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Midiverb II repair

I´ve had a broken Alesis Midiverb II lying around for quite some time now. Just recently I remembered that I´ve always liked Patch #28, even before I dove into ambient and drone. So I thought now was finally the time to open it up and give it a go, especially since not to long ago I aquired a Midiverb 4 from an old friend and thus had a working 9V AC/AC adaptor again which the Midiverb II also requires.


You´d plug in the power adaptor and nothing would happen, neither visually nor sonically. The unit was basically dead, or so it seemed …

Fault trackdown

It´s always a good start to make sure that all the supply voltages are there and within range. The Midiverbs rectify internally so I started at the power adaptor plug and made my way through the PSU unit. Power on all paths was present but at very low ratings. I unplugged it again and did some continuity measurements. I found a short circuit between the output and ground of the 7805 regulator. I took it out and found that it measured okay out of circuit so it itself couldn´t be the problem. I still had to replace it because the input leg simply broke with no real force applied,

The repair

So there was a short circuit on the 5V supply line. I measured continuity across all capacitors and marked al those that seemed short which I then soldered out. Basically I removed all ripple filter caps of the digital ICs. Subsequently I no longer measured a short circuit across the 7805. It turned out that two of the filter caps had shortened. I replaced those with 470pF that I had left over from the Opra 6 repair and put all the other working ones back in.
That was all it needed. Plugged in the adaptor and there it was up and running again.


I noticed that the unit emitted a slight humn and a significant amount of noise when cranked up fully. I then remebered that the device has always been a little noisy. I then found that it might not be a bad idea to check whether the analog circuit could be improved upon. All opamps where LF347 and a little research revealed that the TL074 is compatibel and seemed to have better ratings. So I decided to replace all LF347 with TL074. I used sockets just in case that I where proven wrong and had to reverse it, which didn´t happen. I am quite stisfied with the improvement and really happy to have the unit back in working condition.

Midiverb 2 TL074
Midiverb II w. TL074

BCR2000 USB failure

If all of a sudden you are getting an error message on your Computer stating that your USB device failed then this is most likely due to worn out capacitors of the power supply.
I successfully got my device working again by replacing the two 100uF electrolytics. The original ones where causing ripples on the supply voltage which lead to instable USB signals.

Poly-61 individual voice tuning

When I recalibrated my Poly-61 I had two voices significantly out of tune. Now since all oscillators are digitally controlled and therefore supposed to be in tune, KORG omitted attenuators for individual voices.

Here´s what I found as probable cause and fix: The parts most likely affected by the aging process in the area of voice CV appear to be the Cn02 (n=1..6) capacitors. I could not identify their exact type, but the marking sais they are 10nF with only 1% tolerance, so precision circuitry could be assumed. If I read the schematics correctly then together with the Rn02 resistors they form the input of the antilog. So there are two possible fixes. The first being to replace the capacitors with new high precision 10nF caps. The other fix is what I went for, firstly because I did not have the appropriate capacitors at hand and secondly because I felt that having control over the tuning would be handy. So I replaced R402 and R502 with precision trimpots.

Poly-61 trimpots for inividual voice tuning

Since Rn02 are 15k the best trimpot value would be 20k. I only had 100k at hand so it was a little fiddely to set the right value but it works for now. I still have to get a couple of replacement parts for proper fixes anyway so I will eventually replace all Cn02 with new ones and all Rn02 with 20k trimpots.

Poly-61 repair

Ressurection of a Poly-61

Like some other 80´s polysynths with memory, many of today´s Poly-61s suffer from battery leakage. The problem for most owners without knowledge of the technicalities is that the symptoms come on slowly and do not neccessarily hint at memory. While leaking, the battery may still provide enough power to retain memory while starting to damage the PCB area around it. The device starts acting weird up to doing nothing at all soundwise. The symptoms depend a little on how the synth has been stored and therefore how the acid flowes across the board. If you see a Poly-61 on the used market described as ‘functional’ but ‘holding a steady tone’ it´s most likely a battery dammage.

I purchaised such a unit with the risk of not knowing the exact amount of dammage it had taken. But I was determined to make it fully operable again. After all I thought it couldn´t be worse than the Opera-6 fix.


If you use the information presented here you are doing this at your own risk. I cannot be held responsible for any damage done to your device or yourself caused by your actions! You need the appropriate skills to do work on electrical devices.

The prequel

I looked around for information on the net but could not find much that was useful. Like others I remebered coming across a pretty extensive repair video, but it has been taken down by its author. The rest that is to be found are forum posts that describe the symptoms and take guesses and if they provide a solution it is only the general ‘re-crimp the connectors’ one. Some say that it helped, but never say why. And I wanted to know why …

There are plenty of download sources for the service manual, but the schematics contained are barely readable. It had to do though.


The device initially showed the following faults:

  • Outputs a constant tone
  • Most keys not working
  • DCO2 not working on all voices
  • Number 6 button not working

CPU board fix

First step was to open up the unit and examine the damage on the CPU board.

KLM-509A CPU board battery damage

Interestingly enough the board was missing the battery entirely. The two wire stumps seemed to be the remainders of a first repair attempt.
It would not have made sense to look for certain errors before the general dammage on the board wasn´t fixed. So first step was to remove all parts that where affected by the leak.

KLM-509A CPU board stripped

It should be noted that I also removed CN27B and IC2 after the picture was taken. Next was to clean the affected area. First I used 70% Isopropylic Alcohol and applied a big load of baking soda to neutralize the remaining acid.

KLM-509A CPU board sodarized

After removing the dried up soda with a tooth brush the damaged area looks alot better.

KLM-509A CPU board cleaned

From here we can look for broken traces. With a strong lightsource we can x-ray the board for a visual check. Notice the broken traces in the red marked area.

KLM-509A CPU board x-rayed

Visual examination is not enough though. It is best to verify all traces using a Digital Volt Meter in continuity mode. If at this point one has to be told to use insulated wire to fix the traces then one probably lacks the basic skills to take on this task at all …

As you can see on the picture already I used sockets instead of soldering the ICs into place directly. This helps in finding and correcting faults without putting stress on the chips.

Replacement parts

Sourcing replacement parts was not that hard. Basically all HD14xxx had to be replaced by MOS 40xxx equivalents. Only one transistor had to be sought out of a variety of possible replacements.

Part Original Type Replacement Type Comment
Q11 (2S)C2785 2SC1815 NPN, TO-92, 50V, 150mA
Q10 (2S)A1175 Still available at the shop of my choice
IC HD14xxx MOS40xxx

Battery replacement

I put in a lithium cell insead of the former reloadable type. This requires the load current resistor R70 to be replaced by a diode (1n4148 will do).

The general crimp fix

A heads up in advance: Yes, the board connectors can be the source of many problems on the Poly-61.

However it depends on the general damage, especially in the case of a leaked battery, whether it is enough to just re-crimp the connectors or whether more action is required. In my case the sockets of CN12 and CN27B where badly affected as well and needed to be cleaned to regain full connectivity. The crimps of the connectors where literally falling apart.

Fixing the faults

Constant tone

If a synth outputs sound constantly, chances are high that the VCA is open. Now in most cases the VCA is driven by an envelope generator. EGs need some sort of gate or trigger. The Poly-61 uses SSM2056 envelope generator ICs and they have a Gate input at pin 6. I checked the pins with a DVM and found ~1.3V at five of the six. Now 1.3V is given in the datasheet as the maximum required threshold voltage for the gate. So all these inputs where active. Looking at the schematics you will eventually find that the gate lines are directly connected to buffer IC outputs on KLM-509 via CN12. With CN12 being severely damaged by the battery acid and having barely any connectivity, the affected gate lines where floating. I assume that this causes the 2056 to pull the inputs high internally for some reason. I cleaned the pins of the board connector with a dremel brush and for the lack of replacement crimps came up with a temporary solution until I manage to source the right crimps.

KLM-509A CN12 custom crimping

DCO2 missing

DCO2 for the six voices are provided by IC5  and IC14 on KLM-509. DCO2 was missing on voices 4-6. This meant that one of the chips was doing its job and the other one wasn´t. It turned out that a trace between IC11 and IC14 was cracked, leaving the CS line non-functional.

Number 6 switch

This was in fact an easy fix, because it was also only a cracked trace on KLM-509, but it was the hardest to find out because of the barely readable schematics. In the end I was lucky that I didn´t depend on it more.


That was about it. One Poly-61 back to life. All that was left where the typical restauration tasks like cleaning pots and key contacts and cosmetics.