Haigh (Matthews) Wobbulator

[Electronpusher0]

I have started a new thread to post the design and assembly notes about the Wobbulator pcbs I offered on the forum.

The reason for starting a new thread is that
1 the information on the wobbulator is spread over several threads and I wanted to collect them together
2 If I post the information in the "offered" section where I offered the boards this would eventually get deleted.

My interest in the Haigh Wobbulator started for me with this thread:
https://www.vintage-radio.net/forum/...53#post1296453

This discussed marrying a Haigh wobbulator to an old scope.
David (G4EBT) suggested using an external timebase and suggested a circuit designed by Armstrong.
I analysed this design in this post:
https://www.vintage-radio.net/forum/...6&postcount=15
The Armstrong design uses 4 diodes as a voltage reference, one will be compensated by the base - emmitter junction of Q1 but that leaves the temperature coefficient of 3 diodes in series, 6mV/degC.
The current mirror is more stable with temperature change provided the two transistors are closely coupled thermally.

The original article by Haigh was published in Electronics Today International Vol 27 Iss 13 Dec 1998 available for download at the Worldradiohistory website here:
https://worldradiohistory.com/UK/Ele...1998-No-13.pdf
However, as David has pointed out the original article contains an error.
“Just be aware too, that there's an error in the wiring of the coils to the bandswitch. Correct on the circuit diagram, but on Fig 3 - the PCB wiring diagram - the wires from SW1A to L3 & L4, need changing over”

I describe my first build of my version on home etched pcb and veroboard for the sweep generator here
https://www.vintage-radio.net/forum/...d.php?t=172685

This also has the case I used and a front panel drawing in post 2

The coils and Varicap diode for this project have become unobtanium so substitutes had to be found. For the three highest ranges direct substitutes can be obtained from Spectrum Comms
For the Green coil I used a 10mm black IF transformer from an old transistor radio and disabled the internal capacitor by breaking it with a sharp point.
Spectrum now supply the coils as a set on their eBay shop:
https://www.ebay.co.uk/itm/285096082...8AAOSwY7tjswk4

For the varicap I used a KV1236 which are still available at reasonable cost.

I discovered a possible problem with the original Haigh design around the sweep width control. This is a simple potentiometer that sets the amplitude of the sweep waveform.
In the original design the ACW end of the pot is connected to 0V. Since the CW end is connected to a 6V p-p waveform with a mid sweep voltage of 3V it means that the mid point of the sweep will change as the sweep width is varied.
I decided it would be an advantage to keep the centre frequency fixed as the sweep width is varied. The op-amp used in the sweep generator is a quad device so I used a spare section to derive a 3V offset voltage to which I connected the ACW end of the sweep width pot (instead of 0V). Now when the sweep width pot is varied the mid sweep voltage remains unchanged.

I attach the design and assembly notes, schematic and assembly drawing.

Peter

[Electronpusher0]

Here is the assembly notes in pdf format

Peter

[DMcMahon]

Excellent Peter, will help greatly when I start on mine.

Biggest problem for me is finding any spare time.

David

[Bazz4CQJ]

My 10-turn pot for use as R4 (as per the mod suggested on the Eddystone site) turned up today from China after just 9 days. It’s labelled Bourns, Mexico, and has an extremely nice feel to it; dead smooth. I want to use my H&M mainly for filter work on 455kHz, so it could be nice to have (£4.60 delivered) from eBay.

B

[keland_uk]

Here is link to Eddystone site:
https://eddystoneusergroup.org.uk/Re...Wobbulator.pdf

has some information namely (i'm sure there are others)
- replaced R4 (22k) tuning pot with 20k high-quality 10-turn pot
- replaced R3 (1k) fine-tuning pot with a high-quality 220ohm pot
- replaced R2 (4.7k) with a 3.3k resistor
- replaced R5 (33k) with 15k resistor

[keland_uk]

One query I have is that the original and the "RADIO BYGONES No. 82, April/May 2003" state that all capaciators are ceramic unless stated. On the PCB picture in assembly instructions C3 & C4 are ceramic, the others aren't. Should ceramic ones only be where the higher frequencies are generated? (also seems strange to have polyester on the outputs, but then again what do I know)
Capacitors
C1, C6, C9 100nF (3 off)
C2, C3, C8 10nF (3 off)
C4 1nF
C5 1uF tantalum
C7 220pF
C10 3p3
C11, C12 10nF 400V DC working polyester (2 off)
All ceramic types, 25V working or greater unless otherwise stated

I suppose deep down I'm saying a BOM for this new board would be helpful....
(I've started one if it helps)

[Bazz4CQJ]

Has anyone got a picture of a front panel of the H&M unit?

The old Haigh I have from a silent key sale stuck to Haigh's idea of mounting the PCB parallel and behind the front panel on stand-offs. I think there's a potential limitation with that in so far as once all the wires to the pots and sockets are in place, access to the rear of the PCB is restricted.

If the PCB were at 90° to the front panel, access to both sides of the PCB is available, but I guess we may then be talking about a bigger overall unit.

B

[Bazz4CQJ]

Originally Posted by keland_uk

I suppose deep down I'm saying a BOM for this new board would be helpful....(I've started one if it helps)

That would be good to see a BOM when you finish it.

My board arrived this morning. Now it's not huge, and it doesn't look densely populated, but my one good eye sent a message to my brain along the lines of "and er, who's building that then !?!" .

I think I will use BNC's on all the connectors. Especially for work on filters, ensuring that no stray RF leaks out may be worthwhile.

B

[Cathovisor]

Board arrived this morning Now comes the component gathering.

[Electronpusher0]

Originally Posted by Bazz4CQJ

Has anyone got a picture of a front panel of the H&M unit?


B

This was posted on one of the linked threads but I have attached below.
Peter

[Electronpusher0]

And this is what it looks like when finished.

Peter

[Electronpusher0]

Colin, HG MICKE, has noticed that R11, 1M gate to ground of Q2, is missing on my schematic.

Sorry about that, correct Schematic attached below.

Peter

[Electronpusher0]

Originally Posted by Electronpusher0

This was posted on one of the linked threads but I have attached below.
Peter

The front panel was drawn using Frontdesigner.
If anyone would like a copy of the .flt file please PM me with your e-mail address.

Peter

[Electronpusher0]

Originally Posted by keland_uk

One query I have is that the original and the "RADIO BYGONES No. 82, April/May 2003" state that all capaciators are ceramic unless stated. On the PCB picture in assembly instructions C3 & C4 are ceramic, the others aren't. Should ceramic ones only be where the higher frequencies are generated? (also seems strange to have polyester on the outputs, but then again what do I know)
Capacitors
C1, C6, C9 100nF (3 off)
C2, C3, C8 10nF (3 off)
C4 1nF
C5 1uF tantalum
C7 220pF
C10 3p3
C11, C12 10nF 400V DC working polyester (2 off)
All ceramic types, 25V working or greater unless otherwise stated

I suppose deep down I'm saying a BOM for this new board would be helpful....
(I've started one if it helps)

I am not qualified to answer whether ceramic would be better for the higher frequencies but I do know that the unit I constructed and photographed worked.
Obviously feel free to use ceramic as per the original Haigh design if you wish.

Peter

[Electronpusher0]

Originally Posted by Bazz4CQJ

The old Haigh I have from a silent key sale stuck to Haigh's idea of mounting the PCB parallel and behind the front panel on stand-offs. I think there's a potential limitation with that in so far as once all the wires to the pots and sockets are in place, access to the rear of the PCB is restricted.

If the PCB were at 90° to the front panel, access to both sides of the PCB is available, but I guess we may then be talking about a bigger overall unit.

B

The unit I constructed had the pcb at 90 deg to the front panel as you suggest.

Peter

[Bazz4CQJ]

Thanks for the update and the picture of your very fine looking unit. I'm glad you placed the PCB at 90' as that will confirm my thoughts about doing that.

I suspect that if there was a world league table of wobbulator ownership then the UK would be doing quite well .

B

[Electronpusher0]

Originally Posted by keland_uk

Here is link to Eddystone site:
https://eddystoneusergroup.org.uk/Re...Wobbulator.pdf

has some information namely (i'm sure there are others)
- replaced R4 (22k) tuning pot with 20k high-quality 10-turn pot
- replaced R3 (1k) fine-tuning pot with a high-quality 220ohm pot
- replaced R2 (4.7k) with a 3.3k resistor
- replaced R5 (33k) with 15k resistor

That's the beauty of homebrew, you can experiment. A 10 turn pot sounds an excellent improvement, I cannot comment on the other mods except to note that my pcb uses the same component references as the original Haigh. The sweep generator references all start at 20.

Peter

[Andrewausfa]

I'm just about to start wiring mine up this afternoon. I bought one of the first batch of PCBs in 2021 and finally have some time to assemble the unit. I've used a Pritt stick to lightly attach the front panel graphic so I could use it as a drilling template. This will be removed for a laminated version once assembly is completed.

Andrew

[Bazz4CQJ]

Yes, having bought the 10-turn pot for the main tuning, I'm now very aware that the fine tuning pot will really need to be a good quality to complement it. That is leading me to check out my junk box for some nice old vintage pot; I suspect that much of the modern stuff may not be adequate. I found one old pot which feels silky smooth, but somebody cut the spindle pretty short, so it will need a knob that can cope with that .

B

[Electronpusher0]

Kelland_uk, Chris has spotted another error on the schematic.
There are two R30s, one is on the board, a 1k preset pot, the other is a panel mounted pot of 47K, sweep amplitude, they are different.

Peter