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dtvmcdonald · 2nd Dec 2020, 11:18 pm

A part for my SX-88's dial indicator problem got delayed, so I was able tp get the
RA18C12 on the bench. First I went looking for mistakes I made when recreating
the AGC after the Mad Clipper got at it. There were none.

I did lots of experimenting. My simulations turned out to be reqsonably sane.
The unfixable problem was as I suspected: the 2nd IF stage simply can't produce enough output current to charge up the AGC fast enoufgh above a certain
signal level. Decreasing series resistance values does not help, its in the tube itself.
The only cure would be a big fat transistor emitter flowwer.

That said, of course up to some signal level it CAN and mods do help, quite a bit
it turns out. Here's how I have it at present, presented as a recipe.

1. Remove C159B and C173, both 0.1 uF.
2. change C182 to 0.27uF
3. Disconnect one end of the R81B - R81C chain and check to be sure its in fact 20 Megohm. Replace if bad ... mine was way off.
4.Replace R118 with 4.7 Meg
5. Disconnet the wires that go to the wiper of S6, the AGC speed switch. Stow them away for a moment.
6. Remove the wires that go to the "Short" side of S6 and connect to the wiper.
7. Put capacitors to taste from the Long and Short sides of the switch to ground.
I used a 1.0 uF 250 volt film cap and 1 4.7 uF 63 volt electrolytic, negative side to the switch of course. For electrolytics I recomment 105 degree expensive Nichicons but Panasonics are OK too.
8. Take one of the wires from Step 5 and stick through one of the unused holes in the switch, and solder to the other one. This holds them securely enough.

That's it for changes. The meter will need a rezero. The AGC connection on the back
can still be used to monitor it or you can add even more capacitance across it.

The main difference that I noted is that the fall time to rise time ratio is noticeably higher, dramatically so if the signal change upward if small enough to not overload
the signal amplification chain. If it does overload the unpleasant effects are
smaller.

I will report back later after trying it under many different band conditions. No local
S9+50 signals have appeared yet.

2nd Dec 2020, 11:18 pm	#12
dtvmcdonald Pentode Join Date: Mar 2016 Location: Champaign, Illinois, USA. Posts: 227	Re: Racal RA17C12 AGC A part for my SX-88's dial indicator problem got delayed, so I was able tp get the RA18C12 on the bench. First I went looking for mistakes I made when recreating the AGC after the Mad Clipper got at it. There were none. I did lots of experimenting. My simulations turned out to be reqsonably sane. The unfixable problem was as I suspected: the 2nd IF stage simply can't produce enough output current to charge up the AGC fast enoufgh above a certain signal level. Decreasing series resistance values does not help, its in the tube itself. The only cure would be a big fat transistor emitter flowwer. That said, of course up to some signal level it CAN and mods do help, quite a bit it turns out. Here's how I have it at present, presented as a recipe. 1. Remove C159B and C173, both 0.1 uF. 2. change C182 to 0.27uF 3. Disconnect one end of the R81B - R81C chain and check to be sure its in fact 20 Megohm. Replace if bad ... mine was way off. 4.Replace R118 with 4.7 Meg 5. Disconnet the wires that go to the wiper of S6, the AGC speed switch. Stow them away for a moment. 6. Remove the wires that go to the "Short" side of S6 and connect to the wiper. 7. Put capacitors to taste from the Long and Short sides of the switch to ground. I used a 1.0 uF 250 volt film cap and 1 4.7 uF 63 volt electrolytic, negative side to the switch of course. For electrolytics I recomment 105 degree expensive Nichicons but Panasonics are OK too. 8. Take one of the wires from Step 5 and stick through one of the unused holes in the switch, and solder to the other one. This holds them securely enough. That's it for changes. The meter will need a rezero. The AGC connection on the back can still be used to monitor it or you can add even more capacitance across it. The main difference that I noted is that the fall time to rise time ratio is noticeably higher, dramatically so if the signal change upward if small enough to not overload the signal amplification chain. If it does overload the unpleasant effects are smaller. I will report back later after trying it under many different band conditions. No local S9+50 signals have appeared yet.