TR-808 Kenton retrofit mod issues

Trying to get Humpty Dumpty back together again. Again!

Had my 808 since 1994. Used it on literally 1 song. Tried to install a Kenton MIDI kit in 1995. I gave up, gave the case to Richard Devine. The boards sat around in boxes for years. Finally sent the boards to Jerry Eskue in TX in 2013, he sent them back in 2016, I attempted again to rack it in 2017, never got anywhere. @richarddevine sent me the case back late last year. Finally trying to get this thing to work once and for all…

So to begin, I am focused on the power supply, and next, will dive into the other issues:
• 808 is stuck on Section II instead of I when it first is turned on
• LEDs do not flash when sequencer is started
• No audio output
• MIDI interface not working
• Power draw from Kenton MIDI interface is incorrect

Starting with the working (from David Bivins) 808 PSU I have the following voltages with the main board, sync, & output connected

right side of board:
22: +14.76V (Red)
21: GND (Black)
20: -14.75V (Blue)
19: +5V (Orange)
18: GND (Black)
17: +4.51V (Yellow)

Bottom side:
9: NC
10: +5.5V (Yellow)
11: +5.5V (Yellow)
12: 0V (Red)
13: 0V (Black)
14: 0V (Red)

B596
1: +24-25V
2: +15V
3: +25.81V

D880
1: -25V
2: -15V
3: -25.71V

With just the Bivins power supply, I have the following.

22: +15V (Red)
21: GND (Black)
20: -14.75V (Blue)
19: +5V (Orange)
18: GND (Black)
17: +4.79V (Yellow)

Bottom side:
9: NC
10: +5.67V (Yellow)
11: +6.1V (Yellow)
12: 0V (Red)
13: 0V (Black)
14: 0V (Red)

B596
1: +13.49V
2: GND
3: +5.046V

D880
1: < +100mV
2: +4.69V
3: -9.39V

On my PSU, without the main board connected, I have the following:
22: +35V (Red)
21: +19.9V (Black)
20: +4.7(Blue)
19: +5V (Orange)
18: GND (Black)
17: +4.85V (Yellow)

Bottom side:
9: NC
10: +3.4-5.6 (Yellow)
11: +5.26-6.03V (Yellow)
12: 19.88V (Red)
13: 19.90V (Black)
14: 19.89V (Red)

7805UC
1: +11.79-12v
2: GND
3: +4.79-5V

B596
1: +48-49.56V
2: GND
3: +49.3V

D880
1: < +100mV
2: +4.69V
3: -9.39V

I don’t want to connect it to my main board because I think I’ll toast the CPU with that voltage, if I haven’t already. :confused: My theory is that the voltage going into the B596 regulator and the D880 has a short, because the voltage going out of the regulator is just about the same going into it. I have an additional PSU board arriving in 2 days, but I’d like to fix this in the event I need it. And my curiosity is killing me.

I got the following from Mark Verbos as a diagnosis:

It looks like the ground is funny on the analog side. Make sure the black wire from the transformer is connected to pin 13. Check continuity from pin 13 to pin 1 of the TA7179 regulator chip and then all the way to pin 21. Q1 and Q2 are power transistors that the regulator is controlling to scale the raw voltage to be the DC level that it wants (+/-15 nominally, but it could be %10 off in either way). Look at the upper left and lower right (+ and -) of the D2 rectifier. Those should be the raw unregulated voltage so maybe +/- 21 or something. But how your voltages are now, it looks like you have no ground connection. C12 and C13 are the big power caps (1000uF each) that are good cantidates for replacing because of age. People on synth DIY may poo poo replacing caps willy nilly, but big electrolytics have a rated life span of 15 years, so….

When you check the voltages coming off of the transformer, you should be in AC mode. the black one is the center tap, so ground. It should be something like 24VAC between the black one and either red one.