Revox A77 Capstan Issue - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

cgehansen · 12th Oct 2021, 2:27 pm

I'm working through the board testing each component as I go and have come across a large yellow wire wound? resistor which isn't on my schematic. In the later boards there capacitor C209 (I think as it's hard to read) in its place. The resistor is marked 1k6 125Vm 1%95. I'm getting a very high resistance reading on this part - well over 1k6. I'm not sure what to replace it with as I can't find anything similar online.

ms660 · 12th Oct 2021, 2:40 pm

So far as I can make out, 1.6nF @ 125V 1% capacitor in the service info I'm looking at.

Lawrence.

cgehansen · 12th Oct 2021, 3:04 pm

Yes you're right, thanks!

ms660 · 12th Oct 2021, 3:06 pm

No problem.

Lawrence.

Michael Maurice · 12th Oct 2021, 9:59 pm

They are available but not in one offs. You could try 1500pF in parallel with 100pF.

cgehansen · 13th Oct 2021, 12:09 pm

Thanks for that . It measures fine so seems to be OK. I've ordered replacement diodes as you suggested and a C206 and will see if that fixes it. When I put the board back in yesterday it worked fine for about 5 mins then speeded up, then stopped completely in 7 1/2 ips so I assume something is failing under load. With the old IC201 and/or IC202 on the board nothing moves at all.

Boulevardier · 13th Oct 2021, 1:51 pm

Quote:

Originally Posted by cgehansen

When I put the board back in yesterday it worked fine for about 5 mins then speeded up, then stopped completely in 7 1/2 ips

Does this sequence happen every time the board is powered up from cold? If so, sounds like a heat-caused fault, and these can sometimes be very subtle - microscopic and invisible cracks in tracks, joints, inside components, etc. Do you have any freezer spray?

Mike

cgehansen · 13th Oct 2021, 7:14 pm

The board being cold doesn’t help. It started working correctly after I desoldered and tested the diodes and C206. They all test OK with the multimeter but maybe one of those components is failing in use. Once it’s stopped working it doesn’t fix itself if left off to cool down. I’ve changed all the electrolytic and tantalum capacitors, all the small transistors and both chips.

cgehansen · 14th Oct 2021, 2:42 pm

So after a couple of false starts the board now seems to be working. It has new diodes and a new C206 and that seems to have done the trick. All the old parts tested OK with my DMM but one or more of them were not working when installed on the board.

12th Oct 2021, 2:27 pm	#21
cgehansen Pentode Join Date: Sep 2017 Location: London, UK. Posts: 115	Re: Revox A77 Capstan Issue I'm working through the board testing each component as I go and have come across a large yellow wire wound? resistor which isn't on my schematic. In the later boards there capacitor C209 (I think as it's hard to read) in its place. The resistor is marked 1k6 125Vm 1%95. I'm getting a very high resistance reading on this part - well over 1k6. I'm not sure what to replace it with as I can't find anything similar online. Attached Thumbnails

12th Oct 2021, 2:40 pm	#22
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Revox A77 Capstan Issue So far as I can make out, 1.6nF @ 125V 1% capacitor in the service info I'm looking at. Lawrence.

12th Oct 2021, 3:04 pm	#23
cgehansen Pentode Join Date: Sep 2017 Location: London, UK. Posts: 115	Re: Revox A77 Capstan Issue Yes you're right, thanks!

12th Oct 2021, 3:06 pm	#24
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Revox A77 Capstan Issue No problem. Lawrence.

12th Oct 2021, 9:59 pm	#25
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,224	Re: Revox A77 Capstan Issue They are available but not in one offs. You could try 1500pF in parallel with 100pF. __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

13th Oct 2021, 12:09 pm	#26
cgehansen Pentode Join Date: Sep 2017 Location: London, UK. Posts: 115	Re: Revox A77 Capstan Issue Thanks for that . It measures fine so seems to be OK. I've ordered replacement diodes as you suggested and a C206 and will see if that fixes it. When I put the board back in yesterday it worked fine for about 5 mins then speeded up, then stopped completely in 7 1/2 ips so I assume something is failing under load. With the old IC201 and/or IC202 on the board nothing moves at all.

13th Oct 2021, 7:14 pm	#28
cgehansen Pentode Join Date: Sep 2017 Location: London, UK. Posts: 115	Re: Revox A77 Capstan Issue The board being cold doesn’t help. It started working correctly after I desoldered and tested the diodes and C206. They all test OK with the multimeter but maybe one of those components is failing in use. Once it’s stopped working it doesn’t fix itself if left off to cool down. I’ve changed all the electrolytic and tantalum capacitors, all the small transistors and both chips.

14th Oct 2021, 2:42 pm	#29
cgehansen Pentode Join Date: Sep 2017 Location: London, UK. Posts: 115	Re: Revox A77 Capstan Issue So after a couple of false starts the board now seems to be working. It has new diodes and a new C206 and that seems to have done the trick. All the old parts tested OK with my DMM but one or more of them were not working when installed on the board.