Alignment Tools Help

[OldTechFan96]

Before I get stuck into AM/FM receiver alignment I'll need to build a few tools that are frequently mentioned in service manuals.


Diode Probe:

I found a suitable circuit in Gordon J King's 'Servicing Radio, TV and Hi-Fi Equipment'. What kind of diode would be used here? Would any Ge diode do? I have two Czechoslovakian Ge diodes from an FM detector.

My plan is to assemble the circuit into a small ABS enclosure with a BNC connector on each side so that it can be easily connected in-between probes and an oscilloscope.


Dummy Aerial:

The Advance TP1B termination pad has a built dummy aerial. I was looking at the PDF from Kat Manton and wondered if I could make the DA with just the 20uh inductor and resistor.

I would build it in the same manner as the diode probe but using banana sockets for the signal output.


Alignment Loop Aerial:

Does anybody have a suitable design for alignment purposes? I do have some 1/4mm diameter copper wire which I could use?


Does the above sound like a good plan?


Thanks for reading!

Alignment Loop Aerial:

A few turns of any old wire round a shoe box sized thing will do admirably, a box is handy as a former as it stays where you put it.

[David G4EBT]

I've always just used a loop of wire dangled over or near the radio except where the alignment calls for direct connection to control grids or whatever to align IFTs. As often as not, unless tampered with or any components in tuned circuits such as capacitors have been changed, the alignment isn't out, but if we don't check we don't know, and on sets where a lot of time and efforts has been expended, to not at least check the alignment, seems like unfinished business.

On a particularly troublesome DAC90A which led me quite a dance, after much effort turned out to be an obscure fault. On dismantling the 2nd IFT I discovered that a silver mica cap had corroded connections. It was just clinging on by a thread and attenuating the signal. When I touched it, it dropped off and had to be replaced. To align the IFs calls for direct connection of the generator to the control grids of V2 and V1, but to align the oscillator and RF stage, the maker's datasheet says to use an 8" loop of wire placed close to the radio's frame aerial.

Easy enough to do, but I thought it was well past the time at which I ought to make an alignment loop to make the task a little easier. I'd been using some 3mm veneered plywood for another project and had some offcuts left over, so I made use of them to make a loop. Quite unnecessary I know, but it wasn't much of a task and will come in handy on other occasions. I cut three discs of plywood - two 20cms diameter, and one 9.5cms sandwiched together, around which I wound a single loop of stranded insulated flex, terminated on solder tags attached to two 6BA screws. I made a base from an offcut of mahogany.

In use, the signal generator's output lead is connected to the protruding screws.

Not much more to say about it.

I've attached a couple of pictures and a sketch.

Hope that might be of interest and use.

As I've mentioned the DAC90A, which is a 'live chassis' radio, just to reiterate a note of caution:

Test equipment such as a signal generator, 'scope or frequency counter much not be directly connected to the radio if the radio is powered from the mains, rather than via a mains isolating transformer.

Pic 1: The side with the connections.
Pic 2: The plain side placed near the radio.
Pic 3: A sketch with dimensions.

[OldTechFan96]

Thanks for the input, gentleman.

An idea I have had for an aerial would be to fit two pieces of wood together to form a cross and wrap 4 turns of wire around it. Attached is a crude sketch.

I found a GA202 diode for the FM diode probe, it should do.

[OldTechFan96]

The diode probe and dummy aerial have been built. The dummy aerial only consisted of three components and was easily wired together. The diode probe was assembled on a piece of stripboard.

This was the first time that I've built something and it has been quite satisfying.

This was the first time that I've built something and it has been quite satisfying.

The first of many things I bet!