Old Transistors.

Hello Z, these are early Germanium (rather than silicon ) transistors and I think were only made in metal cans.  Should still be possible to get them.  I suspect very few Germanium transistors (if any) were made with plastic encapsulation.  You cant just swap a silicon one as the forward voltage drop for germanium is much lower so you would also need to recalculate and change the biasing resistors.


Best bet would be to find some new old stock that have been stored somewhere cool.

Hope this helps.

Peter

Thanks Peter, I'll try that.


Z.

There are plenty of equivalents for these but all metal TO1 etc cans.

AC127  could use  e.g. AC176  2N2430  OC140?

AC128 could use 2N2706 

Specs are pretty basic HFE >50 P 340mW(127) 267mW (128)  VCE 16V !!

Plenty of Japanese Germanium made too . most start 2SA.... but not all 2SA are germanium IIRC anything up to 2SA 479 is germanium you would need NPN for the 127 and PNP for the 128.  I have some data (too much to post) so if you find something feel free to contact and I may be able to help with data and connections.

I am not famialer with that model, but I have had a fair amount of broken consumer electronics cross my bench over the years so what follows is general and may not be useful in this case.

I am in a similar state if you need data, though it can sometimes take a bit of finding - I literally  have tens of feet of shelves holding data books for transistors and ICs from various eras on the wall in my dining room (and more rare access stuff in the loft.... )

But unless they have been overheated by being soldered on short leads, the old germanium devices are remarkably reliable in themselves,  partly as the internal dimensions on chip are very large and the fabrication methods quite agricultural by modern, (near quantum mechanical limits) standards. (The youthful naked eye could actually see the metalization patterns on 1960s  devices, now I need glasses)  For the related  reasons ESD is less likely to damage the older devices, party as they are bigger, and partly as they are electrically leakier so charge is less prone to build up.

Quite commonly if an old transistor fails, in a design that was previously reliable, there is another defective component in the process that has stressed it, such as a PSU fault or a leaking electrolytic capacitor increasing the bias current to the base, or a bias resistor that has gone open circuit, again changing the operating conditions. So if you just change the transistor, take care that the voltages are in-spec around the replacement, or you may end up changing it again shortly.

Germanium has a much lower temperature chemistry, so is less tolerant of heating -  by 100C the devices are well on the way out, so when soldering be very quick, and if need be use wet cotton wool buds or a pair of pliers as a heat shunt. Easiest is to leave the leads quite long.

The intermittent knock and work problem is unlikely to be semiconductor, or if it is, it will be associated with a mis-soldered leg. I'd be looking for a hairline PCB crack or a solder joint that reveals the fault when prodded with a dental pick or other sharp point. I assume all switch contacts and potentiometer sliders have be given the electrolube treatment ?

Update - I have the service manual (in Dutch!) - hopefully attached if I have got the forum SW to work  right. Has circuits and dismantling diagrams, which are I hope clear enough.

Happy to try and translate the finer points if needed.

Zoomup,


AC127 and AC128 germanium transitors are available from a number of sellers on eBay, which could save you a lot of hassle with the transistor biasing and would keep your cassette recorder true to the original design.


If you are taking a trip back to the world of germanium transitors, you will probably be looking for selenium rectifiers next! ?

Mike thanks so much for the service manual. You are a STAR.


Hello Ross, the bridge rectifyer is made up of four BY 164 diodes thank goodness.


Bye,


Z.

I have today spent hours looking at a fault on a Sharp 6500 radio cassette player/recorder. The radio works very well. The sound quality is superb. I have had problems with the mechanics of the tape deck. A nylon drive wheel with a rubber tyre was not doing its stuff. I spent hours trying to discover the fault. Eventually it dawned on me that a small rubber drive wheel is missing on the main drive spindle just above the flywheel. Lubricating oil must have attacked it and turned it into a soft black blob. I had removed it with a cotton bud and isopropyl alcohol before I knew just what it was. There is no way to replace it. I may have remodelled the  drive to work. We will see when the glue dries. What a challenging and rewarding hobby.


Bye,


Z.

I have glued slices of cycle inner tube to rubber wheels that have worn down a bit in the past - though it sound like you are rather beyond that.


Coming back to the unir with the poor speed control, looking at the circuit on page 6 shows the speed control transistors TS11 TS10 although the drawing rather obscures the

regulation method and seems to be incorrect when compared to the layout on page7 and a more sensible circuit is redrawn on page13 'the right way up' so it is clearer that most of the motor current goes via TS11 and the diode and  the  other transistor make the regulator and a simple limiter. However, before damning the transistors, I'd verify the voltages are actually varying and it is not just a feature of varying mechanical load, or a scratchy forward/reverse switch. Actully that reg is so crude, silicon devices would probably work without modification, or perhaps a change of resistor value to pull the reference up a bit.

It's sometimes possible to rescue tin cased transistors by applying a high current but low voltage supply reverse biasing the transistor, between case and the other 2 leads (one at a time). This will fuse the tin whiskers clear :)