Service Bulletin affecting 6540 ROM Adapters SN 1-5

It has come to my attention that my Commodore PET ROM adapters may not work in board location H5, H6, and H7 in all Commodore PET 2001 computers. This is due to insufficient input impedance on chip select lines CS1 and CS2 on the adapters. The directions in this bulletin explain how to update the adapter to solve the problem. However, the upgrade requires some delicate soldering. If you are in possession of an adapter that needs this update, I am happy to make the changes free of charge. Mail the adapter to the address below and I will mail it back to you free of charge within one week after it arrives. I am terribly sorry for any problems this may have caused.

Matthew D’Asaro
70 Pacific Street
Apartment 363B
Cambridge, Massachusetts

How to determine if you need the upgrade

1) You do not need the upgrade if your adapter has a serial number greater than or equal to 6. The serial number appears on the bottom of the adapter following the letters “SN”.

2) You do not need the upgrade if you are using the adapter to replace the ROMS at board locations H1 to H4 or the character ROM at location A2. See the table below to determine if this applies to you or not.

ROM Part Number ROM Board Location Update Needed?
6540-010 A2 (Character ROM) NO
6540-011 H1 NO
6540-012 H5 YES
6540-013 H2 NO
6540-014 H6 YES
6540-015 H3 NO
6540-016 H4 YES
6540-018 H7 YES

How to perform the upgrade

1) Replace the 2N2222A transistor in your adapter with a 2N7000 MOSFET. See the picture below for the location of this part. Make sure to orient the new part the same way as the old one. Note that the old transistor may have a different prefix than 2N, such as KN. 2N7000 MOSFETs can be ordered from Mouser electronics, Newark electronics, or DigiKey I will also send you one free of charge if you ask.

2) Bridge the 10K resistor connected to the base of the transistor with a jumper wire to short it out. Again, see the picture below.

3) Connect a 1K resistor across drain resistor for the MOSFET as shown in the picture below. It is a tight fit but it does go in there nicely if you trim the leads short enough and work it in there with a small screw-driver or similar implement. A 1K 1/4W resistor can be ordered from any of the sources listed above, and, as with the 2N7000 MOSFET, I will send you one free of charge if you ask.

4) If you have a schematic on file, replace it with the one below.

Figure 1: Location of jumper to be added and transistor to be replaced

Figure 2: Updated Schematic Diagram download here