Notes on a Hallicrafters S-38B.

[JHGibson]

When purchased, the S-38B was dirty and the black crackle paint was greyish. My first job was to remove the cabinet and wash it followed by refurbishing the crackle as described in my post "Reviving Black Crackle Paint" in General Hints, page 3.

This is an attractive 5 valve AC/DC 120 volt radio having MW and 3 shortwave bands. This model was one of a series starting with the S-38 and having the same general circuit.
http://oak.cats.ohiou.edu/~postr/bapix/S38.htm

The cabinet of distinct modern appearance was the work of a leading industrial designer Raymond Loewy. www.raymondloewy.org/

The S-38's have the valve heaters in series totaling 120 volts so a dropper resistor is not required. I tried to reform the power supply electrolytics, but while the leakage current fell to an acceptable level, they did not come up to full capacity so they were replaced. All the paper caps were leaky and were replaced with polyester types. The resistors were all within tolerance and left in place and the valves all tested good.
The chassis is live but well insulated from the metal cabinet by insulating spacers. These were checked for correct assembly.

At this point the radio was powered up using an isolating transformer and found to work fine so my final step was to align it. The service manual is skimpy on details, it did not note that the chassis has to be out of the cabinet in order to align the IF transformers. Also it did not mention that on bands 1, 2 and 3, the local oscillator is above the signal but on band 4 it is below. This meant that at the 30 mcs alignment point I had to be careful not to peak on the image (2nd spot). To obtain maximum sensitivity at 30 mcs it was crucial to rock the tuning (best done with the bandspread knob) while peaking the signal trimmer.

I obtained the following sensitivities after alignment:
Band 1 (MW). 30 uV @ 600 kcs, 20 uV @ 1500 kcs.
Band 2. 8 uV @ 1.8 mcs, 6 uV @ 5 mcs.
Band 3. 10 uV @ 5 mcs, 8 uV @ 14 mcs.
Band 4. 15 uV @ 14 mcs, 16 uV @ 30 mcs.
These were the levels required to just quiet the receiver. On Band 1 (MW), Hallicrafters purposely reduced the sensitivity by means of a damping resistor in the aerial circuit.
Before and after photos of the restoration are attached.

[Aub]

JH,
That looks like a really nice job you've done there. I'd be interested to know what the valve line up is, being all in series for 120V.

Best regards

Aub

[joe]

Hi Aub

I had a look on www.nostalgiaair.org & it appears to be:
12SA7; 12SK7; 12SQ7; 35L6; 35Z5; & 12SQ7 (BFO & AVC diode) which makes a nominal 118v.
That website was quicker than looking in the loft where I've got one in the "to do" pile. A bit OT but mine came from a dustbin at college where somebody who had one remembered to use an autotransformer - but connected it the wrong way round!

Joe

[Aub]

Thanks Joe, I've bookmarked that site, it looks interesting. I'm still curious about the valves though. They all sould like standard 6.3v heaters but can't be and even if you could wire them up for 12.6v, that still wouldnt make 118v. Are they 19v, as I believe some valves were?

best regards

Aub

[jjl]

Aub

With the most commonly encountered scheme of American valve numbering, the heater voltage is specified by the digits before the letters. Note that a 6 e.g. in 6SN7 actually means 6.3V and 12 means 12.6V. For some reason, loctal valves use 7 to signify 6.3V and 14 to signify 12.6V.
Anyway, with the receiver in question, the total series heater voltage will be 12.6 + 12.6 + 12.6 + 35 + 35 + 12.6 = 120.4 Volts.
Hope this helps.

John

[Aub]

John,
Ah - thanks for that. Sorry I took the thread off topic a bit. I never knew that about heaters and valve numbering schemes.

best regards

Aub