My latest addition - Radialva Groom

[Robert Darwent]

Thanks Robert, the field coil measures 3,200 Ohms which I assume is close enough. I guess I should be thankful that it's not the coil though I'm now running out of places to look.

Yes 3,200 ohms for the coil is fine. Well, as you say, you've either checked/replaced all the relevant resistors, the capacitors have already been replaced, and the field coil is OK. I believe you have also replaced the 25L6 valve, so that covers just about everything in that section of the circuit!

However, looking at some of your earlier comments, you were saying the HT was measured at around 90v which does seem a little low from a 120v supply. I'm wondering if the 25Z6 rectifier is a little tired and trying another might solve the problem? How the negative bias voltage is derived will be affected by the value of the HT in this sort of circuit.

Regards

[Geaney]

I may put this aside for a while and come back to it. I've pretty much replaced all the resistors that could impact the bias on the 25L6 and noting seems to change the voltage from around -20V to the -7.8V it should be. What's surprising is that changing a resistor from 80K to 250K (R13) makes no difference to the voltage measured at the plate of the 6Q7 - which reads around 75V instead of 40V.

At least the B+ looks good at 106V.

I started to think I'd made an error in component replacements or connected something up incorrectly but if I did so far I can't find it.

I'll focus on other aspects of the set for a while and then come back to this later.

Reception on shortwave is superb!

Cheers, John

[Mark R]

Ah yes, I know this syndrome well, fresh eyes in a day or two work well!

Keep going John it is looking really good.

cheers Mark

[Robert Darwent]

I started to think I'd made an error in component replacements or connected something up incorrectly but if I did so far I can't find it.

Hi John,

Suggest you measure the total HT (B+) current and the total LT (through the heaters/dial bulbs). The heaters should be taking around 0.3A and if the service sheet is accurate the HT around 0.13A.

If you've made an error somewhere it will most likely show up with the HT current value. Measure with the volume at minimum and near maximum.

Regards

[Robert Darwent]

Just come across the following set;

       

It is the Braun F56GW model from 1943/44, so a couple of years after the first release of the Radialva Super Groom. The chassis layout is virtually identical but uses a different valve line-up.

Obviously Braun copied, or perhaps even stole the design? This was happening at the time of the German occupation of France during World War II after all.

More details here; http://www.radiomuseum.org/r/braun_s56gw_f56gw.html

Regards

[Geaney]

Yes I'd say the German folks helped themselves to the design. It's unusual since I normally assume German technology to be ahead of most at that time.

Cheers, John

[Geaney]

I may have had a Eureka moment. Last night I spent hours with Photoshop starting to make a dial image from the various scans and photos - this will take a while.

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.

I think I'll see about modifying the wiring to accept the 6Q7 so the set more accurately reflects the only circuit diagram I've been able to find. I appreciate this set was produced in war time and they used whatever was available so this could have been done in the factory - I don't see any signs it was done afterwards.

I also started to systematically identify all the components under the chassis so I would have a ref and I've found some more anomalies which might be down to me being tired or there are other changes I need to investigate. For example in the diagram there are two caps with a value of 0.25uF - I can find only one in my set!

So, some light at the end of the tunnel - let's hope it's not a train coming

Cheers, John

[tiv48]

on ward and hopefuly upwards
rob t

[Geaney]

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?

Cheers, John

[Robert Darwent]

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.

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