Help fixing Boss DR-55 sequencer

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Help fixing Boss DR-55 sequencer

Postby polysixer » Thu May 18, 2017 12:01 pm

Hi! I picked up a busted DR-55 from a friend and so far the analog sound generators are working fine. If I switch to write mode and start triggering sounds they all sound correctly. In play mode none of the steps written gets memorised and I can only hear the HHs triggering and correctly following tempo and set grid. The previous owner already thanged the memory chip to no avail.

I did investigate some things and it seemed the 4011 responsible for the clock generator circuit goes bad sometimes, although the clock seems to work fine if the HHs and tempo are working as they should (am I right assuming this?). I did swap the 4011 for good measure but still kept doing the same. I dug a little more on the sequencer write functions and now my attention is drawn to the delay circuit, which could be responsible for data not getting written. The main component would be Q16 transistor. Am I in the right track or should I look nowhere else? Perhaps the binary counter?

Thanks very much in advance.
Last edited by polysixer on Thu May 25, 2017 2:22 am, edited 1 time in total.
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Re: Help fixing Boss DR-55 sequencer

Postby Mooger5 » Thu May 18, 2017 2:05 pm

According to the service notes, the binary counter sends binary signals to the same terminals whether in play or in write mode. It just counts pulses from different sources depending on the mode . So it seems fine. In Write mode the source is the Schmitt trigger so there should be a pulse train at the collector of Q15. Same at the junction of R93 and R94. 2SC945 are very common japanese transistors. The delay time is set by C46...
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Re: Help fixing Boss DR-55 sequencer

Postby polysixer » Fri May 19, 2017 1:31 am

Mooger5 wrote:According to the service notes, the binary counter sends binary signals to the same terminals whether in play or in write mode. It just counts pulses from different sources depending on the mode . So it seems fine. In Write mode the source is the Schmitt trigger so there should be a pulse train at the collector of Q15. Same at the junction of R93 and R94. 2SC945 are very common japanese transistors. The delay time is set by C46...



Thanks, I'll have a look at those and post the results.
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Re: Help fixing Boss DR-55 sequencer

Postby madtheory » Fri May 19, 2017 10:09 am

Why do people always suspect the silicon? Those parts are often expensive, hard to get, and take a fair bit of effort to replace (lots of soldering). Passive components and solder joints fail too, those are much easier to fix, why not start there? As Mooger5 points out, pulse should be there at Q15 collector, but also resistor junction as well.
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Re: Help fixing Boss DR-55 sequencer

Postby Mooger5 » Fri May 19, 2017 11:18 pm

I agree it´s better to inspect the joints and do some readings before replacing any components, but transistors and ICs fail more frequently. I only mentioned the spot between R93 and R94 because to avoid any typos. It was easier to memorise and to explain than the pin number of the IC number it leads to. Those resistors are actually dissipating very little power.
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Re: Help fixing Boss DR-55 sequencer

Postby polysixer » Sun May 21, 2017 11:38 am

Ok, the hardware shops close today so I spent some time looking at possible broken traces or cold solder joints. I reflowed all solder joints around Q16 area and, to my surprise, in write mode I could record instruments to the steps and hear them memorized as I cycled through the pattern with the accent for example, pressing the stop button (all in write mode). This didn't happen before in any case.

Switching back to play resulted in empty pattern again (just hihats triggering when switched on). This behavior didn't last long though, as cycling power again reverted to the old behavior: no previously recorded instruments in the steps can be heard in write mode. This leads me again to think of intermittent component failure rather than the solder joint itself, but now I'm more puzzled about what could be causing this.
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Re: Help fixing Boss DR-55 sequencer

Postby Mooger5 » Sun May 21, 2017 6:53 pm

Could be a power issue: weak batteries, or a badly soldered diode D5. And the resistor R165 too, which looks like a pullup.
In write mode, each time a step is entered is there a pulse at pin 20 of IC1?
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Re: Help fixing Boss DR-55 sequencer

Postby polysixer » Mon May 22, 2017 4:04 pm

Mooger5 wrote:Could be a power issue: weak batteries, or a badly soldered diode D5. And the resistor R165 too, which looks like a pullup.
In write mode, each time a step is entered is there a pulse at pin 20 of IC1?


Thanks, I'll check those 3 later today. My oscilloscope is on the way :)
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Re: Help fixing Boss DR-55 sequencer

Postby polysixer » Tue May 23, 2017 8:52 am

Ok, so I went all the way to check several things. Replaced D5 with no changes whatsoever. Replaced Q16 and Q15 with no changes whatsoever. I socketed all ICs, including memory chip, to test with replacements. No changes. Further inspecting the electrolytic caps, the whole PCB seems to have been recapped by a the previous owner, save for C39 (next to D5). I still get 5,30V at leg 22 of IC1 mem chip, so I'm not sure if that's causing any trouble. However, intermittent write mode working might indicate a voltage issue affecting IC1, as you say @Mooger5. Replacing the old electrolytic won't do any harm so we'll see how that goes.
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Re: Help fixing Boss DR-55 sequencer

Postby Mooger5 » Tue May 23, 2017 11:35 am

polysixer wrote:If I switch to write mode and start triggering sounds they all sound correctly.


Sorry, I´ve just read your original post more carefully. Never mind the delay circuit or the Schmitt trigger, as they´re OK or there wouldn´t be any signal when in Write Mode.

The binary counter seems like the culprit for now, but I´ll have to check the schematics later.

With the scope you´ll be able to read the pulses shown in the service notes. Until then just refrain from replacing any suspicious components. The transistors and the diode would probably read fine.
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Re: Help fixing Boss DR-55 sequencer

Postby polysixer » Tue May 23, 2017 11:59 am

Mooger5 wrote:
polysixer wrote:If I switch to write mode and start triggering sounds they all sound correctly.


Sorry, I´ve just read your original post more carefully. Never mind the delay circuit or the Schmitt trigger, as they´re OK or there wouldn´t be any signal when in Write Mode.

The binary counter seems like the culprit for now, but I´ll have to check the schematics later.

With the scope you´ll be able to read the pulses shown in the service notes. Until then just refrain from replacing any suspicious components. The transistors and the diode would probably read fine.


Thanks again for your input, very much appreciated. I've tried 2 4520 ICs already and both do the same thing, the binary counter seems fine, as the LED lights up correctly also, at the beginning of a pattern. Start/stop messages from the buttons generate the proper voltages at the pins. But yeah, I agree signalling will be better troubleshooted when my scope arrives. There has to be something altering the timings so data is not being written to memory, my focus is now on caps more likely.
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Re: Help fixing Boss DR-55 sequencer

Postby polysixer » Wed May 24, 2017 1:49 am

Replaced C39 cap with no change whatsoever. The value of the old cap was 47uf and the service manual indicates 22uf so I used that instead, but as I say, to no avail. Also replaced R65 near it but that wasn't failing either. I'm running out of ideas and probably have to wait for the scope to arrive and see things in detail.
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Re: Help fixing Boss DR-55 sequencer

Postby Mooger5 » Wed May 24, 2017 9:59 am

The cap keeps the memory charged for enough time to change batteries. Doubing the capacitance doubles the time it holds the charge. It was probably a factory mod...
Removing the cap and the diode at the same time erases the memory.
Same rhing if the duode is removed for too much time. You'll have to reprogram every pattern.
While doing so check the rhythm selector. Dirty contacts or a bad ground to the pull down resistors might interfere with the programming. And also the play back as the memory wouldnt know ehich pattern to replay from.
In the schematic where it says encoder measure the voltage at the three input terminals of ic1. While selecting the patterns. Zero volts is low and five volts is high.
Pattern one will be LLL PATTERN TWO WILL BE HLL THREE LHL, LLH HHL LHH HLH AND EIGTH WILL BE HHH.
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Re: Help fixing Boss DR-55 sequencer

Postby polysixer » Wed May 24, 2017 10:51 am

Mooger5 wrote:The cap keeps the memory charged for enough time to change batteries. Doubing the capacitance doubles the time it holds the charge. It was probably a factory mod...
Removing the cap and the diode at the same time erases the memory.
Same rhing if the duode is removed for too much time. You'll have to reprogram every pattern.
While doing so check the rhythm selector. Dirty contacts or a bad ground to the pull down resistors might interfere with the programming. And also the play back as the memory wouldnt know ehich pattern to replay from.
In the schematic where it says encoder measure the voltage at the three input terminals of ic1. While selecting the patterns. Zero volts is low and five volts is high.
Pattern one will be LLL PATTERN TWO WILL BE HLL THREE LHL, LLH HHL LHH HLH AND EIGTH WILL BE HHH.


Ok good, I'll check those terminals today and post the results. Thanks a lot.
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Re: Help fixing Boss DR-55 sequencer

Postby Mooger5 » Wed May 24, 2017 10:59 am

No problem. Also clraning the contacts in ebery vonnector might help too. Isnt there a pedal input or something, ig its for step advance or tap tempo it could be it. Good luck.
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