[Hallicrafters] SX-100 troubleshooting

[Roy Morgan]

On Jan 8, 2013, at 10:00 AM, Joe Balestrini wrote:

> Hi All,  I'm trying to get some life out of my SX-100.

A noble endeavor.  Keep at it!

>  I found that the voltage on the plates of V12 (12AT7 - 2nd  
> converter/oscillator in the sub-chassis) is at 25v rather than 90v.

(I did not look up the details of the circuit):
Two possible causes come to  mind:
- plate dropping/isolation resistor seriously drifted high in value
- trouble in the grid circuit causing excessive plate current.

Some more probing with an ohmmeter may be in order.

I have taken the 2nd converter sub-chassis out of an SX-88 in the  
past.  Its not all that difficult but you must:
- note and label each wire  you remove (there are some 6 or 8 or so)
- be careful in threading the wires out through the chassis to get  
them free
- best to replace most or all components under there to avoid future  
removal
- remember to put the bottom plate back on the sub-chassis before  
remounting it and soldering all the leads back in place (you may not  
need to ask me how I know this.)

> ... I did find that adjusting the primary or secondary of T9 seems  
> to have no effect on tuning.  I dread the thought of tearing into  
> that sub-chassis.

Patience may be needed.

These radios have a known malady in the IF cans:  There is a mica  
capacitor across the primary and/or secondary of the thing that gets  
very leaky or shorted.    It is moulded into the base of the IF can  
plastic structure.   The cure is to remove the IF can from the radio,  
remove the mica cap and replace it/them with new dipped mica caps on  
the transformer terminals.   A dremel tool may be helpful in getting  
it out of there.  Think like a dentist.

Get good caps from justradios.com a Canadian couple who have good  
stuff at good prices and send things fast.

I will send to Joe my notes file on the radio ... collected  
discoveries and tips posted to the mailing list over time.  It likely  
has the IF can Mica Cap problem detailed.  Trouble is the values of  
the caps are not on the schematic if I remember right.

To locate plate and screen resistors that have drifted high:
- measure operating B+ voltages at the tube (with a high impedance  
meter)
- power off, put one lead of the ohm meter on the B+ line, and measure  
resistances at the screens and plates.
- take notes to help identify values of new resistors needed and which  
ones to change out  (Bits of blue painters tape and a fine sharpie pen  
help here.d)

Roy

Roy Morgan
k1lky at earthlink.net
K1LKY Since 1958 - Keep 'em Glowing!