Midget Communications Receiver 1

December 2013: Bought this set from the Kalk Bay Trading Post for R 1 150. #25930.

Serial Number 25930

Range 1 coil pack Range 1 coil pack Range 2 coil pack Range 3 coil pack


The "Original Manual" at the Crypto Museum is a combination of the original manual and a four-page supplements created later, when surplus sets were sold to the public. I'd wager that the drawing of the fellow in the trench coat was added at this stage to make the set more appealing -- it's a midget receiver, not a "spy radio".

Errors and discrepancies in the manual

Not only did Clydesdale Supply Co got some values wrong in their supplement, but the component layout on page 7 lists two C6Bs. This kept me busy (and confused) for a while.

By tracing the circuitry I found that the one on the right is in fact C6B. The one on the left is connected to the "black" common rail and to R12A, which makes it C6D. So then, the capacitor labelled C6D in the component layout is...

this one. Your guess is as good as mine. Elimination shows that it could be C8B, but the schematic definitely reads "A". So I concluded that this is in fact C8A, and that there is a further error, the capacitor labelled C8A on the chassis layout is in fact C8B.

This makes sense because:

The schematic on VK2BV's page has C8A and C8B the way I think they should be, but the area where the coils plug in looks wrong -- like someone started with half a schematic and drew that part in incorrectly.

And then there's this. C6E. The component list does not list a C6E. It does however list a C5E which is not on the schematic. So either this is C6E, 0.1uf, and the parts list is incorrect, or this is C5E, 0.05uf, and the schematic is wrong. On the radiomuseum.org schematic (which I can't reproduce here) this capacitor is called C8 and it's 0.05u. C5E it is.


I cut-and-pasted and contrast enhanced and photoshopped the schematic from the manual. It's much more readable now, especially if you print it on A3.


Capacitors often go bad with age. Even Apple Mac computers from 1990-ish have problems, so a set from 1943 will definitely need some work.

The MCR1 uses both mica (doesn't go bad) and paper (does go bad) capacitors. I was worried that I would lose some originality replacing caps, but...

... turns out some butcher's been here before me. The resistors all look original (most with body dot tip marking) but it looks as if most of the capacitors have been replaced. With what appears to be tubular ceramic and poly capacitors (good) with completely the wrong values (bad). C8A is good, C5E and C6A are close, the rest are either five or ten times too small.

Image refdesSchematic refdesValueValue fitted
C1C6D0.1u (error on p20)10n (0.01u)
C2C5C0.05u10n (0.01u)
C3C5D0.05u10n (0.01u)
C4C6C0.1u10n (0.01u)
C6C6B0.1u10n (0.01u)
C7C9A0.003uOriginal (Mica)
C10C8A0.01u10n (0.01u)
C11C5B0.05u10n (0.01u)
C12C5A0.05u10n (0.01u)

The Royal Signals website has some pics of what appears to be an untampered-with MCR1. Serial number is 55792 (as far as I can tell) which explains the more modern resistors.

I always admired the Elektor magazine projects from the eighties, they went out of their way to make good-looking assemblies. I'm not going to the same length, but I had these nice colourful poly caps in stock, might as well use them. OK, the value is a bit high, 68nF instead of 50nF, but a bit more capacitance is not a problem in non-critical (bypass) positions. Unfortunately the only 100nF caps I had in stock are boring yellow boxes :-)

Vacuum tubes "as found"

The slaughter continues. Of the five tubes in the set, only one is of the correct type, and its filament is open circuit.

 Should beWas
V1A1T4Mullard DF96 (pin compatible, higher voltage rating, 25mA heater)
V2A1R5DK92? (hand labelled) (heptode)
V1B1T4Tung-Sol 1T4 (filament open)
V1C1T41S5? (hand labelled)
V1D1T4Haltron 3S4 (centre-tapped 2.8V filament)

The mind boggles.

(In fairness, our DPO also rewired the whole thing to put the filaments in parallel, which means that the 25mA heater and maybe even the 2.8V filament is sort-of-OK, but since the tubes don't use indirectly-heated cathodes, the grid bias depends on the position of the tube in the chain. Which also explains the order that the filaments are connected in -- V1B has the most negative grid bias and V1C has the most).

Datasheers from scottbecker.net, 1T4 and 1R5.

Component List

Having found some errors in the manual, I thought I'd list the component values as I think they should be.

The original manual has strange reference designators (refdes). All components of the same value gets the same refdes, with a prefix starting at A. This makes sense, except that one would then expect that two capacitors with the same capacitance but different voltage ratings should have different refdes, and this is not the case with C8A (250V) and C8B (550V).

In any case, rather than add to the confusion by renaming the components (as Clydesdale Supply Co did), I decided to stick to the original refdes.

C1A 100pBetween coil plug-in board pin 7 and R1A
C2A 50pBetween V1B.2 and V1C.6
C2B 50pBetween coil plug-in board and V1A, next to tuning knob
C2C 50pOn IFT L1A
C2D 50pOn IFT L1B
C2E 50pOn coil plug-in board
C3A 300pOn aerial coil L21A terminals
C4A Fixed cap, not trimmer as manual indicatesOn L2A terminals
C5AC120.05uCircuit board
C5BC110.05uCircuit board
C5CC20.05uCircuit board
C5DC30.05uCircuit board
C5EC80.05uCircuit board
C6AC90.1uCircuit board
C6BC60.1uCircuit board
C6CC40.1uCircuit board
C6DC10.1uCircuit board0.01u on Clydesdale layout (Supplement page 4)
C7AC50.001uCircuit board
C8AC100.01uCircuit board
C8B 0.01uBetween standoff near V1B/V1C/V1D and chassisError: on page 7 this cap labelled "C8A"
C9AC70.003uCircuit board0.0003u on Clydesdale layout (Supplement page 4)
C10A 0.005uOutput transformer terminals
R1A 470kOn terminals next to tuning knob
R1BR8470kCircuit board
VR1Anone250kCircuit board
VR1Bnone250kCircuit board
R2AR1100kCircuit board
R2BR10100kCircuit board
R3AR1618kCircuit board17k on Clydesdale layout (Supplement page 4)
R3BR1518kCircuit board17k on Clydesdale layout (Supplement page 4)
R4AR143.3MCircuit board3M on Clydesdale layout (Supplement page 4)
R4BR113.3MCircuit board3M on Clydesdale layout (Supplement page 4)
R5AR131.2MCircuit board
R5BR41.2MCircuit board
R6AR7220kCircuit board
R7AR92.2MCircuit board
R8AR6270kCircuit board
R9AR2470Circuit board
R10AR5330Circuit board3300 on Clydesdale layout (Supplement page 4)
R11AR12220Circuit board
R12AR368kCircuit board

Resistor Values

I measured all the resistors while I had the whole thing apart, quite a few of them had drifted up in value.

RefdesShould beMeasuresErrorParallel fixUseNew valueError
R2A100k124k24%516 667470k98.1k-2%
R2B100k125k25%500 000470k98.7k-1%
R3B18k33k83%39 60039k17.9k-1%
R5A1.2M1.48M23%6 342 8575.6M1.17M-2%
R7A2.2M2.91M32%9 016 90110M2.25M2%

I like the look of the body dot tip resistors, so I elected to keep them and add a resistor in parallel under the circuit board to tame the drift in the cases where the error was more than 20%. Yes, they'll probably continue drifting. I don't plan on using the set much.


I made a list of all the connections (nets) to help me rewire this thing correctly.

New capacitors (I kept one of the tubular capacitors, C7A, that was the only replacement cap with the correct value. I did measure it to make sure though).

New tubes from eBay, and we're ready to go.


The battery is a dual HT/LT unit with 90V HT and 7.5V LT, using a Type E connector but with different pinouts.

This doesn't match any table I've been able to find. See for example the Battery Equivalent Chart at UK Vintage Radio or the Thompson-Brown Family's information. Only one 90V + 7 1/2V with an eight pin socket, that's not it.

The DZ Birger site has a replica battery, "Battery, Dry HT./LT. 90v/7 1/2V Type No. E.R.W. 1520".

Dimensions are 2 3/8" X 7 1/2" X 2 7/8".

Here are two threads on UK Vintage Radio on building replacement battery packs. And here are some clear instructions and pictures from Radiomuseum.

Or you can make a mains adapter, which is what I did. I only needed to buy two 220uF 63V capacitors, everything else came from the junkbox. The two 470uF capacitors don't need to be 63V, they only have about 20V over them so 35V is fine. I also had a 100u 100V capacitor so I didn't need the trick with the two capacitors and equalizing resistors.

On the filament side I used 1k + 100R variable for R6, this gives about 6.9 to 7.5V and I set it to 7.2 with no load, it drops to about 7.1 with the filaments connected.

[Image] Hit Count hits since 2013-12-17.

Back (This page last modified 2014-10-30)