Juno-60 chorus LFO frequency too fast

[living sounds]

I've got a problem with the LFO controlling the chorus of my Juno-60. This synth has two chorus settings, I and II, both stereo, as well as a mono setting (I and II pressed combined). As I understand it, stereo is generated by creating two LFO signals that are out-of-phase. The modulation frequencies should be set according to the table in the lower right corner of the attached schematic.

The stereo settings work as they should, and voltage swing and modulation speeds measure correctly. However, the mono setting is much too fast, and voltage oscillates only between (roughly) -0.1V and -0.3V, it should be 1.6V PP. I'm only measuring this with a digital multimeter, but in comparison to the other readings (which oscillate between around +- 9V) this is off.

I don't understand the theory behind the LFO schematic... How are frequencies determined? Is there anything obvious to check?

Thanks!

 

[PRR]

Without understanding, or finding the off-image stuff you left off:

TR 4 5 6 7 8, maybe IC3. These seem to interpret off-image signals that set stereo/mono modes into internal VCO changes.

However transistors with 100K resistors don't fail from abuse. I'd start with a strong eyeball looking for dubious solder joints.

History matters. Has it been wet? Has someone worn silk underwear and amber beads while fondling IC3?

 

[zamproject]

to be clear what you show here is just the CV circuit driving the Chorus, and there also a logic circuit coming in

All 3 pos for chorus (I, II and I+II) are "stereo" only change is the CV time modulation, of course when both audio out jack are plunged.
The Juno is "mono" when chorus is OFF
In any case both jack out produce audio

The "I+II" setting are fast anyway, i'm not sure you have a pb with your Juno 60 !!!
I is 2 sec modulation when II is 1.2 sec, i let you calculate the sum of this 2 frequencies

I think its better to have a scop to check CV waves form at the output (39 and 40)

The CV time modulation are not adjustable, but you have a bias adjustment in the chorus board

 

[living sounds]

Thanks! What is not shown in the image is two signals coming in from a logic IC, I thought they bould both just provide a DC signal switchable to on-off to provide for the different modes. So is the timing for the chorus actually itself provided from those tow lines? I've attached the "bigger picture".

I've measured with a scope, chorus I and II take 2.3s and 1.4s to complete a full cycle of a triangle wave, less than 20% off. The I + II setting produce a sinewave at at little over 8 Hz, which could be as much as 10 times the correct frequency.

A while ago I've recapped the synth and installed a midi retrofit. I just remembered that one of the new caps blew (solid tantalum...), C7 probably shorted the -15V, I had to repair the trace. I've added an image of the PCB with the open circuit in red (next post).

Does this narrow down the suspects?

Thanks again!

 

[living sounds]

PCB image

 

[zamproject]

I've measured with a scope, chorus I and II take 2.3s and 1.4s to complete a full cycle of a triangle wave, less than 20% off. The I + II setting produce a sinewave at at little over 8 Hz, which could be as much as 10 times the correct frequency.

Time I and time II are fine on your Juno, so maybe you have to look when both are used and in case when the 2 signal are summed before sended to the chorus board

Also there is R6-R5-C2 short cut thought fet TR1 when I+II are engaged (and grounding positive input of IC1a, TR2)

check voltage at D1/D2

i don't find C7 on the schematic...

 

[living sounds]

Time I and time II are fine on your Juno, so maybe you have to look when both are used and in case when the 2 signal are summed before sended to the chorus board

Also there is R6-R5-C2 short cut thought fet TR1 when I+II are engaged (and grounding positive input of IC1a, TR2)

check voltage at D1/D2

i don't find C7 on the schematic...

Thanks, I'll do that! C7 is a PSU bypass cap, it's in the lower right corner of the first image with a lot of other PSU bypass caps.

 

[zamproject]

i look here for C7 and don't see it ... now it's ok

when looking at the pcb, your blowing cap just cut the TR3 emitter
so this is another point to look at this part of the circuit to complete what i say before

definitely all this section acting when I and II engaged, can't say right now (don't have time for more investigation)
but for sure all this perform change at IC1 and IC2 when summing both signal

Check R8 R9 R10 junction here is where come I and II (respectively R10, R9)

 

[living sounds]

I've checked the junction of D1 and D2, it's +15V without chorus engaged, as well as with I and II engaged respectively. It's -15V with I + II both engaged. I've also re-rechecked many connections for continuity and looked for loose connections, but found no problems. I've also replaced two bypass caps that were the wrong value (someone previously recapped part of the synth), but as expected this didn't change anything.

How can I find out what causes the problem without more or less systematically desoldering and replacing semiconductors? Some of these ICs are old and hard to get, too... A theoretical explanation for the LFO going more than 10 times faster would be great, unfortunately unlike Moog, Roland did not fully explain all the circuits in their technical manuals...


Thanks!

 

[living sounds]

D1 and D2 are all right as well.

 

[soapfoot]

My Juno 60 behaves this was as well.  I was never aware it was abnormal.

 

[living sounds]

My Juno 60 behaves this was as well.  I was never aware it was abnormal.

Are you sure? Could you maybe record a short example with Chorus I and II on at the same time? Thanks alot!

 

[soapfoot]

I made a short 3 second .mp3, but cannot upload it here. PM me an address to e-mail it to.

--b

 

[living sounds]

With the help of soapfoot I found out there wasn't actually a problem.  ;D

The guy who came over to buy the synth said the chorus was malfunctioning...  :

 

[zamproject]

The "I+II" setting are fast anyway, i'm not sure you have a pb with your Juno 60 !!!

I found out there wasn't actually a problem.  ;D

what i suspect  :-X

so every thing solved 

Zam