COMMODORE DATASSETTE RECORDER REPAIRS
                  Latest updates and corrections  5-18-2012

     With such a simple design, there is not a lot to go wrong with the CBM datassette 
recorder. It is essentially an audio recorder with special amplifiers to allow digital 
input and output signals that a computer can communicate with. Commodore outsourced the 
construction of its datassettes and there are several different physical and electrical 
designs, but all conform to the same standards for tape handling and computer interface 
characteristics. The CBM 1531 used with the Plus/4 & C16 computers is essentially the 
same as the more common C2N datassette but the former has a mini-DIN connector instead 
of the edge connector used with a C64.
     The most common problems that I've encountered when working with these recorders 
are 1. dirty R/P (record/play) head, 2. dirty capstan and pinch roller, 3. slipping 
belts, 4. intermittent record/play switch. I've heard a lot about alignment issues but 
in my experience, like the 1541 disk drive, actual alignment problems are rare. 
     Cleaning should be done on a regular basis if the recorder is used a lot. The tape 
door doesn't open far enough to get a good look at the heads (R/P and Erase) or the 
capstan and pinch roller so I use a dental mirror. If the heads and rollers are very 
dirty, I elect to remove the chassis from the case to allow full access. Tape oxide 
residue (not dirt) is what contaminates those surfaces, and solvent on a Q-tip is all 
that's needed for a good cleaning. Alcohol works fine, but if other, stronger solvents 
are used, be aware that many of them "eat" plastic, so you must be careful how you use 
it. Rub across the heads to remove any residue and be sure to clean the capstan (the 
silver pin that pulls the tape through the machine in play and record) and the black 
rubber pinch roller (that presses the tape onto the capstan) of tape oxide residue. If 
heavily contaminated, the roller can be "scrubbed" with an emery (fingernail) board. 
The roller should have a dull appearance. If it's shiny, the surface is probably glazed 
from use and must be resurfaced (emery board or sandpaper) or replaced. 
     The main rubber belt should grip well so the mechanism doesn't slip (motor turns 
but mechanism doesn't move). Belts can be cleaned with solvent to improve their grip 
but if very loose, they must be replaced. A quick check I use to test the main belt for 
grip is to put the machine in fast forward or rewind without a tape, and try to stall 
the motor by grasping the respective supply or take-up reel with my finger. Experience 
will tell what amount of "pull" is normal and when it's time to replace the main belt. 
The replacement must have the same (or -slightly- smaller) circumferance and thickness. 
A thicker belt will not work properly... it may cause faulty operation since it pulls 
harder against a spring loaded mechanism. The smaller counter belt can be pretty loose 
and still work fine... it's not critical. 
     Lastly, the record/play switch is a small rectangular block that is soldered to the 
PC board. It normally sits in PLAY mode until pressed by a lever when the mechanism is 
put into RECORD. When the contacts inside that switch get a bit of oxidation on them, 
the switch can become intermittent or the machine may not work at all. To clean the 
switch, you'll need to disassemble the datassette (three or four case screws) and remove 
the board (two screws) for access to the switch itself. For cleaning, give it a quick 
shot of contact cleaner (I use Caig Deoxit) into either end and work the switch back and 
forth with your finger a dozen times. Do that before you clean the belts because droplets 
from a spray can often go where you don't expect. Whatever contact cleaner you use, be 
aware that some can contain harsh solvents. Use one that is safe for plastics.
     Another switch is used to turn on the motor when buttons are pressed. It's a leaf 
switch on the bottom of the mechanism. The easiest way to clean that switch is with a 
sliver of white paper wedged between the contacts and pulled out as the switch is pressed 
together gently with tweezers. Any residue on the contacts will rub off on the slightly 
abrasive paper. You can see the results on the paper if the contacts were dirty. 
     Now about alignment... the head tilt (called azimuth) adjustment is normally set at 
the factory and the tiny screw on one side of the head mount is secured with a dab of 
glue or paint. Unless that adjustment screw has been turned (breaking the glue bond), 
it's unlikely the alignment will be offset. Don't adjust it if you don't have to. 
Realignment requires an oscilloscope to measure the head high frequency output, which 
must be optimised with an alignment tape. 
     Some repair tests requre that the datassette be powered, and it's safe to run it 
while it's apart using a C64 for power. The motor will run in all functions with the 
computer on, but be aware that it will not run if you have JiffyDOS installed. JiffyDOS 
firmware bypasses the datassette port, so just turn off JD before you power up the 
computer. 

Ray Carlsen 
CARLSEN ELECTRONICS