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I need to know what is the type of this capacitor. Never seen these before. It is polarized (red side is +) and rated 4MFD (4uF I would guess) 25v. I also would like to know what would be an appropriate replacement for this circuit. (See schematic pic below)

I removed the capacitor to test it and what my tester saw is a 15MHom resistor. I've tried to replace it with a 4.7mF, then a 3.3mF electrolytic cap, but it did not behave like the working channel at all. Where I was expecting a 0.2V pp, like the working channel, the reading was like 1V pp.

This is from a Crown IC 150 preamp, right at the input of the phono preamp board. The one on the left channel is damaged.

cap on the phono preamp board

Detail of phono preamp circuit.

Source: ElektroTanya - Crown IC-150 pre-amp service manual

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    \$\begingroup\$ That can't be rated in millifarads. Back in the day mF meant microfarads, today it means millifarads and we use uF for microfarads. Which cap you used, micro or milli? \$\endgroup\$
    – Justme
    Commented Apr 12 at 16:51

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If you replaced it in one channel and that channel now has a higher output than the other it may be that both were bad and your ‘working channel’ isn’t actually working properly. Or you have a problem elsewhere in the circuit.

There’s no reason these caps have to be polarized other than that when this was built it was the only way to get physically small caps at that value. You could replace them both with film capacitors, which are generally considered to sound better when in the signal path than electrolytics (although I’m sure there’s someone who will fight you on that). For what it’s worth, film caps are available in 4.0 uF.

You might want to check the other caps as this appears to be an older device. For example the 500 uF cap between the gain pot and ground is going to affect the gain directly.

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    \$\begingroup\$ What you say is exactly the conclusion I came upon later yesterday. I removed the cap from the working channel to just measure and test it and it appeared that it was damaged. I ended up replacing both with 4.7uF (I only had electrolytics on hands, but I will buy some 4uf film caps) and everything went back to work. The 500uF caps were effectively pretty off. Around 800uF. So They have been replaced with 470uF electrolytics. I will do the same for the rest of the preamp. \$\endgroup\$ Commented Apr 13 at 15:56
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That is an axial-lead wet slug tantalum capacitor. 4uF/25V.

Maybe the problem is elsewhere, or maybe R21 got wildly misadjusted.

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  • \$\begingroup\$ R21 has been checked and is in spec. and has been adjusted equally with the working channel. \$\endgroup\$ Commented Apr 12 at 17:56
  • \$\begingroup\$ Looks more like M39018/01 style electrolytic to me; did they make wet slug in epoxy-endfill style too? \$\endgroup\$ Commented Apr 12 at 20:35
  • \$\begingroup\$ @OrganBender I don't see any reason you would get higher output as a result of that capacitor provided you are testing at some reasonable frequency such as 1kHz. Do you have a service manual for adjustments? \$\endgroup\$ Commented Apr 12 at 20:50
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    \$\begingroup\$ @Spehro Pefhany the reason the output was that high, I understood later after some more investigation, is because the supposedly working channel also had a bad input cap. \$\endgroup\$ Commented Apr 13 at 16:00
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    \$\begingroup\$ @TimWilliams Here is the kind I am familiar with from years ago. They were used for timing capacitors so low leakage was critical. We used film timing capacitors exclusively so didn't have to deal with capacitor leakage problems. \$\endgroup\$ Commented Apr 13 at 17:18
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Your two options are 4.7 uF or 3.9 uF, at 25 V or 35 V. 4.7 uF is way more popular / available. A high end aluminum electrolytic should be fine. By high-end, I mean one rated for 3000 hours / 105 degrees C or more.

At first look, I don't see anything suggesting that exactly 4.0 uF is important, so I'd go with the 4.7 uF.

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  • \$\begingroup\$ As @GodJihyo suggested, these may not need to be polarized. For the moment I put 4.7uF electrolytics on both channels and the preamp works great. \$\endgroup\$ Commented Apr 13 at 15:58

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