Replacement transistors

Hi all,
I am in the process of refurbishing an amlifier out of an NSM jukebox. I will do this by replacing all the transistors and capacitors on the board. I have done this before on my own 'box, a Rowe-Ami MM6 and the difference was astounding. However, I have come up against a problem. Some of the transistors in the amp are of a type I cannot find any information about. They are listed as S7285 TI (which mich be Taxas Instruments. Does anybody have the spec or modern equivalent for this?

Cheers
Malcolm

Hello!
Normally in the Schematic are listed the Types of the Parts. Which Type of Amplyfier you want to
repair?
mfg Willi!

Hello Malcolm

I do not think it's a good idea to replace all the transistors. Transistors have, in contrast to tubes, no aging. The original types are often no more to get. Replacements tend to have slightly different data. If a transistor needs to be replaced, usually it does not matter. If you replace all the transistors, the differences can multiply. In the worst case it may happen that the amplifier is not longer working properly. If that happens, it is very difficult to bring the amp back to work. I know this because I already had some of such amplifiers to repair in the past.

I think you should save the money for replacement parts that do not have to be replaced. Instead, you should give the amplifier to an expert who can check it and repair if necessary.

Best regards - charly49

Why do you want to replace the transistors? Transistors don't age like tubes or electrolytic capacitors. There's no need to replace them unless they are faulty.

Thanks for the advice,
I know that the chances are that it will be a capacitor or capacitors that have gone leaky, I'll replace the elecrolytics first and take it from there. Still curious to know about these transistors though! In the case of my own amp (preamp actually, my MM6 has a valve power amp) it was originally fitted with 5 different types of transistor, none of which were available, but I was able to find the spec for them. In the end I replaced them all with (IIRC) BC147's as well as replacing all the capacitors. As I said, the result was amazing. The quality of the reproduction was vastly improved and the slight noise frome one channel completely eliminated.

Hey Malcolm,

to tell you the correct Type of Transinstor you are looking at, it's nevertheless important to know which Amp aou are working at.

Cheers
Kai

Hello Malcolm

You were lucky that the MM6 - amp sounds good after the rape with the BC147. I suspect that he would still sound better if you had not replaced the transistors. In the amplifier are, as you have seen, five different types. These were not selected without reason. It's not just about the DC- voltage and current limits. There would be thousands of replacements. The types were chosen according to certain other properties (eg internal capacity, linearity within a certain range, noise performance, and others). A replacement type can therefore work well, but the original type would work better. It is possible that de amplifier will be unstable if one uses an inappropriate replacement type in a sensitive stage. Fortunately roweAMI amplifiers are designed to be very stable and there are few sensible stages. With many other amps it is not the case. This is also the reason why I always recommend to have amplifiers repaired by a professional. There is a lot of expertise and experience needed to know which spare parts can be installed without affecting the function.

Best regards - charly49