UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Closed Thread
 
Thread Tools
Old 13th Oct 2017, 9:56 pm   #1
Andrewausfa
Octode
 
Andrewausfa's Avatar
 
Join Date: Dec 2010
Location: St Ives, Cambridgeshire, UK.
Posts: 1,180
Default Sweep generator/wobbulator circuit

I found this circuit by H.L. Gibson in a 1978 RSGB book. It's a 440-550kHz sweep generator, apologies for my rubbish drawing, which I fancied building on a rainy day. Tr1 and 2 are PNP BFX12 which I think I can sub with 2N2906 but it's the NPN OC140 I'm stuck on. They're overly expensive.

Not wishing to stop anyone building fuzz boxes, does anyone have any suggestions for a substitute? I am not good at interpreting circuits and knowing what transistor might work in anothers place. It says 'the multivibrator is supplied from a 9V battery through a constant current circuit TR3 feeding a 5.1v Zener diode..' which I suspect the more astute will know. Unlike me.

Thanks - Andrew
Attached Thumbnails
Click image for larger version

Name:	IMG_3782.jpg
Views:	902
Size:	74.6 KB
ID:	150713  
__________________
Invisible airwaves crackle with life. Or they should do.
BVWS Member
Andrewausfa is offline  
Old 13th Oct 2017, 10:52 pm   #2
David G4EBT
Dekatron
 
David G4EBT's Avatar
 
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,737
Default Re: Sweep generator/wobbulator circuit

I guess it depends on one's definition of 'expensive' is but I'd have thought £3.25 plus £2.25 P&P from a U.K supplier wasn't too over the top:

http://www.buy-transistors.com/transistors/oc140.html

Hope that helps Andrew.
__________________
David.
BVWS Member.
G-QRP Club member 1339.
David G4EBT is online now  
Old 13th Oct 2017, 11:00 pm   #3
MrBungle
Dekatron
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
Default Re: Sweep generator/wobbulator circuit

What are the values of R12 and R18?

It's fundamentally a current source that is used to drive the zener to give a very stable voltage. Try any PNP in there you have. Should be fine. I suspect even a lowly 2n3906 would do the job perfectly well.

You can get 250 of them branded by Fairchild for less than £4. An OC140 is far too expensive for such work

Edit: if you want to test it, just build that part of the circuit with any PNP transistor you have and stick the output of it (i.e. slice the entire left of the circuit off) across your DMM/VOM in milliamps and see what it measures.

Edit 2: the actual circuit is designed for PNP transistors. I find it far more intuitive if you turn everything around and use NPNs.

Last edited by MrBungle; 13th Oct 2017 at 11:06 pm.
MrBungle is offline  
Old 14th Oct 2017, 8:51 am   #4
KeithsTV
Nonode
 
KeithsTV's Avatar
 
Join Date: Aug 2007
Location: South Bradford, West Yorkshire, UK.
Posts: 2,571
Default Re: Sweep generator/wobbulator circuit

The OC140 is a germanium NPN transistor and in this circuit is, as has been said, a constant current source in conjunction with the OA202. You can substitute a silicon NPN transistor, almost any small signal transistor such as a BC108 will do, but you'd need to change the value of R12 and add another diode in series with the OA202. The OA202(s) can be substituted with 1N4148s.

What is the value of R12? If you do change to a silicon transistor the value of this will need to be increased to maintain the same current in the zener.

Keith
KeithsTV is offline  
Old 14th Oct 2017, 8:58 am   #5
KeithsTV
Nonode
 
KeithsTV's Avatar
 
Join Date: Aug 2007
Location: South Bradford, West Yorkshire, UK.
Posts: 2,571
Default Re: Sweep generator/wobbulator circuit

A quick calculation suggests that if you use a silicon NPN transistor for TR3 R12 should be 100ohms. Add a second silicon diode in series with the OA202, 1N4148s would do and change R18 to 6.8k.

This should give a current of around 6mA which should be enough for the zener and the oscillator.

Keith
KeithsTV is offline  
Old 14th Oct 2017, 9:43 am   #6
TonyDuell
Dekatron
 
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,188
Default Re: Sweep generator/wobbulator circuit

As somebody said, TR3 is acting as a current source. Together with the zener it provides a stable 5.1V supply for the oscillator. I don't know how stable you need it, but surely there are cheaper ways to get a stable 5V supply now.
TonyDuell is offline  
Old 14th Oct 2017, 9:58 am   #7
MrBungle
Dekatron
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
Default Re: Sweep generator/wobbulator circuit

Yes. 78L05. Between 6 and 12 pence a pop. That’ll only burn off a tiny bit of power as well in this application which is the voltage drop between the 5v out and 9v input times the current through it (resulting in watts).

It’s not even a very good design because the output impedance of that Zener is isn’t exactly great and there is no amplification via a pass transistor. You’re also kind of shot if you want battery operation here because the zeners VI curve knee usually states quite a few mA through it for any kind of stability so your battery will last a couple of hours. The target circuit might only be consuming a couple of mA but the power supply may be eating 30mA or so to get any reasonable regulation.
MrBungle is offline  
Old 14th Oct 2017, 9:58 am   #8
Andrewausfa
Octode
 
Andrewausfa's Avatar
 
Join Date: Dec 2010
Location: St Ives, Cambridgeshire, UK.
Posts: 1,180
Default Re: Sweep generator/wobbulator circuit

Thanks for the advice. Yes thanks David I did a search last night for OC140 but for mucking about, they're expensive. That's one thing that is slightly frustrating, seeing all these transistorised home brew designs you'd like to build in old books or magazines and then finding the components are expensive/hard to get etc etc. As I alluded to above, for me the second frustration is not having those years of experience in knowing what I can replace these with. I've got various common transistors to have a go with, so I will.

The circuit was published in Test Eequipment For The Radio Amateur and I was happy to read the author, H.L. Gibson. reached his 100th birthday last year. These are all the component values;

R1 about 40k per 10V peak to peak of timebase voltage
R2 2.7k
R3 4.7k
R4 2.2k
R5 2.2k
R6 33k
R7 33k
R8 1.8k
R9 4.7k
R10 56k
R11 22R
R12 68R
R13 1k
R14 1k
R15 47R
R16 47R
R17 47R
C1 0.01
C2 100uF 15V
C3 47pF
C4 47pF
C5 0.1
C6 50uF 15V
C7 0.01
RV1 10k
RV2 10k (on circuit diagram. Says 10R in the text, typo I think)
RV3 1k
TR1,2 BFX12
TR3 OC140
D1 BZY88
D2 OA202
S1 Three way switch
S2 SPST switch
Metal project box (author suggests 7.25" x 4.75" x 2")
Tinned metal sheet for building screen around attenuator
Author builds his on a 3.75 x 3.75" bit of Veroboard (less attenuator parts)
Two BNC or similiar connectors
Tuning range 440-550kHz, sweep 0-30kHz, output 0-50mV

Andrew
__________________
Invisible airwaves crackle with life. Or they should do.
BVWS Member
Andrewausfa is offline  
Old 14th Oct 2017, 10:12 am   #9
MrBungle
Dekatron
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
Default Re: Sweep generator/wobbulator circuit

Thanks for the info. I’ve got a copy of that book here actually and built the FET voltmeter from it.

Slightly tempted to try and build one with off the shelf modern parts. the design isn’t great by the looks. That doesn’t mean it doesn’t work but it’s certainly a large compromise. One of the problems is that the output is going to be fundamentally a very low quality square wave with soft edges due to the oscillator being a multivibrator. That means there is going to be power at more than just the fundamental frequency of the sweep (Fourier’s theorem) so the performance depends on the circuit you’re measuring as much as the sweep generator’s ability. This might make the results a little off.

Also a problem with sweeping at IFT speeds is the compromise between the persistence on your scope and the delta of frequency change. Less of a problem than square waves being generated but still significant distortion results from the sweep speed being relatively fast compared to the wavelength.

With these I find the best thing to do is chuck it together on a solderless board and see what it does. It might work, but as a lot of stuff you see, particularly in PW, it also might not
MrBungle is offline  
Old 14th Oct 2017, 1:46 pm   #10
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Sweep generator/wobbulator circuit

Quote:
Originally Posted by MrBungle View Post
Yes. 78L05. Between 6 and 12 pence a pop. That’ll only burn off a tiny bit of power as well in this application which is the voltage drop between the 5v out and 9v input times the current through it (resulting in watts).
If you delete the zener & replace the OC140 with a 3 terminal voltage regulator in the position that the OC140 is in, you will need the 79LO5 which is the negative regulator. When using these put a 2.2uF tantalum capacitor or similar directly on their input and output lead to ground (in this case positive and the regulators common terminal). Using one will be much better than the OC140 & zener.

PS: watch out on the 79LO5 that the common connection is not in the middle of the package on them (like the 78LO5) its on one side which is a bit of a trap..

Last edited by Argus25; 14th Oct 2017 at 1:55 pm. Reason: add info.
Argus25 is offline  
Old 14th Oct 2017, 2:02 pm   #11
MrBungle
Dekatron
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
Default Re: Sweep generator/wobbulator circuit

Good point!
MrBungle is offline  
Old 15th Oct 2017, 9:06 am   #12
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default Re: Sweep generator/wobbulator circuit

It's a multi-vibrator so you get a non-symmetric square wave output. There is no low pass filter to remove harmonics.
PJL is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 11:12 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.