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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 23rd Dec 2011, 10:14 pm   #1
glowinganode
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Default VMARS RA17 product detector modification

Hi all, I've carried out the product detector modification as described here with good results.
The audio is very clear, and the agc now works properly.
I did find using even a 2pf capacitor provided too much coupling of the if signal, so i just used a couple of twists of wire.
Should pin 7 have a grid leak resistor (22k?), at the moment it's floating about dc wise.
Rob.
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Old 24th Dec 2011, 10:38 am   #2
Chris Parry
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Default Re: VMARS RA17 product detector modification

Pin 7 should indeed have a grid leak resistor to ground. Value 22k sounds suitable. There is one other significant design or drawing error. The 8uF HT decoupler will not reliably decouple to 100kHz I frequency. Therefore it should be shunted by 100nF. Otherwise, the circuit appears sensible. Chris.
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Old 24th Dec 2011, 10:42 am   #3
Chris Parry
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Default Re: VMARS RA17 product detector modification

Typo in my post above: I was trying to say that the 8uF will not reliably decouple the 100kHz IF frequency which is present at the valve anode. Hence the need for extra decoupling which will be properly effective. Chris.
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Old 24th Dec 2011, 12:39 pm   #4
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Thumbs up Re: VMARS RA17 product detector modification

Rob -
Your report is very welcome. That modification is amongst my 'round tuit' jobs: been in that pile for ages. On the basis of your report, it has given me a bit more impetus to get on with it!

Oh, and yes: a Merry Christmas to you and yours.

Cheers,
Al.
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Old 24th Dec 2011, 12:51 pm   #5
glowinganode
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Default Re: VMARS RA17 product detector modification

Thanks guys.
The 8uf capacitor is definately required, as without it hums and motorboats.
I did add an extra 0.1uF decoupling, fitted inside the screened bfo compartment.
It's a neat mod, as only pins 5,6 and 7 require disturbing and can be accessed quite easily.
I didn't have an EK90 to hand, so I used the one from the harmonic generator and fitted an EH90 in its place.
Best wishes to you Al and everyone this Christmas.
Rob.
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Old 4th Jan 2012, 10:16 am   #6
amornummi
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Default Re: VMARS RA17 product detector modification

I tried the 6BE6 as bfo + product detector and didnt like it. Due to "lock in" and distortion.
I ended up putting a 12AT7 in , half as bfo, and the other as audio amp. Detection by diode bridge driven by 75+75 turns added on bfo coil.
The addition of a pnp emitter follower to drive the agc line (from 12 volts only) makes the ssb performance equal to any modern rx.
Also , get double the audio power out by using the "aux" op EF91 in parallel with the main o/p - 0.47 mf between anodes to keep extra dc from o/p tranny.
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Old 4th Jan 2012, 11:04 am   #7
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Default Re: VMARS RA17 product detector modification

Nice one, good thinking with the audio amp.
I replaced mine with a 6AQ5 and an output transformer from a scrap domestic set. I fed it from the anode of the 600 ohm line output valve as there wasn't sufficient drive. This also allowed me to replace the 2Mohm AF control with a more common value 470kohm, the brush had worn down to the metal.
I've boxed the RA17 up now, I want to run it for a bit before I attempt alignment as I'm sure there are one or two gremlins still to come out. I've had some quite obscure faults with this set (it got dropped), including intermittant loom cores, jumping capacitors and a semi conducting insulating sleeve that took a long time to track down. This last fault made a really stringe ringing / whistling noise rather similar to a microphonic valve.
Any further improvements in SSB resolution will be carried out in the from of an add on convertor, as I'd like to eventually return the set to standard.
Is there any chance you could post some circuit diagrams of your modifications please, as then all I need to look out for is a spare BFO.
Kindest regards,
Rob.
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Old 16th Feb 2012, 12:53 pm   #8
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Default Re: VMARS RA17 product detector modification

Circuit of my modified BFO unit at =
http://homepage.ntlworld.com/p.denby...a17proddet.JPG
Serious stuff , complete dismantle and fit B9A socket.
I used a scrap small relay for thin coil wire.
The low pass on the detector o/p cuts at 3 kc/s , so 0.047 cathode bypass restores.
The 560 pf at the o/p anode bypasses some rough harmonics which couple in the valve.
The CT on the BFO winding is earthed , as lots of 100Kc/s present.
Test the input transformer has enough inductance to work at 100kc/s
Voltages marked on cct.
I had to put an additional 56 pf on the bfo tuning for correct freq.
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Old 16th Feb 2012, 2:17 pm   #9
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Default Re: VMARS RA17 product detector modification

Thanks, I'll wait till a spare BFO comes along to play with.
Rob.
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Old 17th Feb 2012, 7:49 pm   #10
Bazz4CQJ
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Default Re: VMARS RA17 product detector modification

Could someone tell me what "lock on" means in this context?

I am spying on your thread, on the lookout for info on product detectors that I could apply to my HRO
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Old 28th Feb 2012, 7:42 pm   #11
amornummi
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Default Re: VMARS RA17 product detector modification

Lock in occurs when the IF input signal causes changes in the BFO frequency.
Due to coupling between signals.
Dosen't need much to upset , especially when both signals nearly same freq.
The 6BE6 has perhaps 20 Db isolation , ring mixer has twice this , plus another 10 Db due to extra audio gain.
The audio out is clean sine wave down to 20 c/s.
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Old 11th Mar 2012, 7:54 pm   #12
amornummi
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Default Re: VMARS RA17 product detector modification

My audio output mod , of putting the aux EF91 in parallel with the main o/p.
You must put grid stoppers or ferrite beads on at least one valve.
I have just discovered oscillations at 300Mc/s in a set I am doing up.
Showed up as distortion , which stopped when either valve unplugged and minus 0.5 v on grid. Poking bypass C or meter leads had little effect.
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