RMI 300B Electric Keyboard

[DrGonz78]

Originally my repair of this instrument included troubleshooting why the organ mode was so faint. Well reading up about the device I have learned the organ mode and the accenter stop tabs are for added effect to the main four voices. They are not meant to played by themselves... Pretty funny when I realized nothing was wrong and thankfully nothing major to repair.

However, I am now working on a dead C# key. Actually it is not completely dead and I am currently trouble shooting still. If I have one of the four voices on like piano or lute, then the key makes no sound. Holding down the C# key when organ mode is engaged is like a volume swell or envelope effect. Meaning the key triggers a sound as if it has no attack but is loud enough with volume as you hold it down making it louder as time goes on. So what I see at the collector of the AC122 transistor is about 1-2vDC when holding down the key, with organ mode. When you turn off organ mode the voltage goes down to like milivolts. I did test the behavior of other working keys and found around 12-14vDC on the collector(AC122) when playing working keys. I observed voltages for C B E and compared them to the C# key. Basically, I am just not done with the troubleshooting quite yet so I am not posting anything difinitively. Have not found any bad solder joints in the C# key circuit and no obviously open diodes, caps or resistors.

Troubleshooting to do later today and post more observed data>>>Here is the link to a similar schematic. Look at the Keying circuit on page 20 and I am going to get a 220ohm resistor tomorrow to follow the full testing of a dead key. (pages 10-11 on service manual)
http://music-electronics-forum.com/t25007/

Question: Do you think I can get by replacing a AC122 with a AC128 transistor in this application? Or at least for testing? I have a bag of AC128 transistors, but no AC122's at all. My source for AC122 is in Bulgaria and takes a while to get them. Schematic say's I could use a 2sb175 as well. Also, transistor cross referencing says to use AC125 too. I can get a good batch of AC125's from the states and is more cost effective. I am just thinking ahead just in case it is a bad AC122. Thanks again as always to everyone here reading and helping with advice, take care.

AC122 transistor data
AC122 Datasheet & Equivalent - AC122 Transistor DataSheet PDF. Parameters and Characteristics. Electronic Component Catalog

AC128 " "
AC128 Datasheet & Equivalent - AC128 Transistor DataSheet PDF. Parameters and Characteristics. Electronic Component Catalog

[doctor]

The small signal transistors (such as the AC122) have as a rule low collector current and high gain. The power transistors (such as the AC128) have as a rule high collector current and low gain. For this reason they cannot be swapped. This is the case.

[DrGonz78]

To start I must correct myself first, when I said I measured at the collector of the AC122 it was really at the emitter. With that said I put a 220 ohm resistor in series with B+ voltage to point D on the keying circuit(refer to schematic). At that point it actually did get the tone generator part of circuit working and I observed voltage at the D junction. This then pointed me towards the keying part of the tone circuit. Next step was to short over C5 to test if that was the problem and sure enough a bad capacitor. Put in a new cap and presto, a working C#4 key! Thanks Doctor for the advice on the comparison of the AC122 to the AC128, much appreciated.