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AMDEK DMK100 gone bad - repair? Special Ef­fects Plugins
Old 4 weeks ago
  #1
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AMDEK DMK100 gone bad - repair?

Hi

my AMDEK DMK-100 has started playing up - i'll do an audio example later -

as soon as Effect is engaged, there's a regeneration going on as if it had a
signal going in. there is evidently a beating with the Delay Time, Intensity is
unusable: completely full-on, and the Input Level seems to have hum.

This is on the High output switching at the back. Low doesn't seem to produce
much level at all.

H,M,L input settings on the front aren't helping. suppose you need the audio
demo for this. please give a heads-up if you are conversant with BBD circuits
using MN3101/MN3005 and this unit in particular, OR can recommend a site to
go to for help. i don't have access to a tech' nearby, nor want expense of
getting it fixed, if i can avoid it!

thanks for any help.
Old 4 weeks ago
  #2
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Not sure I could recommend a particular site.

I've done repairs on several BBD's ranging from a blown preamp, power supply and just the normal need to clean pots and recalibration.
This particular echo does have trim pots which can change the wave shape and things like regeneration. What I'd typically do is connect the unit to a scope then feed it a sine wave and see how well the copied wave looks in comparison to the dry wave. Depending on its age, the caps can drift out of specs and the echo can become warped or distorted. Even if you don't have the schematics or calibration procedure you can usually figure out what each pot does and calibrate the thing.

If you don't have an oscilloscope its not something you should touch. You only need to tweak the pots a little and totally screw things up and possibly damage components.

If you do have a scope you'd connect it to a signal generator. I typically use a keyboard and have it produce a clean sine wave. I then measure the amplitude coming out of the keyboard so I know its not too large or small for the echo unit. Then with the echo volume levels set at about 50% I should be able to get about the same amplitude wave out of the unit that is going in.

After that I'd mark the current trim pot settings with a felt marker so I can set it back where I began before I tried to tweak them.

Then I'd set the echo for a 50% dry, 50% wet so I can see both the dry vs the wet sine wave and compare how the two look. If the copy is distorted, cropped, tilted etc compared to the dry you can take one of the trim pots at a time and see how it affects the wave form.

Of course you want to have your repeats set for one and the speed set to various levels. Chances are it will likely sound best in the center of the range but you'll want to check the speed after making any tweaks to make sure it works across the entire range and not simply at one end of the user knobs. you can wind up with it sounding fine at one end of the knob and having all the bad stuff happen at the other end.

Chances are when tweaking the trims you probably don't need to turn them more then a few degrees in either direction. The trims may also be frozen with silicone glue or locktite. If you choose to break those seals you do it at your own risk of course.

As far as what each put will do, you'll likely have one that affects the number of repeats. You'll want to be sure your user knob is set to minimum then check and see if the echo produces a single echo on minimum and begins to self regenerate in the 50% and above mark.

You'll likely have another trim that tweaks the input gain. You would set it so the input and echo are the same levels without flattening the top of the waves.
The other pot typically changes the shape of the wave. One the ones I've messed with the goal is to get the waves to look ad round and symmetrical as possible so all 4 quadrants of the cycle are identical. The pot can make the wave look unsymmetrical and less round. Sound quality changes when its tweaked too from thin when its off to fat when its correct.

Even in best circumstances the copy a BBD makes is not going to be ideal. In theory it may seem to measure up but when you add in many of the lower quality parts used in BBD's allot of those flaws wind up taking a toll on the sound. BBD's are typically noisy when pushed above a certain gain level and the preamps are not so hot either so they often produce allot of hiss. The copied waveform typically has a good deal of the frequency response missing, especially in the bass and treble frequencies above 6Khz. Still its a common echo many guitarists like as a runner up to tape echo. I own at least a half dozen of them and the best is either the Boss ROD or the Multivox. The RDD-10 has three different speed settings so you can get repeats as long as a few seconds or nearly as high as reverb. The Multivox is can do a wonderful slapback, but not the real long echoes. Its also got a fantastic front end and is designed so it can be intentionally overdriven with a hot preamp setting. I suppose they did it to give a player something similar to hot tape saturation. In any case its pretty cool and acts like an extra overdrive is you don't mind the added hiss.

Personally I quit using all BBD's 10 years ago. I prefer the 24 bit delays which sound clean as glass and have no added noise. They come much closer to sounding like my Space echo which I also own. There are also many different settings you can get from a good digital unit involving multiple delays in stereo that mimic a multi head tape unit. I got spoiled with tape. Having 4 heads that can be turned on and off independently can create some really cool echo patterns.

Like I said, I wouldn't attempt to recalibrate the unit without a scope and to pay to have it done, you'll need to make that decision. I listened to a demo on that unit and took a look at the board. I don't see that unit being worth more then $100 in top working order. Even the SS Multivox I own has three times the circuitry and three times the quality of components and they sell for chump change on eBay these days. What's popular now involves tap tempo so the player can time his echo to the music. The older units are fun to mess with but when they start having issues, I'd say retire it and move on. Far too many great buys out there to be spending good money on a budget build like that.
Old 4 weeks ago
  #3
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Quote:
Originally Posted by wrgkmc View Post

I've done repairs on several BBD's
great info. i'd tend to agree about digital delays. i was just about to sell it.
it's been nice on analog pads from alpha juno, main reason i've kept it,
but i have another one now that i decided to keep instead.

it's worked fine for 20 yrs. then it seemed like the repeat was going a bit
too wild, and now it's - not 'noisy' as in hiss: it's just sort of on-all-the-time,
and perhaps being fed from something on the input control. does seem
completely out of whack switching in and out levels, no porportion at all,
unusable except at one setting, really.
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