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

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Test Gear and Workshop Equipment

Notices

Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

Reply
 
Thread Tools
Old 30th Sep 2019, 12:48 pm   #1
ChristianFletcher
Hexode
 
ChristianFletcher's Avatar
 
Join Date: Dec 2003
Location: South Yorkshire, UK.
Posts: 331
Default Avo Wide range signal generator first inspection

I have AVO wide range signal generator on my desk. I’m just doing an assessment at the moment with a plan to doing the electrical restoration in the future. I have not operated or worked on one of these units before and have a few questions if anyone can advise.

Looking at the circuit diagram it shows a simple bridge rectifier but the unit itself looks to have a long tube which I’m guessing is a western type copper oxide rectifier rather than selenium ? - I will post a picture tonight

The rectifier looks to have about 8 different tapping points so looks different in terms of the connection details than circuit diagram would suggest. I would normally replace a selenium rectifier on sight but this looks more complicated. Has anyone experienced any faults on these units etc. My experience of copper oxide rectifiers is good and not encountered a failure.

Being an AVO it got amazing build quality. I was going replace the two electrolytic smoothing condensers but I wonder about the reliability of the other waxy capacitors as they are not all born equal. I have read previously about damage to the tuning coils burning out and I assume being down to dc getting where it shouldn’t

I was also wondering about the date of manuafcture I am guessing 1950s

So just wondering if anyone has renovated one of these and any hints and tips before I dive in etc.


I’m also interested in what looks like a separate dummy antenna unit. I don’t have one of these and was thinking about building one. I assume they are doing some impedance matching but never used a match pad unless working on car radios which have excessive capacitance due to the feeder. Does anyone have one or a picture of one.

I will post a few pictures tonight as I’m in work at the moment.

Many Thanks Regards Chris
Attached Thumbnails
Click image for larger version

Name:	avo example images.jpeg
Views:	87
Size:	8.0 KB
ID:	191095  
Attached Files
File Type: pdf avo_wide_range_signal_generator.pdf (781.1 KB, 38 views)
__________________
Every Silver Lining Has Its Cloud !

Last edited by ChristianFletcher; 30th Sep 2019 at 12:58 pm.
ChristianFletcher is offline   Reply With Quote
Old 30th Sep 2019, 4:21 pm   #2
Andrewausfa
Heptode
 
Andrewausfa's Avatar
 
Join Date: Dec 2010
Location: St Ives, Cambridgeshire, UK.
Posts: 991
Default Re: Avo Wide range signal generator first inspection

Chris,

The rectifier is a Sentercel, I believe Selenium. I have the dummy aerial for these, I'm at work too so I'll photograph it later. They're a sealed unit.

These took over from the previous AVO All Wave sig gens, date will be on the smoothing electrolytic but I think both of mine are mid-50s.

Andrew
__________________
Invisible airwaves crackle with life. Or they should do.
BVWS Member
Andrewausfa is online now   Reply With Quote
Old 30th Sep 2019, 4:47 pm   #3
ChristianFletcher
Hexode
 
ChristianFletcher's Avatar
 
Join Date: Dec 2003
Location: South Yorkshire, UK.
Posts: 331
Default Re: Avo Wide range signal generator first inspection

Thanks Andrew

Did you do anything to yours regarding recommissioning etc. Could be selenium but just looks more like the copper oxide. It does not really matter just wondering if they put up fight etc. historically had the white onion smoke from selenium more than once etc.

Yes a picture would be nice of the dummy antenna as I want to make something similar etc.

Thanks
Regards
Chris
__________________
Every Silver Lining Has Its Cloud !
ChristianFletcher is offline   Reply With Quote
Old 30th Sep 2019, 6:15 pm   #4
Andrewausfa
Heptode
 
Andrewausfa's Avatar
 
Join Date: Dec 2010
Location: St Ives, Cambridgeshire, UK.
Posts: 991
Default Re: Avo Wide range signal generator first inspection

No problem. I think I stuffed the Sentercel with a 1N4007 and resistor, also replaced all the electrolytic and wax capacitors. It was a few years ago though!

I actually bought two, one after the other rather quickly. The more cosmetically perfect one was missing a knob and a less than perfect one but with a full knob complement came along soon after. I found the one I keep downstairs to be really quite accurate still, checking with a counter. Must have been really useful for the mobile aligner!

Regards - Andrew
__________________
Invisible airwaves crackle with life. Or they should do.
BVWS Member
Andrewausfa is online now   Reply With Quote
Old 30th Sep 2019, 6:31 pm   #5
David G4EBT
Dekatron
 
David G4EBT's Avatar
 
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 3,714
Default Re: Avo Wide range signal generator first inspection

If it's a Sentercel rectifier - a tubular Paxolin jobby - it will most likely have a series of discs in it.

I've attached a couple of pictures of one that I dismantled (in a Heathkit Res/Cap bridge) and replaced with a silicon diode and limiting resistor, (because silicon diodes produce a higher output voltage than do selenium rectifiers, a series resistor is needed to drop the voltage to that desired), but that was a half wave rectifier. Yours is a full wave so will have two connections for the AC input, and two for the +/- DC output. (I'm puzzled that you say it has eight connections).

The first picture is the Sentercel stick before dismantling, the second shows the disgorged contents and the replacement silicon diode and limiting resistor which I re-stuffed the Paxolin tube with.

Before condemning the rectifier, I'd be inclined to first check the AC Voltage input to it (no voltages are stated on the circuit), and compare that to the DC output. I'd expect the DC output to be a little higher than the AC input to it if the rectifier is healthy, but if it's appreciably less, it will need replacement. The easy option would be to fit a 'WO' silicon bridge like the one shown here, which would be more than adequate as it's 400V at 1.5 Amps:

https://www.ebay.co.uk/itm/2-WO4-1-5...4AAOSwDX9b5dGk

Or if you preferred a higher Voltage rating, you can get them up to 1000V.

EG ten for 2.00 here from a UK supplier:

https://www.ebay.co.uk/itm/10x-BRIDG...r=530257631312

If you'd rather steer clear of eBay, RS, CPC/Farnell, Rapid, Cricklewood, ESR - whoever.

The marking on these little round 1.5A bridge rectifiers denotes the Voltage rating: W02 = 200V and so on:

W02 1.5 Amp 200V
W04 1.5 Amp 400V
W06 1.5 Amp 600V
W10 1.5Amp 1000V

The alternative, if you wanted to try to preserve the originality, would be to use say four 1N4007 silicon diodes and a limiting resistor if they'd fit inside the original tube.

It's possible, if not probable, that the two 8uF smoothing caps will be past their best, so it's worth checking their capacitance. They may respond to reforming, or you may want to consider replacing them, though I'm not saying you should. Likewise, before just replacing the tubular capacitors on sight, if you are able to test them for leakage at their rated voltage (with a Megger or similar insulation tester for example), and also check their capacitance, that will show whether replacement is called for.

It would be sensible in any event to replace the two 0.002 filter caps on the AC mains input, which would need to be Class Y caps as they're from Neutral to ground and Line to ground.

Before you dive into it, you might want to see if it's operational. You could try it on a lamp limiter, look at the waveform of the unmodulated and modulated output on the scope if you have one, and check the frequency on a counter, or if not, check it against a radio on a range of stations.

As to the dummy aerial, the component values are shown on the circuit, so it wouldn't be difficult to house them in a small metal canister rather like those that Denco coils used to be supplied it. (I made one some years ago, housed in a small metal salt pot). I assumed that the 70 Ohm resistor on the circuit where the dummy aerial plugs into the attenuator socket is figurative - to indicate a 70 Ohm coax lead, rather than an actual 70 Ohm resistor.

Hope that helps a bit.

Every good wish with it Christian.
Attached Thumbnails
Click image for larger version

Name:	Heathkit C-30U Rectifier[1].jpg
Views:	56
Size:	24.5 KB
ID:	191111   Click image for larger version

Name:	Sentercel emptied and ready to stuff[1].jpg
Views:	53
Size:	34.2 KB
ID:	191112  
__________________
David.
BVWS Member.
G-QRP Club member 1339.

Last edited by David G4EBT; 30th Sep 2019 at 6:41 pm.
David G4EBT is online now   Reply With Quote
Old 30th Sep 2019, 7:50 pm   #6
ChristianFletcher
Hexode
 
ChristianFletcher's Avatar
 
Join Date: Dec 2003
Location: South Yorkshire, UK.
Posts: 331
Default Re: Avo Wide range signal generator first inspection

Thanks David / Andrew

It looks more like a stack of washers mounted on a threaded bar. It was late last night when I had a look so in the light of day the connections are looking more obvious. If you look at the picture the connections at each end of the rod are to the chassis so they are just 0v connections. I did not notice that on first glance. There are 5 connections along the length of the rod would be the 4 bridge connections plus one more that i need trace.

Thanks for the heads up on likely component replacements I would have done as you suggested. I hate changing out these selenium rectifier as I do like to keep my equipment as original as possible but I have learn to my cost the problems with these Selenium units but like most radio components not all part are born equal.

I will probably test the Electrolytics but replace them regardless. Unfortinately the various mega meters I own have a minimum 500 test voltage.

hopefully Andrew has a picture of the Dummy antenna I can copy etc.

Thanks again for taking time to provide such datailed information many thanks it was very kind of you.

Regards Chris
Attached Thumbnails
Click image for larger version

Name:	IMG_1013.jpg
Views:	85
Size:	77.4 KB
ID:	191122  
__________________
Every Silver Lining Has Its Cloud !
ChristianFletcher is offline   Reply With Quote
Old 30th Sep 2019, 8:21 pm   #7
turretslug
Nonode
 
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 2,851
Default Re: Avo Wide range signal generator first inspection

It's very common indeed (usual?) for selenium stack bridges to have 5 connections for manufacturing convenience- think of it as two head-to-head stacks of diodes, each tapped half-way along for the AC input, one polarity of the DC output is then the centre tag, the end two tags linked together form the other pole. It certainly looks as if your specimen is wired that way.
turretslug is offline   Reply With Quote
Old 30th Sep 2019, 10:57 pm   #8
ChristianFletcher
Hexode
 
ChristianFletcher's Avatar
 
Join Date: Dec 2003
Location: South Yorkshire, UK.
Posts: 331
Default Re: Avo Wide range signal generator first inspection

Quote:
Originally Posted by turretslug View Post
It's very common indeed (usual?) for selenium stack bridges to have 5 connections for manufacturing convenience- think of it as two head-to-head stacks of diodes, each tapped half-way along for the AC input, one polarity of the DC output is then the centre tag, the end two tags linked together form the other pole. It certainly looks as if your specimen is wired that way.
Ok thanks for the information. I have a circuit diagram so need to give it some looking at. May power it up gradually after changing the capacitors and have a look at the ripple.

Thanks
__________________
Every Silver Lining Has Its Cloud !
ChristianFletcher is offline   Reply With Quote
Old 1st Oct 2019, 8:38 am   #9
David G4EBT
Dekatron
 
David G4EBT's Avatar
 
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 3,714
Default Re: Avo Wide range signal generator first inspection

Just a thought about the mains suppressor caps.

Given that waxy paper caps in that role have a well deserved reputation for going bang - leastways on the ubiquitous DAC90A for example, would the general view on this AVO signal generator be that (albeit they aren't waxy paper caps), they ought to be replaced, given that they'll be about 70 years old - maybe more? Or are they perhaps considered rather more reliable and best left alone, assuming they pass a leakage test ate their rated voltage on an insulation tester?

I ask that because firstly, 0.002 (2n2) Class Y caps seem thin on the ground, and not tubular axial as I imagine the Avo ones will be, but are invariably radial. Also, how critical is the capacitance value? There are these disc ceramic ones which are 4n7 - not 2n2, described as:

4.7nF class X/Y2 disc:

Radial leaded AC line rated single-layer ceramic Disc Capacitor for line-by-pass applications. The capacitor consists of ceramic disc both sides of which are silver plated. Connection leads are made of tinned copper having diameters of 0.8mm. Coating is made of blue coloured flame-retardant epoxy resin in accordance with UL94V-0. Ceramic is a commonly used dielectric material that can improve capacitor power dissipation.

Complies with IEC 60384-14 3rd edition:

https://cpc.farnell.com/vishay/wko47...00v/dp/CA05845

Also these 2n2 Vishay brand disc ceramic:

https://www.ebay.co.uk/itm/CAPACITOR....c100005.m1851

Then these Wima brand 'box' shaped ones, which though 2n2, have short through hole leads, so maybe not as convenient as the disc ones:

https://www.ebay.co.uk/itm/CAPACITOR...8AAOSwux5YRYLn

I don't wish to hi-jack Chris's thread but I do have a vested interest in asking as I too have an AVO Wide Rage generator, which has slumbered in my loft for at least 20 years. When I bought it, I opened it up and visually it was pristine and unmolested, but it need of restoration as the waveform was distorted. As I had (still do have) other signal generators which work fine, it's a restoration project, like so many others.

Any thoughts please?
__________________
David.
BVWS Member.
G-QRP Club member 1339.
David G4EBT is online now   Reply With Quote
Old 1st Oct 2019, 12:41 pm   #10
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 4,221
Default Re: Avo Wide range signal generator first inspection

These are very basic signal generators so you should expect quite a bit of harmonic distortion. Still a good idea to replace paper capacitors though.
PJL is offline   Reply With Quote
Old 1st Oct 2019, 2:14 pm   #11
barrymagrec
Heptode
 
Join Date: Dec 2017
Location: Morden, Surrey, UK.
Posts: 563
Default Re: Avo Wide range signal generator first inspection

Yes, the older All Wave generator even has a 6H scale to make using the harmonics more convenient.
barrymagrec is online now   Reply With Quote
Old 1st Oct 2019, 6:34 pm   #12
ChristianFletcher
Hexode
 
ChristianFletcher's Avatar
 
Join Date: Dec 2003
Location: South Yorkshire, UK.
Posts: 331
Default Re: Avo Wide range signal generator first inspection

Thanks Everyone. I will certainly change out anything across the mains with a modern replacement as safety is the key. I will also add an earth as it is a metal bodied 2 wire device at the moment. Probably keep the selenium rectifier but fit a fuse in the transformer secondary. I am not planning on using everyday I have modern test gear, but I think it may be an interresting to do a radio repair project using Just tools and equipment available to the repair shop back in the day.

As always many thanks for comment hints tips and advice.

Regards Chris
__________________
Every Silver Lining Has Its Cloud !
ChristianFletcher is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 6:33 pm.


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 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2019, Paul Stenning.