Audio Electronic Circuit Schematics

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Re: Audio Electronic Circuit Schematics

Post by timdrage » Mon Oct 08, 2018 3:18 am

Yes the one with the mini cartridge. Motor runs fine but the entire cartridge holder, tape heads and the cart itself broken beyond proper fixing, has been badly repaired over the years with wrong screws etc so nothing goes together properly any more. Had it kind of working in a nicely broken sounding way but like I say I think I need to hack some other tape mechanism into it.

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Re: Audio Electronic Circuit Schematics

Post by WhiteWarlock » Mon Oct 08, 2018 9:41 am

Do you have this type of "Apollon tape"?
The top problem on those tapes with that unit is the "tape pressure pads"
do you have any squishy foam pads that IC chips are stuck on during shipping?
make replacement pads for keeping tape on tape heads......................
insert under tape in case.......................................
the old pads on those tapes disintegrate over time..................
you can hack something together even if it is simple tape loop in Apollon case
btw felt didn't work as well as IC chip foam
get creative when troubleshooting
best of luck

silly question time....
can you somehow mount the record/play heads so they make better contact with tape?
sounds like your Univox tape delay unit will function with some creativity from your description IMO

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Re: Audio Electronic Circuit Schematics

Post by WhiteWarlock » Mon Oct 08, 2018 3:40 pm

Awesome pedal perf/strip/pcb layout resource site:

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Re: Audio Electronic Circuit Schematics

Post by timdrage » Sat Oct 13, 2018 1:30 am

Yep it's a that same kind of cartridge.l, totally broken tho. Ah the IC anti-static sponge idea is a good call. I've fixed 8-tracks before with some kind of window insulation foam tape stuff too
Can you somehow mount the record/play heads so make better contact with tape?
That was the plan but it's broken or loose in so many places that it barely functions :) I did try attaching a walkman temporarily and that seemed like it would work, I should properly rebuild soon that way :)

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Re: Audio Electronic Circuit Schematics

Post by WhiteWarlock » Sat Oct 20, 2018 11:55 am

BOSS DR55 (Schematics/Manuals/Mods)
DR55 does have an authentic sense of early 80s legendary nostalgia...
being BOSS' first DR-machine and also quite a successful early programmable drum machine.
It has been used by New Order, The Cure, Chris Carter, Sisters of Mercy, Chris & Cosey, Soft Cell and Thomas Dolby.

Boss DR-55: external trigger input mod

By default, the Boss DR-55 does not receive any kind of incoming clock. The ‘FS’ footswitch input takes a latching footswitch that starts and stops the existing clock, but that’s it. Although you can clock other equipment from the DR-55, it would be nice to be able to use an external clock to sync the Boss to, which would allow the Boss to trigger yet more devices with its CSQ and DBS outputs (active on Accented steps only and every step, respectively).

My mod as detailed here does exactly that. By replacing the existing FS jack socket, adding a small circuit, and replacing a jumper, we can safely trigger the DR-55 from an external trigger.

A quick internet search will turn up an existing clock input mod which is simpler to do and requires no extra parts; however, it puts the RAM at risk of damage from high triggers, and it does not sync the Boss’ own DBS output. It also requires ‘arming’ by hitting start before external triggering.

My own mod, though more complex, overcomes all these issues: the trigger input is protected, both the Boss’ trigger outputs maintain their correct functions, and triggering occurs without ‘arming’. The only two functional disadvantages of my mod are that you must set the Boss’ tempo to Fast, and to reset the pattern when stopped mid-way you need to remove the trigger plug. I’m going to blog another small mod which will overcome the latter inconvenience [EDIT: No I’m not! I sold both my 55s, thereby halting this particular project].
The Clock Modification in detail

Below is a diagram which shows everything you need to know about building this mod. Below that is a parts list. Key to this is the replacement FS jack socket; it needs to be TRS (ie. a stereo jack), with single pole changeover switches on the tip and ring contacts. I used a Lumberg KLBPSS3 (datasheet here, Farnell UK stock page here).

The additional circuit can be made very small indeed (3 rows * 8 holes on stripboard), and there is plenty of room for it inside the DR-55, particularly towards the right-hand end. The photos below illustrate my own placement.

There is one jumper to be removed, the one immediately to the right of the Variation switch. The replacement connections for the upper and lower point of this removed jumper are shown in the diagram, and you can see in the photos how I wired this up.

In brief: remove that jumper, solder the two points to two jack pins; build the extra circuit, and solder that to the jack and to the main PCB; replicate two of the pre-existing connections from the jack to the PCB. That’s it. I also stuck a small folded piece of card to the PCB to stop the extra circuit from shorting against components.
Boss DR-55 clock input mod


Boss DR-55 clock input mod revised
Parts list:

1 * TRS 2-pole changeover jack socket – eg. Lumberg KLBPSS3
2 * 47k resistors – I used 1/8W for their smallness
1 * 10nF capacitor – I used a ceramic, again for smallness, but polyester film etc. would be usual
1 * 1N4148 signal diode or equivalent
1 * BC549C transistor or similar standard NPN

Here’s the modified DR-55 (also incorporating my DC supply mod):
clock modded DR-55 overview

clock modded DR-55 overview

And here’s a close-up of the clock mod:
clock modded DR-55 close up

clock modded DR-55 close up


How to use your new trigger input

The new trigger input will accept any positive pulse over a couple of volts. It’s edge triggered, so the pulse can be any length over a couple of milliseconds. The operating principle is to use the DR-55’s existing clock, but to gate it on for a very short duration; normally when the clock is gated off again, the pattern resets, but the new jack socket enables us to disable that by breaking the reset connection when a jack is inserted.

As I mentioned earlier, the Tempo must be set to Fast (ie. all the way clockwise) for correct function. This is because the DR-55’s clock, once triggered, finishes its pulse cycle. If this is longer than the incoming trigger cycle, it will ignore the new trigger; if we set the speed dial to its fastest, we can clock the DR-55 at any rate up to its natural maximum.

The pattern will cycle round as usual, but if you stop mid-pattern, new triggers will continue where they left off. To reset the pattern at this stage, you need to unplug the trigger jack and hit Stop. This is not ideal, I know, and I will be making an amendment to correct this later [EDIT: project halted, see above. I have no current plans to do any further work on the DR-55].

For now though, this mod works fine, as shown in the (slightly rubbish) video below:

Boss DR-55: a 9V DC input modification

One of the drawbacks of the DR-55 as it comes unmodded is the power supply. In its original form, the DR-55 takes only batteries, and though this might be good for reducing cable clutter and having to find yet another wall-wart, it does mean you need to keep a regular stock of fresh AAs, and can guarantee that just when you want to use it, your DR-55’s batteries are too drained for the unit to function correctly.

Luckily, it is a relatively simple process to modify the DR-55 so that it takes a commonly-found 9V DC supply instead. I provide instructions for this below. It’s not the only way to do the job, but this is how I did it, and it works just fine. Modding the DR-55 in this way means it no longer accepts batteries, which means two things: 1) you will need access to a 9V adapter, and 2) pattern data will not be retained on power-off. Given that filling the memory of this humble machine can be done in less than five minutes, and I never use this outside my own home studio, I never found memory retention to be an issue. It would be possible to design a DC input that also catered for memory backup via battery, but I’m not going there.

There are two basic stages to this modification:

Making a 9V DC input: the basic voltage supply circuit
Installing the Mod: adapt some wire links on the output jack and PCB

Because the DR-55’s RAM can be killed by voltages higher than around 7V, we take a 9V input and regulate it down to between 5V and 6V. I chose to use a 5V regulator propped up with a diode to give around 5.6V, but you could also use a 6V regulator and omit D2. The input jack I used is a 3.5mm mono minijack of the kind often used for audio and CV interconnects, mainly because I had lots of them and the holes are easier to drill than the larger ones needed for a plastic-bodied insulated barrel connector. Use whatever type you prefer, but note the polarity of your incoming DC, and don’t connect the +ve to the case… with a tip-positive 3.5mm jack, the sleeve of the input jack is connected to the shell of the socket, so it makes sense for that to be the ground. Some barrel connectors do likewise.

Here’s the schematic:

Boss DR-55 DC input mod schematic

Here’s the final circuit built onto stripboard. It will be panel-mounted using the socket:
DR-55 DC input build

DR-55 DC input build
Installing the Mod

Now we have a simple DC input, we could just solder the +ve and Gnd outputs to the corresponding solder points on the main board – that is, where the battery clip attaches. Black is ground, red is positive. This works, but you still need to insert an audio cable to turn the DR-55 on. I chose to remove that ‘feature’, as there are no longer any batteries to protect from accidental drain. It’s a simple mod that just means a couple of wiring changes.

The diagram below shows the required re-wiring. The audio output socket is wired by default to both ground and audio signal, as well as having two pins wired to act as a switch when a jack is insterted. We want to retain the audio and ground connections, but not the switch. We remove those wires and instead bridge the corresponding points on the PCB.


Here’s a photograph of the full mod (note the wiring):
DR-55 DC input wired and complete


I damaged a track while desoldering the battery wires, which is why the red wire goes to the un-numbered hole next to point 9. They’re directly connected, happily.

Below are some photos of the hole I drilled for mounting the new DC input, and the final appearance when mounted and labelled with cheap Dymo (should have gone with black… oh well):
DR-55 DC input enclosure drilling

So there you have it. My humble DR-55 now works from a regular 9V DC wall-wart supply, and switches on whether or not its audio is connected. The hardest part is putting the DR-55 back together again…

or you can just use old hacked (w/ 9v battery connector)phone charger from cheap Samsung 5v pay as you go track phone...
operating power range from the service manual for BOSS DR55 is 4.5v to 6v
Works for me!

next step external trigger inputs & outputs for each drum sound ... Manual.pdf ... input-mod/
it's obviously not a TR808 or TR909 yet picked it up ages for $20 in the original box :P
replaced bad cd4011UB chip...
for getting the clock working on this old machine
was going to buy the CR55 DIY eurorack then remembered had this old gem buried around here :)
Last edited by WhiteWarlock on Sat Oct 20, 2018 7:20 pm, edited 5 times in total.

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Re: Audio Electronic Circuit Schematics

Post by WhiteWarlock » Sat Oct 20, 2018 7:05 pm

From Fri Jan 02 17:35:00 1998
Return-Path: <>
by with SMTP; 2 Jan 1998 17:34:59 -0000
2 Jan 98 12:34:55 -0500
1 Jan 98 06:56:57 -0500
by (8.8.8/8.8.7) id LAA24931
for synth-diy-outgoing; Thu, 1 Jan 1998 11:45:51 +0100 (MET)
by (8.8.8/8.8.7) with ESMTP id LAA24926
for <>; Thu, 1 Jan 1998 11:45:48 +0100 (MET)
id xma028542; Thu, 1 Jan 98 02:45:06 -0800
From: "nc0839" <>
Cc: <>
Subject: Mod for DR55 that provides trigger in's
Date: Thu, 1 Jan 1998 04:49:38 -0600
X-MSMail-Priority: Normal
X-Priority: 3
MIME-Version: 1.0

I got the schematics for the DR55 this week. In looking over them it
appears that a modification to allow a device like the MAP ADV-MIDI, PAIA
MIDI2CV8 and TR series drum machines to trigger the DR55's voices via
trigger pulses is possible. In bouncing back and forth between the PCB
sheet and the schematic I discovered the pins on the "Logic" chip that
control the firing of voices. I used an analog VOM to verify I had the
correct pins (these being 12, 14, and 16). The meter being analog made it
difficult to pin down the actual voltage of the spike and if the spike is
positively or negatively biased. Another piece of luck developed when I
discovered that the trigger lines on the board pass thru jumper wires just
after the chip (a good 3cm of exposed jumper). This would make lifting the
trigger lines from the chip and providing trigger in jacks instead pretty
easy. I tride the whole thing with a crudely fasioned trigger cable which
seemed to actually work.

For this test I made a trigger cable with a 1/8" phone plug on one end and
an alligator clip on the ground and a wire probe on the lead for the other.
I plugged the 1/8" phone plug into my TR606 trigger out and clipped the
ground to the DR55's shell. I then played a pattern on the 606 that would
fire it's trigger output and touched the prob wire to one of the three
jumpers inside the DR55. Every time a TR606 trigger would fire it would
trigger whatever voice I was messing with. This seems to suggest that using
the DM55 in a setup with a MIDI/Trigger interface or a Roland TR type drum
machine is possible.

The only thing I am not sure of is if the TR trigger is a negative pulse
and if the DR55's internal triggers are biased the same direction? I wish I
had a scope so I could see the pulses from the two machines for
comparision. Analog solutions does a mod for TR/CR drum machines to allow
the internal drum voices to be triggered via jacks. Has anyone ever
attempted a mod like this on a TR? What is the trigger voltage/shape used
in these machines to control the firing of voices?

If I could get the pinout of the "Logic" chip I might be possible to modify
the DR55 to sync to an external source. Say DIN or 24ppq pulses. The
"Logic" chip appears to get it's timing from another IC (IC2 in the
schematics). If it were possible to prevent the timer from advancing until
a pulse occures on a curtian pin, one might be able to make the DR55 accept
external sync (In theory).

P.S. I tried the decay mod for the Kick. It makes a very 808'ish sound when
the resistor is bridged.

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Re: Audio Electronic Circuit Schematics

Post by WhiteWarlock » Sat Oct 20, 2018 7:07 pm

From Fri Mar 1 07:55:08 1996
1 Mar 96 10:54:59 +0500
27 Feb 96 14:04:55 +0500
Date: Tue, 27 Feb 1996 12:27:42 -0600 (CST)
From: Kent Williams <>
Subject: DR 55 sync

I got the DR55 schematics from anthony bowyer-lowe today (thanks anthony!).

This thing is actually a modifiers dream since a lot of the strategic
points to cut run through bare metal jumpers on the top of the board.
All the individual triggers come down to either snipping a jumper, or
clipping a resistor lead.

Sound modifications would depend on you knowing more than I do about analog
sound generation, but most progrably would just involve substituting a
pot for a resistor.

Here's the low down on how to sync it to external clock. Disclaimer --
I've not tried this, but it shouldn't be that tough.

The mode switch (play/write) is a 4 pole, double throw switch. One pole
of the switch changes whether the clock comes from the buttons/foot pedal
(for write mode) or off of the clock flip flop. Out of this switch, the
clock goes to the counter and to clock enable on the SRAM chip

As luck would have it, the switch (play/write) goes out into one of those
bare metal jumpers on the circuit board. All it would take would be this.

1. cut the jumper going to chip enable on the RAM.
2. cut the jumper carrying the clock to the hi hat switch
3. add a wire from the far side of the cut to the CE to the far
side of the hi hat switch jumper.
4. Put a jack in so that it makes and breaks the connection
between the play/write switch and the far side of the CE jumper.

This jack is now an insertion point for a 4 PPQ clock pulse. You
can probably plug a trigger out from a TR X0X right into this jack,
and program the X0X to send a trigger on each 16th note.

If you want to do DIN sync you'll need to divide down to 4 PPQ. This
is going into a pin labeled CE -- does this mean it's a positive going

Before I go and wreck mine, let me ask this: Is it safe to feed
the pulse clocks from a X0X box right into CE on an SRAM?

Anyone in the US (Stuart? Dave?) who didn't get the docs from anthony
drop me a line.
In an expanding universe, time is on the side of the outcast. Those
who once inhabited the suburbs of human contempt find that without
changing their address they eventually live in the metropolis.
-- Quentin Crisp
Kent Williams
(319) 338 6053 (home)
(319) 626 6700 x 219 (work)
(319) 626 3489 (fax)

From Tue Feb 13 11:52:23 1996
13 Feb 96 14:52:17 +0500
12 Feb 96 11:03:13 +0500
Date: Mon, 12 Feb 1996 09:29:39 -0600 (CST)
From: Kent Williams <>
Subject: Syncing a DR55

On Mon, 12 Feb 1996 wrote:

> >I've played with a Dr-55 from a guy that went to the studio.
> I've said it before and I'll say it again - the DR55 is the new 909! (Kinda).
If you believe that, I'd gladly sell you mine for $900 ...

> I've not worked out how to sync the li'l bugger to anything external yet, but I

As was mentioned in passing in someone else's post -- If you plug
a trigger pulse into the foot switch input (I've used 606/707 -- which
is apparently a 5-7volt positive going pulse) and put the 55 into
'write' mode (as though you were going to program in a pattern),
then the 55 triggers on each pulse, sort of.

'Sort of' means that at least on mine, it is sensitive to how fast the
triggers come. Sometimes it will decide to stop and not play any more,
sometimes it will get behind on tempo, but if you fiddle with it, it


The snare sound is gated by the next clock pulse. SO if you send
a trigger on a time step where the snare is supposed to fire, and then wait
a while to send another trigger, you get a continuous white noise "Shhhh"

Has anyone an actual MANUAL for this sucker? What is the foot switch
input supposed to do??? Advance the sequencer during programming? Is this
sucker starting and stopping the sequencer on each pulse???

Someone said they were sending out service manuals for this sucker to me,
but it hasn't come. I'm not complaining, mind you, since this was a free
offer, but I still feel kind of in the dark about this thing.
Personally, I love nothing more than to jam out to a greasy groove. --
A Guy Named Phillip
Kent Williams
(319) 338 6053 (home)
(319) 626 6700 x 219 (work)
(319) 626 3489 (fax)

From Fri Mar 1 10:54:26 1996
1 Mar 96 13:54:16 +0500
27 Feb 96 15:14:27 +0500
Date: Tue, 27 Feb 1996 12:22:56 -0700 (MST)
Subject: Re: DR 55 sync

>I got the DR55 schematics from anthony bowyer-lowe today (thanks anthony!).

>Before I go and wreck mine, let me ask this: Is it safe to feed
>the pulse clocks from a X0X box right into CE on an SRAM?
>Anyone in the US (Stuart? Dave?) who didn't get the docs from anthony
>drop me a line.

If I read you correctly you are throwing a trigger of unspecified and
possibly varying positive voltage at the Clock Enable pin?? If so one would
want to measure the internalpulse/trigger that the unit itself throws at the
CE and than buffer the input jack for the external unspecified pulse/trigger
with a value of resistor that would keep the highest possible external
pulse/trigger to the same voltage level as that of the internal
pulse/trigger generated by the DR-55's internal clock circuit (this would be
located outside the actual microprossecor chip-as *most* microprocessor
chips of the time did not have internal clocks, Z-80, 6500 series etc) the
circuit should be readily marked on the service docs-it would have a crystal
in it-valued for the clock rate.

Another issue here may be the *duration* of the pulse/trigger. One would
measeure this with the aid of a scope and use capacitors to lengthen the pulse.

" everyone knows electronics run on smoke, only they don't run too well when
you let the smoke out of 'em"

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Re: Audio Electronic Circuit Schematics

Post by WhiteWarlock » Sat Oct 20, 2018 7:08 pm

From Mon May 20 16:23:19 1996
20 May 96 19:23:15 +0500
20 May 96 08:15:03 +0500
Date: Mon, 20 May 96 12:49:28 PDT
Subject: DR-55 Bass Drum Decay Mod...

Well, this weekend I met up with a friend of mine who also has a BOSS DR-55 Dr.
Rhythm (groovy, groovy, groovy).

So I say to him, 'Hey, have you modified the bass drum decay?', and he hadn't so
we flipped the top off and had a look inside...

On my DR-55, in the bottom left (near the tone pot) there was a little trim-pot
which varied the kick decay, but on my friends there was only a resistor. Later
study of the service manual shows that only early serial number DR-55s have the
trim pot, so if you've a later rev. DR-55 here's the extremely simple way to
modify the bass drum decay..

The resistor is in the lower left of the circuit board (looking from above),
just below the tone control. 'BD' is stencilled on the PCB next to it, and the
resistor is over a little stencilled trim-pot picture. The following ASCII
diagram should help you locate the right resistor.

l l Tone Pot.
o ^
^==/ <<-- Resistor to be replaced.

Simply snip out the resistor, and replace with a horizontal trim-pot of several
K ohm. The trim pot goes between the three holes in the silly ASCII diagram
above, with the wiper terminal attached the middle/rightmost hole. I used a 2.2K
that I had floating around (what? your parts don't levitate like mine? freaky),
and that gave a reasonable control range...

Of course, I hold no responsibility for anything bad happening from this...

On another point... I'm going to be in London from thursday this week. Any UK
AHers want to meet up and hang out?

Anthony Bowyer-Lowe. Audio Programmer @ Mindscape (UK).

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Re: Audio Electronic Circuit Schematics

Post by crochambeau » Sat Oct 20, 2018 8:33 pm

I get that you're excited about modifying your DR-55, but does that REALLY warrant posting text grab after text grab from an old mail group?

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Re: Audio Electronic Circuit Schematics

Post by WhiteWarlock » Sun Oct 21, 2018 6:43 am

Yes... considering this is an archival thread...
anyone doing web search on DR55 schematics/service manual/mods will find this info
(not that there will be many)
and for some reason my wordpress editor is fuct up right now...
so tried saving accumulated post on my site yet this was far less frustrating
(another thing for doing/solving on the list)
when have extra time will figure WTF and why my wordpress edtor toolbar vanished
and images don't show up plus text is white in the edit window
in the middle of other things at this current moment
needed putting the info somewhere for easy/fast web reference
plus the old hyper real posts are relevant information
it's not as if anyone else is posting schematics/service manual/mod info in this thread recently
it wasn't about as you put it all about being "excited"
more practical reason such as saving info in the middle of working on something...
easy web access in this thread so can review/add info if need be during project
BTW for anyone that is here actually researching DR55 mods...
The 5v phone power supply runs the saved pattern...
yet doesn't seem like it is enough voltage for writing pattern
the loading pattern LED stays on constantly in write mode
making edits doesn't change anything in pattern...
(running some more tests today plus recording the current patterns)
looks like time for digging through suitcases of power supplies for 6v PS
thanks for pretending you actually care :P

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