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Identifying correct resistor value for CS80 power transistor
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Hello everyone,
As I ran into recent issues with my CS80 power supply, I decided to go thru the recapping/4558's swap, that's when i realized that the white resistors connected behind the power transistors (Tr701+Tr702) suffered/leaked in some ways. Before powering things up again, I decided to replace these for future proof reliabilty. Trying to find proper replacement for these, I realized there is some confusion between the reference described in the Part List Manual?and what's physically labelled on the resistor itself: in the Yamaha manual, I think the correct one is mentioned as 'cement molded resistor 5P 1Ω' , whereas it's printed ?' 5W 1ΩK' on the resistors themselves, which shows to my understanding, a big difference in resistance value (1ohm to 1 kohms). I know the Yamaha manuals can be erroneous sometimes, but in the same time since I'm not the original owner of the CS80 I am not 100% sure the actual resistors are that legitimate in there. So before going any further, any advices/experience feedbacks would be very helpful at this step, thank you! |
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The big ceramic resistors are current-limiters for the outputs. The ±15 rails on the CS-80 each pull about 2.5 amps in normal operation. If you had an accidental direct short to ground, this resistor limits the current draw on the output transistors to 15 amps as opposed to a runaway condition if they weren't there. So, they're a safety device for the power supply.? I recently replaced a couple in a customer's power supply?and used the Yageo?SQP500JB-1R part from Mouser. It's a J tolerance which is 5%. Maintain a little air gap between the resistor and big heat sink to help keep things cool. Scott On Sat, Mar 13, 2021 at 3:23 AM <nightmusic01@...> wrote: Thank you Joan, interesting! Yes it would be great if you could double check that, in case nobody else confirms this.Thanks! |
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Hello Scott,
Thank you for sharing this mouser reference, I will get these ones! So this resistor replacement choice answers to my previous question at same time, they are definitely 5W/1ohm resistors as described in the manual, and as Joan also concluded before (then K stand for 10% tolerance). Knowing the role of these resistors is valuable indeed, and now I better understand the heating cause of these, as there must have been some short on the 15V rails in the circuit in first place. Since they're supposed to protect Tr701 and Tr702 transistors by limiting current draw under 15 amps, should I conclude the outputs transistors themselves are 100% kept safe and operating? thx! |
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" should I conclude the outputs transistors themselves are 100% kept safe and operating?" I would expect so, they're pretty?bulletproof and can handle a hefty overload for a short period. As another?point in the matter, do not power-up?the power supply unless it has a load on it. It needs to have the circuit completed in order to work properly. Also the sense lines need to be tied to their respective leads for proper regulation by the 4558's? What you can do if you want to check the ±15 volt side voltages before installing it into the synth would be to put 1k resistors on the lines. You can safely remove the fuses on the blue and yellow wires to disable the?+8.5 and -6.5 volt lines. Then wire the +15 and?+15S to one side of a resistor and the other side to the black analog ground. Do the same thing for the -15 and -15S lines and the single 10.6 volt line. So in the end, you'll have (3) resistors with the ends common ganged to the ground wire with one resistor going to the two brown lines, one resistor going to the two yellow lines, and one resistor going to the orange 10.6 volt line on the connector (it's the red wire coming off the circuit board, but it changes to the orange color at the connector). Then you can power it on and check voltages at each resistor.? On Sat, Mar 13, 2021 at 8:05 AM <nightmusic01@...> wrote:
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If the damaged ceramic resistor is on the -15 volt side, it's likely someone plugged a footswitch into the Foot Controller port on the back of the synth. Doing so will short the -15 volt side to ground. :( On Sat, Mar 13, 2021 at 8:23 AM Scott Rodriguez via <synthrodriguez=[email protected]> wrote:
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"As another?point in the matter,?do not?power-up?the power supply unless it has a load on it. It needs to have the circuit completed in order to work properly"
Waou this info is very useful Scott! ?I made the PSU calibration many times in the past, but always pulled the Molex connector off in order to tie the sense lines together externally, so the circuit was always disconnected from the PSU doing that. So i will follow your procedure 100%, thank you for sharing this with such precise explanations. Besides providing possible inaccurate results on the multimeter, is there any risks for the psu when not recreating the circuit resistance that way? |
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It drives the power transistor output circuit to its voltage limit,?somewhere around 27 volts if I recall correctly..I'm not sure exactly what damage it can cause, but it's certainly not something you want to do. On Sat, Mar 13, 2021 at 9:53 AM <nightmusic01@...> wrote: "As another?point in the matter,?do not?power-up?the power supply unless it has a load on it. It needs to have the circuit completed in order to work properly" |
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That makes sense indeed.
So I won't take any more risks in the future and will do your way from now on. And since these temporary resistors would tie each +15/+15S and -15/-15S lines together, the leads would still be connected to their corresponding sense lines in a safe way. (Always felt stressed when plugging crocodile jumpers inside the molex anyway, since they could easily slip away from the connector and potentially damage the PSU). So first I will order these white cement folded resistors from Mouser, and as soon as I receive them (I think in a week or two, since I live in Europe), I will share some more feedbacks around it. Again, thank you Scott and Joan for your expertise! |
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Scott, The damaged resistor is on the +15 volt line, but the foot switch issue you mention makes me think of another (ghost) short I had some while ago, which happened only when the Yamaha foot controller was connected on the back.Since then, It stayed unplugged the time to deal with that, but never really analysed the cause behind it in the end...Well, one thing at a time, for now it's the PSU first ;-) |
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Since you're ordering parts anyway, just get a new Molex plug and some terminals and make up a little dedicated resistor load bank you can just plug in to the power supply. The plug is Molex p/n 50-84-1090 and the socket terminals are p/n 350536-3. On Sat, Mar 13, 2021 at 11:39 AM <nightmusic01@...> wrote:
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Ah. I re-read your message. What I was talking about was just ruining it without the sense line connected to the power line as some people try to do, that's when you get the circuit driving the transistor outputs to max voltage. If you're say, alligator clipping the two together that would work ok as well. I have a dedicated load bank for all the circuits so it's never been necessary here (and I never did it that way). In that case, you're fine. I'd still buy a plug and wire it so there's a little loop for the sense lines right at the plug for safety's sake. I might even make?one up for myself,?would be good for some quick adjustments rather than pulling out the load bank. Thanks for the?idea!. On Sat, Mar 13, 2021 at 12:06 PM Scott Rodriguez via <synthrodriguez=[email protected]> wrote:
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Great idea! So right, with your p/n references i found the molex plug on Mouser, but the socket terminals gives 3 different results (regular, ‘cut strip’, and also ‘mouser reel’).I dont really get the differences between them all but i presume i should get the regular one, right? thanks |
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I usually get the cut strips, but any will do. I believe?you mean the transistors, not the resistors in your last sentence? If so, for power transistors in the CS-80 I use:
On Sat, Mar 13, 2021 at 2:57 PM <nightmusic01@...> wrote:
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Oh...In my turn I missed this message from you.So yes indeed, using alligator to tie the lead and sense lines together is a quick way to go, but your option of getting a load resistor bank is surely a safest route as alligator clips may always slip out.
Just to clarify, this is the exact method I always used (described at 4:15 in this video):?https://youtu.be/Up-KQHc4LNQ So I would conclude both ways will prevent the transistors from driving out, one way being the easy and quick way (alligator clips), while the other being the most secure method for longest tests (load bank). |
Joan Touzet
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