moog MF 104Z delay turned completely silent

[Frances Rhodes]

hi everyone

i bought an "old" MF 104Z (serial #0288) and i've had some problems with it.
it worked well until it started to pop really loudly when switched on and off, until it finally turned silent.
i opened it, checked the jacks and they all work fine, no broken connections, no wire unsoldered.
i tried the delay only output, and it works as a charm, only the mix output is silent.
also, every control works as it should.
i contacted the moog tech guys because i cannot find any moog licenced repair center around. when i told them about the loud pops i heard when i switched the pedal on and off they thought it might have been a faulty Cmos (used as a switch). i did replace it with a new one and nothing changed.
then, a few weeks later, it finally "died", so i came back to them and they thought it might have been the OTA driving the output that fried, which could have explained the pops, but didn't say much else. i looked and there are maybe 6 different LM13700s in there, and by then they didn't tell me what chip was the output OTA so i could try to replace it.
i bought a new LM13700 but had to let the MF aside for a while because life happened, now that i finally have time again, none of the emails @moogmusic work anymore and i only get delivery failures.

would there be anyone here knowing what LM13700 is driving the mix output, or anyone knowing how i could check all of them to see if one is dead (preferably without removing every one of them)?

thanks in advance,
regards

frances

[Mark Hammer]

Six different LM13700s?  Is that a delay or a phaser?

[Frances Rhodes]

it's the delay!

EDIT: there are 4 LM13700N chips in there.
2 are close to the jacks and 2 others are close to the MN3101, i guess those are for the active low pass in the delay loop.

[Mark Hammer]

Ah.  I gather the OTAs are used for the external voltage-control of things like delay level and feedback, and/or tracking lowpass filtering (to get most usable bandwidth for any given delay time).

Another naïve question:  You wouldn't happen to be using it in a series of pedals would you?  Or does it pop when used entirely on its own - just you the guitar, the delay, and the amp, with nothing else in between.

[Frances Rhodes]

yes, there are CV inputs on the rear panel: feedback - time - mix, one OTA is probably there for the feedback, you're definitely right!

i had pop problems with other pedals before so with the MF, i tried it in series with other pedals (at the end of the chain, just before the amp), then alone, with different amps, different instruments (several guitars and a fender rhodes piano), several cables (factory made and hand made), i even tried it with nothing going into the input, and it poped all the time. the only thing i couldn't change is the power supply because i don't have a spare one.
but only the mix output poped (before turning completely silent), the delay only output never poped, and still works fine!