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|11/9/2002 4:23 PM|
|Boy Howdy||Arion SCH-1 chorus|
Ebay bandito got me.
It worked fine at first, then it starts to distort, followed by a gradual diminution of the chorus effect, which then ceases altogether. At this point, it seems to be dead (I'll try it again later) except for the tone control which continues to work. Any idea what this could be? (good battery, and wall wart, same result)
Any chance I'll get my money back from this guy? He seems to be trying to weasle his way out of the situation. He's "going out of town till the 18th" as of yesterday. I haven't until just now asked for a refund, nor have I entered any feedback on him.
|11/12/2002 5:13 PM|
Your comments indicate that the BBD chip itself (the most difficult one to replace in the pedal) is okay, so hold up at 2nd base for the moment.
BBDs need to have their audio input signal riding on top of a DC bias voltage about a volt below V+. When the bias voltage is set improperly the resulting delay sound is grainy and distorted (small bias error) or nonexistent (big bias error).
Some pedals/units let you tweak this with a trimpot on the board, where others use a voltage divider or some other means to tap the bias voltage off the supply. Sounds to me like something which is governing the bias voltage needed by the BBD is drifting. If it takes time to drift, then it sounds like there is a bad cap somewhere. The most obvious culprit would be in the supply regulation. Try replacing the biggest value cap on the board.
Find out which BBD chip it is, and identify what the DC voltage is on the appropriate pin. If you lack datasheets, most of them can be found in a largish file called BBDementia.zip at my site (http://hammer.ampage.org).
Chances are pretty good you can debug this baby and turn it into both a learning experience and reasonable purchase (if one with a rocky start).
|11/13/2002 1:41 AM|
Hi Mark, Thanks for the reply.
There is a trim pot on the board. Should I monkey widit, before I remove the cap? Would that be an electrolytic or a regular cap. There are a lot of caps on there, and i can't read all of 'em; any hints on how to narrow it down. Please tell me that it's one of the (physically) big ones.
You know that distortion actually sounds kinda good -- before it gets too extreme.
|11/13/2002 4:23 PM|
If there is only one trimpot, there are a number of things it could be, only one of which is the bias setting. If you are able to remember where it was and reset it after monkeying, then feel free to monkey. You will not destroy anything in the process.
Typically, power supply caps are the biggest caps and largest value on the board. It is unlikely to find anything in the 1000uf range for a battery operated pedal, but 100-220uf wouldn't be out of keeping, since a 16v version of either can come in a fairly small package these days. There should probably be only one or two of these on the board since just about any other part of the circuit wouldn't use anything much beyond 10-22uf.
Please note I am not guaranteeing that this IS the specific source of your problem, but it is a possible candidate, given that the problem emerges over playing time and the tone described sounds like a mis-set bias, and is pretty cheap to fix. In some designs, such as John Hollis' minimalist Zombie Chorus (which a number of happy builders here and elsewhere have gotten up and running), the bias is set by the quirks of the op-amps used with no real circuit portion deliberately designed to supply the bias voltage. It may well be that your chorus depends on a similar sort of happy accident.
I guess we'll find out soon enough.
|11/14/2002 2:15 AM|
Well, there are two 220mf electrolytic caps. They are the biggest caps and have the largest value - based on my rather cursory overview. One appears to be connected to the power supply jack.
|11/16/2002 11:16 PM|
I replaced the aforementioned electrolytic cap (same value, higher voltage) ... to no effect. Same as it ever was.
|11/17/2002 12:08 AM|
Okay, that's one thing to cross off the list. Can you tell me what happens as you tweak the trimpot you mentioned? That would give some additional clues.
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