Problem WIth Maxon AD-999

[Bluefinger]

My AD-999 stopped working. There is no clear audible delay anymore. When I turn it on, it sounds like the amp stands far away in a big room .. I don't know how to describe it propperly. It all gets mushy and there is a hint of delay but nothing defined.

Any ideas what this could be and if it might be something I can fix myself?

thanks!

[Mark Hammer]

Um, so how many trimpots are on the board in the one you have? I know the early issue ones had something like 16 or more. They were produced when the supply of 4096-stage delay chips had basically dried up, so Maxon went with eight 1024-stage chips in series, and went to the trouble of providing bias and output balance trimpots for each one.

Note that if the bias on any single one of them is off, then the entire delay path will turn to mush. It CAN be readjusted and restored. The thing is that doing it by ear when there is ONE trimpot is something the average user can do. Doing it by ear when there are 8 such trimpots...not so much.

[Bluefinger]

Thanks ... I'd have to check when I'm at home. So who do you think can do it? Will I have to send it to Maxon?

all the best
BF

[Mark Hammer]

If you have an appropriate audio probe, that would allow you to listen to the output after each bucket brigade chip, then you could do it by ear yourself.

The way that BBD bias works is that if the bias is too high or too low, no audio signal will pass through the chip. If the bias is a little bit too high or too low, then signal will pass, but it will be less than full amplitude and be distorted as well. When you reach the magic spot in your trimpot twiddling where the signal is both clear and as loud as you'd expect, then you've nailed it.

An oscilloscope can help you nail the "perfect" setting, but generally you'd want to have a signal generator too, so that you can compare the waveform you know you've fed the pedal with what you see on the screen, and adjust for least distortion based on visual feedback. What most players can use with reasonable success is simply a guitar plugged in, and their ears. It will take you 95% of the way. Ideally, one would listen to ONLY the delay signal, with no clean signal mixed in, so that the quality of the delay signal could be heard in pure form. I recommend to those attempting such an adjustment to do it through headphones if possible so that there are no distractions. It's not absolutely critical, but you should try and place yourself in the best possible position to judge the sonic quality.

The special challenge with tweaking the AD-999 is that if you can only hear the cumulative output of all eight chips in series, how would you know which particular bias trimpot is off? If you were able to provide some info to any qualified bench tech, they should be able to check and make any needed adjustments. Ideally, such a tech would know at least a little bit about BBD chips. If they don't, feel free to direct them to me and I'll be happy to advise.

Obviously there can be many other sources of malfunction, but bias drift is a common one with analog delay pedals, and generally the first place you look when troubleshooting, simply because it involves absolutely no changes to the board, merely an adjustment. Techs and musicians alike appreciate it when no soldering iron has to come near the pedal.

[Bluefinger]

Thanks a lot!

I'll take it apart and if there is a way to mark the original settings to avois messing things up even more, I might give it a try myself. Otherwise I'll find a tech ...