Servicing an A77
No secrets given away - these are ReelAudios' standard servicing/setup processes and details
(some technical bits are missing - else you'll try to undertake it yourself - and it'll end in tears)
Take the Remote plug out - place the machine face down on the bench - then undo the 4 screws on the rear of the cabinet.
The tricky bit - pull the case off - always a sticky problem on an old machine that has been sitting in a loft for many years.
Turn the machine right side up - Head cover off - undo the 2 reel cover screws and prize the 3 poppers at the rear - cover off.
Carefully, pull off all the knobs another tricky bit without especially snapping bits of the Mains Rotary knob.
Unddo the 2 control panel screws - loosen the 3 poppers at the front and remove the cover.
A Naked A77!.
Note the Circuit Board retaining plate has 2 screws - the left hand screw as shown has a star washer and the right hand screw has none.
A Revox warranty trick that followed on with the B77 and PR99 models.
Simple but effective as the alloy retaining plate shows marks where the star washer has been put on the wrong side/ommitted
or duplicated on the other side.
Get the air compressor and blow the machine out fully - so much dust - bits of old edited tape -
fluff - spongey bits from
the alloy retaining plate - messy job but fully worthwhile.
Clean the outside of the Motor bells with a rag - no fluids - else you'll take the white print off!.
An essential first replacement job -
note we have not powered the machine on yet! -
There are 3 x 0.47uf epoxy resin 150Volt encapsulated Capacitors on the main motor switching board.
These have a habit of cracking up after so many years and catching FIRE! -
They short out once they catch fire and their associated resistors burn out too.
We replace these with modern 400Volt components - and also replace the similarly
suspect Cap on the motor speed control board too.
Originally, the RH tape guide was a fixed pillar made from solid Alloy - (it is an alloy hollow part on the MKI A77).
This collects old tape and needs cleaning regularly during use.
We replace this with a new Roller bearing which improves tape transport immensely.
At the same time the LH Roller bearing is removed cleaned out - checked for wear - then re-oiled.
The LH tape tension guide is also another collector of old tape - sometimes it is also worn down with a large groove.
This needs cleaning and turning so that a new fresh face is presented to the tape path.
Time to power up the machine
(after checking the Mains Voltage Switch is set correctly on the rear of the machine).
Watch the main motor - it should hunt and then stabilise to a fixed speed.
Turn the Speed change switch to 'off' wait a moment then switch over to the slower speed and the main motor will re-hunt and go slower.
If not investigate the speed control sensor close to the main motor and the motor speed control circuit board.
Whilst these mechanical checks are being made, output voltages are checked to ensure that the power supply
board is delivering the correct outputs.
The bulbs and their holders are checked and replaced as necessary.
There are 2 hidden bulbs underneath the Meters to indicate 'Record' which also need checking.
The Tape End Autostop sensor on the A77 is an ORP device relying on the associated bulb to give light
when the leader tape is passing (or when the tape has wound off the trailing spool).
These are very tempramental and need checking carefully along with their associated 2 transistor switching
circuit on the motor control relay board.
If all is well - remove the LH and RH spool platters, remove the brake steels,
remove the brake spools and their associated shims. Do not mix the LH and RH components.
Run the machine in FF /REW whilst dribbling Isopropyl alcohol down the motor shafts from a handy well on each motor
- this takes at least 15 minutes - you should notice old grease changing the colour of the ISO Prop in the wells.
Regularly cleaning out the dirty wells and filling up with fresh ISO Prop will eventually maintain
a clean liquid in the wells.
Clean off the ISO Prop and replace with fine machine oil. Run the machine in FF/REW again for another 10 minutes
allowing the machine oil to dribble down into the bearings.
Clean off the excess machine oil. Now you can run the spools up to speed in REW or FF - press the stop button.
The motors should continue to run freely for at least 90 seconds. If not - repeat the process again.
Re-install the brake spools with the freshly cleaned canvasses on, replace the main counter belt and then reinstall
the cleaned brake steels.
Adjust the brake tensions.
With all the transport cleaned and working nicely, it's time to inspect and clean the heads thoroughly. Use a magnifying glass to ensure
all tape deposits are removed. The slightest spec of old stuck on tape will interfere with performance.
Clean off the Capstan Motor shaft - clean the Pinch Roller thoroughly back to true black rubber.
If you have a Green Pinch Roller - TAKE CARE - these are not Revox true parts and will melt very easily on
contact with ISO Prop fumes / liquid. We do not recommend their use.
This section is for information only.
You will need aligment tapes, frequency counter, oscilator, wow/flutter meter, DB meters
and a host of other specialist equipment. We have kept it short on description as a multitude of checks are necessary
prior to undertaking the electronic setup.
Play an Azimuth Tape and re-align the Play head. Use a blank tape and record some music - adjust the alignment of the
record head to give maximum top on both channels. The erase head needs no adjustment. Re-seal the head adjustment
screws with red Humbrol Enamel paint.
Now we get to the electronic adjustments. Please be aware that we assume all boards are working properly.
You will notice that the old style of large trim pots are suspect to breaking very easily - it is highly recommended
to replace them all as necessary with modern equivelents.
Using a mono BBC test tape adjust the level of each tape output board. At the same time check and adjust the speed
of the machine using a frequency counter (10KHz recommended frequency).
Using an oscilator, record the output and adjust the Bias pots on both speeds/both channels to give maximum
output for the tape in use. Once set - adjust each meter (on source) to 0DB. Adjust the record level pots to give a
consistant +0.5DB on Record.
Play some dynamic music and adjust the EQ pots for each channel and speed to suit the ear. If all of these processes
give poor performance - then either the wear on the heads is too great - or try de-magging the heads -
and start the setup process again.
It may be necessary to re-glue or replace some of the plastic parts - ReelAudio has a wealth of spares available from
over 50 machines which have been broken down just for spares. Having this resource is essential to ensure that we
are able to repair a machine quickly and efficiently at the time of the service (usually with the client in attendance
and being directly involved with all phases of the repair - especially with the levels of EQ adjustment).
Spooling tensions, brake tensions and a host of other checks are also necessary within the repair/setup too.
The long stem potentiometers employed in the A77 are also suspect to replacement - as are the
contacts within the switches to the motherboard.
We hope this short repair synopsis is useful. On average, from start to finish - it can take at least 3 hours
(usually longer) to bring a machine back from the dead to full superb functionality and performance.
The aim is - our clients always leave with a smile - they are full of coffee/information - enjoyed lots of audio chat -
and their prized machine is taken home, now working beautifully.
ReelAudio ONLY recommends the use of NEW Tape for all recordings on refurbished machines. Enjoy your old tapes - clean all
the transport parts after playing each and every old tape!
Please throw those old sticky tapes away if you want new recordings!
DO NOT TRY TO RE-USE THEM! Their use will instantly clog up the tape path components with a shedded sticky mess!
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is stocking new MRL 1/4" Tape @ very competative pricing - give him a call
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