Analog Audio Restorations
The Rust-Glued-On-Plastic Technololgy

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ReVox PR99 MKII Professional 1/4 Inch Tape Machine with NAB Hubs

The Studer ReVox PR99 MKII is one of the finest 1/4 inch tape machines ever produced. They had banks of them in FM radio stations. Mine came from a FM radio station and has a number of broken switches and assorted other failure items lurking. They are typically rode-hard-and-put-away-wet in commercial duty.

Professional machines started out with 1/4 inch tape then transitioned to 1/2 inch, 1 inch and finally 2 inch before the digital age arrived. 2 inch tape supported up to 24 tracks. Recording studios used the more ruggedized Ampex and Studer 4-24 track models. Most of the Beatles albums were mastered on Studer machines. The Beatle's Studer J37 is shown in the left image below. The right image is the final 24 channel, 2 inch tape masterpiece prior to the digital invasion.

Studer J37Revox PR99 MKIIStuder

The PR99 MKII operates in one direction only with 2 tracks.  Frequency response and signal-to-noise is exemplary.

7 1/2 ips    30 - 20 kHz +2/-3dB
                 50 - 15 kHz +-1.5dB

15  ips      30 - 22 kHz +2/-3dB
                50 - 18 kHz +-1.5dB

A complete electronic tear down was required to assess damages. Tear down of the electronics panel and boards is for skilled lab techs only. Don't even think about learning on the job with this puppy. Take lots of hi resolution pictures before the disassembly beast gets fully engaged. The plug-in boards are keyed and well identified in the Service manual. All the point-to-point connections are not well documented and the SYNK board has 2 connecters keyed exactly the same - beware.

PR99 Tear Down

Problems Found and Fixed

Quality Note: All toggle switches and rotary switches wipe the PC board pads with gold wipers for optimim audio signal pathing. I've never seen this before - dieses ist sehr gut !

Professional Calibration Capability

The available calibrations are shown in the following table.
If you have a, MRL reference tape for 7.5 and 15 ips, frequency counter and a calibrated audio signal generator, they are pretty straight forward.

Tach Head AdjustmentVU Meter and Peak LED IndicatorReproduce LevelBias Adjust
Tape Speed CalibrationLine Output LevelOSC Frequency and RF LevelsSYNC Playback
End-Of-Tape SwitchFrequency Response Equalization (3 separate)Bias TrapPinch Roller Swing Arm and Solenoid Timing & Force                                          
Input Circuit Calibration (0VU-+4db=0.775 VRMS)Reproducing Head Azimuth (Fine & Coarse)Record Head Azimuth (Fine and Course)                             Record & Playback Head Height

The beautiful Italian made NAB Hubs arrived with NOS ReVox inserts. Next step is a precision NAB calibration. The machine is back together, all heads and tape guides cleaned and demagnetized.

NAB Hubs

Rear and head block view after all restoration was completed is shown below.

  W7CPA Revox Rear       PR99 Head Block

Calibration  Results

I ordered a calibration tape from Magnetic Reference Labs. These high quality tapes are very expensive these days but I'm glad they are still in business. I've chosen to standardize Ampex 456 Grand Master tapes due to reputation and eBay availability. The  Ampex 456 has a fluxivity requirement of around 340 nWb/m so I selected a similar MRL tape.

7.5 and 15 ips Test Tones Provided (duration in seconds)
1 kHz, 31 (75) s 10 kHz, 31 (75) s 50 Hz, 7 (19) s 100 Hz, 7 (19) s 8 kHz, 22 (54) s 16 kHz 22 (54) s
Fluxivity = 355 nWb/m

Note: the MRL calibration tape is recorded -10db at 7.5 ips and 0db at 15 ips. I assumed they we both at 0db at first and could not adjust properly. Aways read the specs and manuals when all else fails!

15 ips  -  40 to 20k +-1 db
7.5 ips -  50 to 16k -2db +3 db (an odd peak at 14k, head wear?)

The tape head wear was evident at 7.5 ips on CH2 only but not a large enough issue for home play. All I really care about is 15 ips so all is well.
Tape noise is almost non existent - very impressive without any NR equipment.

Calibrated, Installed in the Rack  and Ready to Rock and Roll

PR99 Final

The "Pancake"

I decided to try a new tape from ATR Magnetics. I ordered their Ampex 456-like studio master tape in "pancake" form. Pancake = tape and hub only. One removes the 3 screws from the reel and carefully installs new 2,500 feet of tape quickly. The top screw is a Phillips, the bottom nut is a round thing with barely visible slots for a flat head screwdriver. The pancake is wrapped pretty tight so it doesn't go "blat" and spray 2,500 feet of tape in the air.

W7CPA Pancake 1        W7CPA Pancake 2


The best in-action demos of its functionality are found on You Tube Demo 1, You Tube Demo 2 and You Tube Demo3.
This one for the MKIII model is a hoot -  REVOX GIRL mit too much Rhine wine.
I hope to get my Scotch Irish wife to do a REVOX GIRL - US version.

AKAI CR-81D 8 Track Cartridge Machine

If  you are old enough, you'll remember the ubiquitous 8 Track cartridges from the 60s and 70s. I got a wild hair to find and restore some high end machines to see how they sound on a modern era sound system. The AKAI and Pioneer turned out suprising well and dubbing Stevie Ray Vaughan and Steppen Wolf  CDs to NOS tape was impressive! The major downside is that it is impossible to 100% remove the slight cross talk. The AKAI has a few more khz of top end frequency response and they both are very nice.

The AKAI required only cleaning, lubricating and cross-talk head adjustment. Dubbing from a CD produced amazing results with very low tape noise on the NOS TDK tapes I found on eBay. Its meters do not match exactly and have different ballistics but are just fine for home fun.

W7CPA AKAI High End 8 Track

Pioneer H-R100 8 Track Cartridge Machine

The Pioneer required cleaning, lubricating and one wheat lamp replacement. I am giving the unit to friend for his birthday along with a true geezer rock and roll 8 track collection.
W7CPA AKAI High End 8 Track

8 Track Tape Repair Notes

The 8 Track design is pretty weak and much is posted on the Internet about this fine kettle of fish. The tape is pulled from the center spool, pressed against the heads with cartridge-resident pads and a cartridge-resident rubber or plastic pinch roller that pulls the tape. Collins championed "carts" for radio station commercials using a similar design. If you attempt to use 8 Tracks today that you buy on eBay or junk stores, I offer the following notes:

Do not play a tape without carefully inspecting the following !!!
Repair Notes