AVC Test and Repair

If the AVC line is to be used as a calibration and alignment device, there needs to be a procedure to guarantee that it is working properly.

The first attempt at making this page was heavily influenced by the Gemini AI LLM.
There were a lot of errors, false starts and mistakes made.
You can read it here.
It is preserved because there's a lot to be learned from it, good and bad. Unfortunatly, testing the AVC isn't one of them.

AVC Test

Equipment Setup

• Power off. Radio upside down. Discharge the filter capacitors if you think necessary
• Front panel AVC switch on
• Speakers connected
• Band switch to band 1. Tune to 1400 KHz.
• Connect an RF signal generator to the antenna terminals using the antenna matching adapter found here. Frequency 1400 KHz. Modulation 1 KHz at 80%. Initial amplitude 4 mV p-p (equates to 4 µV into the radio or close to the minimum it can detect).
• The 1st and 2nd RF modules each have a single wire exiting the module on the side away from the power transformer. These two wires go to a common terminal strip lug but the location of that lug varies. In any case, the junction is the AVC line. You will be measuring DC voltages in the range +3 to -15. Adjust your multimeter accordingly.
• Since the AVC line is a DC voltage only (no AC component), lead length is not an issue.
• A scope connected to the AVC line may be useful.

Test

1. Power on
2. With the very low signal generator output, the line should sit near 1.2 V positive
3. Crank up the generator's RF output level. At some point, you should see the AVC node start to go negative.
4. Turn off the enable output button of the signal generator if it has one. The voltage should not snap instantly back to zero. It should smoothly "bleed off" back to 0 V over about a quarter to a half of a second¹.

The Verdict

If the line drops to around -15 V with a strong signal and bleeds off smoothly when the signal is removed, your AVC network is in perfect health.

It is fully certified to act as your alignment calibration device.

If the voltage barely moves (e.g., only drops to -2 V under heavy signal), V8 or V9 is weak, the AVC IF transformer (T6) is misaligned, or a resistor in the voltage divider network has drifted high.

AVC Repair

The part of the page was heavily influenced by the Gemini AI responses here, here, here, and here.

As noted above, a lot of it was wrong.

The repair process is pretty simple. Power off to start.

Unsolder all the wires (3) from the node plus a resistor (R61) and a capacitor (C66).

I found it useful² to use a bunch of clip test leads and a paper clip as a 'bus bar'. Run test leads from all the node components to the paper clip and disconnect one at a time to isolate the source of trouble.

A = paperclip 'bus bar', B = clip to keep test leads from near‐by B+, C = R61, D = C66. Lead length is not an issue

Need to develop procedures for voltage too high, low, not moving at all, etc.

Footnotes:

1. I was using a Simpson 260 which has a damped meter needle so the timing bit didn't work for me. I tried a cheap digital multimeter but that didn't work either: the sample rate was too slow.
2. Perhaps overkill but my first attempt at this resulted in a big ZAP from a loose cliplead and I'm still wondering if I fried something.

Page: /Avc_test_and_repair/Avc_test.shtml

Last modified: Thursday, 23 Jul 2026