Murphy B40D L.O. / R.F. Alignment with scale

[Tyso_Bl]

I've now got this project running well enough to see what else needs to be done, and the most obvious thing is that the indicated frequency on the tuning scales consistently read high on all bands, for instance tune to 7.00 Mhz and the scale will read 7.02 it gets worse on the higher bands, and is less so on the lower bands.

Now I've synchronised the tuning scale drum to the movement of the tuning gang as described in the manual, and the mechanical end stops work as they should, preventing the tuning gang from overunning it travel, so I'm happy that the mechanical side is now OK.

The tuning error seems to have a "non linearity" about it, i.e. it gets progressivley worse at the higher frequency end of the bands. I'm on the verge of doing a calibration plot to try and visualise on paper what is happening.

I've no history of this particular radio, but there is enough evidence of it being "got at", which is leading me to believe that a missasembly of the tuning mechanism has been compensated for by twiddling the L.O. and R.F stage alignment.

I'm unsure as to how to proceed now, is there something I might have overlooked? I don't want to make matters worse by trying to align a fault or mistake in to hiding.

[vosperd]

I think the fact that the receiver has a crystal calibrator and a movable cursor has some significance.
I have found that with many old receivers it's very difficult to get the tracking perfect even following the alignment procedures.
Don m5aky

[G6Tanuki]

You say it's previously been "got at" - could it be that the tuning-scale is not the right one for that particular receiver?

[I'm not sure about the B40D but with some high-class receivers of the 'filmstrip' tuning-scale type the filmstrip was optically matched to the actual calibration of a particular receiver and so interchanging strips between different receivers invariably caused calibration errors. I've seen some such receivers with a replacement calibrated-for-that-receiver filmstrip in an aluminium can fitted on the chassis at the factory]

[Tyso_Bl]

Originally Posted by vosperd

I think the fact that the receiver has a crystal calibrator and a movable cursor has some significance.
I have found that with many old receivers it's very difficult to get the tracking perfect even following the alignment procedures.

I do appreciate that, I dont expect lab type accuracy, but the movable cursor has no hope in places of being moved to the nearest calibrator point (its that far off), as far I'm aware some B40 receivers has to be modified after manufacture due to tracking problems, but I'm assuming that it was the RF stages that were affected, still its a good point and thankk you for highlighting it.

[Tyso_Bl]

Originally Posted by G6Tanuki

You say it's previously been "got at" - could it be that the tuning-scale is not the right one for that particular receiver?
<snip>

I hadn't thought of this! How would I be able to tell? I've assumed that on a low tech radio such as this that there would be enough spread in alignment to null out any manufacturing/assembly tolerances. The only thing I can go on at the moment is that the tuning gang turns 180 deg with the scale drum making its full travel hitting the end stops just as the end of the scale moves past the cursor pointer with the cursors set in the middle.

[G6Tanuki]

Look on the tunig dial itself and see if there is any [hand-written] serial-number that matches your radio?

The film-strip ones I mentioned all had the serial-number of their respective radio photographically included at the leading end of the film, along with a number which was [allegedly] the distance between two marks at the ends of the scale so you could measure it and make sure the film hadn't stretched/shrunk!

[turretslug]

Considering that the CR100/B28 had individually calibrated scale rollers, I suppose it's not beyond possibility that the B40 also had individual calibration on its effectively rather longer band scales. It's a long time since I worked on one of these but setting up the gang variable capacitor drive end stops on some professional receivers is deliberately arranged so that the swing is restricted to prevent end-effects giving strange effects, even reversal, in the final few degrees at the HF end- obviously, getting this even slightly adrift could have a significant effect on calibration.

A long shot but an easy one- what sort of conditions was it stored in, is it worth removing the turret biscuits and leaving them to dry out somewhere good and warm for some days?

[Skywave]

Originally Posted by G6Tanuki

You say it's previously been "got at" - could it be that the tuning-scale is not the right one for that particular receiver?

Good point. I once fell foul of that trap when I had to replace the film strip in a Racal RA-17. I found one in my box of assorted Racal spares. After many hours of struggling to get the calibration right - and failing - I looked carefully at each scale and compared them. Whoops! The RA-17 uses a non-linear scale. The other scale was from a RA-117 - which is linear! Needless to say, I wasn't aware of the difference - but I am now!

Al.

[Dave757]

Hi,

I would have thought that adjustment of the Local oscillator
trimmer caps would have corrected the tuning problem, as these have
progressively more effect at the high end of the bands.

Its relatively easy to log dial errors on the B40, as its IF
is 500kHz, so by loosely coupling a frequency counter to the
LO, the tuning scale can be continually compared with the counter..

You can do this with a lot of sets of course, but it's much easier when
the IF ends in 00 !

Kind regards

Dave

[Herald1360]

That's unexpected- the last IF frequency I would have expected on a marine set is 500KHz!

[turretslug]

Very much so- it almost made me wonder if the designers had some sort of "breakthrough TRF" concept in mind originally.

[Tyso_Bl]

Well no particular matching serial numbers on the tuning scale, thanks for pointing it out, I dont know anything about how the radio was stored in its past, but there are some small water stain corrosion marks between the chassis frame and the underside of the RF unit. I got this radio from the Newbury rally this year, and its been in a very warm dry shack since. I did check it on arrival and it was nice and dry. I've rechecked that I've synchronised the tuning gang properly, and it moved as the service manual say it should.

Last night I got the sig genny out and had a go at aligning band 3 on the radio, (3.9-10MHz). It came up to within about 5kHz after a few goes back and forth. I can tell that the RF stages are now out, so I think I'm going to try a proper alignment and see how that goes.
I've had a closer inspection of the trimmers in the turret, and some are all the way open and some all closed, I know thats always a bad sign!

On a slight tangent how would several receivers be wired together for common aerial working as described in the manual?

[dave walsh]

Forum member QUENIX fron the Netherlands had 11 B40's in 2011-he might know about interconnection! There seemed to be 8 in 2008. Perhaps he's got 14 by now? I recall some impressive photos showing his rig that he put up at one point and there seemed to be a very high level of B40 expertise. Check out his threads.
Dave W

[QWENIX]

Well here I am....... ;-) I follow most (maybe all) of the B40 62b b41 even faz outfit postings. Lots of info about CAW will be found at the radar and communications museum hms collingwood.... I have to look up this myself. First thing which i thought was the aerial (longwire) connrected in series with all primairy hf transformers coils. So the primairy coils of all receivers directly connected. And in the end connected to earth. But I have to check on this.....

[Tyso_Bl]

Thank you for the info qwenix, looking at the circuit particularly the way the aerial terminals are bypassed while changing bands would suggest this. How do you have your radios set up?

[QWENIX]

CAW : still have to look up things or ask my fellow collectors. i'll check on this.


Antennas:
15 meters of cheap laundry phosforousbronze wire sloper type:
no need for expensive braided litz

Almost invisible militairy fieldphone steel wire with two copper litz (braided) isolated with black pvc search for WD-1/TT

Or even Gallagaher cattle springs ()used for cattle fence. These are if stretched 20 meters long (but can only be stretched 5 meters a few in the loft inside home. Some in series.

Tuners (by hand)
Heathkit SA- 2060 antenna tuner
Viewstar antennatuner

DIY 1:9 un un transformer = 450/50 ohms converter for longwire works nice!

Switchgear
Alpha delta
Heathkit
MFJ 1700B

Connectors and coax
a few PL259 (hate soldering these things)
but changed with amphenol bnc adapters
rg 213 + rg58mil c- 17 former ethernet cable

All this can be switched........ and combinated with my racal equipment and murphy B41 and 62B

3 RACAL RA 17L 1 racal 117E
SSB
VLF converter
preselector

etc
best regards Qwenix

[g4aaw pete]

Originally Posted by G6Tanuki

- could it be that the tuning-scale is not the right one for that particular receiver?

Rather an 'after thought' situation, as this thread is nearly a year old.
I currently have two tuning scale drums on my work bench.
One from a B40D & one from a B40C.
They are different.

Beware of swapping scale-drums between receivers!

[Tyso_Bl]

Aha! this is really interesting, I'm going to have a look asap, I've not quite finished the receiver as I've had other distractions recently, but its still on the bench.

You've highlighted two differences, are there any others in the scale markings?

Tyso

[ms660]

For what it's worth, when I aligned mine on all bands to the two + points on the band scales all the xtal marker points were within the cursor adjustment range.

Just out of interest, according to the book of words I have, all the local oscillator coils are the same for B40 a,b,c and d as are their respective padder capacitors.

Good luck.

Lawrence.

[turretslug]

Perhaps the fine oscillator tune control of the 'D variant introduced an additional minimum capacitance consideration over and above that that which would be trimmed out at alignment, very slightly shifting the min-max swing points of the main gang tuning and hence overall law? The sort of thing that wouldn't really be noticed with a less highly resolving scale.