Rickenbacker TR35B amplifier

[redneckgeek]

I have a Rickenbacker TR35B in the shop. It has it's two output transistors shorted. They are Motorola 2N5988 (PNP) and Motorola 2N5991 (NPN). Am having a hard time finding any (at least in small numbers). Does anyone have a clew as to a substitute?

[Enzo]

I just rebuilt one of these not long ago. Is this the amp with a square bar mounted up on standoffs for a heat sink - right in the center of the board?

Your transistors are TO127 case, and that is pretty obsolete. The lead order is backwards from more common types like TO220, TO218, TO247, etc.

If I recall, I used something from stock, mounted to the sink, and arranged to wire them in using jumper wires leaving the transistor legs in the air. If I recall, I wound up moving the transistors to the other side of the heat sink, and I think I remounted a cap or something large that was in teh way. I remounted it to the far side of the board maybe? I could be confusing my memories now.

The transistors themselves are not so critical. The originals are 12A 80v 100w types. I stock a lot of transistors for various things, so I chose from things like 2SC5200/2SA1943 or even 2SC3281/2SA1302. Most any power transistor that meets your spec will work fine.

[redneckgeek]

Rick TR35B work to little avail

Thanks for the information Enzo. This amp came in with a "blowing fuses" complaint. I found the two output transistors shorted on all leads. OK Cool. That would blow fuses. But I checked the mains transformer and wow an open secondary. So I ordered a Hammond transformer and, because of your input, the 2SC5200 and the 2SA1943. The transformer would have had to be made to fit the holes somehow, so I decided to give the old transformer a closer look. I took the cover off and found one of the secondary leads was not soldered when the transformer was manufactured. I soldered it and it works fine. Now to the transistor retrofit. I took the new ones and simply flipped them over and put a 1 1/2 inch wide aluminum plate (same length as the bar and bent the leads (had to lengthen them about 3/4 ") 90 degrees mounting the transistors on the bottom of the plate. I bought some 1" standoffs and raised the plate and bar assembly to make it fit. So far so good. I was so proud of me! And not only that, it was fun ... until I turned it on. I use a current limiter in my shop and when I turned the amp on my 100 watt bulb lit up hard. I have isolated the problem to a short in the main board by disconnecting the pre amp wires. I tested the transformer by isolating it and running it through a bridge rectifier. That worked well. The main board is fed through a connector with 6 leads; the transformer secondary leads (two red and a black {becomes yellow} center tap). The other side of the connector has a brown, black, and white wire. The white and brown are for the speaker and aux. out. The black is to chasis ground. If I just connect the two red transformer secondary wires to their respective places on the main board, I get the "short light" to light.

I have removed and tested all four diodes, all caps, the power transistors (just to make sure). Although I tested the diodes and they tested good in my IC Analyzer, signal test, and ohmeter test, I did put them back in. I also reinstalled the 4700uF filter caps as well as the two smaller 100 and 22 uF caps.

Sure wish I could find a layout for this amp. The only thing I can think of is that I've erred in my installation of the power transistors but I've rechecked it several times. Any further suggestions???

[Enzo]

How about the filter caps. Either shorted? When you were being proud of yourself for inventing a new heatsink scheme, did you insulate the power transistors from it? Hey, I am just askin'. (I sometimes take pride in my work too.)

Are the main filter caps facing the right direction?

SInce Q9, Q13 were shorted, that immediatly adds resistors to the suspect list. Are all those resistors around them OK? The 470 ohm base resistors, the 4.7 ohm driver emitter resistors, the 0.27 ohm ballast resistors.

I hope you are working with no load on the amp. Remove any load until we are sure we won;t blow fuses or put DC on the load. In fact DC can sit on the output without drawing excess current (lighting up your bulb) as long as there is no load.

Clip your meter to a filter cap and watch it while you power up. WHat did the voltage come up to, and was the polarity correct for the cap? Now do the same for the other cap. Same questions. Did both caps come up to about the same?

Lift both 0.27 ohm ballast resistors - that disconnects the power transistors from the circuit. Power up. What happens now? ASSUMING Q9,Q13 are not shorted collector to heatsink, this test should divide the system. The problem either remains and is in the power supply or its distribution, OR the problem lies in the output section.

I am going to assume your tests of Q7-13 are OK, and they are OK. WHen you subbed the parts, did you look at the schematic to notice the PNP was on the +V rail and the NPN are on the -V rail?

[redneckgeek]

I have seen the enemy! He's ME!

The transistors were reversed. PNP to NPN side and visa versa. The thing works great now. Thanks for the help.