Revox A77 restoration

SeleniumFalcon

Thank you, I can do a better job when it's individual pieces being cleaned. There's only so far a Q-tip can be squeezed between parts when it's all one piece. Here are some additional photos.

SeleniumFalcon

SeleniumFalcon

While I was cleaning up the head assembly I came up with the idea of using a Teflon sleeve on the tension arm to see if it would benefit the sound. My reasoning was that any stationary component that the tape travelled along was a source of friction and could cause changes in tape speed. When these potential changes are low in frequency they are called "wow" and when they are high in pitch they get called "flutter". I suppose these names came about because of the tonal qualities they add to certain instruments. I fairly easily found some suitable Teflon tubing of the correct inside diameter (4.9mm) and placed it on the tensioning bar.



Now I could make some wow and flutter measurements using a Tenma meter. This meter allows for four different measurement standards to be used (NAB, IEC, JIS and DIN) and four sets of measurements (wow, flutter, weighted combination and unweighted combination). The process is pretty simple a tone is first recorded (3.0kHz for NAB, IEC and JIS and 3.15kHz for DIN) on a section of the tape (I usually let the counter go up to 120 revolutions) then the tape is rewound and played back and the meter gives the results in %. For Revox they specify less than 0.08% at 7.5ips and less than 0.1% at 3.75ips using the DIN standard. They don't specify whether this is just for playback (this is done using a standard prerecorded tape instead of combining record and playback) or for the harder record and then playback.

SeleniumFalcon

My idea was to first make the measurements without the Teflon sleeve then add the sleeve and see if it makes any difference. In my opinion the DIN standard is the most rigorous and the JIS is the least difficult (in fact it is used mostly for the cassette medium) and the NAB is somewhere in the middle. Using the NAB standard first without Teflon I measured the wow by itself 0.04% and the flutter as 0.09%, if they are combined figuring in the weighting I got 0.05% (weighting is a compensation for the way wow and flutter is heard as opposed to raw numbers). Every manufacturer uses the weighted set of numbers since they are usually lower. The combined wow and flutter without any weighting (called LIN by Tenma) was 0.10% at 7.5ips. The results when repeated with the Teflon sleeve in place was 0.04% for wow, 0.11% for flutter, 0.06% for weighted wow and flutter combined and 0.12% for unweighted wow and flutter combined. So, slightly higher with the sleeve in place.
Now to try the DIN standard and, at first, found that the numbers were slightly lower when using the Teflon sleeve. Ah ha! But then I repeated the complete process and measured approximately the same overall results as with the NAB standard, the numbers were slightly higher with the sleeve in places. So, in general terms there doesn't seem to be any positive results, as far as the tape goes it would prefer to slide past a nice clean rod of polished metal than a piece of slippery plastic. But, the good news is that at least this machine meets the manufacturer's original specification, not too bad for a 50 year old device being put together by a novice.

pitdogg2

Could the Teflon introduce static electricity to the tape Ken? Or is it a toss up between the two?

SeleniumFalcon

Good question! The tape I'm using is back coated (SM911) which is intended to provide a "grippier" surface for either the capstan or the pinch roller (some decks place the rubber pinch roller on the top of the tape and the metal capstan on the underside and other decks reverse the positioning). I'm not sure if static is being generated or not. What would probably make a difference is if the sleeve was able to spin as the tape went past it. This would act more like a "scrape flutter filter" and reduce the sticking and release effect that can cause flutter.

SeleniumFalcon

Now that I've received parts from Moon Audio and Digikey I can continue installing the new head assembly. Instead of reusing the original shielded cable from the record and playback head I would use the Cardas tonearm cable and add shielding. The phono cartridge clips on the Cardas will fit very nicely on the pins of the heads and eliminate having to solder wires. This also lets me reverse the polarity at the heads much easier. So, the first step was to get the small braided shielding over the Cards wire bundle. I ended up using a wooden skewer with the wires masking taped to the end and then fed the skewer through the expanded shield (think boa constrictor and unlucky prey) dragging the wires along. After a second trial it worked and now the challenge was to find a correct fitting shrink tubing and coax everything through it. I ended up spraying a very fine lubricant down the inside of the shrink tubing and carefully sliding the shield/wire through.

SeleniumFalcon

I miscalculated the necessary length of the connecting assembly and ended up resoldering the trimmed off ends of wires. Working with this very fine wire is a challenge, but perseverance and clean living paid off and I got everything working.

pitdogg2

That tiny wire is like trying to solder hair...
A little too long with the iron, poof wire is gone.
Challenging is an understatement

Jetmaker737

SeleniumFalcon wrote: »

...not too bad for a 50 year old device being put together by a novice.

"Novice", lol.

tonyp063

SeleniumFalcon wrote: »

Now that I've received parts from Moon Audio and Digikey I can continue ...

I love getting a digikey or mouser box. It's better than Christmas!

Again Ken, thanks for this thread. It's very enjoyable.

txcoastal1

Jetmaker737 wrote: »

SeleniumFalcon wrote: »

...not too bad for a 50 year old device being put together by a novice.

"Novice", lol.

That was my reaction too, when I read it...

SeleniumFalcon

I've installed the head block back onto the chassis, not an easy job since the same bolts that hold the head assembly to the frame also hold the capstan shield plate and the capstan motor itself. So, you need to use a small flashlight to make sure all the parts line up and then try and seat each of the bolts. Once that is done you have to fasten four additional bolts through just the head block to the frame. Patience is called for and a wooden skewer helps get things aligned.
Now that I've got the head block installed I'm not sure I like how the Cardas leads look on the two heads. Sort of looks like two small metal cupcakes with birthday candles. Opinions?

JayCee

SeleniumFalcon wrote: »

Now that I've got the head block installed I'm not sure I like how the Cardas leads look on the two heads. Sort of looks like two small metal cupcakes with birthday candles. Opinions?

Funny, didn't occur to me but since you mentioned it...yup. Looked at a previous pic you posted and appeared to be yellow, red, etc. Original colors remind me of my Air Force tech days troubleshooting phone lines used to connect TMUX and modems for mainframe computing. Our comm guys taught me "Christmas tree, bumble bee" for Red/Green (transmit), Yellow/Black (receive).

To your question, original colors were a bit more subdued but, a quick look at the assembled R2R and they don't appear to show during use....? Looks easy enough to trace for future generations looking to repair your shoddy (not!) work. I say rock on and consider it inspired by an MTV segment of "Pimp My Ride".

Looks great!

pitdogg2

please do not try to light the candles...

DaveHo

Can't imagine trying to work with something requiring that kind of precision. Hats off to you!

SeleniumFalcon

The next step is to align the record and playback heads with the tape. This is begun by simply looking closely as the tape travels across the path and see if the tape is riding centered across the surfaces. If not there are front and back adjustment screws that raise and lower the two heads.

SeleniumFalcon

Once this is done using a marker the front surfaces of the heads are coated.

SeleniumFalcon

Then a "junk" tape is used to play about 3 or 4 minutes and the wear pattern is examined. The goal is to get a clear vertical pattern with even spacing. As you can see the top of the pattern is wearing slightly better than the bottom. This means the heads need to be tilted slightly back to even up the pressure.

SeleniumFalcon

The next step is to adjust the playback head's azimuth, working to get the head's narrow gap perpendicular to the edge of the tape.



This is done by playing a tape that has 10kHz recorded on both channels and adjusting the azimuth screw on the head plate for the highest reading.

SeleniumFalcon

The next step is to reestablish the correct playback EQ since a new playback head is being used. I removed the fixed resistor in the high frequency negative feedback and inserted two small metal leads. Then a 10k ohm variable resistor was used to begin measuring the playback frequency response using a calibrated tape. Once the correct EQ was measured after several adjustments on the variable resistor until a final value was found. Then a new value fixed resistor was put into the circuit for each channel.

SeleniumFalcon

Now that I'm confident that the playback head is aligned the next step is to align the record head with the tape and the playback head. To do this a 10kHz signal is recorded on both channels and the output is monitored on an accurate volt meter. The azimuth adjustment screw is turned until a peak reading is observed. This completes the head alignment process and now to do the normal tape bias adjustment.

txcoastal1

Getting so close Ken. Do you have a youtube channel and a way to record playback from this wonderful project. I know it's not the same a being onsite, but would be exciting to watch and listen.

Thanks for sharing your project.

SeleniumFalcon

I'm a big fan of youtube, I subscribe to the battleship New Jersey and a few other blogs (there's one where a guy just walks around Japan with a high resolution camera that I like) as well as some binaural recordings. There's also a few audiophile guys and some repair sites that are interesting also classical and jazz record collectors. Also tank overhaul guys are pretty driven (you need lots of money).

SeleniumFalcon

Once I had calibrated the left and right outputs by using a reference tape with 1kHz tone recorded at 275nW/m (nano Webers per meter) to produce 1.6 volts rms I couldn't wait to hear the results. That was three hours ago, it really sounds great and seems to be getting better sounding as everything settles in.

JayCee

Such an awesome journey. Have you kept track of manhours and what, if you had to guess, it'd cost had you hired it out?

Regardless of cost, I'd wager you'd have a hard time finding anyone that would best your efforts. A DIY job well done, priceless! Wishing you many happy hours of listening!!

SeleniumFalcon

Thank you, I appreciate your comments. I try and work on my projects a couple of hours a day, usually in the morning. I think it's interesting to consider the different countries I've obtained parts from. The two reel platters were ordered from India, the motor run capacitors from Australia, the wooden cabinet from Belgium, the replacement heads also came from there. The new brake bands came from America as well as the coupling capacitors and power supply capacitors. Some transistors were sourced in England and America. I ordered new printed circuit boards from Austria and connecting wires and cables from America. The A77 has become the Dynaco ST70 of tape decks, lots of small companies making parts and circuit designs for it. You can convert the mechanical revolution counter for a digital time counter and add wireless remote control if you wanted. There are small companies that make replacement capstan rollers and will refurbish the heads. The Nagravox company produces a wide assortment of parts in various kits and will offer support if a person runs into trouble.

JayCee

SeleniumFalcon wrote: »

I think it's interesting to consider the different countries I've obtained parts from....The A77 has become the Dynaco ST70 of tape decks, lots of small companies making parts and circuit designs for it.

That's awesome and a great analogy. There's an A77 in striking distance listed on CL...been there for 2 weeks. Not terribly far off in S/N from yours if memory serves. Asking ~$1700. I'd love to have a R2R for nostalgia sake but not really a good fit at this time, or realistically, ever...but has the same allure as a sweet classic car.

Jetmaker737

Very nice indeed. I also applaud the tails out storage of your reel!

SeleniumFalcon

A large number of 2 track tapes were made "tails out". This means the tape has to be placed on the right hand reel table and rewound onto the left hand reel table and then played. This is done to minimize the effects of print through resulting from one layer's magnetic field being induced onto the neighboring layer. All of the recent audiophile 2 track tapes are done this way as well.