Gizmo Transmitter Modifications


The Gizmo is an AM modulator available from "Vintage Components" (Tony Williams). It is supplied as a complete package including leads and power supply for £39.95 plus P&P. See for product details.

I recently reviewed this product on this website. The review is available here, and I urge you to read it now, if you have not done so already.

In the review I mentioned that I had carried out a simple modification to my Gizmo to resolve what I felt was excessive background "mush". I had contacted the manufacturers with details of this modification and suggested they implement it to improve the quality of their product, but had received no response. I have to assume that the manufacturers feel that the performance as supplied is acceptable.

I have since been contacted by a few people who have purchased Gizmos (before reading my review) who are also unhappy with the level of background "mush", requesting details of the modifications I had carried out. I have therefore decided to include the details here.

Before proceeding, please be sure to read and understand the following Important Disclaimer.

Important Disclaimer

The modification details on this page were devised by Paul Stenning. The Gizmo is designed and manufactured by Vintage Components. There is no connection whatsoever between Paul Stenning and Vintage Components.

The modification details on this page are not approved by Vintage Components. Carrying out these modifications, or indeed any modifications, to your Gizmo will invalidate the manufacturer's guarantee. Furthermore, Vintage Components will probably also refuse to repair your Gizmo on a non-guarantee (charged) basis.

The modification details on this page are given in good faith and have been tested on one Gizmo. Paul Stenning accepts no liability for any damage or fault caused by carrying out this modification, even if this is caused by an error in the details on this page.

In essence, once you start to dismantle and work on your Gizmo, you and you alone are responsible for the outcome of the work, and for dealing with any problems that may occur with your Gizmo from that moment onwards.

The Problem

The Gizmo consists of a crystal controlled digital phase-locked-loop oscillator circuit and an analogue modulator circuit. Although the oscillator provides a good stable frequency, and the modulator provides a good linear modulation waveform, insufficient care has been taken in the design to prevent noise from the digital circuits getting into the analogue circuits.

Normal design practice in such systems would be to provide separate power supplies for the digital and analogue circuits. Where only a small amount of low current digital circuitry is included, as in the Gizmo, a single supply would be used, with careful separation and decoupling of the supply to the digital and analogue sections. The latter approach is used in this modification.

As designed, the Gizmo uses a single regulated 12V DC supply (from the external adaptor), fed directly to the digital and analogue circuits. The routing of the 12V supply rail on the PCB is such that the analogue circuits are after the digital circuits - the worst possible arrangement!

The Modifications

To carry out the modification, we separate and reroute the 12V DC supply. The supply for the analogue circuits is taken from the full 12V, connected more directly to the DC input socket, and an additional decoupling capacitor is added. The supply to the digital circuits is decoupled with a 470-ohm resistor and existing decoupling capacitors.

To carry out this modification you will need to cut two tracks, and add one insulated wire link, one resistor (470-ohm 0.33W) and one capacitor (10uF 16V axial).

A further modification is detailed, which brings about a slight improvement to the audio quality by removing any RF present at the modulation input of the modulator circuit. For this you will need to add a small 1nF capacitor (disc ceramic or box polyester).

Finally, details of modifications carried out by the manufacturer are noted. These have probably been done to recently supplied Gizmos, but it is worth checking yours while you are at it.

Carrying out the Modifications

Note that the Gizmo contains static-sensitive components. Please take reasonable precautions to prevent static discharge when working on the PCB.

Open the Gizmo unit as detailed in the instructions. The PCB is fixed to the base of the case by means of two sticky pads. Carefully remove the PCB - hopefully the sticky pads will remain stuck to the base and not the PCB. Place the base where dirt and dust cannot fall onto the sticky pads. If your Gizmo has a fixed aerial wire you will probably find it helpful to unsolder it now, noting where it connects.

The numbers in the following description refer to the numbers on the photos below:

Cut the positive 12V power track from the DC input socket to the rest of the tracking as indicated (1).

Cut the positive 12V power track between the analogue and digital circuits as indicated (2).

Solder a 470-ohm 0.33W resistor between the positive (tip) terminal of the DC input socket and pin 16 of the 4060 IC, on the rear of the PCB, as shown (3). Make sure the resistor body is flat against the PCB. Note that this resistor is bridging the track cut at (1).

Solder a length of insulated wire between the positive terminal of the DC input socket and the common side of the output filter DIP switch bank, on the rear of the PCB, as shown (4). When soldering, take care that the insulation does not creep back along the wire, leaving bare wire that could cause a short-circuit. If you have some thin heat-resisting sleeving (silicone or PTFE) and some tinned-copper wire, this would be a better choice than regular PVC insulated wire.

Fit a 10uF 16V (or greater) axial electrolytic capacitor as shown (5). The negative wire connects through an existing hole in the PCB next to a 470-ohm resistor. The positive wire is soldered to one end of a 1k-ohm resistor.

To carry out the further RF filter modification mentioned above, connect a 1nF ceramic disc or box polyester capacitor between the base of the modulation transistor and the negative rail as shown (6). This capacitor needs to be thin enough that it will not prevent the PCB being refitted into the case, and should be laid flay against the PCB.

Manufacturer's Modifications

These modifications will probably have already been carried out:

A 100uF 25V radial capacitor is connected across the 100-ohm emitter resistor of the modulation transistor as shown (7). Note that the negative connection is towards the centre of the PCB. This increases the sensitivity of the audio input so that a lower audio signal level is required for good modulation.

A 1mH inductor is connected in series with the audio input, between the top connection of the input socket and the PCB, as shown (8). This helps to remove any RF getting out of the audio input socket.

A 1nF disc ceramic capacitor is connected across the audio input on the underside of the PCB as shown (9). This helps to remove any RF getting out of the audio input socket, in conjunction with the inductor mentioned above.

Finishing Off

You may wish to test the results of the modification before reassembly. Remember to resolder the aerial wire if you disconnected it earlier. If you added the capacitor at (7), you will need to reduce the audio input level otherwise the result will be very distorted.

Reassembly, as they say in the car repair manuals, is a reversal of the above procedure. You may need a couple of new sticky pads if the originals won't stick.

I am sure you will find that the modification brings about a worthwhile reduction in the amount of "mush" produced by your Gizmo, and therefore improves the audio quality from your vintage radio. I would be interested to hear how you get on with it.


I have received an email from Dick Lee who has carried out these mods to his Gizmo:

I'm writing as I said I would having carried out the modifications to the Gizmo MW transmitter which you kindly put on the website. Maybe it will be of some use to other visitors to your site.

Basically, it works much better now, almost no mush at all.

I can't get the resistor on the underside of the board to fit snugly enough, so the case is now a bit squint, but it works and I don't have to look at it too often!

Mine had some of the manufacturer's modifications but not the choke and capacitor combination which prevents RF getting to the audio input - I wonder what good it will do to try that mod too?

Although the recommended audio source is low impedance from a phones socket, mine works well taking the line audio output from a freeview set top box.

It's ironic, or something, that this makes it possible to hear ancient radio programs, like The Navy Lark and the Carleton Hobbs / Norman Shelley Sherlock Holmes series, which are broadcast in digital quality on BBC7, on ancient valve radios with the same sound quality at which they were first heard. A much better sound to listen to - is it just my freeview box which emphasises the treble to such an uncomfortable level, or is that how it's supposed to be?

Sorry, I digress; Paul Stenning's modifications cure the Gizmo's problem with signal to noise level, and the frequency stability, an original manufacturer's feature, is excellent.

This website, including all text and images not otherwise credited, is copyright © 1997 - 2006 Paul Stenning.
No part of this website may be reproduced in any form without prior written permission from Paul Stenning.
All details are believed to be accurate, but no liability can be accepted for any errors.
The types of equipment discussed on this website may contain high voltages and/or operate at high temperatures.
Appropriate precautions must always be taken to minimise the risk of accidents.

Last updated 14th April 2006.