Mark I Suitcase Preamp Trouble

[Electroharmony]

Hi all,

Having some trouble with my old 73 Mark I Suitcase preamp.  It is the silverface, with sliding EQ version.  

The trouble is, it is only giving output on one side.  I have searched the threads, (thank you for all the info) , and determined it is the preamp:  I have tested this top on a similar suitcase bottom with known working power supply, voltage regulator, amp boards, and 5 pin connector cable, and still only one side is giving output.

I looked at the schematics (100w Suitcase) so nicely provided on this site, but can't seem to figure out where it splits signal to dual-channel. Seems it might split on the Op-amp A3 chip.  Anyone else ever have this problem? Could anyone gently nudge me as where might be a good place to check on the circuit board?  

Thank you kindly

[Electroharmony]

*EDIT*

After checking some more, it appears both sides are working.  However, when vibrato is engaged, it cuts out one channel (when the LED is blinked *off*).  Also on that channel, with vibrato engaged, the signal is directly effected by the position of the intensity pot (at 10 channel is completely silent, at 1 full signal).

Any ideas ?

[Ben Bove]

Well the intensity pot as explained to me, is actually the mixing-in of straight signal.  So it's not affected by the intensity... that's just a signal add up the center.

So it is definitely dead completely when vibrato is engaged.  Have you checked the suitcase with a set of stereo headphones?  This way you can easily hear if both channels are working all the time, vibrato or not as they're separated into each ear.

[Electroharmony]

Checked with stereo headphones.  Both channels working.  Interestingly, even with vibrato off, the intensity pot still controls the signal on Channel A (0 = full signal , 10 = no signal).  

So, if I understand you bjammerz, it almost seems like vibrato is always on channel A, regardless of switch position and is always being effected by the intensity pot?

I've spent some time checking the forums, and found this article - http://www.fenderrhodes.com/service/amp-failures.php - that explains the most common failures were the 4558 IC's.  Does that sound applicable in this situation?

[Electroharmony]

And another quick update...  Just checked all three IC's , and they all have +14V, and -14V at points 4 and 8 , regardless of position of intensity pot.

[Ben Bove]

I'm not the master electronics guy on here, but it certainly sounds like something is dead or shorted where the vibrato signal path is constant on one of the channels.  I have heard of this problem before, not just one one channel, but in general.  May be a technician's fix or if someone can shed light on what would cause that

[Electroharmony]

Thanks for the reply -

Just found a shorted diode in one of the VTL5C1 optocouplers, located at LED1.  Looks like this could be the root of the problem, as this is connected to the output of channel A (or so I think).  Figure I'll change out both of em - just ordered 2 at a cost of $30 delivered to my house.  I'll update once they are changed.

[Electroharmony]

Good news!  That was the fix! So the moral of this thread will be, if you have isolated the trouble to your pre-amp, and are still missing a vibrato channel - check the LED optocouplers, located at LED1 and LED2.  You can buy new ones that work here: http://www.newark.com/eg-g-vactec/vtl5c1/optocoupler/dp/43F885

cheers