[Ron Ramirez]

For a long time now, many radio collectors have cursed the Model 89 and its autodyne oscillator circuit that, sometimes, only works when it wants to (or so it seems).

If you do not mind sacrificing authenticity, I have developed a modification that will permanently cure this trouble with your 89.

Just like the Super 71 I previously discussed here, it involves replacing the Type 36 tube and five-pin socket with a seven-pin socket and 6A7 tube.

The oscillator coil must also be rewound. Details are given in this thread about the Super 71.

A note of warning: Instead of winding 128 turns for the secondary, I strongly suggest you wind 150 turns instead. On my 89 project, it refused to align correctly until I added the extra turns. Once I did, however, it aligned right up to near perfection.

So, it works well on AM; how about the "police" band?

The 6A7 mod gives a vast improvement over the original Type 36 autodyne circuit on the "police" band. Have you ever noticed that on a conventional 89, that the sensitivity audibly drops when the "police" band is selected? Well, the 6A7 mod does not suffer this issue; it is just as sensitive on the "police" band as on AM.

Photos to follow later.

Let me know what you think. In my opinion, this is the production change Philco should have made in mid-1933 when the 6A7 tube became available, but didn't.