Also my replica is passig a "smoke-test"

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Dan Minciu
Posts: 3
Joined: 23 Jul 2021, 20:45

Also my replica is passig a "smoke-test"

Post by Dan Minciu »

Yes, after more than a year after enroll into this group of passionates, I manage to get my ParaSet to an usable form, just receiving at this moment.
It's not quite a replica to keep it on a shelf, it's more an usable replica... I want to operate it.
But, as you can see in pics, it had an enclosure with all necessarily accessories, in a single case.
The reason wich makes me write is the fact I can't have a good reception.
Differences from original build are:
regen tube = 6ZH4(USSR version for 6AC7)
AF tube = 6K4 (USSR version for 6SG7)
normally USSR equivalent for 6SK7 is 6K3 but... I don't find it yet.
So... with 3m of antenna I have very weak receive of an chinese station on approx 7MHz and nothing else.
The headphone level is very quiet(like a whisper, approx 1mW).
And finally: does anyone tell me the level of audio signal getting out from AF tube plate?(measured on a good received station)...

Thank you,
Dan Minciu, YO6DVA
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Parasetguy
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Re: Also my replica is passig a "smoke-test"

Post by Parasetguy »

Hi Dan,

Great you came so far. It is not easy to give an answer to your problems, because you diverted far from the original diagram. So you can't compare your setup to a "original" set.

The tubes you used can be pin-compatible but what about the amplification factor?

3 m of antenna is too little. This antenna will receive well signals in the 6 m band, but not in the 40 and 80 m band. You need 20 m of wire ( 20 m = 1/2 wave on 40, 1/4 wave on 80m).

What about the coupling C between antenne and var.cap.? Smaller = less signal, larger = more signal. Should start at 100 pF.

Then, you use a type of variable cap that I don't recognise. It is air or mica isolated? And is it 98 ~ 102 pf? This determines the tuning range. Regen receivers are, when built properly super sensitive and tricky to tune.

In the audio stage: what rf choke did you use? 60 H gives louder audio than the original 36 H.

Hope this helps,
Kind regards,
Henk - Parasetguy
Dan Minciu
Posts: 3
Joined: 23 Jul 2021, 20:45

Re: Also my replica is passig a "smoke-test"

Post by Dan Minciu »

In audio, it's an output transformer 7k/8R from an old radio. And I'm using 8ohms earphones(stereo16ohms, channels in parallel).
The tubes are given as electrically compatible, not just pin-to-pin, with USA-types.
Also, tubes transconductance is a little greater than orig.6SK7... 6AC7(USSR 6ZH4) has 9mA/V(9000mhos), 6SG7(USSR 6K4) has 4mA/V(4000mhos) versus 6SK7 with only 2.35mA/V(2350mhos)...
Variable caps are salvaged from transistor radios, 125pF poliester/sort of a plastic foil...
Antenna coupling is 100pF...An yes I'm thinking that antenna is a little too short. But access on the roof is....denied by neighbors.
So... My next step will be to try to increase Ra of the output transformer(audio) and another (hopefully better) antenna.

Thank you very much for your time and advice,
Dan Minciu,YO6DVA
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Parasetguy
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Re: Also my replica is passig a "smoke-test"

Post by Parasetguy »

Well, the audio transformer will do, giving enough audio. Test the amplification by injecting an audio signal on point 4 of the second SK7. Use a capacitor 100 nf 400 V in series for DC blokking to the sound source.

Concerning antenna: for receiving be creative. A metal rain gutter will do. Wire, hanging out of a window, washing line. For reception will do. Your 3 m antenna is resonant for 50 MHz, see table below.
Transmitting is a different ball game.

Var.Cap: Not using +/- 98 pf (100) will result in a shift of the tuning range, up or down. I don't know what the influence of the foil cap is to the quality of the tuning circuit (L and C).

I took some measures this morning. Do not have a 125 pf varcap.

Table frequency range varcap-rx-coil
f = 1 / (2 * π * √(L * C))
varcap RX-coil resonant
when value pf ɥH freq. MHz
open 5 23 14,841275
meshed 100 23 3,31861
meshed 158 23 2,6401443
(data from "Hamic" resonance calculator)

If you look at the inclosed diagram, you will notice that the frequency range is not liniar, but logarithmic.
Look at how many scale divisions the 80 m band has (3500~3800) in compare with 40 band (7000~7200).
You can imagine that tuning for weak signals is nearly impossible and you need a good antenna to receive at least strong signals. What you don't put in, won't come out.

Hope this helps
kind regards, Henk - Parasetguy
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Parasetguy
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Re: Also my replica is passig a "smoke-test"

Post by Parasetguy »

That table in my post is hard to read. Here it is again as a picture.

Henk
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Dan Minciu
Posts: 3
Joined: 23 Jul 2021, 20:45

Re: Also my replica is passig a "smoke-test"

Post by Dan Minciu »

Good evening Sir,
Thank you so much for "wasting" your time with a newbie...
Info you provided are extreme valuable for me... I understand also the first table, just moved it into an editor with CourierNew font and...tadaaa. it's aligned back.
AF tube was tested and it needs 30mV to get strong(approx 100mW) sound in headphones...
I have a signal generator wich covers 1 to 110MHz at 1uV to 1000mV amplitude... It's a laboratory toy, cca 6kg of electronics... It's functional but I don't know his actual precision... Trying to suppose errors should be beyond 10%...
With it, I can say 300uV sensitivity in antenna for something barely audible...
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Parasetguy
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Re: Also my replica is passig a "smoke-test"

Post by Parasetguy »

Hi Dan

Quality signal generator:
Be aware that the original Paraset was built with components that had a 10 ~20 % tolerance. There are no alignable components, and the designers were satisfied with the way it was.
They provided for every set a taylor made diagram so the operator could more or less get the tuning in the area that he was supposed to listen. Strong signals from England made him find the broadcast for him.
This is a total different use then what HAMs want.

To know how precise your generator is, have it generate a signal that you can listen to, on another radio. That gives you an idea of its precision. Good enough for the Paraset.

kind regards,
Henk - Parasetguy
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