10-08-2012, 06:47 PM
(This post was last modified: 10-08-2012, 07:08 PM by Robert Darwent.)
(09-08-2012, 08:09 PM)Geaney Wrote: Tonight I went back to the electronics and figured all the problems were around the 6Q7. I went back to the valves originally in the set and in this position was a 6H8. Having checked the pin-outs, these are similar but not the same so some wiring changes were made to accommodate the 6H8. When I put the 6H8 back in I get much more acceptable voltage readings though there is now a noticeable hum in the audio.
Mystery solved then John, having a different valve type in that position obviously explains all the different bias voltages you were getting.
(09-08-2012, 09:14 PM)Geaney Wrote: Here's the work in progress re component overlay ID - still a few to identify as well as one or two that seem to be missing?
I see you've been very busy! Tedious but necessary work, identifying all those components/tracing the wiring, if you intend to match the circuit diagram though.
My set has been on hold whilst a NOS set of valves arrive, particluarly the 25Z6 rectifier. The original one was 'milky' and obviously no longer fit for purpose, unfortunately I didn't have a replacement available.
As soon as it arrives I intend to turn my full attention to the set for a while and will no doubt be able to help you out with the wiring/components around the 6Q7 valve. From a cursory inspection, mine appears to be completely original in that area.
After much investigation of various dropper arrangements and calculations, I'm hoping I've found a simpler solution by using a small mains transformer.
The transformer in question has been mentioned before on the forum by David (Yorkie) in connection with a visit to Bardwell's, a long established electronics shop here in Sheffield, who sell the unusual item;
www.bardwells.co.uk - Transformer TX14 (500mA, 43-0-43 and 8-0-8 volt)
They make the ideal basis of a 90v and 1.5v eliminator, and with that in mind I bought three some time ago. I still have a couple unused and decided to measure the secondaries linked in various combinations.
I found that connecting all the secondaries in series (43 + 43 + 8 + 8) gives about; 112v RMS, and the (43 + 43) gave about; 94v RMS. So that looks promising for supplying the HT and heater/dial bulb chain respectively. Only the inclusion of a small dropper resistor (around 100 ohms at 4W) is necessary to ensure the correct value for the heaters.
The only real concern is that the set has a stated total consumption of 430mA with a 110v supply, so that doesn't leave much of a margin with the transformer's maximum rating being 500mA. However, they are of such small size (H53 x W44 x D46 mm) I was thinking of connecting two transformers in parallel to achieve a 1A rating.
Since the original large capacitor screening can (approximately H100 x W50 x D50 mm) is absent in my set, I intend to bend a simple 'U' shape from aluminum sheet of comparable dimensions and use it to mount the two transformers one abve the other inside the profile. There should still be sufficient space at the very top to additionally accomodate the small dropper resistor, so using the metal screen as a heatsink as well.
Hopefully, if all goes as planned, the set will run cool and have the added bonus of isolation from the mains supply as well. Once fitted back inside the case, the chassis should look almost original in appearance.
Regards
