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Revox A77 MkIV and B77 MkII Cap Recommendations?

S

Segasonic91

Active Member
Hi all,

I will be starting full restorations of these units soon. Are there any recommend BOMs for these units? I have not been able to find any mouser lists so far. I will be ordering from them, so that would be ideal.

Thanks!
 
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No, there is no such list, and moreover it would depend on the version. There are MANY versions of the B77, and even more of the A77, so you will have to find which boards you have and find the part list in the service manual - the good news is that they are very well written, so you will not have difficulties.

My 2 cents on these models:

  • Since it's a restoration project and not a repair, change all the electrolytic and tantal caps, plus the Rifas (replace only with X2 caps) and motor capacitors. But don't change the ceramic, polystyrene and mylar caps: these don't age and there is nothing to gain in replacing them. Plus, they are precision types and replacing them with standard caps will probably cause misalignment problems.

  • Replace the axial capacitors with axial type, not just for aesthetics but because radial capacitors may be higher than the available space. Apart from that, any current model with the same value and at least the same voltage will do. Note that the current electrolytic caps are much smaller than the 50-odd year old original ones but don't worry about that, they have better characteristics. Personally I used Vishay 021 or 031 for the axials, 105°C-rated Panasonic for the radials, and I have had no problem.

  • There are a lot of 3.3 µF tantal caps in those decks. These are hard to find now, but it is not at all necessary to replace them with the same. You can use 3.3 µF electrolytics, or even 4.7 µF electrolytics (the value does not really matter there) which are better quality than the tantal types of the time (which were better than the 'lytics of the time already). Or you can even replace them with Wima MKS2 3.3 µF, which are mylar capacitors so they will be more expensive but better than any polar type (but not important there) and will not age so you won't need to replace them. Now there are other mylar 3.3 µF caps that will do the job too, just make sure when you order that the pin spacing matches the spacing on the PCB.

  • The #1 cause of problems in all the Revoxes I've used so far (and I do mean ALL of them!) is not the caps but those infamous Ruwido trimmers. After all these years, the wiper breaks like glass and they are almost guaranteed to fail as soon as you touch them, or just the card on which they sit.. Change them on sight. Of course, this means you will have to recalibrate you Revox, so you will need proper equipment, tools, calibration tapes, time and experience. You can also measure the value of each trimmer with an ohmmeter before you do anything, and reproduce the same setting, this will not be a proper adjustment but at least you will be in the ballpark.

  • It's not just the electronics that has aged in these decks. You will need to replace the belts (there are 2 of them, for the counter), the bearings, and especially the pinch roller. The latter is especially important to keep the wow & flutter low. You can also consider refilling the capstan oil ring with the proper lubricant - for the B77 it's Isoflex PDP65, for the A77 it depends on the motor you have, check the motor and the service manual. Use only the recommended oil, or the performance will suffer. Check the braking force, the pinch roller pressure and the tape tension according to the service manual.
 
Thank you for that, so very helpful points. Both B77 MKII and A77MKIV need complete rebuilds including pinch roller, bearings etc as you mentioned. It will take a while because parts are expensive, especially from a rip off place like nagravox. Caps will be first, and then the rest as I can afford it. Are there any problematic transistors that should be changed?

Also, I will most certainly be using WIMAs for the under 10uf boards, as I have in my amps.
 
Segasonic91 said:
Are there any problematic transistors that should be changed?
Click to expand...

I would not say that there are transistors that systematically fail, but occasionally I have seen noisy BC108/BC109/BC179 transistors in the input,record amplifier boards. Changing them solved the problem, and it does not require a recalibration. You can replace any or all of them with BC549C (NPN), and BC559C (PNP), which are their modern equivalents. Both Spice simulations and real-life measurements don't show differences even if you replace all the "B" gain versions with "C" versions, which are easier to find nowadays.

Sometimes the TBA231 op amp on the input board fails. The bad news is that it is obsolete, the good news is that you can replace it with a NE5532A which is a better op amp (this is what Revox used in the PR99 which is otherwise the same circuit). However the pinout is completely different, so you will need to tweak the PCB, or buy an adaptor (for example this one - no affiliation or endorsement, this is just an example).
 
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Yes, the NE5532AP is okay. "A" is for the low-noise version, "P" is for the plastic DIP package (other manufacturers use "N" instead of "P", but it's exactly the same). However you will need to connect it to a PCB that was designed for a different chip, which can be messy. I would do it only in case the TBA231 fails. The reference I linked to includes a SMD version of the NE5532 (not visible as it is on the lower side of the tiny PCB), so if you decide to go this route you don't need to buy an addidtional NE5532.
 
So the item from revox-online a drop in replacement? Thank you so much for this information!

Hell, I may even buy more items I need from revox-online. Even with nagravox being local and the horrible AUD-EUR conversion rate and added shipping, I bet revox-online will still be cheaper than nagravox! I need belts, pinch roller, tape guide bearings, the works. I've had them both for over a year and have not started on them yet due to losing my job last year because of covid. The boards are all out of the B77 waiting for caps, though.
 
Thanks jlb2 for those contributions.
I'm trying to fix up an old Mk1 B77....so good to have the info.
You mentioned the Ruwido trimmers....Is there a modern (cheap!) alternative when it comes to replace these please??
(Wipers have fallen from several of them on this deck)
 
Ah, the Ruwido trimmers... They were perfectly good in their time, I wonder if back then there was a way to know that they would age so badly. Yes, you can directly replace them with Piher PT10H for most cards, and PT15H for the big ones on the capstan speed and oscillator cards. This one is strictly equivalent to the original Ruwidos, but a better (more temperature stable) version with cermet track instead of carbon is available too: it is called PTC10H and PTC15H. Of course, more expensive and more difficult to find. Note that many B77 oscillator cards have better trimmers with white ceramic substrate instead of bakelite, these age much better and don't usually fail so you don't need to replace them.

PS: welcome to Audiokarma :)
 
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