AOR Mk3 Thumbwheel Modification

Making use of the thumbwheel push switch on the AOR 8200 Mk3 As a warning, I nearly killed the best (and most expensive) scanner I've ever owned doing this. I must have been mad. Thankfully, it worked. Needless to say, don't winge to me if you kill your pride and joy getting this to work. That said, it was the probing around to find out what to do that was the tricky bit, the procedure itself is easily do-able if you have some experience of soldering and a steady hand. This is a modification to the AOR 8200 to use the push switch on the thumb wheel to act like the ENT/M.in key on the keypad. It could be easily adapted to make the thumbwheel switch act as any of the keypad keys, but for me I wanted it to act as the ENT key so I could quickly put a frequencty on the VFO whilst scanning with one hand without having to dash for the ENT key. Also by holding the thumbwheel in, a frequency can be programmed into memory while the other hand is doing something more useful… This mod came about after the function, key lock and monitor buttons on my 8200 died, and I was forced to have a look inside to determine what the problem was. I thought it was this problem, but it turned out to be a bad via on the side switch board that I fixed using a bit of thin enammeled wire from an old solenoid. Once I'd got this problem fixed, I got to thinking how nice it would be to get the unused push switch on the thumbwheel to do something useful. So I took a deep breath and took the radio apart again and had a probe around… Over optimistically, I though this would be a simple matter of finding where the contacts for the ENT key on the keypad led to, and wiring them to the contacts for the push switch. Hmmm, well, the push switch connects the unused pin on the thumbwheel assembly to 0v, but connecting either side of the ENT key to 0v did not produce the desired result. Damn. Well if it was that simple AOR would have done it, right? Not really knowing how the keypad was wired, I hooked up the scope and had a look at what either side of the ENT switch was doing when it was pressed. One side, connected to the CPU, is at +5v when the switch is open, and the other side is at 0v. These two points have to be connected together to get an ENT key press, nothing else would work. Grounding the side that was at 5v resulted in a right key press on the rocker switch. So, after a little thought, I came up with a circuit based on a PNP transistor switch to connect the two sides of the ENT key when the unused pin on the thumbwheel was grounded. All that was needed was to connect the unused pin on the thumbwheel assembly to +5v via a suitable resistor, and connect this to the base of a PNP transistor to switch the two contacts of the ENT key. Unfortuantely, while probing around for a convenient +5v point and soldering a resistor on to test the theory I managed to damage one of the tiny SMT resistors leading to the thumbwheel and it ceased to function. I almost cried.

yupiteru radio ham scanner aor aor8200 MVT icom uniden

After composing myself and replacing the damaged resistor with one scavenged from an old motherboard, I found a BC557 in amongst my bits and connected up a test using the same enammeled wire I'd used to fix the bad via. After slapping myself and realising I needed a resistor between the pin on the thumbwheel switch and the base of the transistor to prevent effectively grounding the +5v side of the ENT key, it actually worked! I almost cried again. Once I knew that it would work, I replaced the BC557 with an SMT PNP transistor scavenged from the same motherboard, identified with the help of this superb site and a component analyzer. I made a little assembly from the transistor and base resistor, connected it up, insulated it using thin plastic sheet and used small dabs of superglue to attach the assembly onto the CPU board. Once the radio was reassembled, I retired to the shed for some serious scanning…

So, without further ado, here is the procedure to make the thumbwheel push switch act as the ENT/M.in key on the keypad...

Open the case of the radio by undoing these seven screws. If you are going to probe around to use the thumbweel switch to activate a different key on the keypad, you will probably have to remove the CPU board from the case. If you are doing this, unsolder the speaker connections and remove the screws holding the CPU board in place - there is a screw behind the topmost ribbon cable. Disconnect the ribbon cables from the receiver and power boards while you work, they will not stand up to much abuse! Remove the shielding over the CPU by desoldering the points shown in the picture

The connections are labelled as follows:

Your mod should probably look a lot neater than mine, my board round the thumbwheel is covered in flux from replacing the damaged SMT resistor, and the thumbwheel assembly is a little battered from the soldering/desoldering I did to figure this out. I have been using the radio with this fitted for a good couple of weeks now, with no apparent ill effects, but any comments or improvements are greatly appreciated - mail me - even if I can't go through the stress of opening up my radio again to test them....

The Finished Article

© 2005 Chris Bryden