Hi all
Does anyone happen to have service information on any of the Kingsley radiograms? I've had a few across my bench but never managed to find any schematics.

I'm working on a KG525 at the moment so if anyone has any information it would be appreciated.

Thanks...

Valve line-up?

Hi GTC
I managed to find a hand drawn schematic someone had posted. I've just removed an open primary OP transformer on one channel. I've got a few old amps for spares so hoping to find something in them. It uses a 6GW8 into a 6500/15 impedance xformer. Any suggestions on matching OP xformers if I need to?

Cheers.

Not exactly sure what you are asking. Are you asking if you need to find another 6500/15 transformer, or if you need to use that exact specification?

BTW: It may help future restorers to post the schematic here, or at least a link to it.

Yes, looks like I will need a replacement 6500/15 OP transformer.

As a matter of interest, the R value of the good transformer reads 1.6K ohms primary, 2.2 ohms secondary. I tried a 7000/15 I had in the spares bin; the channel came to life, but low volume with distortion at a higher volume. Anode B+ measurements between channels was also different by approx 30 volts. (the good channel being less)

Interestingly, the 7000/15 transformer I tried measured an R value of .470 ohms on the primary and 1.3 ohms on the secondary!

Is there a rule of thumb for measuring resistance values of OP transformers against their rated impedance? I thought is was approx 80% of the value but don't quote me??

Do you know where I can source a replacement 6500/15 transformer from?

Thanks.

The DC resistance value is a function of the wire used (ohms per metre).

With speaker voice coils you can get a good approximation of the nominal impedance by the 75% rule of thumb. So an 8 ohm impedance coil will measure around 6 ohms DC resistance.

As for impedance matching, here is an article explaining how it works and how to measure an unknown output transformer:

http://www.radioremembered.org/outimp.htm

Transformers are not cheap! Generally, the price increases with power rating.

Hammond USA makes a a range of output transformers:

http://www.hammondmfg.com/5caud.htm

Some are available through EVATCO in QLD:

http://www.evatco.com.au/product-category/transformeroutput/

GTC
Good article on OP transformers. Thanks.

So, when I have a known value impedance transformer and the dc resistance measurement is way off for the known impedance values and the amplifier is still working, does this mean that the windings are beginning to break down? What's your experience with this please?

the dc resistance measurement is way off for the known impedance values

I don't understand that statement.

Two identical (i.e. constructed the same way) OP transformers from the same manufacturer may well be expected have the same DC resistance for their primaries and secondaries.

If some windings are shorting together then the resistance will be affected, but you may not be able to detect that accurately. If they are shorting to the core, then that can be detected.

My only experience with OP speakers is that they are either open circuit or okay. When they go open circuit I need to replace them with an equivalent, or as close as I can get to an equivalent.

If the amplifier is still working well, then I don't bother fixing it.

Any chance you could post the circuit diagram on here MacGyver as I have just been donated a KG525 and it would be handy to have something to put in the filing system for future use.

Cheers

The DC ohms of a transformer be it power or audio is somewhat irrelevant; These work on inductance and that is related to to the turns ratio of the primary to the secondary. E.g. I have a Grigsby-Grunow here, Power Transformer 115 Volt primary is just on 10 Ohms DC and it works as it should.

Low resistance on a transformer tricks a lot of people. As Marc said, the inductance is the key factor here and this saves the coil from nuclear meltdown. If you remove the iron core and then power up the coil, expect big problems - it will meltdown in no time at all.

And, just so everyone is aware - if you apply DC to a transformer it will not only not work but it'll get very, very hot.

Transformers of all kinds like AC much better. It is not the voltage itself that makes a transformer work but the constant variation in the applied voltage.