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Author Topic: Hallicrafters S-20A Restoration  (Read 11399 times)
N4NYY
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Posts: 4818




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« Reply #15 on: May 29, 2013, 05:21:20 PM »

Great information!  I will follow up.  My dials are in good shape.  Just the plastic face plate is warped from too much heat in the storage facility where the radio sat for many years.  73, Tom, W4OEQ

If you remove the knobs, covers, front face, and do a thorough spray drenching with Krud Kutter on them, let sit for like 2 minutes, then rinse with water. The nicotine and crud will wash away. Then a light automotive wax, and it will look great. If you have alot of scratches, skip the wax part. Unless you can find a clear wax.
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W4OEQ
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« Reply #16 on: May 29, 2013, 05:53:38 PM »

Great suggestion!  I have some of that cleaner and will try it.  Fortunately, the case is in great shape, considering its age.  Repainting will not be needed.  Tom, W4OEQ
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W4OEQ
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« Reply #17 on: June 03, 2013, 06:59:02 PM »

If you are still watching this thread, here is an update.  The chassis has been cleaned.  All resistors and caps were replaced.  (Well, I left two that I could not reach under the bandswitch.)  Cleaned controls.  Re-soldered cold solder joints.  Did a stage by stage check tracing the signal from my signal generator all the way back to the RF amplifier.  Did a preliminary alignment and checked it out with a 20 foot hunk of wire antenna.  Radio seemed to be working as good as it could be at this stage of the game.  I temporarily installed an old Palomar Pre-selector and was impressed with how it improved the reception of the old radio.  I will do a second alignment tomorrow while waiting on an order for a replacement RF control.  I think I will replace the AF control as well.  Will also check the RF amp tube on my Hickok tube tester to see if it should be replaced.  Found a tear in the speaker paper and will patch it and hope for the best.  As a final step, I will tidy up the wiring and double-check all solder joints.  73, Tom, W4OEQ
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N4NYY
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« Reply #18 on: June 04, 2013, 02:27:16 PM »

If you are still watching this thread, here is an update.  The chassis has been cleaned.  All resistors and caps were replaced.  (Well, I left two that I could not reach under the bandswitch.)  Cleaned controls.  Re-soldered cold solder joints.  Did a stage by stage check tracing the signal from my signal generator all the way back to the RF amplifier.  Did a preliminary alignment and checked it out with a 20 foot hunk of wire antenna.  Radio seemed to be working as good as it could be at this stage of the game.  I temporarily installed an old Palomar Pre-selector and was impressed with how it improved the reception of the old radio.  I will do a second alignment tomorrow while waiting on an order for a replacement RF control.  I think I will replace the AF control as well.  Will also check the RF amp tube on my Hickok tube tester to see if it should be replaced.  Found a tear in the speaker paper and will patch it and hope for the best.  As a final step, I will tidy up the wiring and double-check all solder joints.  73, Tom, W4OEQ

You know, that is great. But those 2 resistors would drive me nuts. I would have at 'em. Be glad you are not me. LOL
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AC5UP
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« Reply #19 on: June 04, 2013, 05:27:04 PM »

We all are.

 Grin
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W4OEQ
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« Reply #20 on: June 05, 2013, 07:25:08 PM »

I, too, am concerned about these few components that have not been changed out.  But, to fix them, I would have to remove the band switch and all associated wires and components.  I do not have adequate documentation of the radio.  No diagram of the band switch and coils.  No service manual.  Of course, I can take close-up photos and draw diagrams.  I may yet have to do this.  The radio does not have any reception on the highest band.  The others seem to be working ok.  I have DeOx'ed the band switch and resoldered as many connections as I can reach.  Before diving into this problem further, I am going to make certain the radio otherwise is working properly.  My major observation to date is that it really is not very responsive unless a long wire antenna is connected.  Six feet of wire on the workshop floor doesn't cut it.  I also note that tuning the osc trimmer is really touchy on the top two bands.  The trimmer screws probably have not been moved in 50 years. I wonder if I should clean each trimmer???  73, Tom, W4OEQ
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N4NYY
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« Reply #21 on: June 06, 2013, 04:38:09 AM »

Of course, I can take close-up photos and draw diagrams.  I may yet have to do this.  The radio does not have any reception on the highest band.  The others seem to be working ok. 


That is what I do. Take pics and make diagrams. Many time, you do not have to complete disassemble the parts. I only disassembled enough to get in place with a needle nose and a soldering iron. In cases like this, I cut the leads to the resistors, and solder the new resistors to the existing leads.

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KE3WD
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« Reply #22 on: June 06, 2013, 06:24:28 AM »

I would first try to get to the connection points on those resistors, using mini clip leads, or tack solder techniques, whatever it takes to be able to take an ohmmeter reading and see if either of the two resistors is actually out of tolerance spec, because if they aren't, such invasive goings on would likely prove to be unnecessary. 

Well, that isn't the first step I'd do actually - first step would be to locate those two zitters on the schematic and ascertain what it is that they are really doing there.  For example, if they are just a voltage-divider pair, then a simple reading of the voltage can tell a lot about their health and welfare as well.  Things like that. 

It has been my experience that not all carbon comp resistors or electrolytic caps for that matter, are going to fail anytime soon.  Some do, but others don't.  This can be due to a few different things, manufacturing is one, but also where and how the component is used in a certain circuit.  Some circuits may hammer the device in operation more than others.  Carbon comp resistors often go out of tolerance due to what's called, "clumping" in which the carbon absorbs moisture over time.  I've noticed over the years that in units that have been kept running at least periodically, the carbon resistors in circuits that make the resistor run hotter as a normal situation also are more impervious to that clumping.  Yet in other cases I've found old carbon comps that ran cold but when checked were still dead on the money.  The decision to replace or not depends on each individual to decide. 


73
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W4OEQ
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Posts: 140




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« Reply #23 on: June 06, 2013, 08:11:58 PM »

What about the little "dog bone" resistors found in the area of the band switch?  These are so old the color coding cannot be read.  Are they any more unreliable than carbon resistors?

Unfortunately, the ones in my S-20R probably cannot be replaced without removing the switch assembly.

73, Tom, W4OEQ
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N4NYY
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Posts: 4818




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« Reply #24 on: June 07, 2013, 05:00:30 AM »

I have only seen dogbones once, and AC5UP had to tell me what they hell they were. If I recall, they are not like carbon comps. If they are high power, replace them with metal oxide. I seem to recall them being wirewound, but cannot recall.

If you are not sure of the value, join Antiques Radio forum, and someone there would know.
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AC5UP
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« Reply #25 on: June 07, 2013, 04:26:37 PM »

The classic dogbone resistor is a carbon rod with the leads wrapped / crimped on each end. Then the assembly is painted. My experience is that they tend to age well and can be very accurate as the lead spacing can be tweaked before they're crimped. This is not to say each one is a hand calibrated marvel of accuracy, but the 60's vintage JA resistors Vinnie found in a Lafayette HE-30 were pretty close to spot on, IIRC.
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KE3WD
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« Reply #26 on: June 07, 2013, 04:47:21 PM »

ALL of the early Japanese carbon resistors were of the dogbone type, carbon rod with wire wrapped around the ends and coated in paint.  Right down to the 1/w watt level.  They aged extremely well. 

The extreme homebrewer can roll their own carbon resistors in this fashion, using common mechanical pencil lead refills, wrapping wire around the ends and then sliding one end while monitoring on the ohmmeter, crimp in place by tightening the coil, then dip in enamel paint.  They work. 


73
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W4OEQ
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Posts: 140




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« Reply #27 on: June 10, 2013, 06:14:10 PM »

This is a duplicate posting that originated on the Antique Radios Forum web site.  I am posting it again here in the hope of finding an answer.

I am almost finished rebuilding my old Hallicrafters S-20R and have discovered that the RF amplifier 6SK7 has no B+ voltage.  It does have screen voltage.  The tube plate pin eight connects to the band switch, and I can verify this connection. The bottom end of the RF coil(s) connects to the B+ line. I cannot find this connection and believe it was lost when I rebuilt the radio. I probably disconnected a wire to get at some of the other components. The B+ comes directly from the power supply and connected somewhere near or on the bandswitch or one of the coils. The schematic diagram describes this as the bottom end of coil T13 and C45D. I have not been able to figure out which is T13 and where the connection point was (should be). Can anyone help me solve this problem? I have not been able to find a service manual for the S-20R. It would have the numbering system for the bandswitch pins and show where the B+ is to be connected. Thanks. 73, Tom, W4OEQ
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W4OEQ
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Posts: 140




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« Reply #28 on: June 16, 2013, 07:00:04 AM »

Well, I am only partly successful with my rebuilding project. There is something wrong with the RF coil section, and I cannot find the problem. I completely dismounted the coils and band switch wafer. Cleaned everything. Replaced the one resistor in that section. Ran dummy voltages using a 6 volt battery. There appears to be a problem with one of the coils but I could not see any physical damage. I did see that there was not perfect continuity in one of the windings......there was a measurable resistance on one winding of one coil. The others did not show any resistance. The problem here is that the B+ feed comes into this winding on this particular coil. If I hook up the B+, it causes real problems, noises, short circuits, etc. I tried skipping that band coil and using the B+ on the other coils/bands. The radio did not like 300 vdc at that feed point but would tolerate 115 vdc, although it still was not functioning correctly on bands 2, 3, and 4. The strange part of this is that if I hook up the B+ where it should be, it sends the same voltage right to the stator of the tuning cap (through the switch wafer). I do not understand why this should happen.......unless there is a short in one of the coils. The wafer wipers seem to be working as they should.

I reassembled the radio without the B+ on the RF stage. The radio works ok on the broadcast band. I have not completed the alignment of the other bands. I may have to operate it without the band where the coil seems to be not functioning correctly.

In summary, I spent 4 days tracking this problem and never got it solved.

Anyone have a suggestion for something I am missing in my analysis? 73, Tom, W4OEQ
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W4OEQ
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« Reply #29 on: June 17, 2013, 06:16:12 PM »

Figured out the B+ question.  Now, it appears that there is a problem with one or more coils or trimmers in the RF portion of the bandswitch assembly.  Do any of you have a junker Hallicrafters S-20R from which you can salvage the RF assembly (or the entire bandswitch and all three sets of coils)?  Thanks.  Tom, W4OEQ
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