Vintage Synth Repair

----- Original Message -----

From: Adrian Corston

To: vintagesynthrepair@yahoogroups.com

Cc: gerryrdahl

Sent: Thursday, August 01, 2013 9:04 AM

Subject: Re: [vintagesynthrepair] Re: CS-70m power supply issue, -15V rail is dead

Hi Gerry,

Thank you for your advice.

I have the +10V and -5V rails working right now - and you were pretty-much spot on with that resistor guess (although for a different voltage rail). Both of the 4.7 ohm 230mA resistors on those rails were open circuit, and once they were replaced I was able to adjust them back to the correct voltages. At the moment I have some temporary non-fusible resistors in place while I wait an order of proper fusible ones to come in.

My remaining issue then is the original problem I first reported, where the +15V test point is measuring +17V (which does not change when VR3 is adjusted) and the -15V test point measures about -12V immediately after power is applied but then this rapidly slides up to approx. +0.8V after about 10-15 seconds.

For now I'm focusing on the +15V circuit.

I measured the 0.22 ohm resistor in the +15V circuit and my multimeter reported 0.5 ohms, which seems reasonable given it's a cheap multimeter and that is a very small value and presumably hard to measure accurately? If it's just a normal 1W 0.22 ohm resistor I can try replacing it, but I really have no idea what the "1P" means - 1W "P"ower maybe?

There is no measurable current flowing on the base of Tr15 and voltage measurements are approx. +26V on the collector, +17V on the base and +16V on the emitter.

The IC3 (2/2) op-amp has +13.22V output on pin 7 no matter what VR3 is set to, despite pin 6 measuring +6.34V. I think this is unexpected since it looks like a voltage follower to me - shouldn't that mean IN- and OUT effectively track each other? I replaced the 0.047uF capacitor but it made no difference. VR3 adjusts the IN+ (pin 5) between +6.66V and +8.72V which is presumably higher than normal because it's part of a voltage divider across 0V-17V, when it would usually be across 0V-15V. If someone could give me some advice about for this part of the circuit, or measurements of what's happening on another functional CS-70m then I'd appreciate it.

Assuming IC3 is actually doing what it's supposed to I guess the next step is to look at Tr18 and Tr16 (and maybe Tr17 and Tr19), however I replaced them all with brand new transistors recently so they shouldn't be the cause of the problem.

Thanks,
A.


On 1/08/13 1:04 PM, gerryrdahl wrote:

Hello,
Have you checked the .22ohm 1 watt (1P?) resistor?
I just repaired a Yamaha CS40M that had that resistor out (.47 on the CS40M) and that was the cause of the -15 volt failure and the other voltages not meeting full voltage (+10 was +8 and so on).

Once I replaced that all came into full power.

Check those out.

GD

--- In vintagesynthrepair@yahoogroups.com, "cpt.zilog" wrote:
>
> Hello guys and sorry to jump in like that but I would suggest to change the big caps as a priority and then check the bridge rectifier again for leakage of AC current.
>
> I had a big problem with a D&R console's psu in which the voltage regulators kept on going Kapput. Turned out that the big Electrolytics were bad and that induced a big stress on the rectifier bridge, which in turn had the consequence of not only letting DC pass as expected, but also AC at the same time !
>
> I did test for AC leakage on the rectifier after the newly installed voltage regulator worked for a couple of seconds before shorting...
>
> After new BIG caps, new bridge rectifier and a 2nd new voltage regulator, everything was sorted.
>
> Good luck !
>