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 Post subject: Hallicrafters SR 160 VFO
PostPosted: Feb Mon 12, 2018 1:41 am 
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Location: Costa Mesa, California
I have been resurrecting my second SR-160. This one had everything wrong with it: 6DQ5's for finals, shoddy workmanship in the previous restoration, mismatched replacement knobs, a missing finals cage cover, and numerous other problems. I changed out the electrolytics last weekend and re-wired the finals for 12DQ6's. This weekend I started alignment. Because the finals were replaced, I first needed to set the bias and re-neutralize the finals. Then I did some initial peaking of the transmitter circuits.

Saturday, I started the VFO alignment. 15 hours later I have it about as good as it will ever be--at least under my ownership. Before going further with this tale of woe--please folks do not spray penetrating oil wantonly around the radio dial mechanisms. Liquid Wrench has its place--just not anywhere near the friction plates for adjusting the dial. It doesn't help the tuning capacitor to be bathed in the stuff. I wiped and dabbed and mopped up penetrating oil for one whole day. It caused numerous problems which really were initially set up by Hallicrafters' dubious design of the whole VFO concept in general.

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The VFO would not adjust to match the dial. If I set the low end with L18, there was not enough capacitance in the trimmer, C127, to get the high end, and then if I added capacitance by tacking on a few p, L18 did not have enough adjustment to reduce the inductance where the tuning capacitor would cover the range. C130 has a range of about 15p to cover 500 KHz. When I say it didn't have the range, I don't mean it needed 20p, I mean it needed 10p or 11p to cover the full 500 KHz. That is about 50 KHz per p. The capacitor itself is not sturdy. It has 5 total plates--the 3 on the rotor are kind of big and the rotor fairly small diameter. The manual tells the operator not run the capacitor into the stop or the plates will fall out. They are held in place by friction--and penetrating oil is not your fiend in a situation like that. A previous owner had tried to "knife" the plates. That also is mentioned in the manual. They say it should only be attempted by an experienced technician. Really? An experienced technician is able to knife plates where .02p is 1 KHz? The knife adds .02p when it is still sitting in the drawer.

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Early on the plates fell out and I started the long process of removing the panel, dials, and tuning capacitor. I initially replaced it with a different variable capacitor, which itself took time to modify so it would fit. That was a failed experiment and the result was a dial even further from reconciling. I ended up soldering the plates to the rotor shaft--not so much as it sounds because the solder would not attach to the plates, but did fill in around the plates enough on the brass to secure them. The heat also burned off most of the penetrating oil.

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VFO 3.jpg
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After repeating the long process of changing the tuning capacitor so the original was back in place, I then needed to figure how to reconcile the dial. I unsoldered or clipped all but one of the capacitors out of the oscillator circuit one-at-a-time and tacked in different values to see if I helped or hurt my situation. Eventually, I got there. C131 and C132 were changed from 510p to 390p and 15p was added in parallel to C130. That allowed the dial to match the frequency with a little room for adjustment at C127 and L18.

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The last part of the process was to try and get the plates of the tuning capacitor in some manner of flatness where the dial was close at a reasonable number of positions. Right now it is good several places at the top and bottom of the range and off up to 5 KHz high or low at the minimum number of locations. When you consider that .02 p is a KHz, then being off a couple of KHz is fairly reasonable--especially if the tuning capacitor has poorly supported plates and someone before decided to try and bend them. Yikes.

I can only say that Hallicrafters had a bad design and the previous owner didn't help.

Norm


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VFO 7.jpg
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 Post subject: Re: Hallicrafters SR 160 VFO
PostPosted: Feb Mon 12, 2018 2:02 pm 
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Joined: Aug Fri 01, 2014 11:18 pm
Posts: 30
Hi Norm,

Sorry about the VFO woes - I can't add much to that. However, there might be an explanation for the 6DQ5 finals if a previous owner modified the rig into a "pseudo-SR-500". The changes between the two are relatively minor and well documented in Walt Cates SR-160/SR-500 document (which I highly recommend in any case). Among other things, there are changes to the PA bias and neutralizing circuits. You might want to keep this in mind when you get to that point. If you need any comparative measurements, I might be able to help out.

Bob, AB1MN


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 Post subject: Re: Hallicrafters SR 160 VFO
PostPosted: Feb Mon 12, 2018 3:27 pm 
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Joined: Sep Thu 20, 2012 3:42 pm
Posts: 381
Location: Salem, OR
Bob,

I have sent you email. Don't know if it is in your spam folder or what. Let's talk.
Sorry about hijacking the thread.

Nice job on the SR-160 project, Norm.

Marc-k7wxk


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 Post subject: Re: Hallicrafters SR 160 VFO
PostPosted: Feb Sat 17, 2018 7:20 am 
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Location: Costa Mesa, California
I finished alignment and the radio is working quite well. It puts out good power and the receiver is very sensitive. I just wish I had the correct knobs and it would be a very fine example. These things do need a spinner--if just to do the alignment. I have been back and forth between the ends of the three bands a hundred times at least.

The only thing that doesn't work quite right is the S meter. I can't quite zero it on 80 and 20 and 40 is about 1/3 up the scale at max zero of the adjustment pot. The resistors seem fine. I changed the one on the back of the meter because it was off a bit. The meter does register correctly on transmit and for receive, it shows good sensitivity--just up scale.

Norm

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 Post subject: Re: Hallicrafters SR 160 VFO
PostPosted: Feb Sat 17, 2018 5:56 pm 
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Posts: 30
Hi Norm,

Since you have already checked the resistors (and meter amp tube?), here are a couple of things to consider regarding the "S" meter reading.

Check the no-signal AGC voltage to see if it is too high with no input. I would expect it to be about -0.2 to -0.3v. If this is the case, the problem is most likely in the meter amp circuit itself. If the voltage is higher, try the following:

Check the carrier oscillator frequency to make sure that it is correct. If it is too far into the filter passband, it may cause the "S" meter to read high with no signal at the antenna.

A second thing to check is IF oscillation. I noted this on two of my SR-160s/SR-500s when the IF stages had been peaked for maximum signal. It appeared to be associated with V10 filament ckt. The addition of a 0.01 bypass capacitor from ground to to pin 4 of V10 eliminated the oscillation. The oscillation may be outside of what you hear and would also show up as a high (negative) AGC voltage with no input signal at the antenna.

Good Luck,

Bob


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 Post subject: Re: Hallicrafters SR 160 VFO
PostPosted: Feb Sat 17, 2018 7:09 pm 
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Joined: Sep Wed 10, 2014 2:01 am
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Location: Costa Mesa, California
Bob,

The AVC voltage was high, which I noticed during alignment, so I tried the by-pass cap on pin 4 of V10 and it solved the problem for the 40 meter band reading up scale as well as reducing the no-signal AVC voltage back to where it should be. Thank you!

I changed the 82K resistor connected to one side of the pot because it measured only 77K. After removing it, I used test leads to insert a 100K pot and then adjusted it to find a good value that allowed a range of adjustment that would more than zero out the S meter. That value was 96K. I ended up with a 100K resistor instead of the 82K and it works well.

Norm

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