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nathanscribe
Silver Member
United Kingdom
376 Posts |
 Posted - 10/03/2008 : 23:54:42
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I'm finally fixing my DR-55 Dr Rhythm, and won't bore you with the details (basically, the patterns write OK but in Play mode they are getting wiped as they cycle) and I suspect that the voltage on the Read/Write pin is too high in Play mode and it thinks it's in Write mode. Or something. Anyhow, my question is this:
The voltage on this pin (20) of the IC is almost 2V, and should be lower - around .65V is recommended on the datasheet. The voltage here is determined by a diode straight from the batteries, a 22u/10V cap, and a 220k resistor. Here's a link to the circuit. I'm looking at top centre, IC1, pin 20.
Anybody any suggestions of the neatest/best way to drop the voltage here? I thought you pedal gurus would be used to working in such confined spaces and would be good to ask...  |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 10/04/2008 : 03:44:20
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Hi nathanscribe!
There is actually no easy answer to this question  The 220k resistor is a "pull-up" resistor that holds the pin at the Vcc voltage unless something grounds it. Looking at the circuit, you need to turn the switch to "write" and then the circuit with Q16 triggers it.
It looks to me like the pin is pulsed, and when it's in "write" mode, the read/write pin is triggered to write in time with the tempo. In "write" mode, the the read/write pin would have 4.5V (read) applied when it isn't triggered, and 0V (write) when it is. Perhaps the "2V" you are seeing is these voltages averaged by your meter? Only way to tell for sure what's going on is to use a scope....
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nathanscribe
Silver Member
United Kingdom
376 Posts |
Posted - 10/04/2008 : 12:24:50
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Thanks for the reply Laurie. You're absolutely right, in Write mode the pin is pulsed; and, oddly, Write mode works perfectly! I can write patterns, switch patterns (remaining in write mode) and add other patterns - and the data is retained. Though, I was under the impression the r/w voltage was the inverse of what you say - that read is low, write high... must check the datasheet again.
It's in Play (read) mode the problem occurs. Switch to Play, and as it cycles, the data is lost. Sometimes I can get a bar out of it before it's all gone. The IC datasheet suggests a minimum value for 'high' of 2V, and a max for 'low' as 0.65 - I'm getting 1.6 to 1.9 in Play mode. That's on a multimeter though; I'll dig out my scope today and plug it in. As far as I'm aware, there should be no pulsing in Play mode, as the switch would leave the pin connected only to the power via the res/cap/diode. |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 10/04/2008 : 14:55:45
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Right you are!
The pin is shown as "read/(bar)write"... the bar above "write" indicates write is active when the pin is 0 volts.
With the switch set to "play", pin 20 should be close to Vcc. Check the volttage at both sides of D5 to ensure that it hasn't failed. I dont have the data sheet to check if any harm could be caused, but I'd probably then replace C39 and swap R65 for 100k and see if that made a difference.
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nathanscribe
Silver Member
United Kingdom
376 Posts |
Posted - 10/04/2008 : 16:45:50
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This is becoming really quite annoying. I replaced C39 (and C42/IC3 just in case); D5 seemed OK so I left it in. I checked the voltage at pin 20, and in play mode it read about 5V (supply is around 6). I tested the unit out of its case, and everything worked as it should. I put it back together, and it worked for about 5 minutes, then decided to fail again.
As I say, when programming, all is well. But in play mode, it loses the patterns as it plays them. Sometimes it will get round a whole (2-bar) rhythm without failing, then lose the odd bit of data - effectively, wiping the occurrences of the drum sounds at whim. Other times, it will just play an empty rhythm - only the hats (triggered, not programmed) remain. What's confusing me is that this only happens in Play, never in Write. If the memory IC was failing, would it do something this odd?
I wouldn't mind so much if I'd bought it like this, but I've had it for a couple of years and it's been perfect, up to a few weeks ago. It's not the batteries either - I put fresh ones in when I first noticed things going wrong. |
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RonNovy
Copper Member
USA
24 Posts |
Posted - 10/07/2008 : 12:22:26
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Don't know if you have this already but this may be of some help...
http://manuals.fdiskc.com/flat/Boss%20DR-55%20Service%20Manual.pdf
[edit]... Just noticed in the schematic that it says R77 and R100 are on the foil side... Maybe the leads are touching another trace on the board or something... |
Edited by - RonNovy on 10/07/2008 12:37:17 |
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nathanscribe
Silver Member
United Kingdom
376 Posts |
Posted - 10/07/2008 : 20:54:11
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Hi Ron - I do have that already, but thanks anyway.
I checked those two resistors - they're fine.
Actually, I have two DR-55s, both now failing... I bought one cheap as faulty, for spares/repair, but I'm sad to see my good one keel over. It'll be something daft, no doubt. |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 10/07/2008 : 22:35:06
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quote:
Originally posted by nathanscribe
Actually, I have two DR-55s, both now failing...
What's common to them? Power supplies? Wall outlet/house circuit? Cables/amp?
Just thinking out loud... |
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nathanscribe
Silver Member
United Kingdom
376 Posts |
Posted - 10/08/2008 : 00:20:42
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They run off batteries only, no DC sockets on them; I plug them into the same mixer (Mackie 1604VLZ Pro); but the one that has been working till recently has been with me for a couple of years, and only decided to flop a few weeks ago.
The other was bought as a dud for experimenting with, and could have been fried in any number of ways prior to arrival.
What's confusing me is that the one we've been discussing ONLY fails in Play mode - which leads me to suspect it's not the memory IC. I may well be wrong though, I don't know what the signs of failure are on those. |
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Dr. Bob
Moderator
Australia
6593 Posts |
Posted - 10/08/2008 : 06:11:07
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I haven't look at the Schematic yet, but,
Does it have a back-up battery for the RAM, or is it Flash RAM based?
Flash RAM has a finite life cycle.
Leaky or Dry electrolytic caps?
A lot of Tantalum's from this era seem to be playing up, I'm starting to see a lot more Shorted Tantalum caps, than I would like to admit.
Just thinking out loud.
Regards Dr. Bob
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RonNovy
Copper Member
USA
24 Posts |
Posted - 10/08/2008 : 07:46:32
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So that's check C39? Check the Write/Play switch too...
It looks like the batteries supply a constant voltage to IC1 weather it is turned on or off... If a power cap has failed then it could drain the batteries pretty quickly which would cause it to fail in play mode and maybe even write mode...
[edit] Something else I just noticed is that the power is switched in the output jack? Check the contacts on the jack. They could be worn or rusted and could probably use a cleaning...
I know D5 was mentioned but check that again too. Make sure you remove it before testing... |
Edited by - RonNovy on 10/08/2008 12:18:23 |
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Topic |
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Voltage drop method?
