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    Building, troubleshooting and testing of these amplifiers should only be
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Intermittent Static on ARC REF 3 Preamp

Vsquared

Member
2014-09-07 8:09 pm
Have you considered the tube sockets?
Yes, I ruled out everything beyond the digital volume circuitry by lifting the L and R output circuits and rerouting the L signal from the digital volume network, to the tube input of the R channel and visa versa. The noise followed the L digital volume network. See image, attached.
 

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  • Swapped L&R channels.jpg
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Vsquared

Member
2014-09-07 8:09 pm
Thanks. :)

Final touch, characterized and selected best matching tubes to the various positions with most emphasis on matching gain to the preamp gain stage, followed by second best on the cathode follower stage. Sounds very nice now.
 

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Vsquared,

My diy Ref3 works fine, I was just wondering how AR did some things. Do you remember what the LT1085 powers and what the output voltage is? Do they draw the -15volts for the CCS from the same source as the voltage that feeds the regulators for the volume control chips. (from the same diodes as the negative heaters)

Thanks in advance.
 

Vsquared

Member
2014-09-07 8:09 pm
Hi Johnmarkp -


Unfortunately I never found the power supply board schematic and can't recollect the details any longer, sorry.


V2
 

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  • Culprit of background noise in L channel.jpg
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  • Assumed Ref 3 Schmeatic (1 channel).gif
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Well folks, this closes the chapter for the culprit of the background noise in the left channel... it turned out to the the Maxim MAX5437 Digital Potentiometers... They were not at all fun to replace, being surface mount devices, but we're all good now. And as for the 2400pF Polystyrene decoupling caps that was seeking, I went with the REL RELCAP-51620, 600v RTE caps from Parts Connexion and had them match all 4 for me. So we're good to go... finally.

I love the mix of technologies from across the ages - valves, DIP and smt....

756984d1557991749-intermittent-static-arc-ref-3-preamp-culprit-background-noise-channel-jpg
 
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I have been replacing the Max 5437 as I've had the identical problem as Vsquared. one of the foil caps has broken in half hence the need for replacement.

The info that I have found on the originals is 0.0024 uf and the same value can be found on a US site called TEDSS but they want $200 to find and ship them which is a little excessive. It's the 0.0024 value that I can't find on other sites and I'm not sure if a higher or lower value would be appropriate. As far as the voltage goes the ones on TEDSS are 160V 2.5% tolerance but I am guessing that these are the same as origonals.

The position in the amp is running in parallel with 4 capacitors you can see them on the attachment marked as C111 and C112

 

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ARC frequently use this in their amps. It's believed that a small value cap in parallel with a large one will yield better impulse response and/or improved high frequency detail, extension, etc. Any decent film cap should work well. The exact value isn't that critical. If you want to get creative you could try a small value teflon cap. Maybe one of the russian teflon caps perhaps.

Good luck, S.
 
With a huge thank you to the participants of this thread, my Ref 3 is back up and running. The problem with mine was exactly the same scratchy/static noise in the left channel as Vsquared experienced. I am capable of doing the work but I am no EE so the diagnosing of a fault is problematic. Replacing the MAX 5437 was quite a challenge and I ended up using a 1mm ball of bluetak behind the chip in order to keep it in place for the initial solder. It was unfortunate that I broke one of the bypass caps but with a bit of advice from S Morley and rayma the problem was solved using some 200V Teflon caps.
I spent quite a long time doing this repair and can appreciate quite how much time and effort Vsquared put into tracking down the problem.
So once again I humbly Thankyou for the existence of this thread without which I would have been screwed.