Germanium transistor sub

Until someone removed it. :( How many of those old TV schematics are available on line, and for free?

General Electric, Sears-Silvertone and other companies did the same thing in the US, but they were usually missing whenever I cracked the case on one.

A piece of 'Spaghetti' tubing over the emitter was common in repairs in the '60s & '70s.

Hi... I can send You a AF126. You pay the postage, the transistor You can have for free.

Bo-Lennart Karlsson Falun, SWEDEN

Hi, What do you think the postage to the US will be? Thanks, Lenny

There seems to be a consensus of opinion that the OP has indeed selected a suitable Ge replacement, however I suspect that even if a Si part had been fitted the consequenses wouldn't be as serious as it not working at all.

Any sort of decent quality radio would have automatic bias common emitter stages, the base bias is set by a voltage divider - that and the value of the emitter resistor determine the collector current.

The self compensating nature of the circuit would tend to ensure a relatively small reduction in collector current, the main problem I can think of is that AGC is often fed at the bottom of the base bias divider, so using a transistor with a higher Vbe may have a similar effect to a large AGC control voltage - very low gain maybe, but totally inoperative; doubtful.

This isn't quite the way I learned it in school.

The kind of bias you're describing tends to keep collector current constant with beta variations. Vbe is taken into account in the design. I don't think a circuit intended for 0.2 to 0.3 volts would work with a transistor needing

Collector current does fall, but there is also a corresponding fall in emitter current wich reduces the emitter resistor volt drop - which goes some way to compensating the higher Vbe.

So it doesn't result in the transistor being completely cut off - as would happen if the emitter went straight to ground without an emitter resistor.

The 'Emmiter resistor in similar radios is nothing more than the DC rsesistance of the AM & FM IF transformer primaries in series.

If the radio is intermitent it does not mean that necessarily the transistor, i think of one probable cause is the variable capacitor besides if the transistor you mentioned is part of the if section then you may check with another radio tuning a frequency so produces an harmonics in the good radio so you know the mixer stage is working then you can inyect a signal with your finger in the base of the first if transistor and a radio station should be heard. Have you tried to resolder all points and verify all paths are in god condition. Hope it can help. rgds

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Well the odd thing is that the radio always starts out working perfectly. It will run consistently for anywhere from ten minutes to sometimes two hours. It seemed that at first operating it at low volume settings may have precipitated the problem but now I'm not so sure about that. However once the intermittent condition begins it continues until the unit is shut off for a period of time. After that the period of time it takes for the intermittent condition to start again after a subsequent power on is directly proportional to the time the radio has been off. I managed to measure all transistor voltages during an "OK" period. I then took similar voltage readings during failure. As expected all voltages on all transistors were affected, however the voltages measured at the terminals of this particular transistor in question are all the same, (about 3.0volts) when the failure condition occurs. It was for this reason i suspected this transistor, (as improbable as this may seem), of developing what looks like a "dynamic" b e c short. Lenny

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Cracked volume control pot JC

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Well the odd thing is that the radio always starts out working perfectly. It will run consistently for anywhere from ten minutes to sometimes two hours. It seemed that at first operating it at low volume settings may have precipitated the problem but now I'm not so sure about that. However once the intermittent condition begins it continues until the unit is shut off for a period of time. After that the period of time it takes for the intermittent condition to start again after a subsequent power on is directly proportional to the time the radio has been off. I managed to measure all transistor voltages during an "OK" period. I then took similar voltage readings during failure. As expected all voltages on all transistors were affected, however the voltages measured at the terminals of this particular transistor in question are all the same, (about 3.0volts) when the failure condition occurs. It was for this reason i suspected this transistor, (as improbable as this may seem), of developing what looks like a "dynamic" b e c short. Lenny

In that case look for C problems

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In that case he could temporarily replace the pot with two resistors, right? And see if the circuit goes intermittent.

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Because it always starts out working, and this is definitely consistent. I really feel that this is an active component problem. Lenny

Maybe, maybe not. (I'm inclined to agree "yes, it is", when you say that the transistor has the same voltage on all terminals.) But you really need to put a 'scope on the unit, and step through it.

If you have the schematics for the radio, then it should be no problem figuring out what is going wrong. There really can't be very much that could cause all three terminals of the transistor to assume the same voltage. Something amiss with the emitter return maybe ? If the base is biased with respect to ground, and the emitter is returned to ground, then you must measure a difference between the base and emitter, equivalent to a germanium P-N junction drop. If the emitter return goes open, then you might well read (almost) the same voltage at the emitter and base, and possibly the collector too.

Arfa

It's a PNP transistor. The emitter has the highest voltage, so it is something opening in the collector path to ground. When it opens, the base voltage rises because the transistor is no longer biased properly.

I haven't got a copy of the circuit to hand, but I'm not sure that I understand what you're saying. A PNP circuit is just the same as an NPN circuit, but with the battery upside down. Ground is ground and bias is bias, applied between the base and emitter. Doesn't matter whether it's a PNP or an NPN transistor, the ground reference is just the opposite pole of the battery. The emitter having the "highest voltage" is basically an arbitrary term as that voltage could be 'higher' in a positive or negative direction, depending on where you are taking to be your measurement reference. Things get a little more complicated to get your head around, where the circuit is powered the 'conventional' way up, but the transistors are in upside down. Maybe that's what you're saying here ?

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I was going to demur, too.

In a junction transistor, the EB junction is forward-biased, the BC junction reverse-biased. The EB junction can be correctly biased even if the collector is dangling. I don't see how opening the collector's path to its intended voltage source would alter the base voltage.