[klondike98]

[skyscraper]

Thanks Bob

Final update for the night. I plugged the rest of the tubes in, and after a while got all the way up to 120 volts. Nothing so far but a fairly loud hum and occaisional pops, thunder and something that might be static. Touching the grids of the 75 and latter 44 result in a buzz, touching the 36 gives just a pop, and toucing the rf 44 gives a quiet pop. Tuning doesn't result in much of a change. I've reversed the connections on the oscillator tickler coil, which did nothing. The volume control adjusts the level of the noise. It also seems like there is a high pitched whine

Any ideas? I'm thinking the 36 isn't oscillating, since only pops get through the rf chain. I have one spare 44 I'll try in the front end tomorrow.

As far as alignment and testing, all I have is an oscilliscope. No function generators or counters. I'm going to probe around the IF section I guess and see if anything is being generated.

[skyscraper]

This morning I disconnected the antenna and hooked a scope lead to pin 2 of the 36 tube. I got a nice looking sine wave, that changed in frequency with tuning. I then, out of curiosity, connected the antenna lead again. It worked! It picked up all of the local stations well with 15 feet of magnet wire, and besides a little noise (shields aren't in yet) it sounded very good. What's more, even after all the coil surgery the dial is almost spot on.

I sent some Bing Crosby over the transmitter and wished I was old enough to drink some champagne

Then I went to disconnect the alligator lead from the 36 (just the lead, not the probe anymore) and it went totally dead, just like before. Ugh.. I reconnected it and it worked fine again. So, it seems like it needs 6 inches of wire on the top of the oscillator primary coil in order to oscillate. I'm totally at a loss as to why that would be.. Any ideas? The cathode bias is 680pf + 6k8 ohms.

Another concern, as I mentioned earlier the transformer has leaked a bit of wax. Under full load it's getting quite warm. Not hot, but warmer than I was expecting. The buzz went away when I tightened the bolts. That isn't normal, is it? B+ is pulling about 65ma, and all voltages are correct.

[Mondial]

Try changing the 36 autodyne converter tube. Its a marginal oscillator circuit at best and unless the tube is in top shape it can be a bit unreliable as to oscillation. I had to try several in mine before I got it stable.

Check the voltage across the wirewound bias resistor. I get about 14-15 V across mine. Much more than that and it might be drawing excessive current. My transformer gets warm, but not uncomfortably hot to hold ones hand on.

[skyscraper]

I don't have any 36 tubes. Watching several on eBay, hopefully there will be some spares in a week.

It's 18.8v on mine. B+ out of the field coil is around 300v, again at 65ma. Is that reasonable?

[Mondial]

Current draw with 18.8 V across the WW resistor is a bit high. Check to see if there is low bias voltage on the control grid of the 42 output. If the coupling cap is leaking or the 42 has excessive gas, it will cause reduced negative bias and high plate and screen current.

You should have approximately the same bias voltage from grid to cathode on the 42 as you have across the wirewound.

I just noticed you have 300V out of the field coil. This is also high. Should be about 260-270 V on the screen grid of the 42.

What is your AC line voltage? Did you increase the value of the first filter cap after the rectifier?

[skyscraper]

Here are all all those numbers. The two new electrolytics are both 6uf, as original.

Line voltage today is 122.5, which probably has a lot to do the high B. The back of the radio calls for 115.

Off the first capacitor, into the field coil is 391v. Out of the field coil, into the second capacitor is a bit over 300v, I don't remember the exact number.
Across the bias resistor is almost 19v, grid to ground on the 42 is 18.2v, screen to ground is 293v.


Thanks for all the help,

Will

[Mondial]

Yes, its probably the high line voltage. My readings were taken at 115 V line. I always run my radios on a Variac, as I also have high line voltage. Its also less stressful on them to bring up the voltage slowly.

You can get an old 7.5 or 10 V filament transformer and connect the secondary out of phase with the line to buck the voltage and reduce it to 112-115 V. The radio will be much happier.

[skyscraper]

I'll have to track down an autotransformer pretty soon. Do you have any suggestions for a relatively cheap unit?

Do you mean just connect the filament winding in series? I have a heater transformer for an old Heathkit. I think it's 6.3 volts.

[Mondial]

I bought my variable transformers at hamfests or flea markets, where they usually can be gotten at a good price.

Yes, connect the 6.3 V secondary in series with the hot side of the line. You connect the primary to the line and neutral. The phase will determine whether the secondary voltage adds or subtracts from the line voltage. If the voltage is higher than the AC input, just reverse the leads to either one of the windings.

You can put the transformer in a box with a line cord, fuse, and outlet, so any radios you plug in will see the reduced line voltage

[Steve Davis]

Will, try replacing resistor number 10 with a 10K or even 7500.

Steve

[skyscraper]

Hi Steve thanks for the tip. I've actually already replaced it with an 8k6, and tried even lower with a pot.
I'm wondering if that's too small, since the reason it's supposed to be lowered is to compensate for moisture in the form, most of which is hopefully gone after the baking. That coupled with the excessive B voltage could be messing with something. I should have three or four 36 tubes to choose from by next weekend. If none of them work, I might try raising that resistor back to 10k. I don't quite understand why a few inches of wire on the coil completely fixes it though..

I have a bit of 'thunder' out of the audio section as well, that isn't affected by the volume control. I think it might be dirty pins or caps, I'm going to clean them all tomorrow.

Mondial, that suggestion is genius, thanks! Wired as you said this transformer produces 115.5v - perfect! I found a fuse holder too, I love the idea of a universal 'safety box.'

[skyscraper]

I got three 36 tubes yesterday, all used but tested excellent according to the seller. I put each one in, the best I can get is oscillation above ~1180kc. All four local stations at the top come in fine.

With a digital radio, I'm able to receive rf+lo at rf+238kc. It's supposed to run at 260kc, is that enough of a problem? I don't have any alignment tools, other than a transmitter and scope. Is it possible to align it without a function generator or counter? It seems like the oscillator could be corrected with the digital radio and transmitter, and the other stages with a scope.

Like before, a jumper lead connected to pin 2 of the 36 makes it all work fine. Any more suggestions?

Thanks,
Will

[Mondial]

The IF tuning seems quite a bit off. You could use your transmitter and gradually adjust the IF trimmers so that the LO freq is 260 kHz higher than the RF input freq. You can peak using the AVC output as a carrier level indicator, measuring with a DVM across the detector output to ground.

It is possible that the IF alignment may have some effect on the LO operation, as the plate circuit of the 36 is tuned both by the IF tranformer primary and the osc coil. I would get the IF tuned to 260 kHz before anything else.

As I recall, you had to rewind the osc coil, so any deviation as to the way the coil was wound and connected to the terminals may be significant.

[skyscraper]

Thanks Mondial, I'll try aligning the oscillator. For future use, what equipment do I need to really do an alignment? Could I get away with only a function generator?

I got the impression from everything I read on the oscillator tickler winding that the particulars weren't critical. Some people seem to get decent results cramming a ball of wire in the center of the form.
With mine, I forgot to bake the coil before rewinding. I did after, not thinking the 200˚ would hurt anything. It melted my insulating plastic, and the coil is now half buried in it. It's still helical with no overlapping turns, so I thought it would be alright. If nothing else works, I'll pull it out again and try again.

Thanks for the suggestions