Location: Boulder, CO
Symptom: Sound in one channel stops working after 20 minutes of use.
I have worked on this jukebox in the past for other reasons; it gets a lot of daily use. When I first arrived, I couldn’t find anything wrong. I could hear sound coming out of each speaker. I checked all of the connections.
We were on the fourth or fifth record, and I was about ready to leave, when it stopped working. It started with static sounds, then after about a minute, the sound was totally gone. After further checking of connections, I learned it was sensitive to vibration. I could tap the final stage of the power amp with the handle of my screwdriver and the problem would change in severity, but wouldn’t begin working completely. My initial thought was there was a bad connection in the socket for one of the power transistors.
I brought the amp back to my home shop and removed each power transistor and replaced the mica insulators and cleaned the pin contacts. I also ordered and replaced the larger electrolytic capacitors in the power supply circuit and the final power amp circuit.
Electrolytic capacitors gradually lose their capacitance with age. I assume this is because the electrolyte paste eventually dries out. In some cases, especially in high voltage vacuum tube amps, the paste will leak out and the capacitor eventually shorts out. This amp is over 50 years old, and although the capacitors seemed fine, it’s just a matter of time.
However, after rebuilding the amp, the original problem still existed. I isolated the problem to Q15. This transistor drives the transformer that phases the final output stage drivers. As a double check, I swapped the channel “A” Q15 with the channel “B” Q15, and the problem moved to the other channel. I don’t know why it’s sensitive to vibration, but there must be a problem with an internal connection inside the transistor case. In any case, it needed to be replaced.
One of the difficulties working on solid state Wurlitzer amplifiers is that Wurlitzer used their own part numbers for transistors, probably even specifying to the manufacturer to print their part number on the case.
It’s difficult to figure out what the transistor is and to find a suitable replacement. Fortunately, with some help from Google and Bing, I found out this same transistor (Wurlitzer part number 125721) is also used in their electric organs. Somebody figured out it matches an NTE121, Germanium PNP Transistor, Audio Frequency Power Amplifier.
I was surprised to find that somebody was still making germanium transistors. (99.9% are made from silicon.) Keep in mind that the forward bias voltage on a germanium transistor is 0.1V, compared to silicon forward bias of 0.6V. This is something I had forgotten.
The replacement part was ordered and the amp is now working fabulously.