The Fisher Saga....Chasing Classic Parts

 

With apologies to Wayne Carini.......

One of the things about working on old equipment is that sometimes the parts are hard to find, especially if they were declared obsolete years, or decades, ago.

Now while most passive components like resistors and capacitors are readily available, some of the semiconductors are not. Diodes are easy to find, and even the infamous "Darlington Power Packs" now have discrete versions available, but most of the Small Signal Transistors have been extinct for quite some time.

The issue then becomes finding suitable replacements, of modern manufacture. Most "US" transistors have a "2Nxxx" number on them, like one of my old favorites, the 2N2222.

Japanese transistors use a completely different numbering system, as explained on Wikipedia:

"The Japanese technical standard JIS-C-7102 provides a method of developing part numbers for transistor devices.^[1] The part number has up to five fields, for example in the number 2SC82DA:

• The first digit "2" indicates this is a 3 lead device ( a diode would have a prefix numeral 1)
• The letters "S" is common for all EIAJ registered semiconductors
• The following letter designates polarity and general application of the device. For transistors:
  • A PNP high frequency
  • B PNP low frequency
  • C NPN high frequency
  • D NPN low frequency
  • E P-gate thyristor
  • F N-base unijunction transistor
  • J P-channel field effect transistor
  • K N-channel field effect transistor
  • M bidirectional triode thyristor
• The numerals following indicate the order in which the application was received, starting at 11
• A suffix letter indicates improved characteristics....."

OK, so that at least tells me about what they are, but to find out exactly what they are, I had to get the datasheets for each (obsolete) part number. The datasheets list all the parameters for the described part number, and from there, you can use the selection guides on the vendor websites (I like Mouser the best) to find currently available parts with matching specifications.

But what if you can't find a datasheet? Remember, these are obsolete parts, and sometimes a datasheet hasn't been scanned and posted somewhere. Well....then you're getting down to hand-to-hand combat. First you look at the schematic, and see what the applied voltages are, and make a best guesstimate as the the current and power dissipation required.

This is the schematic for the right channel of the power amp :

At this point I've found current replacements for all but three of the original transistors, but it's going to take a bit longer to decide what to use for the last three.....