period of time then the decay takes over with an RC time constant. This works as follows:
?
FAST AGC setting
In this mode, hang AGC is disabled by Q2, and the AGC decay is set by R55 (1K) and C52 via R65 and R64. You probably should leave this as is.
?
MEDIUM AGC setting
In this mode, both hang AGC and AGC decay are active. Q10 turns on and the hang AGC time constant is set by the parallel combination of R146 (27K) and R45 (330K) and C42.
The AGC decay is set by the parallel combination of R52(47K) and R53(10K) and C52.
?
SLOW AGC setting
In this mode, both hang AGC and AGC decay are active. Q10 turns off and the hang AGC time constant is set by R45 (330K) and C42 only.
The AGC decay is set by R52(47K) and C52 only.
?
Suggested Mods.
In my receiver, the SLOW AGC setting was very slow - somewhere around 4 seconds. The MEDIUM setting, however was too fast at less than 0.5 second. I have adjusted the values as follows to give around 1.5 seconds on MEDIUM and around 3 seconds on LONG with a longer decay to avoid the abrupt AGC cutoff. I ended up with the following values - but feel free to vary these as you like.
R45 - 180K
R146 - 180K
R52 - 82K
R53 - 100K
Another possibility is to fit small trimpots so the times can be easily adjusted.
?
The receiver is now much more pleasant to listen to on SSB and CW signals, especially when using MEDIUM AGC.
?
Cheers,
Peter - VK2AN
?
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Re: Racal RA6790 Hang AGC problems
On 2/19/2025 1:21 AM, Peter E via groups.io wrote:
I assume you just burn the .bin file to an eprom them swap it out?
If so, could I get a copy of the 1, 10 and 100Hz firmware?
I'll send it to you directly. 73- Dan
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Re: Racal RA6790 Hang AGC problems
Hi Dan,
?
I assume you just burn the .bin file to an eprom them swap it out?
If so, could I get a copy of the 1, 10 and 100Hz firmware?
?
My eprom burner is pretty ancient and only takes ascii files, but I might be able to loan one for a while.
?
Cheers,
Peter
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Re: Racal RA6790 Hang AGC problems
On 2/17/2025 2:04 AM, Peter E via groups.io wrote: Aaah that makes some sense. So "Hang" AGC is only used on the long time constant it seems.
I am wondering then, why it is neccessary to have R146 and Q10 at all? Peter, sorry I was working from the 1792 notes, in the GM Q10 obviously comes into effect on the medium speed position as well. ?I think I have three different versions of firmware .bin files if you wanted to try different ones.? Favourite for me is the set with 1, 10, and 100 Hz tuning steps as well as the LF extension. 73 - Dan
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Re: Racal RA6790 Hang AGC problems
Aaah that makes some sense. So "Hang" AGC is only used on the long time constant it seems. I am wondering then, why it is neccessary to have R146 and Q10 at all? Maybe it was used on an earlier version of firmware? or maybe something used for remote control?
?
Anyway, I hope I can workaround this by just adjusting the value of the resistor R53 in the decay AGC section for medium.
?
I am on a trip interstate at the moment so I wont be able to get back to it till the end of the week, but if I get it working well, I'll post something for future reference. It certainlty would be nice to be able to get into the firmware....
?
Cheers,
Peter - VK2AN
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Re: Racal RA6790 Hang AGC problems
On 2/16/2025 12:29 PM, Peter E via
groups.io wrote:
Thanks Dan.
There is actually nothing wrong with the AGC as far as I can
tell, its just that the Hang times are wrong. I am beginning to
suspect that Q2 is being turned on, thus bypassing the effect of
R146 but only when its on Medium AGC.?
Peter, Q2 OFF selects Hang mode, and I think should be ON for
short and medium.? Q10 should be ON for short AGC along with
U12a.? U11a should be ON for Medium, and both U11a and U12a OFF
for long with Hang.
If you can find the big RA1792 Maintenance manual, Chapter 5 has
a very clear explanation of the AGC stuff on the A4 board,
slightly simpler in the 1792 without Q10, but basic operation is
the same.? I always go to that finding it clearer than the GM
book.
I have had 4042s that didn't latch, but that is easy to see with
DC levels.
And I don't think there are any firmware versions that affect
AGC.
73 - Dan
ac6ao / g3ncr
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Re: Racal RA6790 Hang AGC problems
Thanks Dan.
There is actually nothing wrong with the AGC as far as I can tell, its just that the Hang times are wrong. I am beginning to suspect that Q2 is being turned on, thus bypassing the effect of R146 but only when its on Medium AGC.?
Why would it be doing this? and it is not as per the explanation in the manual.
Could there have been something added to the control code in some versions of this receiver?
Cheers,
Peter
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Peter, the circuit is a bit more then those few resistors. The hang time and the decay time are
separately controlled by signals from the latches in 4042s U4 and U13, you need to check that they are sending the correct control signals to the switching transistors Q2 and Q19 and the 4066s U11 and U12.?? It looks to me like something is not switching
when it should.
I was sorting some 1792s recently that had AGC problems and found sending a string of short CW pulses from a signal generator a great help in fixing them.?
73 - Dan
ac6ao / g3ncr
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Peter E