Revox A77 restoration

[Deleted User]

I recently received a newly manufactured set of brake bands from a company called Studio-Sonics in Schaumburg, Illinois that seem to be well made. I've already ordered a rebuild kit from Nagravox in Australia but with the reduced air freight it might take a while to arrive. I can use the kit to refurbish deck number two. Now I have to add some tape to the bands to eliminate any unwanted noise from braking.

[Deleted User]

I received two 626-ZZ bearings and can complete the tape guides, one in front of the heads and the other after the capstan/roller. Now both will be rotational which should reduce tape wear.

[Deleted User]

Several years ago I bought an original unused capstan roller that was still in the Revox part package. After cleaning up the roller arm, pressure plate, arm shaft and other parts the capstan roller assembly can be installed. One of the parts was fairly scratched up so I decided to paint it.

[Deleted User]

The A77 allows the owner to fairly easily edit tapes. A small lever is moved to a click position and the tape comes close enough to the playback head to be heard, but not close enough to touch the spinning capstan. That way the tape can be moved left and right until just the right spot is reached and edited with non-magnetic scissors.

[Deleted User]

Here's a photo with everything except the brake bands installed.

[Deleted User]

Here's a photo of the finished input/output rear support bar. This seemed to take forever to get in shape. The RCA panel mounts, DIN connector and speaker connectors all used pop rivets for attachment and had to be drilled out. Never an easy chore. After plenty of Q-tips and Flitz the bar started looking better, I'm going to scrap the original RCA connectors and try and make some of my own using phenolic as a base material. Once this bar is complete I can connect AC to the deck and actually see if it works. The A77 uses a safety interlock which prevents anyone turning on the deck with the cabinet removed. The interlock pins are part of this bar's connections as well as the remote control socket which requires a "dummy" plug to allow the transport to function.

[Deleted User]

After I installed the input/output plate, bypassed the safety interlock (don't do this at home, kids) and connected remote connectors 1 and 2 to allow the transport to function. Now the moment of truth, switching on the power and threading a tape. Three problems showed themselves: 1. the deck wouldn't stay in the play mode 2. the deck wouldn't rewind or fast forward and 3. the tape revolution counter wouldn't advance past 30. Other than that at least the capstan motor spun. The problem turned out to be the motor on/off switch which I had installed by looking at another deck to see its wiring connections. This switch was different and I changed the wires so the motors were actually getting AC voltage instead of being switched off. The revolution counter needs to be repaired or replaced, the only suitable replacement needs a new toothed belt which are very common on eBay.
The next step is to set the capstan roller's pressure and make sure the pressure plate had the correct gap. The pressure should be around 1.2kgm (2.6lbs) and the gap should be around .5mm. I placed a 2.5lb weight on a string attached to the roller's axis and adjusted the spring compression until the roller separated from the capstan shaft and the tape stopped. Then I had to adjust the solenoid's position to get the spring plate gap at .5mm. This changed the pressure so I had to go back to the pressure adjust screw and reset the pressure. It took a few back and forth adjustments but it went very smoothly.
Now to check the capstan motor's rotation speed.

schwarcw

WOW! A very impressive labor of love. Ken's technical skills are second to none when it comes to R2R.

Great photos too!

txcoastal1

Getting so close ...exciting!!

[Deleted User]

Adjusting the capstan rotation speed is done in two parts. A frequency counter is connected to the motor's tachometer with a 10 second gate setting. Then an inductor that is part of the discriminator circuit is adjusted until a frequency of 1600hz is reached. Then a +21vdc is applied to the speed regulator which switches the playing speed from 7.5ips to 3.75ips and the frequency should drop to 800hz, if not a potentiometer is used to reach that goal.

[Deleted User]

Now that the transport seems to be working correctly I can take a listen. I'll feed the output from the playback head directly into an external tape preamp, the De Havilland 222. From this I'll go to the headphone amp and to my old Polk headphones. After repositioning the playback head cables out of the front of the transport away from the motors instead of from behind the noise level drops dramatically. Things sound very good, plenty of clarity and immediacy and some really amazing details from an older tape. There's some chatter coming from the capstan motor that I'll check into. I might have damaged the bearing during installing it. Tomorrow.

[Deleted User]

Here's a photo of the direct head connections.

JayCee

KennethSwauger wrote: »

After repositioning the playback head cables out of the front of the transport away from the motors instead of from behind the noise level drops dramatically. Things sound very good, plenty of clarity and immediacy and some really amazing details from an older tape.

....I might have damaged the bearing during installing it.

Can't remember if you answered this earlier. Is rerouting the cables a documented thing with enthusiasts or did you decide to do it? I wonder how enlightened the engineers of the day were to cable's matter, etc, in assumptions/experience back then.

Fingers crossed that the bearings aren't toast but, based on your knowledge/experience/posts, I think you know what you speak of! Usually, IME, the gut knows.

[Deleted User]

Quite a few tape enthusiasts opt for external electronics and I've seen the cables brought right across the front of the deck and up the chassis. I've also seen cables attached to the circuit boards and routed out the rear of the lower assembly. The problem is the close proximity to three motors and induced humming. The stock cables that come with the A77 seem fairly standard for the era. In fact the output cables that go from the output amplifier up to the RCA outputs between the motors isn't even shielded. Various shields are placed on the mother board and around the capstan motor and the playback head has the "trap door" shielding, but not sure why not use shielded cables throughout.
The finest example of the quality of cables was done by Ampex on the 351. Since the 351 uses large, high impedance heads >1k the capacitance of the cable leading from the head assembly to the first stage of gain has to be very low. Ampex built their own cables to get the lowest capacitance possible. Then they added a series of shield layers one overlapping the other in such a way that no hum field is picked up. If I had a wish I would ask for a 300' reel of that cable, it would make some interesting RCA cables. A bit bulky, but good sounding.

JayCee

Very interesting. Thanks for the detailed response.

I may be repeating something I've said before, so excuse the forgetfulness, but I remember my first experience w/R2R and multi-track.

I'd just seen Dire Straights in concert. A few weeks later I was walking down the hall in the military audio club I worked in and heard Mark Knopfler singing Sultan's of Swing. Just him. And, it was only him. Walked into the room and just a bunch of company reps. "Where the hell is Knopfler", said I. They told me he'd stopped by and they recorded him. A bit later, I was let in on the joke. They had a multi-track recording and were able to turn tracks on/off. Pretty wild to hear him, the drums or guitar all by their lonesome. Not sure if those kinds of recordings/equipment are common to mere mortals? The Mainz Kastel Audio Club was packed w/every brand and item conceivable. I truly was a high school kid in an audio candy store.

So, that's my R2R experience, and the spark that prompted following your thread.

SIHAB

Amazing work KS!

[Deleted User]

Here is a photo of a record head and a playback head both for 4 track (also called quarter track) made by AM Belgium. They produce extremely high quality replacement heads for Revox and Studer.

[Deleted User]

I replaced the bearing in the capstan motor with a very low friction version and the noise I was hearing went away. I then rechecked the tachometer reading and the 7.5ips was fine, but the 3.75 was off. After making adjustments to the trim pot I still couldn't get it to 800hz. I checked the +21vdc that switch between the two speeds and the voltage was +21.93vdc so I adjusted the power supply voltage to +21.0vdc and remeasured the 3.75ips reading. Now I was able to readjust the trim pot to exactly 800hz. But then the 7.5ips was too low, so back to adjusting that. Changing the motor's bearings caused the rotation to change, which is reflected by the tachometer readings. There is a more recent version of the speed control board that uses ICs that eliminates the 800hz trim pot.

mhardy6647

ooh, I have a pair of those "old Polk headphones" from when they were half-price-saled (Polk F&F sale).

Are they old?

PS That's a sexy tape EQ/preamp!

[Deleted User]

I like the sound and comfort of the Polk headphones, the only problem is the headband assembly began to come apart on both sides.
The deHavilland 222 is a really versatile unit, there's an all tube "Ampex" input and a hybrid solid state/tube input when an additional 10dB is needed with low output heads. It can switch between IEC and NAB playback and has separate treble adjustments for each. It uses carbon resistors and Russian vitamin Q capacitors for a kind of melted butter-like sound. Designed by Kara Chaffee who couldn't be a nicer more helpful person who certainly knows how tube circuits work. Plus it's very reasonably priced and well built. A person could use this with the built in direct head connections of a Teac 6010 and have an amazing tape playback system.

[Deleted User]

@JayCee your story reminded me of the US Army's tape club when I was stationed on Okinawa. As a way to raise money for the NCO's charity they offered a tape dubbing service. They had this really huge catalogue of lots of music of all kinds with tear out labels that you could buy. Then all you had to do was buy a blank open reel tape from them (BASF tape) and select which recordings you wanted and come back in a week and they would have dubbed the selections onto your tape at no additional charge.
There were small shops, off base, that would make copies of phonograph albums. They had dubbing machines that would "cut" a new record for about half price of what the record would sell for in the PX. Not a great deal of copyright protection in that part of the world in those days. I don't think I ever bought any records that way they didn't sound as good as the "store bought" versions. But, I do admit to going there to see what was new, they were a couple of months ahead of the PX in getting new stuff.

JayCee

That's cool. Didn't come across anything like that. My first Jackson Browne and Jethro Tull albums Hold Out and Songs from the Wood, respectively, were loaners from the Lindsey Air Station library and they ended up on TDK SA CrO2 tapes. Loved my TDK! Even though I didn't "pay" for those copies they were gateway drugs to future purchases I may never have made if not for finding them. Flash forward....good old Far East...I was stationed in Korea and the electronics market in Seoul, reportedly the largest in the Pacific Rim, had tons of bootlegged stuff.

JayCee

First tape deck I owned was a cheap Technics 2 head deck, RM-215???...beautiful dancing FL meters. I worked for Matsushita and purchased it on sale; wrote myself an additional "vendor's discount". My best friend was a spoiled only child and had a Nak Dragon. He'd record tapes for me from his VAST LP collection and is responsible for the majority of my music background. Closest I came to a 3 head wunderkindt was in the mid 90's. Purchased this Sony deck and it currently resides in the closet at the ready in case I have a cassette emergency. Will never part with it. The NAD pre, keeping it company, is one of my favorite budget pres from my first days on CP and is respectable in it's own right. It'll keep the Sony company and also a backup for other emergencies....or building another rig I can't fathom right now.



Edit....found the Technics deck. Bit the dust many years ago but we had quite the love affair...RS-M216:

[Deleted User]

The RCA connections on this deck were beyond restoration, too tarnished to salvage. So I used some phenolic sheets and new RCA jacks from Parts Express and made some new ones. One of the original is on the lower left.

[Deleted User]

This is with the new RCA assemblies installed on the deck's chassis subassembly.

JayCee

KennethSwauger wrote: »

...phenolic sheets...

Had to look it up...
https://www.merriam-webster.com/dictionary/phenolic
phenolic noun

Definition of phenolic (Entry 2 of 2)
1 : a usually thermosetting resin or plastic made by condensation of a phenol with an aldehyde and used especially for molding and insulating and in coatings and adhesives

— called also phenolic resin

[Deleted User]

Early terminal strips and sockets were made of Bakelite which is a type of phenolic material. The first printed circuit boards, in the '60's, were a paper derived phenolic. Today chances are if you see a tan PCB it will be a phenolic type.

oldrocker

Mad skills you have....

[Deleted User]

And tweeters.

guyincognito

This is so far outside of my "realm of knowledge" but I love watching your progress on this restoration. Keep up the good work!