Boss DR-55 Repair

[bitrex]

I'm trying to repair this vintage programmable analog drum machine on my
bench. It was water damaged, and it looks like it's been sitting in a
basement and hasn't worked in a very long time. I've cleaned up the PCB
and underneath the rust residue it looks OK, and I've replaced all the
electrolytic capacitors and removed any signs of corrosion. It's
showing signs of life now, and occasionally produces sounds that seem
vaguely correct, but still isn't working properly.

Here's the schematic:

http://fa.utfs.org/diy/boss_dr55/dr55_schematic.jpg

It's a pretty crude machine by today's standards - there's no
microprocessor. Just what seems to be a CMOS static RAM chip that the
patterns are manually bitbanged into by the user in "write" mode, and
then clocked out of in "play" mode. The logic outputs of the RAM IC
then trigger four different analog sound generation circuits on the
right...for example the bass drum seems to be based on a transistor
phase-shift oscillator, etc.

The first big problem I see with my scope is that the there's no clock
coming out of the clock generator when I try to set the machine to
"play" - tracing backwards it looks like the output at pin 3 of the flip
flop made up of two sections of a 4011 (what's the deal with those
symbols?) is oscillating at about 4 Hz, and the nominally active-low
inputs of the flip flop are totally unresponsive to button presses or
manually pulling them low with a jumper.

On the scope the oscillation looks like a slow "inverse" exponential
decay from high to low, followed by the output of the flip flop slamming
low for a little bit and then rapidly coming back up and repeating. It
looks like the on-off duty cycle is about 90%.

Any idea what could be going on here?

[Phil Allison]

** Have you cleaned and checked for continuity the normally closed contacts on the Foot Switch jack?

All the switches are likely to have some corrosion and/or grime on the contact surfaces so need a few drops of WD40 or similar.


... Phil

[mrob...@att.net]

In sci.electronics.repair bitrex <bit...@de.lete.earthlink.net> wrote:
> I've cleaned up the PCB and underneath the rust residue it looks OK,
> and I've replaced all the electrolytic capacitors and removed any
> signs of corrosion.

Did you notice that C42 (in the clock circuit) is non-polarized?

> The first big problem I see with my scope is that the there's no clock
> coming out of the clock generator when I try to set the machine to
> "play" - tracing backwards it looks like the output at pin 3 of the
> flip flop made up of two sections of a 4011 (what's the deal with
> those symbols?)

A 4011 is nominally four NAND gates - inputs A and B, output
not (A and B). not (A and B) = (not A) or (not B). I don't know why
they drew three of them as invert-then-OR and one of them as NAND, but
it's logically valid.

> is oscillating at about 4 Hz, and the nominally active-low inputs of
> the flip flop are totally unresponsive to button presses or manually
> pulling them low with a jumper.

The section labeled F/F seems to handle the "start" and "stop" front-
panel buttons as well as the foot switch. I would expect pin 3 of the
4011 to go high and low as those controls are operated, but not to
oscillate like that.

Part of the feedback for the flip-flop goes through the foot-pedal
jack socket. Are you sure that socket is OK and not crudded up? Also,
are the "start" and "stop" switches really open circuit when not
pushed?

If you completely pull the 4011 out of the circuit, apply an 0V/5V
square wave clock to where pin 11 was, and the inverse of that clock
to where pin 10 was, I think it should make a lot more noise than it
does now.

Matt Roberds

[Mike Tomlinson]

En el artículo <55fb77d5$0$5643$4c5e...@frugalusenet.com>, bitrex
<bit...@de.lete.earthlink.net> escribió:

>

>The first big problem I see with my scope is that the there's no clock
>coming out of the clock generator when I try to set the machine to
>"play" - tracing backwards it looks like the output at pin 3 of the flip
>flop made up of two sections of a 4011 (what's the deal with those
>symbols?)

They're the standard symbol for logic gates.

https://s-media-cache-ak0.pinimg.com/736x/a1/72/a6/a172a616d71c340624f75
15b8b4e6d3a.jpg

> is oscillating at about 4 Hz, and the nominally active-low
>inputs of the flip flop are totally unresponsive to button presses or
>manually pulling them low with a jumper.

Pin 3 (the output) is connected via R95 to two switches. How sure are
you that these switches are ok? Could they be corroded inside?

Lift one leg of R95, does the output signal shown on your 'scope at 4011
pin 3 improve?

Swap out the 4011?

--
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(='.'=) Bunny says: Windows 10? Nein danke!
(")_(")

[bitrex]

On 9/18/2015 12:27 AM, mrob...@att.net wrote:
> In sci.electronics.repair bitrex <bit...@de.lete.earthlink.net> wrote:
>> I've cleaned up the PCB and underneath the rust residue it looks OK,
>> and I've replaced all the electrolytic capacitors and removed any
>> signs of corrosion.
>
> Did you notice that C42 (in the clock circuit) is non-polarized?

Yep, it's been replaced with the appropriate non-polarized cap.

Thanks for getting back to me, I'll give your suggestions a try.

[TTman]

er.
>
> On the scope the oscillation looks like a slow "inverse" exponential
> decay from high to low, followed by the output of the flip flop slamming
> low for a little bit and then rapidly coming back up and repeating. It
> looks like the on-off duty cycle is about 90%.
>
> Any idea what could be going on here?

The output of F/F pin3 should be static, either 0 or 1. Pressing START
will set pin 3 to HIGH and enable the oscillator IC3 pin 11
D18/R78/C41 ensures the flip flop powers up in the STOP state.
For IC3/3 to oscillate at a few Hz, something around there is faulty/broken.

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[bitrex]

On 9/18/2015 11:59 AM, TTman wrote:
> er.
>>
>> On the scope the oscillation looks like a slow "inverse" exponential
>> decay from high to low, followed by the output of the flip flop slamming
>> low for a little bit and then rapidly coming back up and repeating. It
>> looks like the on-off duty cycle is about 90%.
>>
>> Any idea what could be going on here?
> The output of F/F pin3 should be static, either 0 or 1. Pressing START
> will set pin 3 to HIGH and enable the oscillator IC3 pin 11
> D18/R78/C41 ensures the flip flop powers up in the STOP state.
> For IC3/3 to oscillate at a few Hz, something around there is
> faulty/broken.

I found that _both_ the pushbuttons were faulty and the IC had failed.
How annoying.

[mrob...@att.net]

In sci.electronics.repair bitrex <bit...@de.lete.earthlink.net> wrote:

> I found that _both_ the pushbuttons were faulty and the IC had failed.
> How annoying.

Hey, at least you found it. Thanks for posting what the result was.

Matt Roberds

[bitrex]

Yeah. The switches are that membrane type where a conductive bit of
carbon shorts two contacts together. They can be pried open by drilling
into the case a bit with a Dremel tool. It looks like over the years
the metal contacts had formed a layer of oxidation, preventing a good
contact from being made.

Replacing the 4011 stopped the oscillation.