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Author Topic: Hallicrafters SX-99 bandswitch  (Read 2191 times)
K3STX
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Posts: 961




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« on: July 08, 2013, 06:54:56 AM »

The Hallicrafters SX-99 I am fixing up has suddenly developed a problem. I replaced electrolytics/paper caps and all was well (actually, some scratching like silver mica disease and/or a really dirty sensitivity control), but it was working for a couple of days just fine.  A couple of nights ago i saw smoke, and looked underneath and there was arcing at bandswitch S1B. Arcing at all four band positions. I am thinking some sort of short, but looking at the schematic I am confused. I understand FILLED circles mean "solder points/connections" for wires and OPEN Circles indicate the open switch positions. But in the schematic on the coils there are open circles that are NOT at the switch. Some are marked, A, others B, C, or D. What does this mean? On one of these (for coil L6) at position C the open circle is at the GROUND. Does this mean it IS or is NOT grounded? And are all these coils linked to the same common ground?

Since I suspect a short to ground at my bandswitch I have to know what should be hooked and what should NOT be hooked-up at the different bandswitch positions. I will post a jpeg of this area on my website www.k3stx.comunder the BOATANCHORS tab. ANY help would be appreciated.

paul
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W9GB
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Posts: 2600




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« Reply #1 on: July 08, 2013, 11:07:32 AM »

When you read the American radio schematic diagrams from 1940s and 1950s ... GANGED Rotary Switches were very common.

The Band/Frequency rotary switch being the most complicated.
A rotary switch is a midcontact switch part of the schematic with the contacts arranged in a full or partial circle. Instead of a pushbutton or toggle, the mechanism used to select the contact moves in a circular motion and must be turned.

ROTARY SWITCHES are specified by POLE (P) and THROW (T)
The Ceramic or Phenolic WAFERS -- MAY have 2 separate switch mechanisms on one wafer.

The SUFFIX on the Switch Number, such as S1A, would refer to Wafer "A";
S1B, would refer to Wafer "B";
===
CORROSION, DIRT, and WEAR are common issues with these switches after 60 years.
Spray contact cleaners are POOR approaches for these switch mechanisms.  Too much cleaning fluid CAN cause issues with early phenolic wafers (expand), that result in more problems.

Dental picks, Q-Tips, foam VCR head cleaning swabs work best,
wetted with Caig DeOxit or equivalent solvent.
==
Takafumi Ueno (Japan) Restoration of SX-99
http://www.w9wze.net/Restorations/SX99/SX99restoreUeno.html

W5KCM Restoration Photos
http://www.flickr.com/photos/wb5kcm/sets/72157605484479311/

===
Regarding your IF CAN (coil) questions, I will have to pull up the schematic
Picture is worth hundreds of words ....
« Last Edit: July 08, 2013, 11:28:32 AM by W9GB » Logged
G3RZP
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Posts: 4391




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« Reply #2 on: July 08, 2013, 12:58:04 PM »

I don't understand the schematic as posted. On bands 3 and 4, how is the RF amp plate connected to the tuned circuit? I suspect that there are features not shown on the schematic.
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K3STX
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Posts: 961




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« Reply #3 on: July 08, 2013, 01:34:00 PM »

i have now included the entire schematic on my site. I was really just pointing at the open circles and whether as "open" it meant GROUNDED or NOT GROUNDED.

paul
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K3STX
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Posts: 961




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« Reply #4 on: July 08, 2013, 08:30:10 PM »

I might be getting somewhere. With Variac I powered it up REAL EASY on band A (position 1, BC band). No smoke up to 105V and hear stations well. All is fine. On Band D (position 4) I got smoke at 80V. Then on band B (position 2) I started to see smoke at 80V, heard lots of "crackling" and saw the bands on resistor R6 bubbling up! At Band C (position3) i got smoke, but not on resistor R6, now it was on bandswitch wafer itself. Gotta check out C22 to see if it is shorted. But what are the OPEN circles marked A, B, C, and D??

paul
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KE3WD
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Posts: 5694




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« Reply #5 on: July 09, 2013, 05:48:44 AM »

You should place a standard AC light bulb, about 100W, on the Line between your Variac and the receiver. 

When turning up the Variac slowly, the lightbulb will light brighter in events where the unit is drawing too much current.  --  And eliminates the "smoke testing" you are currently doing.

It is also possible to "Bring it up on the bulb" and take voltage measurements in order to find out where things are not right inside the unit under test.  Don't expect the full voltages on the schematics to be there if the Variac is turned down below 120V and the bulb is in the circuit, but do expect to be able to follow trends that will show you where the trouble is really happening. 

A 120VAC rated switch across the light socket terminals will allow you to easily switch the bulb in and out of circuit. 

This is a very important piece of test gear, every unknown piece that runs on 120VAC that  crosses my testbenches is first "brought up on the bulb" to see what's going on.  Shorts inside the piece will only light the light bulb brighter. 

Saves many components from failure caused by the one bad one, saves troubleshooting time, saves $$$, saves sanity. 


73
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K3STX
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Posts: 961




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« Reply #6 on: July 09, 2013, 07:49:09 AM »

Thanks, I've got a "dim bulb tester", but never thought of using it with the variac. Since there does not ever appear to be smoke at 60volts maybe I will take some measurements. (AFTER I replace the fried resistor). I'll see if C11 is shorted.

paul
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W6OU
Member

Posts: 184




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« Reply #7 on: July 09, 2013, 10:08:17 AM »

The open circles are the same as black circles. It indicates a terminal on a tuned inductor. There are four band tuned circuits. All of the tuned secondaries have the bottom side grounded. If S1B (front) is arcing then there is abnormal current flowing through it. If R6 is overheating then excessive B+ current is flowing. Unplug the receiver, disconnect one end of R6, and use an ohmmeter to check the resistance from one side of where R6 was to ground as you switch between bands. Repeat at the other side of where R6 was. You may have to wait a while for the ohmmeter reading to stabilize. If the resistance reading decreases, it indicates an abnormal short.

Caution: You may be overheating the primaries of coils L4 and L5.  If they eventually burn open you will have a major repair job.
« Last Edit: July 09, 2013, 10:35:20 AM by W6OU » Logged
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