1977 VINTAGE MXR SCRIPT LOGO DYNA COMP HELP & DEBUGGING

[ammalato]

Hi Everyone,

I have a vintage script logo 1977 Dyna Comp that needs some love. It bypasses fine, but not there is no effected signal.  I checked the switch and wires and all that jazz, they're not the issue.  I'm wondering if it's a dead component.  The pedal has an old 1/8" power jack someone added to it which immediately leads me to below it was either hit with reverse polarity or too much juice. I removed the jack and wired it up stock (I have a New '76 Reissue as well for reference).

It seems like the IC (metal can CA3080) isn't getting the proper "Control Current" on Pin 5, and I'm not sure if the IC is shot or there is something else leading to it.  Here are my readings (I haven't compared them side by side to my reissue yet). All were measured with the black probe lead to ground

IC (CA3080)
   
Pin 1:        0.01 vdc
Pin 2:   4.55 vdc
Pin 3:   4.56 vdc
Pin 4:   0 vdc
Pin 5:   0.01 vdc
Pin 6:   2.76 vdc
Pin 7:   9.12 vdc
Pin 8:   0.01 vdc
   
Q1   
C   9.12 vdc
B   1.97 vdc
E   1.5 vdc
   
Q2   
C   6.92 vdc
B   2.76 vdc
E   2.2 vdc
   
Q3   
C   9.06 vdc
B   0 vdc
E   0 vdc
   
Q4   
C   9.06
B   0 vdc
E   0 vdc
   
Q5   
C   9.12 vdc
B   9.06 vdc
E   0.01 vdc

[PRR]

> Q5   
> B   9.06 vdc
> E   0.01 vdc

That transistor is dead.

[ammalato]

Hmmm.  The transistors are all labeled C1849.  I found a few on ebay, but I'm wondering what will work in there?  I have some MPSA13's on hand, and Radio SHack isn't too far.  Any recomendations?  What if I was trying to keep it as original as possible?

[Le québécois]

2n3904 will work just fine (easy to find at RS).

[ammalato]

Well I'll be d*mned! 

$1.27 at radioshack, and 5 mins with a soldering iron and this Vintage Dyna Comp is back in the game.  You guys rock!

Thanks!

[kurtlives]

Well I'll be d*mned! 

$1.27 at radioshack, and 5 mins with a soldering iron and this Vintage Dyna Comp is back in the game.  You guys rock!

Thanks!

What was it exactly?

[ammalato]

As PRR suggested, the Q5 transistor (a C1849) was dead so I bought a 2N3094 from RS. Before I even soldered it in I just touched the legs of the new transistor against those on the old one and the effect came on, so all I had to do was replace it. Easy enough. :-)

[PRR]

Funny thing, though: I don't see how Q5 B-E junction could be blown.

True, the pedal is 35 years old and these were surely lowest-bid parts, so it may just have been a bad transistor. Perhaps leaky epoxy case and it took 35 years for enough air to get in and rot the silicon.

> What if I was trying to keep it as original as possible?

That transistor is not a "tone thing". Q3 Q4 put the peak of the audio on a 10uFd cap, Q5 buffers this slow control signal to the '3080 control pin.

2SC1849 is no mojo-part. It is pretty much the 2nd cheapest thing they could get at the time. (Cheaper transistors were used in AM radios where you can't have and don't want large gain per stage.)

If, as you say, simply shunting the dead device with a live part works, you could tack the new part underneath where nobody will see it, or even micro-solder a SMD part to the underside of the dead transistor's body. However I would let it be.

[ammalato]

Perhaps over voltage or reverse voltage? There was an old 1/8th inch jack wired in there for power without diode protection or filtering so I was thinking someone put the wrong plug in there one day and killed the pedal. Would it make sense that Q5 would bite the dust as a result?