Analog Audio Restorations
The Rust-Glued-On-Plastic Technololgy
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ReVox PR99 MKII Professional 1/4 Inch Tape Machine with NAB HubsThe
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.
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
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
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.
Problems Found and Fixed
- Both VU meter lamps burned out - new ones installed
- Non US standard AC power cord missing - found a NOS on eBay
- CH2 and CH1 SYNC switches are unrepairable - too much stinking broken plastic, NOS stock found in the UK and replaced
variable speed push button has been removed and a permanent OFF state
via solder jumpers has been installed - what idiots they must have had
at that station - replaced
- Both READY/SAFE toggle handles cut off by some
num-nuts at the station, still can be worked but looks stupid, NOS
found in the Germany - replaced
- One UNCAL push button broken, ordered NOS from Switzerland, replaced
four tape tensioner poles worn, replaced to 2 outer guides, pair
under head are riveted but small vise grips and milspec duct tape
let me rotate without scratching.
heads (record and playback) are worn a bit. I'll decide to replace
after all back together and tested - super expenive, might be overkill
for home toy use. It's also possible to get them refurbished (lapped)
factory assembly error resulted in shorting the two 2400 uf caps on the
Output Board. This burned two 10 ohm resistors (R5 & R6), no other
damage was done due to quality ReVox design. When you remove any boards, always
perform a thorough visual inspection. This short happened as a result
of simply unplugging the board so it was very, very close to a short
from the factory.See these end to end caps in the picture above on the left side.
Reproducing Head (Playback) is way out of alignment and there was a
missing spring on the pinch roller swing arm. Some unskilled person
must have hosed this up at the FM station. The head wear is
unfortunately is little trapezoidal do to this adjustment error. The
red locktite was still intact so someone thought they did a good job
- Push Button spring pads worn out on STOP and PLAY, too hard to push - replaced spring pad assembly and applied DEOXIT to pads
Remote Control dead, only PAUSE worked (B77 and PR99 versions are
totally compatible) - Phillips 47uf electrolitic capacitor had morphed
into a 0.2 ohm resistor - easy fix.
many calibration operations (the MPU repeat function was very handy here), the supply reel brake is now weak. I
ordered 2 new brake hubs and tension springs - this is starting to get
- The tape head cover had holes in it for a non standard splice block, replaced
- The first rotary tape guide with a bearing had hair and dirt on the shims. I cleaned and lubricated the brass shims lightly.
all the calibration runs, the optical tape position sensor started
binding during high speed fast forward, throwing off the digital
position indication. The tape block must be removed to remove the
assembly. I simply pulled on the drum to remove the shaft from the
optical sensor disk - a press fit. I cleaned and lubricated the entire
shaft with synthetic lubricant. Push the shaft back on the optical disk
by pushing it through the brass bearing, now clean the end of the shaft of
all oil, then push on the optical disk, leaving a very slight
up-down gap. Now when I spin it with a Dremel tool - no binding -
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 CapabilityThe available calibrations are shown in the following table.
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 Adjustment||VU Meter and Peak LED Indicator||Reproduce Level||Bias Adjust|
|Tape Speed Calibration||Line Output Level||OSC Frequency and RF Levels||SYNC Playback|
|End-Of-Tape Switch||Frequency Response Equalization (3 separate)||Bias Trap||Pinch 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|
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.
Rear and head block view after all restoration was completed is shown below.
Calibration ResultsI 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
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
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?)
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
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.
DemosThe 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.
CR-81D 8 Track Cartridge MachineIf
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.
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
Pioneer H-R100 8 Track Cartridge MachineThe
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.
8 Track Tape Repair NotesThe
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 !!!
- Is the pinch roller a melted blob? Some early models used un-curred rubber - a huge mess potential lurks.
the pads still exist? Felt pads on metal strips eare easy to
inspect and replace. The form types usually crumble when pressed.
- Is the metal foil splice still intact? 80% of the ones I tried always failed at the end of the track.
- Most NOS tapes need repair work, don'be fooled.
- Replacement pads, (foam or felt) are available on eBay. - amazing!
- Replacement metalic splices are available on eBay and use can cut your own out of foil backed duct tape.
better cartidges can be opened with little tabs but some must be broken
open and glued back together with hobby store plastic glue - worked
- If the tape is binding, you will polish (destroy) a tape
spot and it sounds like crap. You have to take the cartridge apart and
clean/lightly lubricate the pinch roller and reel posts. The original
lube dries up and binds. Press and spin the pinch roller with clean
hands when done to very the tape pulls easily.