Revox A77 switch-on surge. - UK Vintage Radio Repair and Restoration Discussion Forum

Ted Kendall · 2nd Apr 2023, 5:31 pm

A while ago, somebody on the forum came up with a modification to stop the switch-on surge on the Revox A77 from bashing the VU meter pointers against the end-stops. I've searched but can't find the thread. Does anybody remember it?

DMcMahon · 2nd Apr 2023, 7:13 pm

Could it be Post 38 of this linked Thread ?

https://www.google.com/url?client=in...xJeZRLDUaM0lHp

David

Ted Kendall · 2nd Apr 2023, 8:35 pm

Yes, thank you, I think it is. I'll peruse the manual...

...which I've now done. C103 damps the feedback path in the 21V regulator, and I should think the increase in value from 3.3u to 220u changes it from under-damped to over-damped. The only snag I can think of at the moment is that the capstan may take longer to stabilise, as the rail will take longer to come up. I'll try it.

MeerKat · 3rd Apr 2023, 7:41 pm

Ted you need to see post 43 of that thread which describes revision 1 of the mod, an improvement on the original and the one I settled on. Let me know how you get on.

Ted Kendall · 3rd Apr 2023, 9:41 pm

Thanks for pointing this out, Andrew - the final version looks pretty good at first sight. There is a German aftermarket product which tackles this problem by muting outputs and grounding the VU drive. I think your approach is rather more elegant, and cheaper, though less easy to fit. I'll give it a go.

MeerKat · 4th Apr 2023, 12:39 am

No problem Ted, I was pleased with the final result of my musings, only 4 components and once Dx reverse biases then the components become electrically 'invisible' to the regulator circuitry.

As mentioned the only drawback is you still get needle banging if you switch speeds without a pause at 'off', that could be cured by extra components to quickly discharge Cx but that would have needed a small circuit board.

I had considered a temporary short on the VUs but that would have been more complication. I would be interested to see the German product, is there a web link?

Ted Kendall · 4th Apr 2023, 12:44 am

I'm not terribly good with links, but try Googling revox-online. All sorts of goodies there...

2nd Apr 2023, 5:31 pm	#1
Ted Kendall Dekatron Join Date: Jul 2008 Location: Kington, Herefordshire, UK. Posts: 3,464	Revox A77 switch-on surge. A while ago, somebody on the forum came up with a modification to stop the switch-on surge on the Revox A77 from bashing the VU meter pointers against the end-stops. I've searched but can't find the thread. Does anybody remember it?

2nd Apr 2023, 7:13 pm	#2
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,259	Re: Revox A77 switch-on surge. Could it be Post 38 of this linked Thread ? https://www.google.com/url?client=in...xJeZRLDUaM0lHp David

2nd Apr 2023, 8:35 pm	#3
Ted Kendall Dekatron Join Date: Jul 2008 Location: Kington, Herefordshire, UK. Posts: 3,464	Re: Revox A77 switch-on surge. Yes, thank you, I think it is. I'll peruse the manual... ...which I've now done. C103 damps the feedback path in the 21V regulator, and I should think the increase in value from 3.3u to 220u changes it from under-damped to over-damped. The only snag I can think of at the moment is that the capstan may take longer to stabilise, as the rail will take longer to come up. I'll try it. Last edited by Ted Kendall; 2nd Apr 2023 at 8:47 pm.

3rd Apr 2023, 7:41 pm	#4
MeerKat Hexode Join Date: Jan 2016 Location: Ramsgate, Kent, UK. Posts: 287	Re: Revox A77 switch-on surge. Ted you need to see post 43 of that thread which describes revision 1 of the mod, an improvement on the original and the one I settled on. Let me know how you get on. __________________ Andrew Illegitimi non carborundum

3rd Apr 2023, 9:41 pm	#5
Ted Kendall Dekatron Join Date: Jul 2008 Location: Kington, Herefordshire, UK. Posts: 3,464	Re: Revox A77 switch-on surge. Thanks for pointing this out, Andrew - the final version looks pretty good at first sight. There is a German aftermarket product which tackles this problem by muting outputs and grounding the VU drive. I think your approach is rather more elegant, and cheaper, though less easy to fit. I'll give it a go.

4th Apr 2023, 12:39 am	#6
MeerKat Hexode Join Date: Jan 2016 Location: Ramsgate, Kent, UK. Posts: 287	Re: Revox A77 switch-on surge. No problem Ted, I was pleased with the final result of my musings, only 4 components and once Dx reverse biases then the components become electrically 'invisible' to the regulator circuitry. As mentioned the only drawback is you still get needle banging if you switch speeds without a pause at 'off', that could be cured by extra components to quickly discharge Cx but that would have needed a small circuit board. I had considered a temporary short on the VUs but that would have been more complication. I would be interested to see the German product, is there a web link? __________________ Andrew Illegitimi non carborundum

4th Apr 2023, 12:44 am	#7
Ted Kendall Dekatron Join Date: Jul 2008 Location: Kington, Herefordshire, UK. Posts: 3,464	Re: Revox A77 switch-on surge. I'm not terribly good with links, but try Googling revox-online. All sorts of goodies there...