'Figaro Special'

[Yorkie]

Yorkie, can you post a link to that firm in Bognor Regis who supply speakers?  They sound a useful firm to know

Yes, it's from a firm called 'yourspares' and can be found at this link:

http://www.yourspares.co.uk/parts/ys117258/altai-general-purpose-elliptical-speaker-4w-8-ohm-L001.aspx?utm_source=google&utm_medium=shopping&utm_campaign=Google%2BProducts&gaw=agid:15560861837,c:46439495957&gclid=CjwKEAiA27G1BRCEopST9M39gykSJADQyqAlDHNmdfsXijFoG7IcMSVpvtcdOPc-W37BJzfFELEvxRoCmALw_wcB

Good eh?

A lucky find, I thought.

(Haven't checked out their other stuff - they do spares for domestic appliances).

Hope that helps.

[Yorkie]

All coming along nicely David: must make an article me thinks !!

Gary

Yes, I don't think a Figaro has featured before, but I'll have to see the job through to the end first!

Bulletin articles are to such a high standard that it becomes a little daunting to not  wish to let the side down and lower the tone. ('scuse the pun!).

[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.

[Yorkie]

Despite this little set being very lively and bringing in several stations at good volume, as I said earlier, it was clearly built 'down to a price' rather than 'up to a standard' and I'm wondering if the hum I'm experiencing is a feature of the shortcomings of the design, rather than an actual fault with my particular set. Hum does seem to be a common complaint with  'Figaro' sets from what I've gleaned on internet. I note the absence of a cathode bypass cap and maybe the addition of one would help, so I'll try that. Also, perhaps an additional electrolytic on the HT line might help.

I've attached a couple of simple amp circuits lifted off internet which feature ECL82s. Both show a cathode bypass cap; the first one shows a 50uF cap on the HT line - the second shows an 8uF.

Easy enough to try.

[Radio Fixer]

I don't expect the cathode bypass cap to make any difference to the hum. But the negative feedback maybe there to cancel some distortion so that could be worse. The extra on the HT line might help. Could be wiring and screening to the switch on the volume control, assuming the switch is there. Could be heater cathode leakage in the audio pre-amp as I just had in a Murphy.

Gary

[Yorkie]

Thanks for reading and for your helpful comments Gary.

Yes, the switch is on the volume control.

It's not the ECL82 valve - I've swapped it over with the one in the other Figaro set which doesn't perceptibly hum and it's fine in the set.

Realistically, once restored it's not going to be in daily use so it's not a big deal, but it does feel a bit like 'unfinished business' to not try to reduce or eliminate the hum.

[Terry]

I'm in agreement Gary about the additional negative feedback due to the lack of cathode bypass capacitor and the addition of extra HT decoupling to the triode section.

It might be interesting to just dab a suitable electrolytic across that cathode resistor with the volume at minimum - I assume that  the volume setting makes no difference to the hum - and see (or, rather, hear!) what happens. If the hum level remains the same it would point to the smoothing as the culprit but if it increases, this is caused by the removal of the feedback and the resulting increased gain pointing to the origin being in the triode section.

Other possibilities are routing of the heater wiring - is this twisted or rely on the chassis for the return path? Temporarily moving the common connection to the ECL82 might help.

As you have two of these sets, side by side visual inspection of the physical layout might show why one hums worse than the other ...

[Refugee]

I had some 100Hz hum on a valve amp and it turned out to be earthing of the smoothing capacitor.
The cure was to earth the smoothing capacitor to the physical 0V terminal on the mains transformer. You might need to insulate the can from the chassis.

[Yorkie]

Oh dear, it's curtains I'm afraid!

Before progressing any further, I've just switched the set on and it went dead.

The 0.4A integral fuse is still intact, but on examination, the primary of the mains transformer is open circuit.

That's effectively written the set off as beyond economic repair as a rewind would be quite out of the question.

As the transformer is very specific to this set, there's no prospect of a substitute.

Grrrrr, a bit of a bummer when I was on the last lap.

Oh well, it will look pretty on the shelf.

Thanks anyway chaps for reading the thread and for your advice and encouragement.

[Yorkie]

Egg on face time - I might as well own up!

I unplugged the radio to look further into the auto-transformer problem and the resistance readings looked in order, as did the new flex I'd fitted, and the13A  plug, and the double pole switch on the set. I plugged the set into a different 13A socket and it set came to life. Tried my soldering iron in the socket I'd been using and it didn't heat up. The live terminal inside the twin switched socket had come loose. So, everything is back on track.

Furthermore - I've discovered what was causing the hum level to be higher on this set than the other one. In addition to the ferrite rod antenna, it has about half a Metre of wire which - when the set is in the cabinet, is pinned around the inside to give additional pick-up. While I've been restoring the set it's been draped round the chassis. Ideal for picking up 50Hz Hum!  I've move it away from the chassis, and the hum is minimal. The other set with the plastic cabinet doesn't have the wire aerial - just the ferrite rod.

I must be losing my grip!

Encouraged by this change in fortunes, I've fitted the new magic eye, which works nicely, but there isn't much swing in AGVC voltage, except on very strong stations, but neither is there on the other set.

So, I'll check the alignment next and see about casting some knobs.