I needed to make a replacement
test probe for my spectrum analyser because I'd mislaid my usual
one when recently rearranging the workshop. I wanted to check
the alignment of an HQ170 receiver on which I'd been working
and whilst waiting for delivery of the parts I looked at my R208.
I removed the receiver from its case but before plugging it into
the mains supply checked the mains input panel which gives the
connections for 100 to 250 volt input. Surprisingly this was
set to 210 volts which I suspect was the voltage tapping used
by its last military user. Although the set has been in the hands
of at least one radio enthusiast since then, pound to a penny
no-one ever looked under the cover to check the mains setting.
I would imagine that at least one valve is low in emission from
being over-run, but nevertheless I plugged it in and checked
At this point I had no inkling
of the history of the R208 and assumed it was just an ordinary
government surplus receiver designed by experts and fully meeting
its design criteria but, after over 70 years, just needing a
little tweaking to restore perfect operation. As you read on
you'll get a feeling that all was not well back in the early
1940s. It would be easy to redesign the old set but that would
ruin its originality so I'll bash on and make the best of it...
Although the receiver
responded with crackles when the wavechange switch was operated
there was very little background noise except when the BFO was
turned on. I fitted a 20dB attenuator to protect my signal generator
and connected it to the aerial terminals. The set was clearly
not only a bit deaf, but the local oscillator didn't appear to
work fully across each waveband. On the 10-20MHz band I needed
100mV to produce a response at something like 29MHz and at other
frequencies in that band I suspect the oscillator wasn't functioning.
Maybe a bad 6K8 mixer/oscillator, or a resistor gone high/condenser
leaking... or both.
In the band 20-40MHz I could
hear rough responses (at least on the lowest band the signal,
although weak, was clean), both main and image signals with 10mV
input. One of these, I can't say which until I've read the technical
spec was stronger but not much stronger than the other. On the
40-60MHz band I could hear a signal at 58MHz with maybe 5mV input,
but this needed more input voltage as the frequency was decreased
and soon disappeared at below 50MHz.
The frequency coverage (10MHz
to 60MHz) of the R208 is a bit peculiar, but could be explained
by the fact that many WW2 communications receivers were limited
to 20MHz, and a few reached 30MHz, but apart from specialized
aircraft receivers, the low VHF band was inaccessible.
Dealing with the wayward local
oscillator.... a simple trick to test the local oscillator is
to place a wire aerial from another communications receiver near
the chassis and so I did this to look for the presence of the
R208 local oscillator. By using an S-meter you can quickly see
whether the signal is good or bad, (for example a constant level
or fading out). I once used this method to diagnose my RA17 extreme
deafness. A low emission EF91 VFO was cutting out before the
tuning condenser reached full mesh. A replacement valve fixed
the RA17. If your monitor receiver doesn't exactly match the
band being checked you can tune to a harmonic of the oscillator.
According to the R208 handbook
the intermediate frequency is 2MHz and the local oscillator is
on the high side of the incoming signal on all three wavebands
so, for example, an input of 25MHz requires the local oscillator
to be 27MHz putting the image at 29MHz. Both the 25MHz and the
29MHz signals will be heard when the set is tuned to 25MHz and
their difference in level will be a function of the tuning and
tracking of the RF amplifier coils. So if the mixer is wrongly
tuned to 29MHz the image will be received strongly. Equally,
one can hear and therefore alter the tuning of the receiver to
25MHz by inadvertently setting the local oscillator to 23MHz.
This will result in tracking difficulties.
Before testing the valves I'll
first check the voltages at the test panel. These should be close
to the values shown in the handbook.
Here are a couple of sets
of measurements. The second test was with a better HT voltage.
I checked the readings
at the test panel (see First Test) and Anode V2B' reading was
about 142 volts (= the HT voltage), and varying. No doubt the
hum from the speaker was connected with this variation so I wired
a 22uF 400v capacitor across the metal-cased condenser C1B and
the hum dropped considerably and the HT rose to 175 volts (see
Second Test) Clearly the old condenser is past its best. Looking
at the circuit diagram there are two similar condensers C1A and
C1B acting as reservoir and smoothing respectively for the HT.
C1B was bad so where was C1A? This is located underneath the
power supply chassis (below) which is fastened to the main chassis
by four 2BA screws. To detach the power supply it's convenient
to remove first the horizontal bar between the front panel and
the rear frame. This done, the power supply can be unscrewed
and turned upside down to access the reservoir condenser. I used
the trick of connecting the condenser in series with a resistor
across a variable HT supply whilst monitoring the current by
measuring the voltage across the resistor (in this case 20Kohm).
With 200 volts applied the current drawn was initially pretty
low but as I watched it rose steadily to about 1mA. Increasing
the voltage to 400 increased the current to 4mA. It then very
slowly dropped and stuck at 3.7mA. I then tested the smoothing
condenser. This gave roughly the same results so leaving them
both disconnected, I wired a couple of new 22uF 400v electrolytics
as reservoir and smoothing. The HT was then 175 volts, still
a bit low.
Here's a view under the power
supply chassis.. Many of the parts are for 6 volt battery option
using a vibrator. The previous owner had removed the Mains/Battery
switch and some of the wiring and the vibrator.
Under the main chassis
there are a further two metal-cased condensers. These are 2uF
and rated at 350 volts. I tested these and to my surprise, both
were in excellent shape with only tens of microamps leakage at
300 volts. Both measured very close to their marked values of
2uF with an ESR reading a small fraction of an ohm so I left
these in place.
The decoupling condensers are
pretty substantial 70+ year-old components and quite unusual
but swapping these might help reduce the demands on the HT rectifier.
I tested the electrolytic condenser at the bottom of the picture
(clearly not original 25uF 12vw component) and found it was marked
50uF and measured 51.87uF with an ESR of 0.39 ohms which is remarkable
as it will date from 1958.
While I had the chassis upside
down I checked all the resistors. All were high by varying amounts,
but apart from a couple were not too bad but as none were open
circuit or dramatically high I left all intact.
The next test was to investigate
what appeared to be the local oscillator dropping out, and sure
enough whilst listening on another receiver, the oscillator was
giving S9 over about a third of the two higher ranges and for
only a quarter of the lowest range. I'd already swapped the two
6K8 valves around so my initial thoughts were that the problem
may be low HT or a bad component in the 6K8 oscillator. It's
quite possible the previous owner had changed the mains transformer
tapping to deal with this?
I shelved the local oscillator problem
for the time being because I couldn't easily read the dial markings
because of tarnishing and I needed to read the markings in order
to align the set. The Muirhead dial was best removed for this
job. It's held in place by a screw securing the drive output
to the tuning condenser shaft. I had to use a torch to see the
screw head which needs to be turned to a convenient point for
slackening. You'll need a long thin screwdriver to access the
screw. The dial mechanism has a tab which locates in a metal
block but before it can be pulled off the small plastic bandmarker
needs to be slackened (one screw is sufficient, allowing it to
be turned out of the way of the dial). Once the assembly is off
four screws are removed to free the metal dial which can then
be cleaned with Brasso. Once cleaned it was easy to see the numbers
The dial, partly cleaned before
deciding to remove the metal plate and get rid of all the tarnishing.
This meant removing the salmon-coloured finish to produce a shiny
At some time it might be a good idea
to apply a matt finish to help with readability.
Whilst sorting out the
dial I noticed that the RF gain pot wasn't connected. I swapped
over the wire from the centre where it was soldered to the ground
wire and connected it to the end of the wiper. This made the
receiver totally deaf because someone had not only disconnected
the pot but they'd completely removed the resistance wire so
the wiper was rubbing on plastic.
It soon became obvious what
had happened. The pot must have seized and broken its winding.
This may have been shorting to the chassis so had been removed.
Why not replace the pot? Well the pot is directly in front of
the coils for the RF amplifier so there wasn't room to get it
out. Two of the coils had been broken from their mounts in an
effort to remove the defunct pot but still there was half an
inch too little space so they'd given up.
The solution was to break off
the spindle. This is brass rather than steel so was easily broken
off with a large pair of pliers. I was then able to extract the
remains and fit a replacement. I had a small WW2 US pot small
enough to fit in place. The new pot is 5Kohm rather than 2Kohm
so I fitted a small resistor to shunt the resistance down to
the correct value.
I'm probably revisiting
something that may have happened back in 1958 when Sputnik 1
was in orbit?
The RF gain potentiometer had
failed but there wasn't enough room to remove and swap it even
though the two coils directly behind it had been forced from
their mounts. The securing screws for the coils can only be accessed
once the whole RF front end chassis has been unbolted from the
main chassis and all of its cable harnesses unsoldered. A daunting
Maybe the resistance wire had
become jammed in the wiper and shorted to surrounding metalwork
so the wire had been painstakingly removed?
The solution was to break off
the brass spindle, giving half an inch extra clearance, which
enabled the pot to be detached.
Below, the new potentiometer
installed and wired up.
It took 60 years to fix.. but
what about other problems?
Above is the coil pack. Left
to right RF amplifier, Mixer input and local oscillator and top
to bottom, 40-60MHz, 20-40MHz and 10-20MHz.
The coilpack above was typical
of receivers designed in the 1930s. It's a utilitarian design
also found in things like the AR77
but not in newer receivers aiming for better performance and
easier servicing such as the R206.
As is the case for all superhet receivers it's vital to accurately
track the oscillator and RF amplifier stages. The problem is
the oscillator is set at 2MHz away from the RF stages but the
tuning condenser must peak the coils right across its whole range.
To track the tuning beehive trimmers are adjusted at the HF end
and the coil slugs at the LF end of each range. This process
must be repeated until the receiver's maximum RF amplification
is always achieved at any setting of the dial.
I connected a long wire aerial and initially
checked the lowest band 10 to 20MHz because this was completely
dead, whereas the two highest bands were active over a small
dial rotation. I suspected the wavechange switch and after spraying
it with switch cleaner and rotating it backwards and forwards
a few times the lowest band sprung into life and loads of broadcast
stations appeared. The type of switch used in the R208 is "self-cleaning"
so any slight tarnishing is scraped off as the switch is turned,
but after 70 years a little help is needed hence the use of switch
I turned on my signal generator and
after 20 minutes twiddling the lowest band was nicely aligned,
as was the IF strip at roughly 2MHz. The first IF transformer
had an intermittent but as I looked for a means of removing it
I noticed its two fixing nuts were very loose. I tightened these
with a 4BA nut spinner and found the intermittent had magically
Now that the dial was clean I could
read the dial markings and I was able to see that the oscillator
was cutting out at 33MHz and 53MHz on the two higher bands. This
might have been a bad 6K8 or a low HT voltage although the latter
isn't too bad now that the HT is cleaned up. I removed the RF
amplifier and the mixer/oscillator valves and checked the valveholder
pins. Anodes and screen voltages were all present, reading 195
volts. As the HT is 175 volts with all valves in place, I suspect
the HT rectifier (a full wave bridge) maybe has more resistance
than ideal and is adversely affecting the HT rail. This tends
to support the reason for the mains selection tapping of 210
volts. I might change the setting to something in-between...
say 230 volts rather than 240 volts. Before that I'll swap any
leaky decoupling condensers because I imagine the oscillator
problem could well be due to poor decoupling which will reduce
Most of the decoupling condensers are
very large black things marked "H.C" and "O.F."
with either "41" or "42" (=dates 1941/1942
?) which, surprisingly, all tested as perfect. I changed a 75Kohm
resistor that measured 92Kohms but this didn't fix the local
oscillator. I may use an external HT power supply to see if this
works... it didn't.
Below, I've indicated the various
measurements made (with the HT measured at 182 volts). Not easy
because the underside of the valveholder is buried under the
coilpack and it was easier to stick a short length of wire into
the socket with the 6K8. I tried various examples of the valve
and ended up with one that tested 100% on my AVO valve tester.
The voltages are what might be expected and suggest insufficient
feedback to maintain oscillation. The lowest band works over
the whole dial but you can see the local oscillator amplitude
is dropping at the LF end (ref. the grid voltage). The cathode
bias increases as the oscillator amplitude drops because the
triode is drawing more current (going from circa 2.6 volts to
3.1 volts). Also, the triode anode voltage drops as the oscillator
amplitude drops ( from 125 volts to about 114 volts and then
81 volts when it's stopped oscillating)
Waveband & dial setting
Stops at 33MHz
Stops at 53MHz
6K8 triode anode
6K8 grid, grid1Triode
When the local oscillator has
maximum amplitude the grid voltage is minimum (ie. -2.5 volts)
and when it stops the grid voltage increases (ie to +2.7 volts).I
disconnected the R208 HT feed at the HT choke and measured the
current at 66mA then connected an external HT supply. This was
adjusted to provide 250 volts and the receiver drew about 70mA.
The internal HT supply provides about 180 volts. The local oscillator
did manage to get down a little further with the increased HT
but only improving to see 46MHz. At 200 volts this went to 47MHz,
but I need to get the oscillator down to at least 42MHz so a
further study of the circuit diagram is needed to try and identify
a rogue component. I
checked all the resistors around the frequency changer and the
condensers (decoupling, grid coupling and feedback) for value
plus any leakage and all were fine. Shorting out the cathode
resistor slightly increased background noise level but only affected
the local oscillator cut off point by a few KHz and made the
cut off more abrupt. I suppose the next step is to change the
feedback condenser C12A from 150pF to say 330pF and see what
effect this has...
Changing parts in the R208 local
oscillator area is tricky because of the position of the valveholder.
Its buried beneath wiring and wedged between screening plates
and the wavechange switch. As I studied access I noticed someone
had been here previously because there was an earth solder tag
with solder but no component or wire and the condensers had been
cut off and resoldered in place. Also, although the grid blocking
condenser and the feedback condenser are both supposed to be
identical, one was correct and the other (the grid coupler) was
200pF instead of 150pF. Oddly that was going to be my next step...
Instead I fitted a 125pF and put a 330pF in place of the 150pF
feedback condenser. Checking the 20-40MHz range proved that this
had absolutely no effect on curing the oscillator cut-off problem.
Looking at the oscillator on my spectrum analyser however showed
something rather odd. The total tuning range of the oscillator
was correct (22MHz to 42MHz) although cutting out with the tuning
condenser two thirds closed and of course not matching the dial.
Maybe the padder is too high? This was accessible so I checked
it and found C8A, the 0.002uF condenser measured 1800pF. Thinking
about this I reckon even at 1800pF it must be too high (because
the full tuning range was achieved with two thirds of the available
rotation) so I fitted a handy condenser marked 830pF. To my surprise
the oscillator amplitude remained steady right across the band
and tuned 21 to 40MHz. After twiddling the slug and trimmer this
ended up at 22-42MHz... perfect but very puzzling. Almost as
if the wrong tuning condenser is fitted, a wrong coil or it's
very degraded in some way. Anyway a new oscillator padder fixed
the problem so two ranges are now correct.
Looking at the dial markings the tuning
range for the highest band marked 40-60MHz only covers two thirds
of the dial. Because of the findings on the 20-40MHz range I
should really check the oscillator padder so that the frequency
coverage matches the dial markings. The lowest frequency of full
dial rotation is indeterminate bcause the dial markings stop
at 40MHz. Also, at this stage, although I aligned the 10-20MHz
band, I'll recheck this with the spectrum analyser. Because of
the padding condenser puzzle on Range 2, I thought I may have
confused the main RF input to the image.
Just a recap...that oscillator padder
change from 2000pF to 830pF for the 20-40MHz band is really odd.
The implication is the tuning condenser has too great a range.
Maybe I should check this? Physically, the tuner has three identical
sections and the user manual confirms 3 sections each providing
110pF swing. Maybe some calculations are necessary to indicate
what's happening? I cleaned the tuning condenser contacts and
checked the tightness of all the securing screws in case one
was loose and extra inductance was creeping into the equation.
Some screws were loose but tightening them didn't have any effect.
After some rough calculations I worked
out that the 830pF padder capacitor would work perfectly given
around 0.47uH for the oscillator coil and, for the RF amplifier,
a coil of about the same value would tune 2MHz lower. The answer
must be in the oscillator coil inductance. If this is different
to the original, perhaps from ageing (ie. a change in characteristics
of its former or the iron dust core) it might explain what's
going on. Another explanation, a rather slim possibility, is
that the wrong coil was fitted to the oscillator circuit in the
factory. But now, I'm happy with Range 2 (albeit puzzled) I'll
look next at the 40-60MHz band...
I must have spent a few hours
working on the 40-60MHz band. Using my DSA815 I could see the
local oscillator take a dive into the noise at much the same
dial position each time I tried something new. I tried a different
grid coupling condenser, a different feedback condenser and added
extra ground wires but each time, although the amplitude and
maximum frequency would alter somewhat, the thing cut out as
the dial was rotated lower than around 50MHz. Then I noticed
that twiddling the RF amplifier coil and trimmer dramatically
affected the amplitude and exact frequency of the loss of local
oscillator output. It seemed that at a specific point on the
dial the oscillator output was being sucked away.
I looked again at the wiring. Because
of the high frequencies involved and the mechanical layout dictated
by the physical sizes of the parts, the earth wiring is quite
long and, practically speaking, must be part of the tuned circuits.
Certainly, in the highest frequency range this extra earth wiring
must be a significant part of the tuned circuits. Then again..
perhaps the 6K8 and the immediate electrical circuitry is being
asked to do a job beyond its capability. Could this explain the
reason for the dial markings being incomplete in this range?
Also, it's quite possible that a previous owner has made modifications
to the local oscillator wiring?
Maybe the adjacent 20-40MHz oscillator
coil is resonant and as the 40-60MHz coil is tuned it passes
through a resonance point of the 20-40MHz coil which is disconnected
from the tuning condenser? Maybe I should short out the coil
and see if this has an effect? It didn't.
Shorting the adjacent coil didn't have
any effect on the local oscillator problem because scrutiny of
the wavechange switch revealed it was probably a shorting type
anyway. This type of switch uses a grounding wiper to deselect
the desired coil rather than select the one required. There is
however a slim chance that because of the relatively long RF
connections in the coilpack some sort of tuned circuit remains
even when the unused coils are grounded.
Why all these problems you may wonder?
Well the answer is that the 6K8 wasn't designed for use at VHF
and to get it to run at 60MHz must have been a challenge for
the designers. The coilpack and its connection to the valves
is not ideal. For frequencies under 30MHz it's a classical design
but, because the coil sizes are relatively small at 60MHz, the
wiring has assumed a significant portion of the tuning. Not only
that, but the condensers used in the circuit intrinsically include
some inductance so swapping these for different types affects
the way the oscillator behaves and there's evidence that at least
one previous owner has struggled to get the set working so any
further work involves undoing previous remedial work that didn't
achieve its object.
During testing I noticed the response
of the IF strip was strange. I could see the local oscillator
signal flanked by main and image responses but the RF responses
were doubled up giving the impression of five signals rather
than three. Perhaps I should fix this before proceeding...
I checked the IF strip and found that
having tuned it by ear it wasn't too bad, but extra adjustments
removed the double humping and resulted in the shape below.
I used a sweep of 1.8 to 2.2MHz with
my home made probe connected to the final IF transformer output.
The results are qualitative rather than quantitative because
the probe is very lossy. The curve shows a bandwidth of 80KHz
at 30dB down and at the image frequency around 2MHz the IF gain
is 40dB down.
Note that the wavechange
switch has extra contacts which are used to modify the IF transformers
for the 10-20MHz band.(see circuit diagram). I suppose this was
done to give adequate gain in the two higher ranges whilst not
too much gain in the lowest range.I guess, with a little extra
work the response below for the 10-20MHz band could be improved.
I then carried on with trying
to improve the lowest local oscillator frequency in the 40-60MHz
band. By experimentation I'd got this down to 45MHz = reception
at 43MHz. I'd noticed that that as the dial was tuned lower the
oscillator amplitude would drop very suddenly giving the impression
that an absorption effect was present although continuing to
tune down didn't resurrect the oscillator output. When the cut
off point was reached I could back off the tuning slightly then
changing the tuning of the mixer would increase the oscillator
amplitude back to its normal level. Lowering the oscillator frequency
then showed a slight improvement with cut off taking place at
a lower frequency than before. The overall impression was the
two tuned circuits... oscillator and mixer were crossing over
at a point on the dial so that their resonance was coupled through
the 6K8. As I'd had trouble getting the tracking right for the
middle band 20-40MHz, I wonder if bad tracking between the oscillator
and mixer is the reason for the oscillator cutting out. It may
be that the problem is being aggravated because the 6K8 characteristics
result in significant unwanted coupling at VHF, whereas at lower
frequencies the coupling is not important.
For perfect tracking the oscillator
coil should track the mixer coil such that it is always exactly
2MHz higher. Given the wrong coil inductance in either the oscillator
or the mixer there may be a point on the dial where the two are
at the same frequency. At this point the oscillator output is
absorbed by the mixer coil. That would certainly account for
the abruptness of the oscillator output cutting out and the shift
in this changing as the mixer coil is trimmed. A way to prove
this might be to lower the Q of the mixer coil? If this works
it will be a lot easier to adjust the tracking.
To support this idea I'd noticed
that shorting out the 6K8 cathode resistor would increase the
oscillator amplitude by maybe 30% but the oscillator cut off
effect was even more pronounced.
Above is the final layout of
the parts for the two higher local oscillator ranges. The oblong green capacitor measured about 830pF
and replaces the 0.002uF original padder. In the centre wired
together are two capacitors in parallel, being 100pF and 270pF,
because 370pF was the best value for the padder for the highest
range. This is in place of the original 1000pF. I had to put
back the 3-30pF trimmer when I found the 2-9pF trimmer didn't
allow the dial to match the lowest frequency. I had to fit a
different iron dust core when I found neither the one fitted
nor the correct one which I found in another coil worked well
enough to maintain oscillation. I also had to raise the frequency
of the oscillator by a couple of MHz by adding the extra copper
wire. The tuned circuit was formed, not only by the coil but
by connecting wires and, to some extent the surrounding metalwork.
The final result was that the dial markings of 50MHz and 60MHz
now correspond exactly to local oscillator frequencies of 52MHz
and 62MHz. The oscillator will run down to about 45MHz before
cutting out, not quite achieving the 42MHz in order to tune the
receiver to the lowest dial marking of 40MHz. Maybe a selected
6K8 would manage the extra few MHz?
I'm a little suspicious of the
40-60MHz coil. This seems to be wound over paper wrapped around
the former and its turns are spread wide apart unlike the mixer
and RF amplifier coils which are relatively close-wound. Maybe
the coil has been replaced at some time?
Looking at the picture of the
dial above, the lowest band has a 1000pF padder and this works
fine but I don't think a 0.002uF padder sounds right for the
20-40MHz band and as the highest band is spread much more than
the two lower bands its 1000pF padder sounds too big as well.
Although an 0.002uF part was fitted I wonder if this was a documentation
error and the correct part should have been 1000pf? Also a 1000pF
padder for highest band might be useable with the 2-9pF coil
trimmer and dust core. In fact the trimmer was specified as 2-9pF
in the schematic and parts lists in bold print but a 3-30pF was
factory fitted. It does strike me as a failure to embody manufacturing
changes ie. the 0.002uF should not have been fitted and the 2-9pF
trimmer was probably later re-specified as a 3-30pF trimmer and
duly fitted? A mystery we'll never solve....
Finally I'll check the front
end amplifier and mixer to see if their tuning range can be adjusted
to match the dial readings? Here we are below after slight tweaking
of trimmers and coil slugs...with a word of explanation in case
you haven't seen this type of scan before..
The tracking generator was input
to the R208 aerial post and a high impedance probe placed on
the 6K8 mixer grid. The tuning control was set to 10MHz then
20MHz for Range 1, 20MHz then 40MHz for Range 2 and 40MHz then
60MHz for Range 3. You can see the peaks of the RF tuning for
low and high settings for each range together with a spike which
is the local oscillator running 2MHz above the tuned frequency
(except for the 40MHz setting for Range 3 which is missing the
oscillator). The final pictures are around the mid-band position
of Range 3, with normal tuning then slightly lower just before
the local oscillator disappears into the noise.
Of note is the fact that the
amplification was pretty well flat whilst tuning across each
range from low to high. Interestingly you can see the response
curve broadening and overall gain dropping slightly as the frequency
increases because the ratio of capacitance to inductance gets
When I air-tested the receiver
the two lower bands worked exceedingly well.
It was just about at this point in proceedings
I learnt of something interesting. The R208 was designed at a
time when valves good for frequencies over 30MHz were in short
supply. Developments in radar and other things had priority and
newer WW2 valves capable of running at VHF were as scarce as
hen's teeth. Although the requirement for the R208 was important
it had to take a back seat and this meant that the designers
were stuck with the 6K8 mixer. By careful selection and probably
a lot of effort the R208 managed to get through its manufacturing
stage but with the proviso that the performance of the highest
frequency band might be a bit hit and miss. I understand a second
radio company was brought in to help but they gave up and as
I'm finding out over 75 years later the 40-60Mz band is well
nigh impossible to fully commission and back in 1942 a decision
was made to make do until a later completely new receiver became
available... this being the R308, looking very R206 MkII'ish
See more R208 pictures