HRO 'Set Top Box'
 

This is my HRO ‘Set Top Box’, first discussed in a thread about HRO product detectors a couple of months ago. This is at an ‘early prototype’ stage and contains only the new PSU for the receiver and the DFM. The front panel is a just a ‘sacrificial’ scrap of plywood, pending decisions about what will finally end up in the box and what the front panel needs to include. The unit is a little taller than I would have liked, being dependent on the size of the HT transformer. The HRO itself is (slowly) being completely rebuilt, but is now at a stage where I can start applying power.

When the HRO is finished, the next item to go in the box will be an audio processor module with a bandpass filter from 200Hz-2.5kHz and which also includes an ‘expander’ circuit. That is actually an old Maplin kit, dating back to those distant days when they did good stuff. It was originally designed as an 'add-on' for use with an Eddystone EC10. Later on, I might get around to putting a product detector in. I hope there will be enough space on the front panel for a small, front-facing loudspeaker.

I think the black paint will need to be replace by a suitably respectable grey.

Re: HRO 'Set Top Box'
 

Looks like a great project! Please keep us posted.

Re: HRO 'Set Top Box'
 

Thanks Phil. I pinched the idea from a guy who had done something similar with an RA17 and wanted a DFM on it. That was some years ago and the DFM was made of individual ICs, so that alone took up most of the box. In my case the DFM is a module, so small, it takes ~1%?

I'm wondering what to do about the HRO's crystal filter; that is really only much use with CW reception. Considering the large volume it occupies, I think something more appropriate to AM/SSB reception could be done in terms of selectivity. Other than that, I'm leaving the RF and IF stages strictly as per original.

B

Re: HRO 'Set Top Box'
 

I'd go with a modern audio DSP filter module. They seem small, cheap-ish and plentiful. I like DSP and it works very well, the noise reduction can be really impressive and no need to modify the radio's RF or IF circuits.

A really narrow xtal filter at IF is great for CW, but the radio needs to have very low drift!

Re: HRO 'Set Top Box'
 

Could you perhaps still keep the narrow xtal filter but apply some degree of treble-boost after it?

In the spirit of Dr. Robinson's "Stenode Radiostat"...

Alternatively I'm wondering if a relatively simple DSP sampling at the IF could be used: a Raspberry Pi and a small colour LCD panel could then give you both DSP noise-reduction and a "panadaptor" type display over a 5KHz band-window.

Re: HRO 'Set Top Box'
 

Mmmm, I'm afraid I'm going to have send off for the "Ladybird Book of DSP Filtering" ???. That is an area which has passed me by, but I'm still at a stage where the options are still completely open at the moment.

The Maplin AF unit (contributed by a guy called Robert Penfold and written up in some detail on the old Maplin magazine) uses five IC's and RC filtering to get the job done; (2) 1458C, CA3080E, 741C and a TBA820M. It was discussed briefly on this Forum back in 2014 https://www.vintage-radio.net/forum/...d.php?t=111583, but otherwise, has passed in to total obscurity as far as Google is concerned. I built it ~20 years ago.

Anyway, I'll continue the re-build leaving the filter aspect until everything else is up and running, which will give me some time to catch up with the times! Many thanks for the inputs, which I'll certainly be looking into.

B

Re: HRO 'Set Top Box'
 

This product is the sort of thing I had in mind:

https://www.sotabeams.co.uk/dual-ban...odules-ssb-cw/

I've no experience of these, and other suppliers are available!

Re: HRO 'Set Top Box'
 

Good grief Phil, the shape of those bandpass curves make the module I have look pretty pointless. I have no interest in CW, so one of the SSB versions would suit me very well. That looks like another line item for the Xmas wish list ;D. Interesting that most of his customer feedback comes from people who have fitted it to modern gear. Thanks for that info.

B

Re: HRO 'Set Top Box'
 

Yes, I agree they are impressive! DSP can also incorporate such features as an auto-notch filter, which can dynamically track and notch out a tuning whistle automatically. The one in my FT-857 is unbelievably effective. It makes listening on the HF bands a relaxing pleasure rather than the usual stressful effort.

Re: HRO 'Set Top Box'
 

I have both the Sotabeams filter that Phil highlighted and this one from Cumbria designs. On the whole I prefer the Cumbria Designs one, the kit is an easy build and I find the "Denoise" function very useful.

Hugh

Re: HRO 'Set Top Box'
 

That does look good, Hugh, but so good it's currently listed as 'sold out'! I'll mail them and see what's happening. Thanks.

B