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Hallicrafters S-72 - Printable Version

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Hallicrafters S-72 - BrendaAnnD - 09-25-2013

Anyone else have experience with these? Mine's an S-72r. Not sure what the difference is between the different versions.

I'm SUPER impressed with this radio. 8 tubes, two IF amps, BFO (though I have yet to figure out how to use it). Seperate 1L6 oscillator and 1U4 mixer. Coverage all the way up to 30 MHz, and gain is good all the way up! Rock solid LO, too. No drift whatsoever, but then, there's not a lot of heat in these radios. This thing spanks my T-O's and every other portable SW I've ever owned (tube types, and most ss types)

RE: Hallicrafters S-72 - morzh - 09-25-2013

Terry I think knows them well.

RE: Hallicrafters S-72 - Radioroslyn - 09-25-2013

I had one back in the '70's good little set but it does use that 1L6$$$$.
Terry

RE: Hallicrafters S-72 - BrendaAnnD - 09-25-2013

I was in error on that part. It uses a 1R5! It must be the combination of the 1R5 as separate LO and 1U4 as mixer that allows it to reach 30 MHz so well. Can't do that with a 1R5 in a T-O!

RE: Hallicrafters S-72 - Arran - 09-25-2013

A separate mixer and oscillator is always the way to go, Canadian RCA/G.E liked to do that on their middle to higher end models and it really makes a difference. Hallicrafters definitely had some good engineers working for them, they managed to create some very good performing sets at a budget price, something that their main competitor National had a hard time keeping up with. National had some excellent sets, but the bang for your buck was not as good as it was with a Halllicrafters.
Regards
Arran

RE: Hallicrafters S-72 - marty - 09-25-2013

Hi All;
Brenda Ann, What is a T-O ?? Time Out..
THANK YOU Marty

RE: Hallicrafters S-72 - Arran - 09-25-2013

Any brand Z Transoceanic model.
Regards
Arran

RE: Hallicrafters S-72 - BrendaAnnD - 09-26-2013

Just got this radio yesterday, been playing with it, replacing parts as needed. I've found several modifications that were made in the past, things like a .001 cap across the primary of the mixer transformer for the AM band, and another from the plate of the BFO to ground.. sorta defeated the purpose of having the BFO in there at all.

Now to what I need some help with: Despite my best efforts, I cannot get the antenna input stage to track above 1100 or so. It's good to that point, but gets farther off the farther I go up the dial. 1400 tunes at about 1550 (looking at the signal on my scope), and it won't catch 1500 at all, still too much capacitance. I've replaced the cap between chassis and B-, that didn't change a thing. Taking that .001 out of the mixer definately gave me better gain (and the ability to actually tune the mixer input!). Everything else looks OK, just can't get that last bit of tracking. Icon_sad

The other problem is that something is going into some sort of parasitic oscillation.. in the LO or mixer stage... I can't tell which (two separate tubes), but if I put my finger on the LO section of the tuner to load it, then let go, the parasitics go away for a while. Guess I could try replacing the 1R5 and see if that helps, but somehow I doubt it will. So, if anyone has any pointers, or has dealt with these problems on this radio, please let me know.

RE: Hallicrafters S-72 - Mondial - 09-26-2013

Had a similar parasitic problem with a radio using a 6SA7 with separate oscillator excitation. Sensitivity would drop suddenly until you unplugged the tube and replaced it, at which point it would work properly again.

Finally connected a spectrum analyzer and found the 6SA7 oscillating at 200 MHz !!. Cured the problem by placing a ferrite bead on the grid lead right at the socket.

I dont know if it is the mixer or osc that has the parastic in your case, but you might try beads on the grids of each. If you don't have any ferrite beads, you could try a 10 ohm resistor in series instead.

As to the tracking problem, is it the antenna stage or the osc that is causing the problem? Does the osc track the dial calibrations, or does it deviate from the indicated RX freq at the high end of the scale?

Do you have a link for the schematic of the radio?

RE: Hallicrafters S-72 - BrendaAnnD - 09-26-2013

The LO tracks pretty well, it's only off by perhaps 10 KHz at the top of the band. As I was going to sleep last night, I thought of something to try (can't really hurt much). I'm going to see if bending the plates out on the high end of the antenna section will give the edge I need. This is one of those tuners that's actually made to do that, with the slits every few degrees on the outside plates. Wish me luck.

I'll see if that will help (the resistor in the grid circuit). Just a bit tight in that part of the chassis, though. Interesting all this happens ONLY on the AMBCB.. the other three bands are actually very good.

RE: Hallicrafters S-72 - marty - 09-26-2013

Hi All;
Brenda, I had a thought from reading Your posting, Since You said that some one added extra caps to the BFO.. I would look elsewhere for more "added Caps" where not needed, or different value caps I.E. replaced with the wrong value..
THANK YOU Marty

RE: Hallicrafters S-72 - BrendaAnnD - 09-27-2013

Well, I did find more "modifications". For instance, a .01 cap between the screen grid and control grid of the mixer tube that obviously didn't belong there (it was a 'newer' cap with long leads just wrapped around other components on either end). Removing that stopped the parasitic oscillations. Gain is great on all bands, but I still can't get any adjustment on the high end of Band 1 for the antenna. Seems closer, though, since I spread the plates on the end section of the antenna tuner.

Also recapped the set completely. That didn't really change much, other than the AGC works a bit better. And I lost one thing: The BFO no longer oscillates. That's driving me crazy, too. It's a simple Hartley oscillator. Everything seems good, voltages OK, coil is good, caps are good. Circuit is correct.. even put in a new 1U5, but still nothing. This is most odd since it was working before I started. :/

RE: Hallicrafters S-72 - marty - 09-27-2013

Hi All;
Brenda Ann, I would put back the things You took out until You get the BFO to work again.. Once that hopefully happens, then try to determine why what you put back in makes it work, and see if there is something else affecting the circuit, so, it won't work as it should..
THANK YOU Marty

RE: Hallicrafters S-72 - BrendaAnnD - 09-27-2013

The BFO is a completely isolated circuit on this set. The only coupling is via stray inductance into the 2nd IF. I'm perplexed that it ever did it's job at all.. Icon_smile At any rate, nothing I removed is anywhere near that circuit, which is literally an island unto itself, with the only connections to the rest of the radio being the filament and B+ connections.

RE: Hallicrafters S-72 - marty - 09-27-2013

Hi All;
Brenda Ann, could the wire that You speak of to the 2nd IF, gotten moved and so changed its (BFO) ability to work or transfer its information..
THANK YOU Marty