Audio Transformer for Strad 511 radio

[telstar]

I am working on my Strad 511 radio (1950) and would like to obtain a replacement audio transformer. The present one seems to measure very high ohms 12.6 k, when it should only read 200 ohms (according to Trader service sheet 1014). Alright, this is not the impedance value, but this does not seem right. No wonder that I could not get a buzz from the volume control!
There are voltages on the valves, but there is no o/p at all.
The valves are 7S7, 7B7, 7C7, 35L6GT, and 7Y4.
I have some audio transformers in stock, but it might be a task of experimenting of which will match the 35L6GT with the speaker, but if someone has a suitable one that is made to match then please let me know?
Many thanks
Mike

[Ed_Dinning]

Hi Mike, you need a transformer somewhere between 35 and 40 :1 for a 3R speaker.

Rewind is also possible;12.6K for primary resistance is way too high

Ed

[Cobaltblue]

Hi Mike

What did you measure it with?

Some modern meters seem to get confused by inductors.

Is there volts on the anode of the 35L6GT ? it should be near the same as the HT

Did you check you get a scratching noise as you connect a l.5V battery to the speaker.

Not saying the output transformer isn't at fault just I would do a few checks before replacing it.

Cheers

Mike T

[Glowing Bits!]

The valves should be removed before testing the primary.
If the valves were removed prior to testing, there is likely to be a break in the primary coil with a carbon build-up as the gap becomes a spark generator, this will result in odd readings.
A way to prove if this is happening would be to use an old Megger insulation tester (magneto 500v), the reading after fast cranking should drop, the tranny then would need re-winding.
I hope the above makes sense, it's a bit complex to explain the procedure.

[telstar]

Hi. Many thanks everyone. I have measured it with my Maplin Digital Meter. Will try to measure it with the valve removed next.
There is 184 volts on the anode and 75 volts on the screen grid. The 7C7 also measures 75 volts on the anode
I think that I have the first edition of this radio as the circuit does not follow the one on Trader service sheet 1014.
I got a scratching noise when I switched to d.c volts and measured on the anode of the 7C7 valve.
I have a Megger analogue device, but the pointer no longer seems to move as it should (even with a new battery).
The 7Y4 has 210 volts on its cathode, and that matches the details on the diagram.

[telstar]

Hi. I have done some more work on the Strad radio. The transformer did turn out to be O.K at 200 ohms. I took some incorrect readings, but it did not change the fact of no output. The valves are O.K. One thing that was off putting was that there was a large resistor in the power supply measuring 12.6 K ohms, and I wonder if it was supposed to be 10 K ohms, as something was mentioned about this in the manual 1014 ? Anyway, I replaced it with a Maplin 3 watt 10 K type, and the voltage did increase on the H.T line.
The audio coupling capacitor from 7C7 to 35L6 used one of those blue & grey plastic devices. Wonder if this has gone faulty?
Will let you know.
Mike

[telstar]

Still no luck with the Strad 511 radio having any audio output. I measured 80 volts on the supply rail that is applied to the I.F & F.C valves. No sound when the centre tap of the volume control is touched, and yet when measuring voltages on the anode of the 7C6 valve a sound is heard from the loudspeaker (click).
Both scale lamps (3.5 volts @ 0.15 amps) suddenly stopped working. I know that these are in series, and replacing them did not bring back the illumination.
I wonder if the heaters on my valves are in series (I have the early model in this range). The valves on the later one on Trader sheet 1014 are in parallel.
Is there a manual on this site (for download) with the circuit that suits my radio?
All the bothersome wax paper capacitors have been removed and new caps installed, and all the resistors that had gone way out of spec have also been replaced.

[Cobaltblue]

The heater arrangement on the 511 is a little odd can you tell us the full valve complement ?

Does it have a mains transformer?

The circuit I am looking at uses 6.3V valves except the output valve 35L6GT but they are just fed off different taps on the mains transformer.

The HT sounds really low so either the output valve is pulling too much current, the rectifier is tired or something else such as low capacitance in the smoothing circuit.

If the dial lights are not working have you got a partial short on the 6V heater tap? that would make the rectifier look tired. It would stop the rest of the set working as well.

Cheers

Mike T

[ms660]

80 volts HT to the mixer and IF sounds about right.

Lawrence.

[cathoderay57]

Looking at Trader Sheet #1014, 80v HT is as per spec so that should be OK. You can get this Trader Sheet (and others) here: https://www.service-data.com/search....&search=Search . The first step should be to find out what has gone wrong with the heater supply. AC volts test across the heater terminals to the valves and/or the scale lamps is needed. With the set disconnected from the mains, also remove the rectifier and check its dc resistance between the cathode and heater pins - if there is a heater-cathode short or near-short the HT supply would have appeared across the heater supply and taken out the pilot lamps and possibly some of the valve heaters. Let's hope not - the 6X5GT rectifier is designed to operate in this type of circuit and normally will withstand high heater/cathode voltages without exhibiting this failure mode. Cheers, Jerry

[Andrewausfa]

The speaker voice coil's not jammed up is it? You had issues with the audio transformer but I can't see a mention if that was confirmed working. Currently dealing with a stuck speaker myself

Andrew

[telstar]

Hi everyone
The valve line-up is 7S7 7B7, 7C6, 35L6GT, 7Y4
I managed to sort the pilot lights out, the wiring was very flimsy, so I installed some that I bought from the NVCF. Upon switching on the set, both lights returned to normal.
Also there was a dreadful hum from the speaker, but this later stopped and I could hear m.w stations accompanied by modulation hum (a bit rare on a.c radios).
I wonder if I should attach a 3 core mains lead to the radio? From what I have read though, the mains transformer is double wound, and probably would not need an earthed chassis?
At least I have made some progress.
I have cleaned the pins of all the valves apart from 35L6GT as they are nickle silvered and get very bad contact.
Will let you know more later
Cheers
Mike

[telstar]

Hi. The loudspeaker is not to blame as I have substituted two different loudspeakers and I still get the hum problem.
I am in the process of checking the cathode by-pass capacitor (35L6GT) to see if it is leaky. It is in a can of h.t capacitors, it could be the problem?
Cheers
Mike

[Andrewausfa]

Hi Mike,

It was just a thought as I couldn't see where you'd mentioned the audio transformer was 100% working but you had just a click from the speaker. I've been working on a Philips set which was very rusty on the speaker coil centre-piece which made me think yours, like my Philips, may be a jammed speaker. I managed to get it moving slightly then stuck a signal from an audio signal generator up it and it freed up completely after a while.

Have you radically moved any wiring? That might cause hum.

Regards - Andrew