Revox A77 aficionados in Scotland ? - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Boulevardier · 21st Jun 2021, 2:24 pm

Quote:

Originally Posted by boy1904

Quote:

Originally Posted by Station X

OL indicates infinite resistance, so either your probes aren't making good contact with the end caps or the fuse is intermittent.

You could put both fuses back and measure the DC voltage wrt ground on all four end caps
caps.

Okay so can you measure the DC voltage on the end caps using the multimeter also ?

Yes, but make sure you switch meter to the DC Volts range first.

Station X · 21st Jun 2021, 2:26 pm

Quote:

Originally Posted by boy1904

Quote:

Originally Posted by Station X

OL indicates infinite resistance, so either your probes aren't making good contact with the end caps or the fuse is intermittent.

You could put both fuses back and measure the DC voltage wrt ground on all four end caps
caps.

Okay so can you measure the DC voltage on the end caps using the multimeter also ?

Yes, but you need to measure the DC Voltage with respect to the other side of the supply. Not having the service data to hand I don't know where that is, but you might be ablt to find it by experiment. Chassis metalwork is always a good starting point.

ms660 · 21st Jun 2021, 2:33 pm

There are various copies of the schematic and service manuals here in varying quality, FOC or registration, I'm looking at service manual 4 (not the MK4) if that's any use:

https://www.vintageshifi.com/reperto...oogle_vignette

Lawrence.

DMcMahon · 21st Jun 2021, 3:40 pm

The fuses are AC voltages from the secondary windings of the mains transformer.

For fuse F103 the winding voltage is shown as 22VAC, for fuse F102 the winding voltage is shown as 24VAC. To measure these AC voltages at either side of the fuses you would have to connect the other side of the meter (set to AC voltage) to the respective other end of the winding that connects to the respective bridge rectifier (shown as connection D5 for the 22VAC and connection D7 for the 24VAC).

Do not know what sort of reading you might get if you tried to measure DC voltage either side of the fuses but it would be misleading.

I would say probably easier just to measure the +21V & +27V DC voltages at the smoothing capacitors C101 & C104 ( +ve connection) respectively, assuming they are practically accessible, with the meter negative lead connected to 0V/ground

David

Station X · 21st Jun 2021, 4:08 pm

C101 is on the wrong side of the voltage regulation circuit for measuring the DC voltage across it. Better to measure it across suitable interconnect points. AD1 and AD2?

DMcMahon · 21st Jun 2021, 6:51 pm

On 2 schematics I have checked the +ve side of C101 connects directly to AD1, AD2 is the 0V so appears identical to me to what I suggested ?

David

ms660 · 22nd Jun 2021, 10:34 am

Yes, measuring from ground to the +ve of the reservoir capacitors will give the B+ supply voltages.

Lawrence.

Mikebee88 · 19th Oct 2021, 4:56 pm

Sorry this response is a little late and you may have sorted your A77 by now but I own and have fully restored four of these machines. I live in Dunfermline and would be pleased to offer you any practical assistance if you'd like to get in touch.

21st Jun 2021, 2:33 pm	#23
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Revox A77 aficionados in Scotland ? There are various copies of the schematic and service manuals here in varying quality, FOC or registration, I'm looking at service manual 4 (not the MK4) if that's any use: https://www.vintageshifi.com/reperto...oogle_vignette Lawrence.

21st Jun 2021, 3:40 pm	#24
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	Re: Revox A77 aficionados in Scotland ? The fuses are AC voltages from the secondary windings of the mains transformer. For fuse F103 the winding voltage is shown as 22VAC, for fuse F102 the winding voltage is shown as 24VAC. To measure these AC voltages at either side of the fuses you would have to connect the other side of the meter (set to AC voltage) to the respective other end of the winding that connects to the respective bridge rectifier (shown as connection D5 for the 22VAC and connection D7 for the 24VAC). Do not know what sort of reading you might get if you tried to measure DC voltage either side of the fuses but it would be misleading. I would say probably easier just to measure the +21V & +27V DC voltages at the smoothing capacitors C101 & C104 ( +ve connection) respectively, assuming they are practically accessible, with the meter negative lead connected to 0V/ground David Last edited by DMcMahon; 21st Jun 2021 at 3:46 pm.

21st Jun 2021, 4:08 pm	#25
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Revox A77 aficionados in Scotland ? C101 is on the wrong side of the voltage regulation circuit for measuring the DC voltage across it. Better to measure it across suitable interconnect points. AD1 and AD2? __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

21st Jun 2021, 6:51 pm	#26
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	Re: Revox A77 aficionados in Scotland ? On 2 schematics I have checked the +ve side of C101 connects directly to AD1, AD2 is the 0V so appears identical to me to what I suggested ? David

22nd Jun 2021, 10:34 am	#27
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Revox A77 aficionados in Scotland ? Yes, measuring from ground to the +ve of the reservoir capacitors will give the B+ supply voltages. Lawrence.

19th Oct 2021, 4:56 pm	#28
Mikebee88 Triode Join Date: Mar 2014 Location: Dunfermline, Scotland, UK. Posts: 23	Re: Revox A77 aficionados in Scotland ? Sorry this response is a little late and you may have sorted your A77 by now but I own and have fully restored four of these machines. I live in Dunfermline and would be pleased to offer you any practical assistance if you'd like to get in touch.