Help with Mu-Tron III Capacitors

[Audiotrove]

I have an early serial number original unmodified Mu-Tron III (probably built in 1973 or 1974) which pre-dates the widely available schematic.  Mine does not even have a circuit for the PS-1 power supply, it's strictly a 2 x 9V DC box.  Instead of 4558 chips, it has 3 dual opamp metal cans (which date to January 1973) and a transistor.  So it's definitely not the standard circuit I've been seeing all over the internet.

It's making a high-pitched whine (a recent development) and I'm going to replace the electrolytic caps and see if that fixes it.  I'm trying to keep this pedal as original as possible, so I want to do the most conservative/minimal repairs as I can.  I wanted to keep the exact values on this pedal as it is slightly different than the later versions and I'm having a hard time locating the right capacitors.  I'm not sure it matters if the values change slightly.  I haven't pulled any of these off the board yet to try to check values but I assume 50 year old electrolytics need to be replaced.  Here is what I actually have on the board:

1)  My "C1" cap is a polarized axial electrolytic cap that reads "5mfd 25v".  The schematic calls for a 4.7uF cap, so 5uF is within tolerance but I don't know if this little difference matters or not.

2) My "C8" cap is a polarized axial electrolytic cap that reads either "2.2mfd 50v" or "22mfd 50v".  2.2uF is consistent with the schematic and it's about the same size as the 5uF cap so I'm assuming it is 2.2uF.

3) My "C9" cap is a weird little egg shaped rainbow colored radial with no specs printed on it.  It must be tantalum.  The schematic calls for a 4.7uF in this spot.

4) My "C10" cap is a non polar axial electrolytic cap that reads "15uF 25VDC", which is consistent with the schematic.

I have a decent handle on electrical circuits but I'm wading into new territory here and am looking for guidance and what parts to use to replace these caps.  I don't want to modify the original tone.  Tips for manufacturers, values, material, sources, etc., for replacements would be extremely appreciated.

-rb

[PRR]

> "5mfd 25v".  The schematic calls for a 4.7uF ... this little difference matters or not.

No. Since it is not a sharp filter, you can not hear a 6% difference. And a 5uFd in those days could be -20%/+50%, so 4uFd to 7.5uFd.

I think what you want is "looks alike", parts of size shape and color similar to the originals. 10% value differences don't matter to the ear, only to a buyer's eye. 

[Scruffie]

Your 'rainbow cap' colours should be the code, read it like a resistor.

I assume the transistor is in the power section, battery switching or some such.

[Audiotrove]

Thanks for the info!  To answer some questions:

1) All 3 of the electrolytics are black.  At least one is labeled EIC.

2) The colors on the tantalum cap are (from the top down: yellow, purple, green, and it has a big white oval on the front.)

3) The transistor is a 2N5172.  The collector comes out of Output A (Pin 1) on the 2nd Dual Opamp. The emitter goes to the Vactrol/optocoupler.  The base looks like it's tied into the "Range" circuit. 

I had a very hard time reading the codes on the opamps.  Like the Vactrol, the actual part numbers are either obscured or removed.  It almost looks like somebody intentionally wiped off the codes on top of the opamps, but after much careful looking and using a jeweler's loupe, I was able to read them.  They are Signetics N5558T opamps with a date code of 7304, in case anybody's interested.

[Audiotrove]

The tantalum capacitor color value calculator I just found says 4.7uf 16V which is totally consistent with the schematic.  Much thanks.

I've read a lot bad things about tantalum caps.  What would you replace this with?

[Scruffie]

Oh okay, the transistor is on current duties in the feedback loop, obviously they decided it was superfluous.

Tantalum are bad from an ecological stand point although I don't think a single one is going to change much if you can live with it on your conscience, that they auto-die when put in backwards and that they don't tend to last as long as standard electrolytic capacitors. So if you want to keep things original, just replace it with another tantalum, you'll probably get 20 years out of it and change.

Never heard of an N5558 'twould be before my time but a google says the 1458 was designed to be interchangeable so it's a dual 741 type.

[Audiotrove]

Again, for historical purposes, regarding the Op Amp:

I actually found the old 1972 Signetics catalog with the N5558T on the internet.  My research skills are stronger than my electrical engineering skills. 

The "T" signifies the metal can, they also had a plastic IC version which was a "V" code at the end.

It is, in fact, as you indicated, a 1458 equivalent. The Motorola equivalent is MC1458G.  There was also a National Semiconductor equivalent LM1458H.  It is a dual 741, which I guess is a 747.

Signetics later changed it to an MC1458H metal can, which looks to have identical specs.

I think Musitronics only used these metal cans pre-1975, as by 1975 the schematic shows "type 4558 I.C." as the chip and there is no transistor.  There is a picture floating around on the internet of an early Mu-Tron III circuit board with the transistor and one (presumably) RC4558 chip (I guess it could also be an N5558V) with 2 metal can opamps, so mine must be even earlier than that.

[willienillie]

Tantalum...don't tend to last as long as standard electrolytic capacitors.

I thought tantalums lasted longer than aluminum electrolytics.  Is that just shelf life but not working life?