'Figaro Special'

[Yorkie]

I’ve made good progress with this little set – I fitted a new volume control today as the switch was duff on the original. Luckily I had one of the same size and shape, but with a plastic shaft rather than metal, which is no bad thing. The radio works nicely on all three bands, albeit I’ve not yet checked the alignment. I’ve also yet to fit an NOS DM70 magic eye which I have to hand.  I also intend to cast two new knobs - (I've turned two 12mm brass ferrules, drilled out to 6mm, which will be embedded into the knobs for the 6mm shafts of the volume control and tuning shaft).

So there's light at the end of the tunnel with this little project. However, there is a pronounced hum level, albeit not so noticeable when stations are tuned in. I’ve replaced all the paper caps and the reservoir/smoothing caps. I’ve removed the frequency changer and IF/Detector valves to check if the hum is still present, and it is, so it’s emanating from within the audio stage.  It does only have a half-wave rectifier rather than full wave, so the hum will be 50Hz rather than 100Hz. I’m wondering if that could be a facet of the hum problem?

I’ve looked at the component layout and wiring of the plastic case one I have – basically the same chassis, but with a much lower hum level. Can’t really see anything that stands out. Obviously it’s a very basic set – almost to the point that I can imagine that they designed a set which worked well, then started removing components till it stopped working, then put the last part back!

I’ve attached a copy of the circuit and wondered if anyone has any thought of things I might try in order to reduce the hum level. Extra decoupling caps etc?

Sorry the circuit is rather indistinct - it's the only one that there is, from the 1968/69 R&TV Book.

There’s a thread from 2013 on the VRF in which there is reference to hum:

http://www.vintage-radio.net/forum/showt...hp?t=88284

It can be heard very clear at this youtube link:

https://www.youtube.com/watch?v=4MNjjuThAa4

There’s a Figaro with plastic cabinet here on youtube in which the hum level is much less. in which the hum level is audible towards the end, with no stations tuned in:

https://www.youtube.com/watch?v=VCtfv_Kqr-k

(It seems to have had an accident with an electric iron or some such thing!).

Incidentally, as to my earlier post about the confusion with the auto transformer tags, I’ve confirmed that there is a separate secondary winding just for the heaters, which supplies both 6V3, and 1V4 (for the DM70).  The tags shown on the picture of the transformer don’t correspond with the circuit. (Tag 2 on the circuit is 220V AC into the primary, whereas Tag 2 is in fact 1V4 on the secondary). I’ve drawn a sketch to explain further.