My latest addition - Radialva Groom

[Geaney]

If you change the surge limiter to 10 ohms instead of the 33 ohms you are using at present you should find the voltage across the heater chain much closer to the 70v you are looking for, doing this also changes the capacitor value needed to 10.03 uF bringing it into tolerance.

Since a 10 ohm surge limiter produces a 3v drop instead of the 10v you originally allowed for, the resistor will only need a 0.9W power rating. So using a 3 or 5W type it should run much cooler and you will probably not need to make ventilation holes in the screening can.

Regards

Thanks Robert, I got carried away with the goal in sight so the dropper is now in the can - but I have some 10 Ohm resistors so I'll make the change over the weekend. The only hassle is getting the nuts on to the bolts that hold the can in place - not a lot of room to work among all the coils!

Soon I will have to tackle the glass dial issue - that's going to take some magic to find a workable solution. Thanks for posting the scan.

Cheers, John

[Robert Darwent]

I got carried away with the goal in sight so the dropper is now in the can

Hi John,

Looking good, you've made an excellent job of hiding eveything away! Once the chassis is back in the case it will look completely original. Just got to get myself moving to catch you up with my set now!

Best wishes and regards

[Robert Darwent]

Soon I will have to tackle the glass dial issue - that's going to take some magic to find a workable solution.

Regarding restoring your dial John, as you will be aware the problem with making a reproduction of a glass dial that has white or very light legends is not being able to print 'white' unless you are lucky enough to own one of the very few printers that can, such as those made by Alps (now obsolete unfortunately).

The following discussion may prove helpful though, where there is a description of using various embossing powders to create 'raised graphics' on decal (water-slide transfer) paper with the aid of an heat-gun.

http://www.radiomuseum.org/forum/dial_sc...inter.html

I have yet to attempt this method myself but it looks very promising and the results achieved look excellent. With respect to the Super Groom dial, the white areas could be isolated and printed by themselves to produce a waterslide transfer that is then applied to the original glass, so replacing the missing legends. Obviously the graphic would need to be made as a mirror image so the 'raised graphics' could be applied behind (on the inside of) the existing dial glass.

Regards

[Mark R]

This set is looking really good John, Robert has provided some excellent pointers.

cheers Mark

[Geaney]

The following discussion may prove helpful though, where there is a description of using various embossing powders to create 'raised graphics' on decal (water-slide transfer) paper with the aid of an heat-gun.

http://www.radiomuseum.org/forum/dial_sc...inter.html

Thanks Robert, it's a very interesting discussion on rmorg - I believe some experimenting will definitely be needed. I've got some clear decal paper from a previous project so I can start tinkering. I suspect the final dial will be in two 'layers' - one for the non white items using a conventional decal approach and a second layer with just the white items on it. Hopefully putting two decal layers is a doable thing - I'll find out.

Cheers, John

[Geaney]

If you change the surge limiter to 10 ohms instead of the 33 ohms you are using at present you should find the voltage across the heater chain much closer to the 70v you are looking for, doing this also changes the capacitor value needed to 10.03 uF bringing it into tolerance.

Since a 10 ohm surge limiter produces a 3v drop instead of the 10v you originally allowed for, the resistor will only need a 0.9W power rating. So using a 3 or 5W type it should run much cooler and you will probably not need to make ventilation holes in the screening can.

Regards

Thanks Robert, I got carried away with the goal in sight so the dropper is now in the can - but I have some 10 Ohm resistors so I'll make the change over the weekend. The only hassle is getting the nuts on to the bolts that hold the can in place - not a lot of room to work among all the coils!

Soon I will have to tackle the glass dial issue - that's going to take some magic to find a workable solution. Thanks for posting the scan.

Cheers, John

I had a 10 Ohm 10 watt resistor so fitting that has done the trick. Heater volts now just around 69V and no discernible heat being generated.
Thanks again Robert.

Cheers, John

[Robert Darwent]

I had a 10 Ohm 10 watt resistor so fitting that has done the trick. Heater volts now just around 69V and no discernible heat being generated.

Pleased to hear that changing the resistor has improved the low volts and the heat issues John, you're most welcome!

Now you have the chassis running well from a 120v supply, could I ask you to take a few current readings for me when you have the spare time?

I'd be most interested to know the HT current being drawn at the junction of the anodes of the 25Z6 rectifier and C26/C27 on the circuit diagram please. Not having my chassis up and running yet (waiting for the arrival of a NOS 25Z6) I can't as yet measure it myself.

From the service sheets I am guessing it will be in the 10 to 15 mA range depending on the volume setting. Having a 'ball-park' value from your set though would assist with the diode/resistor dropper I'm calculating to run the HT from in my set.

Regards

[Geaney]

I'd be most interested to know the HT current being drawn at the junction of the anodes of the 25Z6 rectifier and C26/C27 on the circuit diagram please. Not having my chassis up and running yet (waiting for the arrival of a NOS 25Z6) I can't as yet measure it myself.

Regards

Hi Robert, no problem I'll do that though I should mention that I'm not out of the woods just yet. I was checking voltages and the bias for the grids on the 6Q7 and the 25L6 is shown as -7.8v on the top of R12. Mine reads about -19V. Last night I checked all the relevant resistors (R6 through R14) and I replaced R6, R7, and R13. R6 and R7 were way higher than spec but R13 measured 80K versus the stated 250K. After all that the voltage at the top of R12 remained the same!! I may get some time to work on it today so I will keep at it.

Oh, and I've lost medium wave but the other two bands work fine. Hopefully that's 'switchery' though it's possible I've pulled a wire off in among the coils around the wave selector as it took a fair bit of messing about to get the electrolytic can re-fitted since one of the mounting nuts is underneath those coils - hopefully it's nothing too serious.

Cheers, John

[Robert Darwent]

Last night I checked all the relevant resistors (R6 through R14) and I replaced R6, R7, and R13. R6 and R7 were way higher than spec but R13 measured 80K versus the stated 250K.

Hi John,

As you have already checked R6 through R14 and replaced those resistors out of tolerance, the only other thing connected across that part of the circuit is the field coil (marked EXC on the diagram) with a specified resistance of 3000 ohms.

This set is quite unusual in having the field coil connected in 'parallel' instead of series with the heater/dial bulbs supply. Because of this it will have an effect on the negative bias arrangement. Check its resistance, hopefully it will be fine and the problem located elsewhere.

Regards,

[Geaney]

Hi John,

As you have already checked R6 through R14 and replaced those resistors out of tolerance, the only other thing connected across that part of the circuit is the field coil (marked EXC on the diagram) with a specified resistance of 3000 ohms.

This set is quite unusual in having the field coil connected in 'parallel' instead of series with the heater/dial bulbs supply. Because of this it will have an effect on the negative bias arrangement. Check its resistance, hopefully it will be fine and the problem located elsewhere.

Regards,

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.

Cheers, John