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Revox B77 Restoration

337alant

Negatively Biased
I recently bought a very nice example of the Revox B77 its in excellent condition and the heads are excellent with very little wear, the downside of this one is its only the 3-3/4 & 7-1/2 IPS version but these decks still sound excellent at 7.5”ips

The B77 is a consumer deck classic and has RCA phono In and Outputs, the PR99 mk 1 is virtually identical but was meant for professional use with XLR in/outs

These machines are very robust being built on a aluminium chassis and all circuit boards are removable for service so they are relatively easy to work on

IMG_2208 by Alan Towell, on Flickr

B77 by Alan Towell, on Flickr

Before switch on I took the back off and had a look round for and leaking capacitors or split Evox Rifa X2 caps that may burn out on switch on, anyway all was well and the X2 cap on the mains input had been renewed.

So I switched it on and everything worked as it should. I did notice some channel imbalance and the pots and switches were scratch so want cleaning

To get at these pots and switches you have to strip down all of the bottom panel as they are all screwed into the audio interconnection board so you also have to remove all of the audio cards and all interconnecting wires. Tip it to take lots of photo’s so you know how everything goes back together

To cure the scratchiness you clean the gold plated pads on the circuit board and the switch connections, work the switched back and forth multiple times after spraying with Deoxit .
There is another item to clean on this board its the speed slider switch which has multiple connections that get tarnished so need cleaning and lubricating, I used deoxit then fader lube

IMG_2282 by Alan Towell, on Flickr

IMG_2285 by Alan Towell, on Flickr

IMG_2289 by Alan Towell, on Flickr

IMG_2397 by Alan Towell, on Flickr

IMG_2323 by Alan Towell, on Flickr
 
That looks like a very tidy example, though what is going on in IMG_2289, is that corrosion at the bottom right?

I’ve always wanted a nice Revox A or B77, though have so far never justified buying one!
 
Yes Tony that's a bit of corrosion on the board, I scraped away the mask to see how bad it was but the tracks are still in tact so OK I normally just tin them

I paid £450 for this B77 so not a bad price really
 
Recapping the Audio power supply and tape deck control cards


Like all Revox R2Rs they are 30+ years old so capacitors in particular need to be changed

The Revox used all good quality caps Frako & Phillips electrolytics and tantalum caps

The tants and Frakos have a habit of failing short so are best replaced with a modern day equivalent

For the ctape deck control boards I tend to replace all caps with Panasonic FC

And for the audio boards Nichicon FG and Elna silmic2 for all caps in the signal path

Where possible I replace old ceramics with COG types

You can replace all Axial type caps with Radial ones

Make note of the polarity of all the caps before changing and I print off a copy of the Schematic and the board layout before I start

Another part worth changing is all of the Trim Pots as they are very tarnished and some just fall to pieces and can make the calibration process frustrating so best replace them with good quality Piher type. Tip when changing a trim pot do them one by one and check the resistance of the old one and set the replacement one to the same value so you are somewhere near with all settings when you do the calibration

A good source for Revox parts is https://www.revox-online.de/

Reproduce Amplifier Board before and after
recapped with Nichicon Fine Gold and Elna Silmic2 for all caps in the signal path, also all trim pots replaced

IMG_2358 by Alan Towell, on Flickr

IMG_2373 by Alan Towell, on Flickr

Record Amplifier board

Again Nichicon FG and Elna silmic 2 caps and trimpots replaced

IMG_2294 by Alan Towell, on Flickr

IMG_2313 by Alan Towell, on Flickr
 
I bought a (supposedly) refurbished 7.5ips/15ips 2-track B77 on eBay a few years ago for £550 I think, but I returned it for a refund as it had a few (probably minor) issues that made me a bit jittery as I have no experience or ability to service kit like this and it's too heavy and fragile (and expensive!) to send back and forth to repair shops, and the learning DIY curve would have been far too steep for my dad and I. I do sometimes regret not keeping it as it made much better sounding recordings than the Teac 7.5ips/15ips 4-track A3440 that my uncle kindly gifted me over a decade ago, but on the other hand I'm relieved that there's one less piece of hifi equipment in the house requiring regular upkeep! Every time I see a thread like this I start to get the R2R bug again....I must stop looking at them!
 
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Yeah its really hard to find a R2R that doesn't need some sort of attention these days
The Tascam BR20 where one of the last models made so they tend to be a good buy if you can get one at a reasonable price
The TEAC 3440 is a 4 track all in the same direction machine that is really mean for musicians

Alan
 
Tape Deck Control Board
This board came with an additional DIY board that had been patched into it
Turns out this add on board is to change the momentary Pause function to a latching pause function
I didnt like this bodge up so I disconnected it.
Turns out you can actually get a plug in board that does the same thing from Revox online so I have ordered one of them, looks a much more professional option
https://www.revox-online-shop.com/z...pausenschaltung-fuer-revox-b77-pause-77-r0023

IMG_2246 by Alan Towell, on Flickr
IMG_2247 by Alan Towell, on Flickr
IMG_2253 by Alan Towell, on Flickr
 
The Power supply
Changed out both bridge rectifiers for 6A versions
Chnged main power supply Caps for 2200uf Vishay caps
The reg on this board is only a 7820 reg wonder if this would be better with a LM 317

IMG_2259 by Alan Towell, on Flickr

Alan
 
Ok I will look into than thanks
I cant find a data sheet or pin out for the TBA231A though ?

I found one and had a look but don't now have the link.. google it.. It's a crude mid 70's dual op amp in a 14 pin package but specs are pretty crap by any modern standards.

It was only really in the late 80's that decent op amps became available and later still that we got proper PNP transistors on the same substrate as the NPN ones...
 
Thanks Jez I am going to park that mod until I have it fully working and calibrated as I would like to hear the difference this mod makes without any other changes
I have some LM4562 op-amps which I like very much when I have tried them previously so I will give them a go when I work out the pin out merge between the 2 op-amps

Alan
 
Thanks Jez I am going to park that mod until I have it fully working and calibrated as I would like to hear the difference this mod makes without any other changes
I have some LM4562 op-amps which I like very much when I have tried them previously so I will give them a go when I work out the pin out merge between the 2 op-amps

Alan

LM4562 should be just fine:)
All those 4558's are asking to go as well if they are in the signal path! I once thought of updating the electronics in my PR99 MkIII but never got round to it...
 
Ah Jez those PR99 Mk3 are complete rubbish, if you give it to me I will dispose of it for you so you don't get charged at the skip yard :rolleyes::D
 


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