Hi, I’m working on one of my Juno 6 which is not working. I swapped the CPU boards between a working Juno and the problem definitely resides on the CPU board. I’ve think I’ve tracked the problem to the region circled in red. I think it’s this because in the working 6, the oscillating signal on R412 is not present in the nonfunctional 6. I’ve checked the coil for continuity and it seems OK. Voltage at TP2 is correct. I have two questions:
What is the circuit in red for? What does it do?
Do you have any suggestions as to how I should repair the synth WRT what to check?
According to the information gathered in the Service Manual, this appears to be a VCO. The input voltage is the Bender voltage, and the output to the two 8253 counters is a variable frequency between 1 and 3.5 MHz.
Hi Didier, Many thanks for your advice. I shoud have spotted that on the schematic flow chart!!! I checked TP2 and it is fine. I’ve replaced all the electrolytics so they are out of the equation. I’ll start desoldering and checking the transistors. I’ll let you know how I get on. Thanks again, Craig
So I sequentially desolder and checked TR58 to 63 and tested each using my Peak DCA 75. Turns out TR 60 (a 2sc2603 Y ) looked a bit dodgy…Hfe of 1. Ordered a replacement, which I might add was a little hard to locate a viable source. I may also order some KSC2383Y which appear to be good replacements for the 603. once these arrive I’ll update you all. Best wishes, Craig
This is the master DCO clock, if this is not outputting a fixed frequency in the MHz range you will have no sound. The two 8253 timers divide this master clock to give you squarewaves at the musical pitches when you press a key down.
Good news boys and girls. I replaced TR 60 (a 2sc2603 Y ) on the CPU board and the synth fired up. It’s working well. Now I’ll add Tubbutec midi and as we say up North; ‘the job’s a good un’.
