Ferranti 925 mains lead

[poppydog]

In the pics you can see the red dot on the side of rectifier, this is the output which the wire from that point goes to the + of smoothing cap c25 denoted by brown wire with white tipex dot on the end. Attached to this point on cap is also 1 end of r21, the other end of r21 attached to the + of c24.

poppydog

[ms660]

Originally Posted by poppydog

In the pics you can see the red dot on the side of rectifier, this is the output which the wire from that point goes to the + of smoothing cap c25 denoted by brown wire with white tipex dot on the end. Attached to this point on cap is also 1 end of r21, the other end of r21 attached to the + of c24.

poppydog

If that's the case and the large component that's connected to the switch is R22 then R22 is in series with R21 and therefore it wouldn't be the same as the schematic, 1k @ 0.034 Amp (your last heater chain current measurement) equals 34 Volts, 34 Volts + 68.3 Volts (that you measured at the input to R22) equals 102.3 Volts which is what you measured at the rectifier.

Lawrence.

[poppydog]

Ok Lawrence thanks for that, will have to try and sort this out tomorrow now.

Regards Poppydog

[GeoffK]

The rectifier + cathode has one end of R21, R22 going to + C25, three connections.

Other end of R21 ONLY goes to C24 + and switch S8 (m = mains)

Other end of R22 goes ONLY TO junction on switch S6 (B = battery) and S7 (M = mains)

The mains battery switch over selector.

[GeoffK]

Originally Posted by GeoffK

Other end of R22 goes ONLY TO junction on switch S6 (B = battery) and S7 (M = mains)

The mains battery switch over selector.

Correction, R22 goes to outside tag on S7. The junction S6,(battery) S7,(Mains) goes to Pin7 (heater) V4 and C22+.

[poppydog]

Why is r22 getting so hot as it is at the moment? also something that I've never done before was check to see if r21 is hot and it is, its boiling. Will have a proper look at this later.

poppydog

[GeoffK]

Check the resistors with V4 removed, make sure the set is fully discharged by leaving it switched off before removing or refitting any valve. There is a serious short circuit to cause the HT resistor to get hot. Check the output transformer by touching it, is it hot, do not leaving it running for long like this. Check C18 HT smoothing capacitor.

[ms660]

If R22 is passing filament current it will get hot....Isquared*R

If R21 is dropping 34 Volts as previously reported then it will get hot....Vsquared/R

The circuit should be wired/connected as per the schematic.

The brown wire with the white dot from the rectifier's output (as you described) should be connected to the junction of R21&R22&C25, there should be no other connection to that junction, in your photo you can see that the the brown wire with the white dot is connected to the end of R21 that's connected to the mains power supply HT switch S8.

Lawrence.

[poppydog]

So its looking like i have connected 1 end of r22 to the wrong smoothing cap, i will move it and take some more voltages.

[poppydog]

With the end of r22 moved the voltages are as follows

output of rec=90.8v
into r22=90.8v
out of r22=6.01v
pin 7=6.01v

R21 is cold now, r22 is still red hot and the volts on pin 7 are up a bit but not enough.

poppydog

[ms660]

Measure the resistance of R23 and measure the DC voltage across it.

Lawrence.

[poppydog]

This sounds fairly good with the change I have made so far though doesn't it? I also noticed the rectifier is getting fairly warm now, also had a biggish tickle off of rectifier trying to work out just how hot its getting. R22 is very hot, put a wet finger on it and it sizzles. Will look at r23 in a bit.

poppydog

[poppydog]

Originally Posted by ms660

Measure the resistance of R23 and measure the DC voltage across it.

Lawrence.

R23 out of circuit=53.6r voltage across it= -2.52v

[ms660]

Taking that into account those measurements, the low voltage out of the rectifier would point to the rectifier itself assuming that the reservoir capacitor is of sufficient value, let's see what others think.

Lawrence.

[poppydog]

The capacitance is stated as 32uf but I have got 22uf in there as that was what I had here, has that upset things??

poppydog

[GeoffK]

The grid bias seems about right. There is nearly 4 watts (90.8-6.01=84.79 V/1900=~0.045mA*0.045=0.002*84.79=3.81W) being dissipated by R22 so it will run hot if not sufficient size, but should not be excessively hot. 22uF rectifier smoothing should be just about ok. Check C21 if it is the original 500uF and C22 for leakage that could draw more current through R22, disconnect and fit temporary 100uF 10VDC min if you have spares handy.

[ms660]

Re: Post#35:

Possibly not, but if you do decide to replace the existing rectifier with a silicon rectifier I would fit a 33uF for C25, and the same value for C24.

Lawrence.

[poppydog]

Thanks for all the replies. I will carry out the suggestions mentioned by Geoffk then decide what to do.
I have to decide wether or not to carry on with this as there will almost certainly be a lot more components to buy and I'm unsure what state the valves are in and as I said in a previous post, the radio itself is in very poor shape.

regards poppydog

[poppydog]

Any recommendations what diode/s I could use in place of the old rectifier please?

poppydog

[Mike. Watterson]

Originally Posted by Herald1360

TYou should then see about 7.5V at the DL94 pin.

Actually 6.8V is closer to the ideal on mains. The 7.5V is a nominal battery voltage. The filaments are nominally 1.4V parallel, 50mA series, but 1.35V +/- 5% recommended by Philips on mains. The actual series current isn't 50mA due to ballast resistors and significant DL94 anode and screen currents in the filament.