Adding a Product Detector to an HRO-MX

[Ricardo]

I am thinking about adding a product detector to my HRO-MX to see whether it makes any significant difference to the ease of resolving SSB signals.

I want to keep the modifications as minimal as possible and they need to be readily reversible.

I have found a design using a 6BE6 valve on the James Millen Society website which looks quite promising. Here is the link: http://www.isquare.com/millen/Articles/hropd.html


Does anyone have any experience of using this particular design? I am uncertain as to what changes (if any) are required to the existing circuit around V7 (6B7), 2nd detector, first audio valve in the HRO-MX? The article about the product detector says that its output connects to the volume control in the HRO which means that V7 must be retained. I assume that the existing BFO connection from V8 to V7 is disconnected and re-routed to the product detector.

Any advice or alternative circuit suggestions would be welcomed.

[ms660]

I think there's a mistake in the articles description...." the BFO ON/OFF switch is rewired to select the appropriate detector output being fed to the volume control"

AVC switch maybe...

Lawrence.

[Radio Wrangler]

A lot of the value in having a product detector lies in taking the BFO signal away from the AGC detector. This means you can have AGC running with SSB and CW, though you may want to switch appropriate time-constants in.

This allows you to have plenty of BFO injection. I'm not sure about a 6BE6, I'd probably go for something at least single-balanced like an ECC81 or 82

David

[Ricardo]

Thanks for the replies.

Lawrence, As you say I think that the author most likely actually used the AVC switch in the HRO as switch S1 in his circuit. The BFO On/Off switch is only a single pole switch.

Having studied the HRO circuit more closely it appears as though the Pentode section of valve V7 (6B7), 1st audio amplifier can be left un-modified and the switched audio signal from the new detector circuit(s) connected to the volume control as the article author says.

David, Thanks for your comments. It seems to me that the 6BE6 based circuit in the article has the features that you have described and I happen to have one in my junk box, so I might go ahead and experiment with it.

[G3PIJpeter]

Have a look at the paragraph:

"... Incidentally, because the old AM receivers were designed
for several volts at the detector, a product detector
using the original pentode BFO and the IF signal
coupled by a small capacitor to the suppressor
grid works very well."

. . . in the VMARS newsletter article at:

http://www.vmarsmanuals.co.uk/newsle...test_entry.pdf

- Peter G3PIJ

[Ricardo]

Hello Peter (G3PIJ),

Thanks for the reference to the VMARS newsletter article. I am not too sure quite what product detector the article is talking about? I thought the HRO circuit uses a diode envelope detector for AM and the CW OSC (BFO) is not coupled to the suppressor grid.
[The 6BE6 circuit in the article by K9EUI linked in my original post has the BFO connected to the oscillator grid.]

David (Radio Wrangler),

I have now found a few ECC82 double-triode valves so if there is a better circuit design using that valve I'd be interested in seeing the details.

[ms660]

"... Incidentally, because the old AM receivers were designed
for several volts at the detector, a product detector
using the original pentode BFO and the IF signal
coupled by a small capacitor to the suppressor
grid works very well."

Maybe that means disconnecting the existing BFO's suppressor grid from chassis and connecting it to the IF out via a small value capacitor.

Lawrence.

[G3PIJpeter]

The sixth circuit diagram down the page at

http://www.noding.com/la8ak/f12.htm

shows the modification. It uses the BFO itself as a product detector.

- Peter

[Radio Wrangler]

Double triode product detectors, I've seen several, but off the top of my head I remember one in George Jessop's book of amateur radio circuits. A browse through the tech Topics CD could be successful, too, I think.

The real benefit is an having a separate detector to the one used for AGC.

David

[G6Tanuki]

The problem with trying to retain AGC when using the diode detector of a 'classic' receiver that's been retrofitted with a product-detector is that the traditional diode-AGC detector is designed to average the carrier and of course the carrier's not there with SSB!

The "RF" component of SSB varies at audio frequency, which the time-constants of the traditional receiver's AGC circuit are not really able to handle - so you get an effect where the first few syllables of speech horribly overload the receiver, and noise comes-up in the gaps between subsequent sentences.

For SSB you need a fast "attack" on the AGC line to de-sense the receiver in the first few milliseconds of the appearance of the audio signal, followed by a long (half-a-second at least) 'hang' time between the signal going-away and the receiver's sensitivity coming back.

It's all about the time-constants - something which in the past has caused a lot of disappointment to owners of classic receivers who've tried to fit product-detectors and still use the original diode for AGC.

[Bazz4CQJ]

You may find it useful to search Google for articles dealing with adding a product detector to the AR88. I seem to recal that there were a good deal more of these than for the HRO, and there may be useful ideas/insights to steal from them. I think there were articles for other (later) military receivers (the RA17 ?) that were in production before SSB became the norm. One guy built a DFM and an "all mode adapter" in a single external box for the RA17).

This is a project that has crossed my mind a few times, and I spent some time researching it, but it has never made it to the top of the "To Do" list. I'll retrieve the file from the archives and see what I can find.

B

[Bazz4CQJ]

The article on the RA17 is here http://g3ynh.info/Racal/RA17adap/RA17adp1.html one of five parts.

As can be seen, this addresses all the issues that apply to the HRO, including that of the AGC speed. Using the external chassis, which sits neatly above the main RX (a "set-top box"!), is a neat way of being able to experiment with a plethora of options with only limited changes to the HRO itself. Of course, poor old Dave Knight had to wire up a bag-full of IC's to produce a DFM, whereas a module can be bought online for a few quid now. I think some form of AF processor could also be added for listening to SSB (filter and expander?). Also, if you are not wedded to the original HRO PSU, something rather better could be squeezed in to the same box.

Of course, my own HRO has been undergoing a "Mid-Life Update" which started in ~2010...and is a "work in progress" .

B

[Jon_G4MDC]

I put a DSP dongle on one of mine. Was that the 1st HRO with a USB connector? Probably not.

It turned up some very interesting results - but sadly it was not an HRO anymore. I have removed it now before being struck by some rather potent lightning.

[Stockden]

The circuit that Peter drew attention to is originally from Technical Topics in Radcom. According to this page on the same site it was described in Jan 2004 issue (p.45) and again in Jan 2005 (p.75.).

I should have both those issues somewhere so I'll try and dig them out and have a look.

Hugh

[G3PIJpeter]

With reference to the 'RZP circuit, would the BFO act as an AM detector if the oscillation was killed - possibly by grounding the cathode via a 150 ohm resistor bypassed by 0.01 mFd?

The BFO on/off switch could be re-routed to switch this R / C in and out of circuit and thus meet Ricardo's original aim of: "... I want to keep the modifications as minimal as possible and they need to be readily reversible".

But it all depends on the answer to my question above.

- Peter G3PIJ

[Ricardo]

All,

I am continuing to follow the discussions with interest and have yet to decide how to proceed with any potential modifications.

There is a lot of information about modifications to other vintage receivers on the internet for improved SSB reception, but not too much that I can find specific to the HRO apart from the G3RZP circuit. I'd like to see the associated Radcom Technical topics articles, but my RSGB membership lapsed a long while ago and the most recent Radcom magazines that I have date from 2001.

G6Tanuki, I appreciate the important advice regarding the need to modify the AGC circuit for a fast attack and a long hang time if it is going to remain operational during SSB reception.

Bazz4cqj,

Dave Knight, G3YNH's set-top adapter unit for the RA17 is very impressive and I quite like the idea of a separate external unit. However even if the parts for such a design are obtainable now it is a more extensive construction project than I have in mind at present.

John G4DMC, I am not quite sure what you mean by a DSP dongle? I have come across a number of kits to convert an 455khz IF radio signal to a 12 kHz audio signal that can be input to a PC soundcard and then processed in software. It might be interesting to play with something like that, but my preference would for a standalone converter which doesn't require a PC to be connected.

[Bazz4CQJ]

Originally Posted by Ricardo

Dave Knight, G3YNH's set-top adapter unit for the RA17 is very impressive and I quite like the idea of a separate external unit. However even if the parts for such a design are obtainable now it is a more extensive construction project than I have in mind at present.

Ah, I'm not at all suggesting any kind of replication of that unit, but rather, the idea of having an external box as a "playground" where could experiment with whatever you want. That might simply be an "SSB circuit", or whatever else you fancy.

For instance, good SSB reception requires some stabilisation of the HT supply; certainly, I found it quite chirpy without stabilisation, which I achieved simply by adding a neon lamp withing the existing chassis, but I like the idea of putting a new power supply in the box, together with a small loudspeaker.

During the course of this thread, promted by re-reading Dave Knight's article, I managed to dig out a 17" wide chassis which I kind of forgot that I had and it sits nicely on top of the HRO. I started back in 2010 to replace every component in the set (which had not been used for decades and was very, very dead). It's not back in action yet, but maybe this winter ?

B

[radiotechnician]

I too have an HRO. It is in a *rack with the power supply, and has also a case of coils.

Here are the handwritten notes from then. My HRO was retrofitted with modern tubes as per E H TROWELL , R.S.G.B MARCH 1957.

The additions are in a grey box connected by the cables.

I use an Icom 746 Pro now.

*The rack will probably be turfed out, after I pass. The rats will grab the 746 and dance away laughing. Haven't switched on the HRO in years.

VE7ASO

[Jon_G4MDC]

I also have a ~1938 HRO fitted with "modern tubes" and lots of mods.

It came with associated paperwork - most of the articles published between
1947 and 196x-7x when people lost all interest and stopped publishing.

There are notes on scraps of cornflakes packets, letters and replies to National themselves etc etc. I see all this as personal to the previous owner so I'm not letting it out, nor am I touching the set either - because it is what he made it. And it should stay that way.

I reckon it's marginally stable - if he could fly it and enjoy it - that's great.
Against an original which i also have - I would call it "tweaky" but I haven't used it daily and so not followed its progress and learned it's quirks.

[Stockden]

I've now managed to locate the TT articles referred to earlier but sadly neither of them adds much to the discussion. The first is mainly the continuation of a "debate" (an argument might be a better description) over the merits of the HRO and the second is really an exposition on product and envelope detectors. Neither of them contain any explanatory text relating to the diagram.

I'm happy to scan either or both articles and add them to the thread if you wish but I'm afraid that it's going to be about a week before I'm able to do it.

Hugh