[Hallicrafters] SX-100 troubleshooting

[hallicrafterssr2000 at k9axn.com]

Good morning Roy et Joe:

Thanks for the reminder about the internal capacitor in the transformer. 
Did you record the value of the capacitor in your notes?  Don't have a good 
one to test but would like the value for the record.  BTW, if one has 
failed, the other sideband should oscillate.

Roy, is the transformer in the 88 the same PN as the 100 and were there any 
paper caps under the sub-chassis in the SX-88?  I believe the SX-88 was 
built four or five years before the SX-100.  The SX-100 series did not use 
any paper under the sub-chassis.

Kindest regards Jim K9AXN

-----Original Message----- 
From: Roy Morgan
Sent: Tuesday, January 08, 2013 10:05 AM
To: Joe Balestrini
Cc: Hallicrafters at mailman.qth.net
Subject: Re: [Hallicrafters] SX-100 troubleshooting


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!



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