Ferranti 925 mains lead

[poppydog]

Does it make any difference which way round I wire a plug onto the mains lead on this Ferranti 925 radio as both wires are the same colour (brown)??

Regards
Poppydog

[Graham G3ZVT]

None at all that I can see looking at the circuit.

I generally like the live side to go to the switch, which would mean the neutral will go to the voltage tap on the transformer.

[poppydog]

Thanks rambo1152, I wanted to ask just to be sure.

Regards Poppydog

[poppydog]

This is a 915 (I think) portable battery/mains set that I am trying to get going. I have replaced the ht smoothing caps (1064 trader service sheet) C24-C25, removed the valves for heater filament testing and checked r21 22 and 23 are in spec.
With the valves still "out" i wanted to check that the heater voltage was ok, I have got 96v on pin 7 of v4 DL94 and this slowly rises if left on for any length of time. Nothing appears to be getting warm ie transformer, rectifier or ht smoothing caps, r22 used to get quite hot before I changed the smoothing caps c24-c23.
I haven’t done anything else to this radio.
I am powering this thing up via the mains.
poppydog

[Herald1360]

The set derives the filament supply from the HT supply via R22 current limiting resistor. With no valves in circuit and no filament current to drop the voltage, the DL94 socket will see near enough HT+ on the filament pin connected to R22. To get an idea of what the filament chain voltage would be you'll need to connect a 150R 5W resistor from the "hot" filament socket of the DL94 to chassis. You should then see about 7.5V at the DL94 pin.

It'll all be a bit approximate since the currents in the series filament chain in normal operation get complicated by the cathode currents of the valves concerned!

[poppydog]

Thanks for the reply Chris, I did try a dummy load on pin 7 but I only tried with a 2k..
I dont have a big wattage 250r here at moment but I tried a 200r 5w this morning and I'm getting 6.50v which is good news.
I'm now concerned that c22 that is only 12v rated has had a rather large voltage up it but not much current, so do i replace that now as well??

poppydog

[poppydog]

I’m still having difficulties with this thing regarding the heater voltage reading on pin 7 of v4 (dl94), at the moment I’m getting around 4.10v, all valves in, which is too low and r22(1.9k) is red hot though it is still in spec. C22 and C6 have been replaced as they were very bad and C21 (filament by-pass caps) tests fine, but something still appears to be pulling the voltage down.
I'm getting 68.9v at 1 end of r22 and dropping to 4.11v on the other side so there is a 34ma approx draw, is this high?
I haven’t a clue where to look next.

Regards Poppydog

[ms660]

Valve filaments in this receiver are 0.05 Amp, the resistor should normally be dropping around 95 Volts, measure the rectifiers output voltage at the rectifier and take it from there.

Lawrence.

[poppydog]

Thanks for the reply Lawrence, original rectifier still in place and i'm getting 104.2v out of it.

poppydog

[ms660]

I'm using the schematic from radiomuseum, R19?

Lawrence.

[poppydog]

The sheet i'm using is the 1064 trader sheet as mentioned in post 4 as I am sure its a 915. If you haven't got trader sheet I can email it across to you.

poppydog

[ms660]

I can get the trader sheet no problem, the schematic I was using was the R&TV Servicing one from radiomuseum.

Will report back.

[ms660]

104.2 Volts at the rectifier's output and 68.9 Volts at the input to the dropper (R22) suggests a high resistance connection between the two.

Lawrence.

[poppydog]

ok, r21 have anything to do with it?

poppydog

[ms660]

R21 is the HT filter resistor it's not used for the filament supply per se.

The filament circuit on mains is basically....transformer>>rectifier>>R22>>S7>>valv e filaments in series>>R23>>transformer.

Lawrence.

[poppydog]

Its now made me suspicious of my wiring to the 2 smoothing caps I replaced, but I cant see what I've done wrong, I haven't really done this as it should be.
Because the radio is a bit of a turd i have just lashed it up really just to see if I can get it to work, thought the experience might be good for me..
Is is worth changing the rectifier?

poppydog

[ms660]

Do the voltage measurements again....voltage output at the rectifier, voltage input to R22, voltage output from R22 and the voltage at pin 7 of the output valve, all with respect to chassis.

Lawrence.

[poppydog]

Will re-do voltages in a bit and post.

regards poppydog

[poppydog]

Voltages are as follows:
Output of rectifier = 102.1v
R22 in = 68.3v
R22 out = 4.12v
Pin 7 = 4.12v

I have also included some pics, as I said you will have to excuse the cobbling together of everything.

[ms660]

Check to make sure that R22 isn't connected in series with R21, it should be connected to the rectifier as per the schematic.

Lawrence.