How does one interpret the polarity of these capacitors? These are just two leaky examples that I removed from a small amplifier board with Germanium transistors. The transistors are all SBxx types which would suggest that it came out of some Japanese made item. Modern electrolytic capacitors ones have stripe down the side for the negative terminal, but I have not seen makings like these. Does the arrow point to the positive or the negative terminal? (i.e. perhaps suggesting that positive is at the opposite end to where the arrow is pointing?)
I ask because I had initially assumed positive, but, after I had already replaced a couple, I then started to realise that on the PCB in question, the indicated legs actually sit on the common rail which also connects to the output and driver transformer bodies and the screen of the input coax wire. This would, in turn, suggest that this is a ground or negative rail, which is further re-enforced by the fact that the connecting wires are either black or white. The red and orange are on the opposite rail.
On the other hand, I have seen a radio with Ge PNP transistors where the common rail was connected to the positive terminal of the battery. This has me wondering whether the same might be the case with this amp board, but then, what about the polarity of the input coax? The board is small enough to have come out of a portable battery powered radio so I guess it is possible that the RF section might also be oriented with the same polarity otherwise you would have a situation where you would be connecting a positive screen to a ground on an external source which would make little sense, nor would having a black wire mark a positive rail.
The evidence would therefore seem to suggest that the arrow points to the negative terminal. What do others think?
The arrow has a great big + sign on it - how much clearer an indication of polarity do you want?!
I concur!!
Does the arrow point to the positive or the negative terminal?
Indeed, positive! The cap clearly identifies the +ve lead, thus in turn, you know the -ve, I agree with the others, there is absolutely no ambiguity.
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As this thread is in danger of becoming exactly like the facebook post as demonstrated here, with everyone stating the obvious, and as the question has been answered, a rare occurrence at Vrat, the thread will be closed.
Edit:
Reopened by request from OP
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Thank you for confirming the obvious and my apologies for what looked like a stupid question! Perhaps my focus on the capacitor itself was at fault and I asked the wrong question. I do actually concur that the marking points to the positive terminal and that would be the obvious conclusion. What gave rise to my confusion is, that in that case, then how to make sense of the wiring? If those terminals on the capacitor are positive, then as it stands the positive goes to ground and negative to the power rail and therefore black would have to connect to battery positive and red to battery negative which is the opposite of what one might expect.
Hence my wondering later in the post whether a circuit based around germanium transistors might have its power feed negative with respect to ground?
I can accept that the capacitor terminal is positive and orient the capacitors accordingly, but then how do I power this board? I was hoping that someone familiar with circuits based around germanium transistors might have some insight. I have reverse engineered the circuit and drawn it in Kicad. The diagram is attached. I can't guarantee there are no errors, but I did carefully check the orientation of the transistors and electrolytic capacitors and have assumed that the symbol points to the positive pin. As they say, a picture is worth a thousand words.
Updated image:
The circuit shown uses PNP transistors hence ground is more positive than the other power rail. In effect PNP circuits can be thought of to be powered from a negative power supply.
The circuit uses silicon transistors, but is very similar to the type of amplifier used in transistor radios of the '50s and '60s where germanium PNP transistors were the everyday choice - germanium NPN transistors being much more expensive with a limited choice of devices.
John
John,
Thank you for spotting that error in my diagram which I have now corrected! All of the transistors on the board are germanium types. The output pair are both 2SB33 and both of the earlier stages are 2SB56. Kicad seems not to have germanium types in its list and I forgot to change the legend.
Thank you also for confirming my suspicion. One other piece of advice which I heard some time ago and which has just come to mind was to follow the arrow in the emitter to determine the direction of current flow using conventional flow, i.e. positive to negative. Doing that also indicates that the rail connected to the red or orange wires would be negative with respect to the common/ground connected with the black or white wire. I vaguely remember working on a germanium transistor radio quite some time ago and seem to recall that the battery positive was connected to the common or ground, which at the time surprised me until I had more fully understood the circuit. I just wanted to re-assure myself that my memory wasn't playing tricks. I have another radio with germanium transistors waiting to be looked at so I wanted to refresh my memory on this matter.
I had wondered why germanium transistors tended to be mostly PNP types. However, since the first experimental transistor was a PNP type, subsequent commercial ones would also follow suit and be PNP types. The NPN type apparently didn't arrive until a bit later with the invention of silicon transistors, by which time germanium PNP types would have become popularised in radios of the era. By the time NPN transistors were made, silicon had by and large replaced germanium which would certainly explain the limited choice of germanium NPN types available and their higher cost. However, I digress.
I appreciate your reply as it does seem to confirm my suspicion as well as that my recollection is evidently correct and I can proceed with some confidence to re-cap and power this board.
Just an update: the amplifier has now been re-capped. Most of them had leaked gunk and needed replacing. Also, the couple of caps I put in the wrong way around have now been re-installed the correct way around. I had previously checked the transistors for leaks/shorts and the transformers for o/c and all tested good. A current limited power up test at 6V (with black wire to positive and red wire to negative) showed the amp drawing 14 milliamps quiescent which, if I recall correctly, is in about the right ball park for a small germanium transistor audio stage. Injecting an audio signal with the sig gen then gave a good clear and loud output through an 8 ohm speaker. Job done.
No idea what it came out of, but it might come in handy some day.
Thank you for the contributions.
