HRO Junior (much modified)

 I rediscovered this amateur-modified HRO the other day after confusing another with this one. I knew I had one based on miniature valves but found octal types when I removed the cover from what I thought was this one. You can see that one here..

The controls are straightforward. On the left the black knob is a 3-position rotary mains on/off switch fitted in a hole marked "ANT". Why not a toggle switch? Possibly there was a plan to include a standby position for using the receiver with a transmitter but never implemented and instead relying on the B+ switch? AVC on/off is above an RF gain control. On the right there's the audio gain control, noise limiter adjustment, "C10" which appears to be a BFO adjuster and what may be the original B+ switch. Nothing remains in the hole marked "CAL" so it's possible these latest modifications are a second incarnation of the original receiver. The very heavy metal cover is present.
 
 
 

 Here's an R106, a standard HRO Senior Model M and comparing the front panel with the modified set above there are some interesting differences. First of all.. is the panel original?

If you look carefully the "B+" engraving over the ON/OFF toggle switch is present therefore the modified receiver does carry an original HRO front panel, but it has no hole for an S-Meter, and the absence of an S-Meter means the set was an HRO Junior.

It's a rack-mount version with a very heavy steel front-panel and maybe the owner didn't bother fitting an S-Meter because of the difficulty in cutting a hole for it, although metalwork generally wasn't an issue because a lot of the chassis has been cut away and replaced with a new section.

 Below you can see how a new section of aluminium chassis has been used because the original probably had too many large holes for the modern, mainly B7G valves, to be fitted. Was the design based on an article published in a 1950/60s radio magazine or a creation of the original owner? The fitted coil, H619 is marked for top band and 80 metres. All the valves carry labels giving their emission details.
 
 
 

 The valve layout is shown here. Without tracing the circuitry (but judging by the number of valves) I first suspected the set must be a double superhet, however a brief look under the chassis tells me it's a single superhet. I then found the original circuit which explains everything including its earlier design and subsequent changes... a pair of EF93 RF amplifiers followed by a 6BE6 mixer with a 6C4 oscillator to produce an IF passed via a mechanical filter at 455KHz to three 6BA6 IF amplifiers. The second 6BE6 is used as a BFO and product detector. The EB91 is an AVC rectifier and noise limiter. There isn't a mode switch making the circuit a dedicated SSB receiver with a huge amount of gain.

The Z77 is disconnected from the circuitry (was it an AVC amplifier or a noise limiter?). The ECC81 is an audio amplifier feeding the 6BW6 output valve.

See the 1974 spec for the MF455 filter

 Quite a tidy layout under the chassis. The mounting screws for the LF choke (on the left) are very loose and might indicate it's been removed in the past then roughly replaced after discovering it was faulty? You can see an early silicon rectifier diode between the smoothing condensers at the bottom left (plus a second under a condenser). Before powering the receiver I need to check the reservoir/smoothing condensers and other HT components to determine that there isn't a bad short-circuit. I also need to figure out the reason for several disconnected components.
 

 A power supply is included in the modification. It's based on a heavy Radiospares Standard Mains Transformer marked "100mA DC".

 Looking at the HRO coils in my collection it appears that the ones marked "SSB HRO" are the examples that came with this set. I discovered this once I'd traced the emails relating to its purchase in 2002.

Whilst pondering over the origin of this receiver I was searching for some information on a Hallicrafters SX23 and came across a set of hand drawn circuits of a receiver and recognising the output stage, a 6BW6, I realised the information must have accompanied this HRO. It seems I bought it from Gordon G3XEW in October 2002 for £27.50 and it weighed 27Kgm (that's about 50p per pound).

The design is loosely based on the G2DAF Communications Receiver (see "Deisign Notes" below) and was operated by Harry, the late G3IOW before being returned to Gordon. Harry, who died aged 90 in 2006 was blind from birth.

See the Design Notes on the SSB HRO.

 See more HRO receivers

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