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I'm repairing an old (circa 1960) console hifi system for my parents. I'm no expert but also not totally incompetent. It originally came from Sears, I do not know the model number, nor the power output of the unit. Attempts to find any schematics online have failed.

However, it's pretty clear to me the problem:

The audio amplifier just produces a very loud hum with the signal mixed in softly below it. Investigating, I find the last large capacitor on the amplifier board is literally blown apart.

Capacitor is about 1.5" tall and 1/4" in diameter. It's made of rolled paper inside. Very much looks like a firecracker that blew apart.

Sadly, the only remaining plastic I can read, I can make out "35v" and the final digit "9" (at the end of what was a longer number). And, it reads open with a meter, so I have no clue what it's original spec was.

Just looking for some procedure for replacing it - best guess, process of elimination, most commonly used rating for radio amps of the era, or other advice.

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  • \$\begingroup\$ Can you reverse-engineer any of the schematic? \$\endgroup\$
    – Hearth
    Nov 4, 2018 at 0:25
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    \$\begingroup\$ Must be a transistor unit if capacitor is 35 volts. \$\endgroup\$
    – Marla
    Nov 4, 2018 at 0:27
  • \$\begingroup\$ Any other paper capacitors (assuming, a photo would help) of this vintage are definitely bad and should be replaced as well. I suggest watching some of Mr. Carlson's videos. \$\endgroup\$
    – rdtsc
    Nov 4, 2018 at 1:15
  • \$\begingroup\$ If it was an electrolytic, at 35V and size 3/8" by 1.5" (the closest I can find), typical capacity would be 100 uF (and polarity matters, it has to have a marked positive and negative end). This, from a fifty-year-old catalog... \$\endgroup\$
    – Whit3rd
    Nov 4, 2018 at 3:23
  • \$\begingroup\$ extract a bit of schematic, such as what active devices and Rs and Cs and Diodes are attached to the blown cap, \$\endgroup\$
    – analogsystemsrf
    Nov 4, 2018 at 5:01

2 Answers 2

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It's probably an electrolytic, to state the obvious. You said the "last large capacitor on the amplifier board". Are there other capacitors of the same size on the board? If so then you could copy-paste another one in there if the specifications are still legible.

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Capacitors of the same technology and voltage have volume roughly proportional to the capacitance rating. So, find another similar 35V cap which is readable, divide its capacitance by H*D² (H = height, D = diameter) and then multiply by H*D² of the blown part. The error of such estimation is smaller for bigger capacitors.

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