Hallicrafters SX-62a

			Restored underside.

Schematic found on BAMA



				56k ohm resistor way out of tolerance. The resistor looks new.
Most all the capacitors have been replaced. Now on to the Resistors.	There are Resistors tucked way under the RF decks. Any resistor operating in the RF sections (before the IF) will receive Carbon Composition. See this explanation.	Here is a carbon composition resistor under the RF deck. The carbon film (upper left) resistor is a DC B+ plate resistor. No RF through this resistor.	There are two RF tubes. The larger socket is for the mixer/first IF.	Those wafer switches are made of Unobtainium. BE CAREFUL and GENTLE!
A new capacitor inside the RF deck.	This is the side shot of the RF decks.	Two RF sections on the right and the Mixer/1st IF on the left.	Progress to date. 08-25-09

Today was Fire Up day. First was to clean all the controls with contact cleaner and a tooth brush. I put the unit on the dim bulb tester and applied line power. It crackled a few times and started to receive Dr. Laura on 730 am. A few pops and crackles more I started to check the tube voltages. I don't like crackles.

Each time I touched V-5 plate with a test lead the radio would hum loudly, stop receiving and draw high current. Quickly snapping off the power switch I pulled V-5. Back on with the power switch Dr. Laura's scolding of some youg woman came loudly through the bench speaker. Looks like V-5 is a bad tube.

After checking all the wiring and highlighting each tube pin circuit I determined there is no restoration wiring errors. HEY, It does happen once in a while!

I found a bad tube that did not show bad in the tube tester. That goes to show that every good tube in a tester is not particularly good for the application. In this case V-5 (IF) 6SG7 did not show a short that pulled the whole receive sensitivity down. I found it by checking the bias voltages noted in the repair manual. The screen grid and Plate were at the same potential. That did not show up in the tester.

I also replaced the resistors inside the first IF can and the discriminator can (transformers).

IMG_1870.JPG (116591 bytes)

Discriminator Transformer can.

IMG_1872.JPG (144149 bytes)

With three new resistors.

IMG_1873.JPG (124839 bytes)

The first IF can. There are no other resistors with in any other transformer or coil cans.

So far the radio receive on all bands and modes. With an alignment this radio should increase in sensitivity and dial scale accuracy.

Next, I will replace the slipping tuning dial cord, tension spring and do a little cleaning and lubrication of the tuning mechanics. Then I will start the alignment.

Alignment

The alignment is rather straight forward except for the crystal IF filter. Adjustment can be a bit tricky. I used the RF signal generator and output meter as prescribed in the alignment procedure. Then after some research on line I found some ideas on how to allow my sweep generator to help me visualize the crystal response.

The challenge was getting the sweep generator to just below the IF frequency. The below response curves are shown with the sweep frequency increasing and decreasing. So it appears to have a double hump.

Selectivity	Broad	Medium	Sharp
Normal
Crystal

Test setup.

IMG_1899.JPG (964801 bytes)

Sweep Generator

IMG_1900.JPG (891186 bytes)

O-scope

We had a minor set back this week while this unit was burning in. The audio output transformer developed a short from center tap B+ to ground. The short does not show up with an ohm meter but defiantly is verifiable by an amp meter and simple disconnection from the circuit.

It is better for a problem to arise in the shop other than back at the owners home!

This out of the chassis test was performed to prove to my self that the transformer is actually bad. It is my form of an "Onoff" test. that is On or Off. With the transformer out of circuit the radio plays fine, through the head phone jack. When the transformer is grounded and the primary center tap connected the failure occurs.

There should be total isolation between the primary and secondary or ground. But this test shows there is not. An ohmmeter shows infinity ohms.

This is the audio output transformer removed from the chassis. The ground is established with the white jumper and the B+ on the center tap lead with the red jumper.	The Center tap is connected directly to the B+ right at the 5U4 rectifier tube. A current meter shows 700ma draw during the fault.	Pulling the lead off of the B+ draws an arch of around 3/8 of an inch and restores the B+.
This is the new transformer and amp meter showing 60ma on center tap. This transformer is rated for 70ma.	Wiring table for this Hammond transformer. The 6V6 likes 10k of impedance in Push Pull configuration.	There is no under chassis transformer picture. The new transformer fit easily on the side of the chassis. The original above the chassis has been abandoned in place for the vintage look.

This rig sound great. If I come across one at a swap meet or hamfest I will pick one up for my self!

Happy DXing!

Hit Counter

Contact me including your thoughts and comments.

Please see the LINKS page. I have collected many resources over the years.

135,912 unique web site visitors (14,499,000 hits) from October 2004 through August 2011.

Copyright © 2004 - 2012. All rights reserved.