Roberts R66

[ppppenguin]

These battery valves are very sensitive to overvolting their filaments. Their nominal voltage is 1.4V, maximum must be at least 1.6V as a fully charged cell can do that. As Mike said, more than 2V can blow them open circuit. His recommendation of 1.35V is a good one. Don't expect to see a glow, except perhaps in a dark room.

There are 2 resistors between the rectifier and filaments (R15 3.9R, R16 5.4R made up of 3.9R and some resistance wire) which seem likely to drop your measured voltage to about 1.4V. Measure at C29 as Mike suggests, or at S7 or at the filament pin of one of the valves.

[LyntonP]

Just re checked the LT voltage. Across C29 I get 1.18 volts.

[ppppenguin]

That should work but it's a little bit low. Equivalent to using a rather old 1.5V battery. If the set is working then you can leave it. Otherwise the first thing to do is make sure you're on the correct mains tapping. Measure your mains voltage and set the tapping accordingly.

If that's OK, then check R15 and R16. Note that the resistance of the meter leads and the quality of the contacts they make will be significant when measuring low value resistors. You can check the meter leads by shorting them together and subtracting that reading from any measuement. It will often be around 0.4 ohms. Apart from that you're looking at replacing the 3 electrolytics C29,30,31. Modern 2200uF parts will be a lot smaller than the originals. Finally MR2 could be replaced by a couple of 1N400x diodes. Make sure you don't end up with too high a filament voltage after all this.

Reading the general notes see clip from trader sheet.

[LyntonP]

I think the radio has been listening to this thread as it has now gone silent again. Time to get the multimeter out again!

Lynton

[LyntonP]

Returning to the radio today first step was to re-do the diode and resistor rectifier for the HT. satisfactory conclusion with the HT a couple or three volts below 90v. Then considered the LT but on closer examination I’m not sure if I have found something amiss? The selenium rectifier Has five connections. Two go to the transformer, one goes straight to ground (not shown on diagram) and the other two which are the DC should both go to the junction of R16 and C31 (Traders sheet). On my radio one goes to the junction mentioned but the other goes to the junction of R15 and C29? Is this a mistake or intentional factory mod?
I took some pictures as best I could but please excuse the soldering around the HT diode/resistor as I was still experimenting!
Lynton

[Terry]

That sounds somewhat weird! The outer two tags being linked together is normal - it is what completes the bridge. [1] Without the circuit it is difficult to judge what, exactly, is going on. I assume from the component numbering that R15 and R16 are connected together - is that right?

If you could post a snippet of the relevant part of the circuit, together with component values, it  would make it much easier for me and others to help you - at the moment it is a case of guessing in the dark!

As for the voltage required by the valves themselves, 1.3V is a good choice to aim for.

I know that Michael Watterson went into this in some considerable detail some while ago but I'm not sure if it was here, on VRAT or UKVRRR.

[1] Looking at your pictures, the configuration of this bridge looks somewhat unusual. if the green tags are AC, one of these is at the end of the stack, which is odd. The usual connection order is : +ve, AC, -ve, AC, +ve with the two positive tags linked together as I said.

[ppppenguin]

Terry, it's a biphase rectifier, not a bridge. At least that's what the Trader sheet says. Clip attached. The 3 smoothers are all 2500uF, R15 is 3R9, R16 is 5R4 subject to a note in a previous post.

[pwdrive]

There's only one +ve (cathode) connection tag on the LT rectifier, and the two "spare" spare ones are a through connection in the rectifier so far as I can make out.

Lawrence.

[Terry]

Thank you, Jeffrey - my fault for jumping to conclusions! The situation as described by Lynton is bizarre! Alternate half cycles leapfrogging two stages of smoothing and missing out on the voltage drop of 9.3Ω (or even more!)

I think the best plan of action for Lynton is to measure the voltage across C29, rewire the rectifier as per the Trader sheet, check that the values of R15 and R16 are correct, measure the voltage across C29 again and report back.

If the first voltage reading is significantly above 1.4 volts and has been like it for some time, it doesn't bode well for the condition of the valves.

[LyntonP]

Here is a photo of the rectifier removed from the radio. I have tried connecting 2 diodes but am struggling to find the correct value for the dropper resistor does anyone know this of the top of their head?
I removed all the valves to avoid damage, and V3 is an oddball. It should be a DK96 but the valve in place is a UX1 D7B2. I cannot find this listed as an equivalent in any valve data?
I will search for the posts mentioned for further info.
Also struggling to load a photo of the valve website says file is too large. I will try and reduce the size.
Lynton