I got a 147 Amp in which I was told "It quit working". This amp has the diode bridge on the circuit board and it is missing the 8 Ohm limiting resistor into the bridge. One lead of the PT secondary goes nowhere. Looking at the schematic I don't see the resistor. On my 147, the bridge uses terminal strips and the resistor is there.
Was there a running change to these amps?
Yes, there are a few running changes to these amps. The 8 Ohm limiting resistor appears in the mid/late 1960s. Early 147/122 amps don't have them. It's easy enough to retrofit one. Early 147/122 amps will often have two diodes in series for each leg of the bridge (8 in total) for better voltage rating with early silicon diodes.
The switch to diodes mounted on terminal strips instead of a phenolic eyelet board occurs in the early 1970s, maybe ~1973.
To know what the disconnected PT lead is for, it would help to tell us the wire color.
In terms of other variations, some amps had "death caps" across the 120VAC lines from the factory, and some did not. Some amps have two PT primary fuses, one in front, and one soldered in back.
Thanks Dave - I see it in the 147-247 SM. Not in the 145-147 SM. There is a spot for it on the board. Still wondering how it "just quit working" when there is a part missing.
David and DB Dave - thanks for the response. I was uncertain if the earlier amps had the resistor. Looking at the diode board it became obvious that the resistor was missing. Customer got back to me - "Oh yeah - I forgot I had removed it for another amp".
David and DB Dave - thanks for the response. I was uncertain if the earlier amps had the resistor. Looking at the diode board it became obvious that the resistor was missing. Customer got back to me - "Oh yeah - I forgot I had removed it for another amp".
Ah, well that was confusing since earlier versions of these amps were made without those resistors. I never heard of anyone removing one, but people without access to the proper parts they need do lots of weird things.
Edit: Looking back at your original post, I was confused by your saying that one lead of the PT secondary "goes nowhere." I'm guessing you meant that it was still attached to the board, but was not connected to the bridge since the 8 Ohm resistor was missing.
Didn't check the year. I looked for pictures on the web but didn't see any which showed the board. Yes - the PT wire was attached to an eyelet on the upper corner which didn't have anything attached. From what I saw, the resistor attaches here and runs diagonally down to the other "node" of the bridge.
Jim
Yes, the resistor connects one PT B+ secondary wire to one of the bridge inputs.
The confusion for me was what was meant by "missing" since this resistor is, indeed, missing from earlier 147 amps -- from the factory. It is standard to retrofit them on those early amps when they are serviced.
You may have a transitional amp where the schematic on the side hadn't been updated, but the resistor had already been added. Or it might be an older amp that had the resistor added later (and then removed).
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