RA17 Meter

[pwdrive]

I've been following Al's (skywave) post on the other forum with interest, I don't own a RA17 but I do have a RA117.

Looking at an online manual for the RA17 the original version seems to have only two switch position settings for the S meter, one position indicates the current at the detector the other position indicates the AF output on the 600 ohm line.

When it's set to the detector position it would appear to show the current when the BFO is switched on also, this would (I think) make it difficult to get a useful received signal indication when trying to copy a CW signal.

The RA117 has a three way switch as opposed to the two way switch in the RA17, the extra position is used to connect the meter to monitor the 1st IF amp and as such should be BFO immune.

This led me to thinking of a reasonably simple mod for the RA17, it would mean changing the two way switch for a three way one and fitting a resistor and a preset pot.

On the RA117 meter system (the extra switch position) the preset pot (1k) is fitted between the bottom end of the 1st IF screen bleeder network and chassis the slider of the pot then goes to one side of the meter via the meter switch, the other side of the meter (via the meter switch) is connected to the cathode of the 1st IF amp via a 6.8 k resistor.

It should work ok, maybe....anyone got any ideas on that?

I will try and scan the relevant bits of the RA117 schematic to show the detail.

Lawrence.

[pwdrive]

Scan enclosed for that part of the receiver circuit, "E" and "F" on the meter switch connect to their respective points on the Ist IF amp.

Note: the RA117 has an extra mixer stage (4th mixer)

Lawrence.

EDIT: Full schematic here: http://koti.mbnet.fi/~ijl/ra117sch.gif

[Skywave]

Hi Lawrence,
Not wishing to duplicate what I have written elsewhere, but there may be others here, following this thread, who haven't read the thread elsewhere to which you refer.

One modification to the RA-17 that does eliminate the 'BFO signal appearing on the R.F. level metering' is the conversion of the BFO to a product detector. I fitted such (based on a 6BE6) to my RA-17W. That mod. was primarily fitted to improve the resolution of SSB - which it does very well. The disappearance of the 'BFO signal appearing on the R.F. level metering' just happens to be a fortunate by-product.

The technical bit . . .
The modulated SSB signal still passes through the A.M. detector and its load and filter, so the resultant current that is fed to the R.F. level metering is a measure of the instantaneous amplitude of the SSB signal. The modulated SSB signal is simultaneously and separately demodulated in the product detector (using the original BFO oscillator cct.) and the resultant audio is thus extracted. The carrier injection signal thus never 'escapes' into the rest of the circuitry of the receiver. A simple switching arrangement (ganged to the 'BFO' H.T. on / off switch) simply selects the appropriate detector source: A.M. or SSB - and feeds that selected audio to the A.F. amplifier, via the A.F. volume control.
The one real problem in implementing the modification is simply one of fitting all the components that comprise the P.D. into the existing BFO housing: there is very little room to spare and a good deal of 'shuffling things around' in that housing is necessary. So it's more than a 'five minute job', but well worthwhile and once fitted, you tend to forget that the receiver's metering ever 'worked' in a different way, plus the improved SSB resolution. You can set the I.F. gain control to any 'comfortable' setting, depending of the strength of the received signal, noise & QRM, etc. Turning down the I.F. gain and advancing the A.F. gain, when receiving SSB, then becomes a thing of the past.

Al.

[pwdrive]

Thanks for the info Al, so far as I know the American version of the RA17 had the same metering system as the RA117, I have an SSB adapter type RA63G for the RA17, schematic etc is enclosed for anyone that's interested.

Lawrence.

[Skywave]

One modification to the RA-17 that does eliminate the 'BFO signal appearing on the R.F. level metering' is the conversion of the BFO to a product detector.
The one real problem in implementing the modification is simply one of fitting all the components that comprise the P.D. into the existing BFO housing: there is very little room to spare and a good deal of 'shuffling things around' in that housing is necessary.

And here is the finished item. It is shown connected, out-board, to its parent RA-17 for final checks and adjustments, prior to fitting it. The solder reel simply makes for a convenient stand: the valve - a 6BE6 - is inside the centre of the reel!

[attachment=10523]

[attachment=10524]

Al.

[pwdrive]

Where does the PD valve live Al?

Lawrence

[Skywave]

Same place as where the original EF91 was: the valve becomes a 6BE6. You can't see it in the photo., 'cause it's inside the solder reel that is supporting the assembly!
The mod. does not require any mechanical changes to the original BFO ass'y.: just fitting replacement & additional components, except for the frequency-determining components, which remain the same.

Al.

[pwdrive]

Very neat solution Al.

Lawrence.

[Alan Douglas]

Pretty tricky, making the second photo a mirror image.

[Skywave]

Pretty tricky, making the second photo a mirror image.

"A mirror image": I'm a bit puzzled by that comment. All I did was to take one photo and then rotated the assembly through 90°, moved the camera position by another 90° (totalling 180°) and then took another photo.: no cunning photographic or image-manipulation took place.

Al/