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Racal RA6790 Hang AGC problems

 

Hi Everyone,
I have done an alignment on this receiver and it is working really well, but the only slight gripe I have is the hang AGC time constants. On the Long setting, it is incredibly long - at least 4 seconds, however Medium is so short that the AGC "pumps" noticeably on SSB signals.
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I thought it should be a simple matter to adjust some resistor values to something more useful, like maybe 1 second for Medium and 2 or 3 secs for long, but it is proving to be much more difficult than I thought.
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According to the manual, R45 (330K) on the A4 board sets the LONG time constant, and the parallel combination of R45 and R146 (27K) should set the MEDIUM time constant. I have changed R45 to 220K and it does seem to have shortened the long time constant, but changing R146 to a much higher value has no effect whatsoever on the medium time constant. In fact, removing the resistor altogether, still has no effect on the medium time constant?
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Does anyone have any information on what is going on here?
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Please note also, that I want to keep the "hang" AGC and not disable it as per the article by Dallas Lankford.
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Cheers,
Peter - VK2AN
 

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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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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

From: RacalRadio@groups.io <RacalRadio@groups.io> on behalf of Dan Rae via groups.io <danrae@...>
Sent: Monday, 17 February 2025 4:01 AM
To: RacalRadio@groups.io <RacalRadio@groups.io>
Subject: Re: [RacalRadio] Racal RA6790 Hang AGC problems
?

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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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
 

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?
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Anyway, I hope I can workaround this by just adjusting the value of the resistor R53 in the decay AGC section for medium.
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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....
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Cheers,
Peter - VK2AN
 

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
 

Hi Dan,
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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?
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My eprom burner is pretty ancient and only takes ascii files, but I might be able to loan one for a while.
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Cheers,
Peter
 

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
 

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.?
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73-Graham
g0nbd?
?
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Yes just checked ..hang only engages in long?
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The agc is a little clunky compared to the 1778 ..but that radio was profiled for operator use?
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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 ..
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73 Graham?
?
 

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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

From: RacalRadio@groups.io <RacalRadio@groups.io> on behalf of Graham via groups.io <g0nbd@...>
Sent: Saturday, 22 February 2025 8:56 AM
To: RacalRadio@groups.io <RacalRadio@groups.io>
Subject: Re: [RacalRadio] 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?
?
 

On Fri, Feb 21, 2025 at 11:54 PM, Peter E wrote:
definitely engages hang AGC in medium
Hi Peter?
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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.
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Without knowing the? designe aims, hard to tell if its good, bad or indifferent, the large jump from medium to long is 'novel?
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?mods to what is a classic receive
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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 -?
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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.
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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 !?
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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?
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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.
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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.?
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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.
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With no Tx/Rx mute , it is possible to mute the? 6790 Rx by using the? diversity Agc function -again an? external? circuit?
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73-Graham
g0nbd?
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Hi Graham,
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There is a guy in Germany selling Plessey SL621C's for about 28 Euro. Is that the one you are referring to?
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Cheers,
Peter - VK2AN

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