AVO 8 Mk2 - A.C. problem - UK Vintage Radio Repair and Restoration Discussion Forum

vahakuja · 6th Jan 2005, 3:30 pm

I have an Avo8, which has been on shelf for a longer time. Now, when "reactived" it, I met the following sympton: The intrument is low on all A.C. Ranges but is correct on D.C. Ranges. For example when input voltage is 230V (A.C.), the instrument displays abt. 70V /(range 250V). There are now visible faul signes in componets.
Could somebody help and advice possible reason for this sympton ? I have the schematics.

BR Ake

jim_beacon · 6th Jan 2005, 3:39 pm

Hi Ake,

firstly, the service manual is here:

www.g1jbg.co.uk/service.htm

Secdondly,

my first thought would be the rectifier, but these very rarely fail. It may be a common resistor gone high.

Jim.

SPCh · 6th Jan 2005, 5:37 pm

I have never had rectifier problems on any of the AVOs I've collected.

HOWEVER I have had to replace them in all 3 of my father's meters.

They had been used extensively for television servicing, and thus "exposed" to voltages around LOPTs. I'm not aware that he used the AC ranges very often, at all, and he wouldn't have CONSCIOUSLY used them for drawing arcs from PL81 top caps, or EY51s, but he may well have done so INADVERTENTLY.

Moral: Treat 'em right, they'll last forever.

pmmunro · 8th Jan 2005, 3:22 pm

The Model 8 Mk2 uses a Westinghouse copper oxide rectifier in a squat, black cylindrical package mounted on a piece of srbp strip fixed under the current transformer. I have know this to develop the fault you describe. These were supplied with braided pigtail leads attached which may be an indication that heat directly on the tags of the rectifier package is not good for the rectifier elements which are in the form of copper discs about 3mm in diameter.

One possible cure, if you're not concerned about historical accuracy, or accept that repairs are part of this, is to replace it with a bridge of four OA95 germanium diodes. This does not seem to call for recalibration.

One point to watch is the vulnerability of the pointed hexagonal studs which press against the battery contact fingers. The leverage on these studs, due to their length, makes it very easy to break the moulded bakelite mounting plates for the ohms adjuster pots. (A possible remedy is to make a new plate from two thicknesses of GRP printed circuit laminate bonded with epoxy resin, using the original spacer pads cut from the moulding).

vahakuja · 8th Jan 2005, 8:54 pm

Yes, the rectifier stack looks very special and the construction is what your told. I was wondering might the rectifier be from selenium but its good to know what it really is.
My intention is use this Avo in my radio restoring works so your advice to use OA95s suits me well. I don't have OA95s available just now but I can make first test with some other diodes. Is it necessary to have germanium or are SI diodes sufficient? (Just to test)
The bakelite mounting plates seems to be OK but I'll recheck this area too.

Thank you to all of you about your support.

Ake

Ed_Dinning · 8th Jan 2005, 10:02 pm

Hi Ake, you will need to use a Ge rectifier as Si has too high a forward volt drop and will affect the cal of your instrument. Some of the small schottkys (BAT***? series) may do as they have low forward drops.
Ed

pmmunro · 9th Jan 2005, 2:50 pm

OA95 germanium diodes are available from Farnell (Stock No. 368 039) at 50p each. They will accept credit card orders but have a minimum order value of £15 if this form of payment is used.

I suggested OA95 diodes as these are used in later versions of the Model 8, but it is very likely that OA91s would be just as good and that schottky diodes would are suitable if you have a suitable standard with which to check calibration. (OA91s are available from Maplin at 37p).

vahakuja · 11th Jan 2005, 6:59 pm

It seems that this AVO fault is quite tricky. I have replaced the original rectifier with four OA 95s (actually with AA117s from Telefunken) but A.C. voltage display indicated same as earlier! (about 25% of actual). So the original Westinghouse rectifier is not the basic fault reason.
Anyhow, my intention is not yet give up, so therefore all hints are very welcome.

BR Ake

vahakuja · 12th Jan 2005, 10:38 pm

A.C display problem is now solved. Faulty componed was "The switch contact assembly" in D.C. Switch cam (Contact OP was in closed postion when the DC switch is in A.C postion; must be open).

BR Ake

6th Jan 2005, 3:30 pm	#1
vahakuja Triode Join Date: Feb 2004 Location: Halikko, Finland Posts: 25	AVO 8 Mk2 - A.C. problem I have an Avo8, which has been on shelf for a longer time. Now, when "reactived" it, I met the following sympton: The intrument is low on all A.C. Ranges but is correct on D.C. Ranges. For example when input voltage is 230V (A.C.), the instrument displays abt. 70V /(range 250V). There are now visible faul signes in componets. Could somebody help and advice possible reason for this sympton ? I have the schematics. BR Ake

6th Jan 2005, 3:39 pm	#2
jim_beacon Retired Dormant Member Join Date: Mar 2003 Location: Bishop's Waltham, Hants, UK. Posts: 939	Re: AVO 8 Mk2 - A.C. problem Hi Ake, firstly, the service manual is here: www.g1jbg.co.uk/service.htm Secdondly, my first thought would be the rectifier, but these very rarely fail. It may be a common resistor gone high. Jim.

6th Jan 2005, 5:37 pm	#3
SPCh Rest in Peace Join Date: Nov 2004 Location: Antwerp, Belgium Posts: 345	Re: AVO 8 Mk2 - A.C. problem I have never had rectifier problems on any of the AVOs I've collected. HOWEVER I have had to replace them in all 3 of my father's meters. They had been used extensively for television servicing, and thus "exposed" to voltages around LOPTs. I'm not aware that he used the AC ranges very often, at all, and he wouldn't have CONSCIOUSLY used them for drawing arcs from PL81 top caps, or EY51s, but he may well have done so INADVERTENTLY. Moral: Treat 'em right, they'll last forever.

8th Jan 2005, 3:22 pm	#4
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,813	Re: AVO 8 Mk2 - A.C. problem The Model 8 Mk2 uses a Westinghouse copper oxide rectifier in a squat, black cylindrical package mounted on a piece of srbp strip fixed under the current transformer. I have know this to develop the fault you describe. These were supplied with braided pigtail leads attached which may be an indication that heat directly on the tags of the rectifier package is not good for the rectifier elements which are in the form of copper discs about 3mm in diameter. One possible cure, if you're not concerned about historical accuracy, or accept that repairs are part of this, is to replace it with a bridge of four OA95 germanium diodes. This does not seem to call for recalibration. One point to watch is the vulnerability of the pointed hexagonal studs which press against the battery contact fingers. The leverage on these studs, due to their length, makes it very easy to break the moulded bakelite mounting plates for the ohms adjuster pots. (A possible remedy is to make a new plate from two thicknesses of GRP printed circuit laminate bonded with epoxy resin, using the original spacer pads cut from the moulding).

8th Jan 2005, 8:54 pm	#5
vahakuja Triode Join Date: Feb 2004 Location: Halikko, Finland Posts: 25	Re: AVO 8 Mk2 - A.C. problem Yes, the rectifier stack looks very special and the construction is what your told. I was wondering might the rectifier be from selenium but its good to know what it really is. My intention is use this Avo in my radio restoring works so your advice to use OA95s suits me well. I don't have OA95s available just now but I can make first test with some other diodes. Is it necessary to have germanium or are SI diodes sufficient? (Just to test) The bakelite mounting plates seems to be OK but I'll recheck this area too. Thank you to all of you about your support. Ake

8th Jan 2005, 10:02 pm	#6
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: AVO 8 Mk2 - A.C. problem Hi Ake, you will need to use a Ge rectifier as Si has too high a forward volt drop and will affect the cal of your instrument. Some of the small schottkys (BAT***? series) may do as they have low forward drops. Ed

9th Jan 2005, 2:50 pm	#7
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,813	Re: AVO 8 Mk2 - A.C. problem OA95 germanium diodes are available from Farnell (Stock No. 368 039) at 50p each. They will accept credit card orders but have a minimum order value of £15 if this form of payment is used. I suggested OA95 diodes as these are used in later versions of the Model 8, but it is very likely that OA91s would be just as good and that schottky diodes would are suitable if you have a suitable standard with which to check calibration. (OA91s are available from Maplin at 37p).

11th Jan 2005, 6:59 pm	#8
vahakuja Triode Join Date: Feb 2004 Location: Halikko, Finland Posts: 25	Re: AVO 8 Mk2 - A.C. problem It seems that this AVO fault is quite tricky. I have replaced the original rectifier with four OA 95s (actually with AA117s from Telefunken) but A.C. voltage display indicated same as earlier! (about 25% of actual). So the original Westinghouse rectifier is not the basic fault reason. Anyhow, my intention is not yet give up, so therefore all hints are very welcome. BR Ake

12th Jan 2005, 10:38 pm	#9
vahakuja Triode Join Date: Feb 2004 Location: Halikko, Finland Posts: 25	Re: AVO 8 Mk2 - A.C. problem. Now solved A.C display problem is now solved. Faulty componed was "The switch contact assembly" in D.C. Switch cam (Contact OP was in closed postion when the DC switch is in A.C postion; must be open). BR Ake Attached Thumbnails