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Philco 70 - Tubesforme - 03-26-2017

I recently finished a complete restoration of a model 70, the early version # 184427. Everything went smoothly. This was a complete rebuild, from the Mershon filters, rebuilding the bakelite blocks, replacing the resistors with a set from Steve Davis, speaker recone and replacement of the volume control with a NOS. The radio played fairly decently. Then I aligned it and the radio really perked up.

One minor problem. We have stations located at 570, 590, 610 and 640 Khz. Below 640, no stations come in. 640 is only 25 miles from me and it too needs to have the volume turned most of the way up. From there up all stations are loud and clear. I only have a 20' wire attached for the aerial. Grounding the chassis didn't help. I rechecked the alignment and was super careful to rock the rotor as I set the 600 Khz trimmer. Nothing changed. I have replaced the oscillator tube 2nd det and 1st IF tube with NOS tubes. No change. Possibly I just need a longer wire antenna, but I doubt it because my 21, with the same wire, works fine at 640.

In 50+ years I have not repaired or restored a model 70. The alignment instructions I was using were off the internet and the paper read Philco Radio & Television Corp., Model Nos. 70 (below ser.#B22,000) Any ideas?  

Thanks guys.

RE: Philco 70 - Radioroslyn - 03-26-2017

Would tune your 70 to 550kc and listen for the oscillator at 810kc or so. Sounds like the oscillator is dyeing at the low end. If so rewind the feedback winding.

RE: Philco 70 - morzh - 03-26-2017

When you aligned it, was everything good across the dial?

RE: Philco 70 - Radioroslyn - 03-26-2017

OBTW Does adjustment #7 change the frequency at the low end of the dial? If not we're back the the osc coil again.

RE: Philco 70 - Tubesforme - 03-27-2017

(03-26-2017, 09:43 PM)Radioroslyn Wrote:  OBTW Does adjustment #7 change the frequency at the low end of the dial? If not we're back the the osc coil again.

#7, the 600 Khz trimmer does vary the location of the signal as I vary the dial, not as much as I think it should, but it does vary the signal. I did check the coil for continuity as well as the oscillator voltages with a VTVM and all seem to be within tolerance.

RE: Philco 70 - Tubesforme - 03-27-2017

(03-26-2017, 08:50 PM)morzh Wrote:  When you aligned it, was everything good across the dial?

Everything is great across the dial. The volume would drive you out from 710 up. One problem is the 710 signal is so strong that it covers the dial from 700 to about 750. Also I have noticed that this radio doesn't tune as sharply as I thought it would, however I think that is due to lack of AVC and the strong signals in this area. the 710 signal is super strong and close by. I haven't checked, but believe 710 is a clear channel. The lower two stations are a distance away and barely audible.

RE: Philco 70 - Tubesforme - 03-27-2017

(03-26-2017, 09:43 PM)Radioroslyn Wrote:  OBTW Does adjustment #7 change the frequency at the low end of the dial? If not we're back the the osc coil again.

I forgot to state that the local clear channel 640 does come in clear, but isn't as loud as the local 710 station, which is lower power than the 640 station. 640 is 50KW. Below 640 I can't receive anything except noise. I can move the 640 station on the dial up or down, slightly, using the #7 600 Khz trimmer.

RE: Philco 70 - Radioroslyn - 03-27-2017

> I can move the 640 station on the dial up or down, slightly, using the #7 600 Khz trimmer.

I would think you should be able to move it 100kc or better. Not quite sure how much slightly is?
If you have a scope you can look at the osc output at the grid cap of the 24A mixer. Check the amplitude.
I think you'll find it's dropping off as you get lower in frequency

RE: Philco 70 - Tubesforme - 03-27-2017

(03-27-2017, 04:22 PM)Radioroslyn Wrote:  > I can move the 640 station on the dial up or down, slightly, using the #7 600 Khz trimmer.

I would think you should be able to move it 100kc or better. Not quite sure how much slightly is?
If you have a scope you can look at the osc output at the grid cap of the 24A mixer. Check the amplitude.
I think you'll find it's dropping off as you get lower in frequency

Additional notes. I found that we have stations located at 570, 610, 640, 670, 710 and up. I can't receive either 570 or 610. From 640 up, everything seems to work. I did discover that adjustment #7, the 600 Khz trimmer doesn't seem to move the frequency as much as I thought it did. I adjusted it while rocking the dial. 640 has lower volume than any other of the stations. 710 works, but the volume must be raised considerably. All stations from there on up are very loud. I don't have a scope. I wasn't able to move the station located at 640 more than 10 Khz either way from its frequency. For both 640 and 710 the volume must be turned up well over half way. On any other station that level of volume will drive you out of the room. The only equipment I have is a DMM, VTVM, signal generator, signal tracer, variable AC isolated supply and tube tester. The signal generator is only one year old. I did not connect a capacitor in series with the generator lead as the Philco instructions call for. The generator instructions call for use of a resistor from 100 to 200 ohm in series with the hot lead. I did not use a resistor this time, when adjusting the trimmer.

RE: Philco 70 - Radioroslyn - 03-27-2017

See post #4

RE: Philco 70 - Tubesforme - 03-27-2017

Ok, maybe the osc. coil is defective, but why does it test good and why do I have good grid voltage at all frequencies? I just noticed one other thing. While I have great volume, over the dial, I find that it is slightly difficult to tune in a strong station because the tuning is broad. I can turn the dial through about a 30 Khz range on strong stations. I suspect that the oscillator is working correctly, however maybe not at the correct frequency, because the average super has about -5 to -25 volts on the osc grid. On mine the grid, using a VTVM reads between -21 to -25V throughout the tuning range. Yes, a scope or frequency counter would help, but those I no longer own. The oscillator is definitely working. Correct frequency???

RE: Philco 70 - Tubesforme - 03-27-2017

(03-26-2017, 08:49 PM)Radioroslyn Wrote:  Would tune your 70 to 550kc and listen for the oscillator at 810kc or so. Sounds like the oscillator is dyeing at the low end. If so rewind the feedback winding.

I might agree with you if It wasn't for the fact I have a great osc grid voltage at all frequencies. If it is the coil, I can't rewind it at my age anymore so would have to find someone to do it. I have checked for a replacement coil and no one had one.

RE: Philco 70 - Tubesforme - 03-27-2017

(03-26-2017, 08:50 PM)morzh Wrote:  When you aligned it, was everything good across the dial?

yes, it was.

RE: Philco 70 - Tubesforme - 03-27-2017

(03-26-2017, 08:49 PM)Radioroslyn Wrote:  Would tune your 70 to 550kc and listen for the oscillator at 810kc or so. Sounds like the oscillator is dyeing at the low end.

I tried your suggestion and at 600kc the signal was the strongest, then it dropped off until I approached 810kc and the signal was back again, only not as loud as at 600. The 600kc was weaker at 810kc, but was present.

RE: Philco 70 - Tubesforme - 04-06-2017

The problem turned out to be the 600 K.C. trimmer capacitor! Thanks for all the info. Michael (tubesforme)