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Author Topic: Hallicrafters SR-160 VFO Unstable  (Read 8228 times)
VK7TK
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« on: August 10, 2010, 05:20:45 AM »

Am restoring above transceiver, all Ok except VFO is so unstable it requires constant re-tuning to keep speech intelligible. Freq counter shows it zipping all over the place. Have replaced tube with no success. All components in VFO I spray with freezer have an effect on frequency. There is in parallel with the main tuning cap a 47pF NPO and a 18pF N470 tubular ceramic. Is it likely to be these or more likely a resistor? Where can I get these types of caps?

Regards VK7TK
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KE3WD
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« Reply #1 on: August 10, 2010, 07:07:08 AM »

If you were to replace all those critical caps with the new mo' bettuh caps, you'd have to realign the doggone thing and then if you hit those new caps with freeze spray, it would go out of tune just as before. 

Freeze spray shouldn't be used here.  ANY cap will change value in this type of circuit when frozen, due to the contraction factor at that very low temperature. 

Older VFOs were designed to be at or near their calibration point at full operating temperature.  This is why any written service data about performing a calibration starts out with letting the unit warm up to full operating temp before attempting alignment. 

Older hams know about leaving the older equipment ON all the time if keeping it on frequency is a goal.   That keeps it at a more steady temperature state. 

In your situation, the first thing I'd suspect is mechanical things that have contacts in them.  If the VFO uses a variable capacitor at the "knob" then I would suspect the brass brush contacts in the variable capacitor to need a wee bit of Deoxit 100% applied sparingly to each brush.  These are generally brass brushes that are readily seen when examining the central shaft of the variable cap.  Typically there will be at least one contact brush for each section.  Also the ends of the shaft where the end bearing and shaft bearing rotate, place one more wee small drop of the above in each of those.  Rotate the cap full travel at least 10 - 20 times immediately after application of the good stuff to effect a mechanical wiping action while the chemicals are still active. 

it is important not to overdo the amount of chemical used here, or even that can detune the circuit. 

If you don't have the little red bottle of Deoxit 100%, my preferred for this purpose, the "standard" Deoxit spray may do the trick, although it is not 100%.  Don't spray directly from the aerosol can, though, as it is simply too easy to end up with way more of the chemical than needed, which, as stated already, can actually serve to DEtune and make things worse.  Spray onto a small pointed wooden stick such as found on a cotton swab when cut with the diagonal cutters to a point, then use that point to transfer just that wee drop of the good stuff right into the area where the brass brush contacts the capacitor shaft. 

It is quite often the case that simply cleaning a variable cap restores both proper operation and calibration as well, BTW.  Not always, but certainly what to do first. 

Also, resoldering the connections first is always a good idea, especially AFTER CLEANING and lubing that cap and you find no joy. 

But changing the temperature will ALWAYS result in changing the frequency.  That's just one of those pesky laws of physics that we mere mortals cannot bypass.

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AC5UP
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« Reply #2 on: August 10, 2010, 07:39:35 AM »

I'm assuming you've been here: http://bama.edebris.com/manuals/hallicra/sr160/ and have eyeballed the VFO circuit.

As a general rule a bad cap can knock an oscillator off frequency but you won't see it "zipping all over the place" as you noticed on your freq counter. The more likely culprit is the 0A2 voltage regulator failing to supply stable B+ to the 6EA8 in the VFO. Hang your DVM on the plate of the 0A2, locked scale, to see how stable it is. If OK, do the same on R115 (1,000 Ohm) and R111 / R112 (5600 / 27k) and watch for jitter in the voltage. Those three resistors appear to be 1/2 watt jobbies and if they dissipate any significant juice they could be ready for replacement.

The book doesn't have a parts list, but see if you can cross reference CR5 labeled as a 'Varicap'. This rig seems a bit old for a cap diode, but if that were to go flaky.............

R82 and R83 (3500 10W / 470 2W) are also prime suspects, especially R82. Any hint of degradation in those two resistors would be a bad thing as the same regulated line feeding the VFO feeds the TX / RX mix oscillators.

BTW: An old trick is to first observe the stability of the 0A2 then tap around under the chassis with a plastic stick pen. Bad carbon comp resistors often fail by way of the carbon pill inside shrinking (or cracking) away from the component leads. A little mechanical provocation can tell you if someone has gotten loose inside.
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W4OP
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« Reply #3 on: August 11, 2010, 04:50:00 PM »

Your problem is almost undoubtedly the dogbone temperature compensating caps.
They will become erratic and cause freq jumps along with drift.
Nebraska Surplus sells some, but they are becoming  increasingly rare.
I have repaired a couple SR-400 VFO's that behaved exactly as you describe.

Dale W4OP
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VK7TK
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« Reply #4 on: August 16, 2010, 07:38:34 PM »

First thanks to all those that replied. I appreciate the time that you put into it. I checked the voltages all around the VFO and they are rock steady. I sprayed cleaner on the VC brushes and the VFO now appears more stable but still not good enough. Next step is to tap and measure all the resistors, then I will look at ordering some dog bones. Regards and 73s
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W5RKL
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« Reply #5 on: August 17, 2010, 08:21:06 AM »

Have you measured the frequency drift over a typical warm up period?

A typically warm up period can be between 30 and 45 minutes. The SR-160 factory stability specification is within 300Hz after warm up period.

Just my 2 cents worth.

73s
Mike

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VK7TK
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« Reply #6 on: August 19, 2010, 10:50:08 PM »

Thanks Mike for your interest. Cant measure the drift as its jumping all over the place. It depends on what time you look at it. This is after 2 minutes or after 60 minutes, so in this case a drift measurement would be rather meaningless. Its jumping kHz each time
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AA4PB
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« Reply #7 on: August 20, 2010, 05:13:52 AM »

If the frequency is "jumping" rather than drifting slowly then it is likely a capacitor. It is normal for gradual temperature changes to cause a gradual drift in frequency. Changes in the tube filiment voltage supply (due to changes in the 120V line voltage) can also cause fairly quick changes in frequency. Years ago I had a case where the VFO would drift up/down in frequency as the wife's iron cycled on and off.
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Bob  AA4PB
Garrisonville, VA
WB0SNF
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« Reply #8 on: January 07, 2019, 06:41:04 PM »

Hi.  I had the good fortune to acquire a 160 about a week ago.  Didn't work.  First thing I downloaded the SR 160 supplemental by Walt Cates, a goldmine of info.  I replaced the first three critical caps and fired it off.  Played but jumped around and drifted like crazy.  Found that the dog bone cap on back of vfo tuning cap was not connected properly, resoldered.  Checked both ends of dog bone.  both ends were bad.  Much better but still drifty.  Cleaned bandswitch again, this time with alcohol and q-tips, very carefully and cleaned relays again. That made a huge difference.  It  is now just as stable as the other radios of that era.  Lots of fun to use.  Good luck with yours.  wb0snf
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ZS5WC
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« Reply #9 on: January 07, 2019, 11:03:58 PM »

I have two and are very stable.
Make sure RIT switch is off..(Amazing feature for this vintage rig..), maybe it is on and switch / pot is dirty.

73 de William
ZS4L / ZS5WC
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G3RZP
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« Reply #10 on: January 08, 2019, 08:34:46 AM »

Another problem with rigs of that age are that carbon composition resistors tend to go high - or even very high! like 10kOhm going to 500kOhm - in value. They can also get very noisy, and in a VFO, could well produce the symptoms you describe.
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W9FI
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« Reply #11 on: January 08, 2019, 09:40:30 AM »

Another problem that Hallicrafters radios of that vintage can have is a bit of corrosion in the ball bearing races of their variable capacitors.  This can be fixed by spraying that area with contact cleaner, cleaning out the gunk with a clean rag, and then lightly regreasing the bearings.  I once had an SX-111 receiver that went from an unstable to a rock-solid condition after undergoing that treatment.

GL & 73, Jim  W9FI
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WA4JQS
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« Reply #12 on: January 10, 2019, 09:37:04 PM »

I agree with Dale W4OP about the dogbone caps.. i have restored a couple 150's and 160's . since the oa2 voltage is stable and the carbon resistors in the vfo have not gone high in value. it would seem the caps are causing the problem. do the tapping things.. you will be surprised just what you will find by doing that sometimes. 73 Tony WA4JQS
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WB0SNF
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« Reply #13 on: January 11, 2019, 08:49:49 PM »

Hi again.  Here's some more on my SR150.  I noticed if it's out of the cabinet it is drifts more. .  I listened to it drift around the calibrator signal and for whatever reason I blew on the tuning capacitor and it promptly drifted off about 2 kc.  I assume it was from the heat of my breath and not cringing from some disagreeable odor.  Put back in the cabinet it was   quite good.  Put muffin fan over finals, which created an internal draft,  which I usually do, and it started drifting again.  I guess the vfo tuning cap is extremely sensitive to temp changes.  Maybe needs a damp chaser like the SX101.  Any ideas.  Good luck, wb0snf
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WD0GOF
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« Reply #14 on: January 16, 2019, 11:15:52 AM »

All of the SR series transceivers have extremely stable VFO's if properly restored. Everything you ever wanted to know about the SR-160 or 500 is found here:

https://wd0gof.files.wordpress.com/2018/11/sr160-sr500-supplemental-final.pdf

73 Walt
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