Philco 70 output transformer
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Hello all, new on this forum. Restoring a 70 someone gave me years ago to keep me busy. All caps restuffed, new resistors except the 51k at oscillator, some new wiring, and cleaned up.
Ordered new 7k 8 ohm transformer to push 8 ohm speaker - might rebuild the original later. In the meantime, the radio plays good, but checking the output transformer seems it has shorted turn in secondary. I measure raw resistance 453.1 ohms primary, but only .02 ohms .02mh on secondary. Does anyone know what they measure on one that is good? 5k perhaps? Speech is fine, but music is garbled, I am using an 8 ohm speaker at the moment with the original output, big mismatch, don't think it should be this garbled though. I did take out the field coil with 3100 ohms to feed D wireround point.
Edit: I meant to say secondary of around 5 ohms.
(This post was last modified: 05-07-2020, 04:29 AM by radio1.
Edit Reason: Mistake
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Hi and welcome,
The primary resistance seems good, it wouldn't be anything close to 5000 ohms. I would check the speaker for a rubbing voice coil. At low volume levels it can cause distortion and or garbling sounds.
Some thing else to bear in mind the early model 70's were non avc sets and the power detector circuit does not product the cleanest audio. You may want to listen w/a tracer and see how it sounds @ different volume levels. The "volume control" is a rf and if gain control so as you adjust it it's changing the level of signal going to the detector. It's not adjusting the gain of the audio amp as it is set to a fixed level.
Bad ground connections @ riveted points on the chassis is something else to have a look at.
Happy Hunting!
GL
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
(This post was last modified: 05-06-2020, 07:32 PM by Radioroslyn.)
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I am using a good 8 ohm PM speaker, it is the secondary ohms I am looking for, 90 to 1 ratiio should be around 5 ohms, but wanted to know if someone measured this secondary 2673 T output transformer.
(This post was last modified: 05-07-2020, 04:30 AM by radio1.
Edit Reason: Keyed wrong
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Welcome to the Phorum!
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According to the speaker chart in the library, it should be a K3 with a voice coil impedance of .7 ohms and a primary out put transformer DC resistance of 450 ohms. It doesn't give the secondary DC ohms value, but it would be low. I would measure mine but it has a replacement transformer in it.
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Mine seem correct on the primary side - 453.2, but shows .02 on secondary. I saw the Philco chart also.
(This post was last modified: 05-07-2020, 04:47 AM by radio1.)
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Your transformer is fine, go back and re-read post number 2.
Steve
M R Radios C M Tubes
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Okay, will go back through it looking for bad grounds. I get 0 ohms from ground of volume control to ground of wire round. I am aware of the oddity of this volume circuit, and low volumes affect the detector's cathodes as well, non linear region possibly. I did notice the inner 5k of the dual volume does not track linear with rotation checking with ohm meter, and I don't see much change there until almost half rotation, then everything is fine. Have not disassembled the volume to clean, just cleaner through the holes. After recap I noticed a low level hum, almost like a ground loop hum, not lytic loud hum, I was running 40mfd's and it was not evident, running 10's now and it is, but might point to bad ground somewhere. I ran one first position 10 and last a 40mfd and it was better than two 10mfd lytics. You can feel it in the speaker, so something is not right. This is an early non avc unit serial 850023. I will check my work also looking for errors. Thanks.
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Take the cap apart, rotor and stator separately, soak in dishwashing soaped water (not together), take a long soft brush, gently brush inside. Rinse well, blow with air (be careful with airguns, better use canned air), dry completely.
Reassemble, lube.
The dial could be washed with mineral spirits. Try on a corner first.
Find the grommets. The ones you have are fully flat. The grommet should be 1) a bit thicker than the metal insert, 2) allow the inserts to go through and 3) be able to go through the openings in the chassis without being torn apart (heatgun on low helps by softening it). Renovated Radios might have them . If not, bu a suitable grommets in Lowes and use rubber washers to pad if not long enough. This is a bit tedious to reassemble but I did that a few times.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
(This post was last modified: 05-07-2020, 10:44 AM by morzh.)
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Does the warped dial cause problems? You can a new repro from Radio Daze. Or, I had reasonable success by removing it (on my 71), putting in on a flat surface with a piece of sheet steel on top of it, then heating the steel with a heat gun and then putting a weight on top until cool. You have to be VERY careful not to melt the dial, though. Then, of course, it has to be re-riveted to the metal ring.
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If all goes well, I'll put a new dial in it. I am taking this radio in phases to see if there is a show stopper I might not want to tackle, but I think it is restorable with my skills. I carefully examined all exposed coils with bright light and magnifying glass to see if any NC disease, they look great and all ohm out good. I removed the last covered detector to see its shape and it looked perfect also, I will re wire if I have to.
About cleaning the cap, do you clean the micas as well? There's black hardened buildup on the shaft which I tried to get off, maybe the soak will with careful brushing. I have some good neoprene grommets that will suspend this cap tightly. I will rewire the entire cap, those are gone also.
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These are fairly simple radios.
The only potential show stopper is the power transformer, and even that is rather hampers than stops it.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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The hardened build up is probably the original lubricant that they used on the bearings and the grounding wires. I'm not sure why it is so black. I've always wondered if they mixed some powdered graphite into the grease to help with grounding. I have been experimenting with that myself. The problem is, it takes a pretty high concentration of graphite to get any conductivity in the grease. then it's more of a paste than a grease. I've been using that on the grounding wires/springs on the last couple of radios.
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