Oberheim OB-8 will no longer start up after PSU repair

Synth Repair
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Hi there, I have an Oberheim OB-8 that I am trying to get it going and I am posting this to see if anyone has encounted these problems before - which do not seem to be documented online.

This is an early OB-8, with a Panasonic keybed and a factory added MIDI board. It has the B-5 ROM chips in it. Apparently something fell onto this Oberheim, and the top board was extracted and moved to one with an undamaged front panel. I have been closely examining this board for cracks or broken solder joints or cables, and have found none.

When it arrived it had some of the standard problems that are covered in many repair videos already. The LED’s all came on and responded but there was no sound. After some time the synth started to respond and worked over MIDI with my Arturia keystep, although some funky things would happen when knobs got turned.

The power supply had seen some previous work, with the filter capacitors replaced. However, the original rectifiers were still in place. I replaced them with new bridge rectifiers, including one rated for a higher amperage on the 5V line. There was some damage on the board from the original 5V rectifier, which was overheating the board. It doesn’t look pretty, but as far as I am able to tell, there are no shorts. The voltages are testing perfectly across the synth.

After this repair however, the synth stopped making noise and the LED’s on the front panel were no longer illuminated. Only the bender panel lit up, and in some different patterns when the power is cycled. None of the buttons respond anymore. It will not go into test mode, and does not respond to the reset prodecure.

I have been cleaning sockets, cleaning molex connectors, checking every connection and everything has tested okay. I have tried replacing the factory RAM chips with Synthchaser’s battery eliminator RAM chips (and removing the original battery as per that upgrade). I have replaced many CMOS chips at this point on the CPU board. I have acquired a TTL/CMOS tester for the remaining chips, and all have tested okay.

With the MIDI board unplugged, I am able to get 10V out of the DAC. When this board is plugged in however, it drops to 0V.

Yesterday, I tried removing the MIDI board altogether and relocated the Microprocessor and TTL chip to their original sockets. After this change - the synth started to respond more normally. Front panel LED’s lit up. Buttons responded to pushes. There is some flashing on the LED’s. There is some sound, including the noise generator and some sounds from the oscillators. It is going through autotune and passing it pretty quickly. This is all happening intermittently.

Anyone ever encounter these kinds of issues with an OB-8? Most videos and blogs I have seen haven’t had any serious issues like this and have only had to undergo routine matience.

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I recently repaired a Matrix 12 which had intermittent boots issues. Turns out the last person to work on it had put a wrong logic IC in the chip select circuit so double check all IC’s are what they re supposed to be.
Also I have just repaired an LM-1, it had bit rot in the eproms. Have you tried reading and then verifying them? The LM-1 would boot 1 in 4 times when the wind was right and a rotten cell produced a correct read.

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here is a video where I check the eproms. They would not verify at the normal 5v.

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Hi,

I haven’t worked in an OB8 but I hope the helps somehow:

When repairing a Mono/Poly that suffered from a bad PSU voltages and after an extended troubleshooting, i identified and replaced some damaged chips(mostly op ams) and the keyboard started working but I came across a weird issue where the envelops were not working properly and there were weird noises in the output.

long story short, i found that the new op-amps (authentic ,same part number as the old/bad) in the ADSR circuit had different/higher gain. Swapped it with an original IC from another section on the boards and it worked properly.

The point is: double check the chips you’ve replaced and verify whether or not they’re function as intended in the circuit.

Addition tips,

1- Verify the CPU is running by checking the clocks, Reset and the data / address lines using an Oscilloscope.

2- if your Oscilloscope have multiple inputs, connect them to multiple key/strategic points simultaneously ( eg , voltage regulators, logic gates,..) and cycle the power to look for any abnormalities.

3- isolate the boards and start with bare minimum (main) to see if the CPU runs properly and add the rest one-by-one (thern the power off and make sure the caps not holding power when you do that)

4- sometimes i get misleading readings from my multimeters and oscilloscopes due to bad Ground /Reference points. The physical connections can be good but the circuits might be isolated,

Good Luck

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Thanks for taking the time to write a reply. The moment this came in with the ‘lights on - no sound’ symptom - my first thought was the ROM. I recently got a Korg DW-8000 that was doing the same thing - it would occasionally make some random sounds and have random LED behavior and wacky voltages in the logic circuit. In the end, a new EPROM chip from Syntaur got it going.

I have not checked the EPROM’s yet, but it is next on my list.

I will double check my IC’s, but all of these symptoms were happening before any IC’s were replaced. I do not believe I have made any mistakes as I have been very carefully removing them one by one. You never know though.

Thanks for sharing that video! I will check it out and report back.

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Thanks for the reply! I appreciate the input, these symptoms can happen on any synth of this era - this is still helpful and getting the wheels turning in my head.

I have been very carefull removing the IC’s as I go through it, one by one. I have only been replacing chips known to fail, RCA, CD series (4051’s). You never know though, I am human after all and can still make a mistake. I have all of the original chips as well, I have been keeping all of the original parts together in case they are needed.

Thanks for the tips with checking the clock on the scope. I have a Rigol DZ1054 with four channels - should be more than enough to do some of those tests. I will check it out and report back.

It’s really too bad they decided to put the PSU on the same board as the processor circuitry instead of its own board - but luckily the 5V regulator did not totally destroy the traces.

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Having repaired an OB-Xa and have one sitting here waiting for repair I’d like to give some hints, which should relate to the OB8 too:

#1) There is a 5.6V supply for the 4051. Verify that this voltage is really 5.6V. If it is lower, then the signals maximum voltage might be too close to 4051’s positive supply input, which makes the 4051’s die like flies.

#2) The OB-Xa I repaired suffered from severe oxidation of the IC socket contacts. As a first attempt, I removed all logic ICs on both control boards and reinstalled them. That helped somewhat, but not permanently. My final repair was as follows: I removed all (!) ICs from the control boards and cleaned the sockets with a contact spray (in my case, Caig D5), cleaned all IC pins with a glass brush, and then reinserted the ICs.

Assumingly I will have to do this on the next OB-Xa too…

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Thanks for the reply!

I’ll verify this voltage. I have replaced all 4051’s but still have the originals which I can test now.

I have seen a few dirty IC pins, but have been cleaning every socket as I go.

I have seen a few OBX and OBXa repairs where almost all or all of the chips on the Processor board had been replaced in the end. I have been through almost all of it at this point, replaced many RCA and CD series chips, and all 74LS chips have been testing okay in the tester. I have installed the Synthchaser battery eliminator RAM chips, removed the battery, and replaced the bridge rectifiers. At this point, I think the EPROM chips might be corrupted or suffering bit rot. I have a EPROM reader on the way and will verify this soon.