TM504 - who moved my transistors?


Good day,

While working on a troublesome plugin, I had cause to open up my TM504 to check the power transistors (as one does). I am surprised to find that Q22 and Q30 (an NPN from one middle slot, a PNP from another) have been cleanly relocated from the PCB to a location about 6" away, and heat sunk to a chassis member there. Fancy color-striped wire was used, and heat shrink etc: it looks like a pro job. Anybody know what's going on here?

And you're going to ask me if they are the OEM PNs; that will require more disassembly.

Dan G

Hi Christian,

This arrangement should be factory-standard. My TM504 is configured the same way,
and Fig. 2 Exploded Parts Diagram (rev B from 1974) shows item 34 (identified as
Q22 and Q30) as being attached to the chassis near the transformer.

I imagine that Tektronix discovered that the bracket used to heat-sink the remaining
four transistors and the two diodes could not quite dissipate enough heat if all six
transistors were attached there at full power.

Is your unit an early serial number? My bracket (item 78) has no provision for mounting
Q22 and Q30 at all.


Mark Vincent


Dan is most likely right.about relocating transistors that are too hot where they were originally. I have not looked at the schematic or have this device. It makes sense to do this modification when needed. I did a modification of moving the two series pass transistors in my Pioneer SA-9800 to the frame where two holes are to reduce the heat problem of these two transistors.That greatly(!) reduced the temperature of the transistors. I did make sure to use an insulator and good quality compound on each side of the insulator because the collectors are not grounded. This is a known problem in this amp even after fully restoring. I know this Pioneer model is not a scope. The principle is the same no matter what device it is, reduce too much heat by relocating or adding more heatsinking. If your TM504 is working as it should, leave the modification alone. Adding compound to aid in the heat transfer is your decision. The S/N of models would likely be the reason for your transistors being mounted differently than other S/Ns.



I'm not presently where I can look at it, but I opened my TM504 a couple of weeks ago while evaluating electrolytics and pass-transistors in my TM for ordering replacements. Fortunately those in the TM504 were OK. I, too, was surprised to see two of the pass-transistors mounted on a rear member. These may have been those for the high-power slot, but I do not remember. It sounds like yours are in the same locations.

Bruce, KG6OJI