Unknown Marine Receiver, RX24

 I bought this "WW2 Receiver" in mid-March 2015 from a fisherman in Ramsgate.

It certainly appears to date from WW2 or the period shortly afterwards as it uses typical components from the 40s.

Can ayone suggest its original service type number? Everything inside smells like a government surplus radio shop of the 50s and 60s....

Thanks to Jonathan Hobbs M0ZGB who just bought a similar receiver and identified my RX24 as a Type 97/MX lifeboat receiver made by Coastal Radio of Edinburgh.

See pictures of Jonathan's radio....

It looks like a piece of Rexine has been glued to the front panel, so I might consider removing this to see if there are any clues hidden underneath.

Yes, well a small clue.. under the bottom left corner was stamped "RX24", and generally the colour is a pale green, like the outer case, but I need to remove lots of screws to complete the job. In fact there were so many redundant screw holes under the rexine, clearly lots of bits have been removed... I suspect mainly labels like the pair remaining?

 Basically it's a receiver covering the high end of the medium waveband and the trawler band but with what appears to be an eight way preset mechanical tuning arrangement similar to that used in the T1154. Once I'd detached the front panel for refurbishment (see below) I found the number "10D/974" marked on the side of the mechanism so this, plus the white tuning disc visible through the hole in the panel are indeed identical to the parts used in the T1154. The white plastic card shows 5 preset stations but the knob over the dial lamp has 8 settings and there are 8 sections in the mechanics.

Top centre is a dessicator whose end shows a colour depending on dampness. The thing above the earth terminal is a lamp with a slotted shield which can be rotated.

 As you can see above, there's an RF stage followed by a frequency changer, hence the 3-gang tuning condenser.

Under the valve covers I found a 6K7G RF stage, 6K8GT frequency changer, 6K7G 1st IF amplifier, 6K7GT 2nd IF amplifier, 6Q7GT detector/amplifier and a 6V6GT output stage.

There's a replacement RS output transformer and, bolted on the rear of the case, a mains power supply using RS parts.

 Above.. slightly odd positioning of the grey metal condenser which is actually used to connect the earth socket on the front panel to chassis. The green/yellow and brown wires are from the added power supply at the rear of the case. Originally power would have been supplied via the front panel connector. See later for power supply details.

 Here's the result after I removed as many screws as I could before peeling off the Rexine having decided to repaint the panel.

Loads of redundant screw holes where labels were originally fitted.

 As you can see there's a single clue, the mark "RX24" punched into the aluminium panel. At least I thought it was aluminium, but when I removed the panel it was much too heavy for aluminium and I guess it's nickel plated brass. Checking the case, what I'd thought was steel is actually brass.

The original paint finish is poor and was partly removed when I attempted to get rid of the glue used to stick the plastic covering

 The unsightly hole behind the tuning knob is peculiar. Presumably, in the original design the space had been filled with something? The inner edge of the hole is painted so the hole must be original and not a later mechanical modification.

There are screws visible which are adjusters for setting the click-stop mechanism. Five of these are marked, but there are eight click stop positions. One possibility is there might have been a clip-in disk with a hole for the tuning spindle plus one or more holes through which you could set the click-stops.

Another possibility is the rear of the tuning knob may have had a disk screwed to it with perhaps a tuning scale on it, plus a hole for adjusting the click stops. This is the option I'm considering implementing when the panel is refurbished.

The label at the top right (see the first picture) is engraved "S.W." and "M" "1" to "5".

When I unscrewed the label there were 5 frequencies neatly inked on it, also a number under "M" viz. 1 = 2182 Kc/s=137.5; 2=1647.5Kc/s=182; 3=1715 Kc/s=175; 4=2754 Kc/s =108.

This suggests the hole was covered by a scale marked in metres as I'd outlined above. Another point to consider is sealing. Whatever method was used to cover the hole it would need to be more or less sealed to exclude ingress of moisture.

 There are four holes above the tuning hole which might originally have mounted a vernier or marker for a dial-mounted scale?

Now some pictures after further dismantling..

The rear of the front panel

 View into receiver after detaching the front panel for cleaning and refurbishment. Note the coil at the bottom right. It's marked "Wearite" and has the number "6". These were widely sold to home constructors in the 1950s. I checked the Wearite specification.

RF coil PA6 is 37.5uH and covers 91 to 261 meters (3300 to 1150 Kc/s) with a 450pF tuning condenser trimmed with 60pF.

Oscillator coil PO6 is 27.45uH and is used with the PA6 and padded with 900pF and tuned with a 450pF tuning condenser.

The three gang tuning condenser has three equal sections each having 11 stator plates, which I reckon means each section is 450pF.

 Details of the T1154 click stop mechanism including the tuning disc (see below) which had been fitted back to front and marked to show five preset tuning positions. No... it's not a picture of the trawler band radio after refurbishment. It's a picture of the T1154 transmitter like the one from which several parts must have been scavenged?

 

 

 Below is the dial for the click-stop mechanism. The one selected for the trawler band receiver is the dial at the top left and has been cut to fit against the mechanism.

 

 

 Many sets are abandoned by their owners when something nasty happens. In this case the tuning condenser coupler was slipping so that tuning was virtually impossible.

 

 
 Someone had attempted to tighten the four grubscews and broken the screwdriver slots. The remedy meant removing the front panel, detaching the click-stop mechanism and drilling out the broken screws (or finding a replacement coupler). As the front panel was detached for refurbishment it didn't take long to remove the click-stop mechanism. Three broken grubscrews came out with pliers and the last I had to drill out and re-tap the hole. Proper 4BA screws suitably shortened to avoid fouling adjacent metalwork will ensure the problem never happens again.

 The T1154 dial click mechanism. Seven stops plus a brake pad for stiff tuning. The spindle selects the specific preset tuning position or the braked setting.

 Freshly painted front panel. Green hammerite because I have two aerosol cans of the stuff.

 Above: Speaker and handles refitted after light cleaning.

I scrapped the original front-mounted power connector. It was so brittle it disintegrated when it was removed.

Below: Mostly re-assembled but still waiting for inspiration re- the tuning control hole.

Above:  I found a suitable dial which neatly filled the space. Not a very common part... it's from a BC1066 receiver.

Lots of screws required to fill unused holes...

Below: Here's a picture of a BC1066 showing the dials.

When I bought mine around 1960 it was brand new and unused and cost me about 15/- or 75 new pence.

It used a pair of 958 acorn valves plus a 1D8GT, an amazing diode triode pentode on an octal base.. All that remained buried in the junk box was one of the two dials.

 Now try and identify other parts used in the receiver that were commonly used in specific WW2 radio equipment.

Below is a picture of the power supply that must have been made by the first owner of the receiver. I removed it from the rear of the chassis where it was fitted with self tapped screws so that I could modify it slightly. Note the EZ40 valve holder which is mounted on 6BA screws secured to the tinplate and the home-made clamp for the smoothing condenser.

I wanted to remove the old mains lead and fit an IEC connector.

 I usually fit an IEC connector to old sets if the mains lead needs to be replaced.

As I have countless old computer power supplies I 'll generally remove one either complete with mounting metalwork cut from the PSU case, or if the new location can be drilled I'll fit it there as you can see below (the end of the power supply had this detachable panel).

 

 

 I noticed the mains transformer had been drilled for self-tapping screws for convenience of mounting.

I'm not sure if this practice is a good idea but I'll welcome comments.

Below is a smoothing condenser fitted at the end of the receiver chassis that fried and got extremely hot when power was applied.

 

 

 The modified power supply just before fixing it back to the rear of the receiver case.

Mains is carried through to the front panel on/off switch and the heater and HT supplies are carried by the other four wires. A 1 Amp fuse protects things.

The new IEC connector can be seen on the right. The safety earth pin is connected to the chassis.

Instead of re-using the self tapped screws I threaded the holes in the brass case and used 4BA screws.

 Once assembled I plugged in the set and tried it. There was nothing heard from the loudspeaker except a hum. Poking around inside I found I could hear a burbling sound when the grid of the LF amplifier was touched with a screwdriver. The problem was simple to locate. The screening cover on the last IF amplifier valve was shorting the grid to chassis because the rubber insulation on the wire to the top cap had perished.

Once I moved the offending cover the set burst into life, but all I could hear was mush and burbling until I'd turned off my network camera, then a dozen stations at the top end of the medium waveband could be heard. The overall gain is way down so I suspect a proper alignment is needed.

The next step was to set the receiver up on the bench with a signal generator and figure out the lack of gain. The first thing I found was the disconnected overheating condenser was still getting very hot because the isolated case wasn't isolated. After removing the condenser instead of just curtting its earth lead the speaker volume doubled. I'll fit a new one later.

I checked the frequency coverage and noticed lots of responses so clearly something is amiss. Roughly speaking I was able to tune 1.18Mc/s to 3.2Mc/s but I noticed multiple responses for particular inputs. Sensitivity was hopeless being roughly 30uV at 3.2Mc/s and 500uV at 1.19Mc/s. Maybe the IF amplifier is poorly tuned? A check revealed the IF response was very broadly tuned and centred on 437Kc/s which is strange. On the side of the three transformer cans is written IF Type M400B 456/475Kc/s so a new setting of 465Kc/s should good as a starter.

Once I'd reset the IF slugs the sets performance was transformed and you could now hear a decent level of background noise. For some reason, perhaps internal trimmer capacitors in poor shape, all the transformers were way off tune. In fact one doesn't want to peak so I'll need to remove it and see what's wrong, but for the moment I'll continue alignment.

I noticed a rather odd effect. With the signal generator connected to the aerial socket I could hear lots of background noise, but sensitivity as far as the test signals was poor. After peering into the front of the set I found a black covered wire from the RF coil had come adrift from its chassis connection. Reconnecting this improved sensitivity and tuning the trimmer across the coil improved things again. Now I can just hear a couple of microvolts of test signal instead of 100 microvolts.

I rechecked the tuning range. Now the set covers 1.16 to 3.17 Mc/s or 258 to 95 meters (the coil spec was 261 to 91 meters). I'm not sure what the IF design setting was but I guess 465Kc/s is OK.

The next day I decided to find out why the top core of the second IF transformer failed to peak. There are three transformers in total and the other two were OK as was the lower core of the middle one. I noticed a silver mica condenser across two of the transformer pins so that was a good place to start. I unsoldered it and examined the value. It said 120pF but my trusty capacitor tester said is measured only 12pF, so presumably that was the problem... the lost capacity had put the resonant frequency too high to allow the coil to be tuned to 465Kc/s? I fitted a 100pF replacement and confidently switched on but discovered the transformer still failed to peak. In fact, if anything, it was worse so I fitted a 30pF which seemed about the optimum value. At this point I was a little mystified so added various capacitors across the pins of the transformer. I found I could move the tuning position of the lower coil which peaked OK, but the top coil just would not tune to 465Kc/s. At this point I had plenty of stations coming in from my long wire so decided to call it a day and finish off by measuring the various electrode voltages of the valves for the record.

I noticed the 2nd IF amplifier cathode was sitting at around 6 volts and varied by a volt or two as stations were tuned in and the AGC line was working OK. The anode and screen voltages of the RF amplifier, mixer and the two IF amplifiers were all satisfactory, but I noticed the cathode voltage of the 1st IF amplifier was sitting at zero volts. This is odd because the cathode resistor is 1.5kohm. Maybe the coupling condenser has failed short-circuit or maybe the valve isn't drawing any current? As I had a spare 6K7 at hand I swapped this for the 6K7GT. Once the valve had warmed up the set was transformed. Stations were hugely stronger, the AGC couldn't cope and there was loads of distortion. I checked the cathode voltage. This was still zero volts, but then I noticed that the link between the cathode and G3 was missing. Someone had unsoldered the link and connected the cathode directly to ground, hence the cathode was zero volts because it was connected to chassis.

I moved the link back to its correct position and found I got around 3 volts at the cathode. I then checked the tuning of the IF transformer and found to my surprise it tuned differently. Maybe fitting a 100pF capacitor at the IF coil would now work? I removed the 30pF and soldered in a 100pF condenser and would you believe it, the top coil peaked beatifully. I checked all the IF settings and all were now peaked correctly. My long wire was now severely overloading the receiver so now I'll have to go through the AGC line and check all the resistor values and decoupling condensers to get a high enough bias to remove the overloading.

I might test the old and new 6K7 valves later to see exactly what was happening.

I'm no wiser. I tested the 6K7GT which looked pristine, and it checked out at 80%. The replacement is an all metal valve and I didn't replace the screening can. It's possible that the grid lead which has lost most of its insulation was part shorting out. Now its connected without any stress because the screening can isn't fitted maybe that explains the problem? Strangely the distortion has disappeared. I checked the AGC line and it's minus 17 volts on strong signals, hence the valves are backed off.

Comments welcome...

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