[morzh]

Ed

I don't know if the radio is for 110/117V and you might have 120V, so some bucking might be in order, but Zenith is known for using smaller transformers than the rest, so it might be natural for it to get warmer.
Again, it is an old radio and probably should not be driven hard

Another way to deal with it, a quiet fan, 12V or so, exhaust or blow doesn't matter. Aim it at the hottest part, although general circulation action is also helpful.

[EdHolland]

Thanks Mike... I'll check a few things today, including line voltage, and the current it is drawing. Operation seems perfectly normal with no hum or distortion, and good clear FM. It doesn't need to run hard to fill the room with music.

[Eliot Ness]

Ditto what Mike said Ed.   I'd try running it on a variac at about 110 and if it runs cooler make up a bucking xfmer for it.

[EdHolland]

So - a little data. I measure our line voltage this morning. assuming my multimeter is accurate, mains in is running around 123 - 124 Volts. That is quite an increase from 117 V.. I measured line current at 0.77 A, and that remained stable over a good period of operation. So this tells us a total power ~94W.

Poking around the audio amplifier chassis suggested the HT was not so far out of spec for the service sheets I have. Voltage on the 6V6 cathodes (self bias arrangement) was 12 V where 15 V is called, so these may be running a little hot, but I need to get the grid voltages as well for the full picture.

I'll do some calculations and try a bucking transformer next time we are here. I should also get a variac, as I've become sucked in to this ancient electronics business for good

[morzh]

Ed,

A variac is a handy thing to have in any radio workshop, so you might as well get it.
Then run your radio off it before you go for the trouble of making a bucking setup.
Measure the total power but also measure the temperature of the transformer with a thermocouple, both without and with the variac over time.
Give it a rest in between so you start from the same point, this will show you the difference.

The only really vulnerable part is the transformer: tubes can take the heat and even if running a tad hotter, they will still work fine, considering you don't use the radio as they used to, you might still get years from them. And unlike the transformer they are easily (and for most radios today also inexpensively) changed.
Not so with the transformer.

[EdHolland]

Understood Mike - thank you

[EdHolland]

I'm returning to this radio for a couple of reasons.

1) Mrs H wants to be able to play records (33rpm LPs for the most part), and purchased a small turntable that will fit in place of the 78 RPM changer.

2) The power transformer runs hot, and I would like to know why. This could be as simple as arranging to reduce the line voltage, but I'd like a definite explanation.

To begin with, we'll tackle the first question. Since the phono was via the Zenith "Cobra" design, there is a connector on the chassis that takes an audio input. It has 8 pins in the pattern of an octal socket, but also a pin in the centre space, offset to ensure correct orientation. While I could use the connector from the small pre-amp chassis, I rewired the whole thing and would prefer not to mess with it. So, I have another plan - take an octal plug, remove the spigot, and add the offset pin. The Octal plug I found on-line looks to have enough material in it to support a pin in a suitably sized hole. I can use the socket as a template to get the location. No electrical connection is required from the pin for this application. The only thing I have not checked precisely is that the regular 8 pins are exactly octal pattern, but they looked very close.

With a working plug, I just need to combine the L+R channels from the new record player, and perhaps include some attenuation, since the Zenith's input seems very sensitive.


Cheers,

Ed

[Arran]

Ed;
  You could try to reduce the input voltage to the power transformer, I have tried this on one set, via a Variac, because the field coil on the speaker was running a bit warm, did not seem to help much, though changing the cathode bias resistor on the 6F6 tube did help. I reduced the line voltage from 120 all the way down to 110, did not make a difference.
Regards
Arran

[EdHolland]

Thanks Arran,

I see references elsewhere to engineering lower input voltage, e.g with a bucking transformer. Worth testing but it may not make all the difference that I think is necessary. The power transformer eventually becomes hot enough that I can't keep my hand on it. From memory, the radio draws around 1 Amp at a line voltage that may run around 125 Volts at times. The other thing I want to look at is whether the O/P stage or something else is drawing more current than it should from the HT. It will have to wait until I can visit the set again, as it is in our other place. With current "Stay at Home" restrictions, it might be a while.

[EdHolland]

Well the phono connection idea is going to work nicely. An octal plug with the centre spigot removed mates perfectly with the outer ring of 8 contacts in the oddball socket. Now to add the off-centre locating pin

[EdHolland]

OK, I should get some pictures of this, as the next step is also working nicely. I was able to drill a hole in the plug at the correct location. Separately, I found some brass tube on the junk shelf that is a good fit to the contact in the socket. I drilled out the hole in the plug, making it an interference fit to the tube, and also rolled over the end of the tube to form a radius so that it plugs in smoothly. This plugs neatly into the radio, and will enable connection to the audio section without modification of the chassis.

Cheers

Ed