Discussion in 'Yamaha' started by EmuMannen, Sep 2, 2017.
Attached is the BOM as Mouser Shopping Cart. Anything else before I place the order?
Hi again everyone, here is a quick status update. I have tried to source the BOM for month in EU but there was always something on back order so I finally ordered from US. Digi-Key had everything in stock and delivered to my door in less than 3 days! Thanks Digi-Key you are just fabulous! I was still hesitant about C802 & C803 on the Electrolytic Capacitor Board but I finally went with the initial suggestion by avionic. It came with 4 legs and almost the same pattern as the originals BUT the pin out was totally different! The negative pole is where the positive are on the original and the positive pole is on pin #1, that is 90 degrees from the negative pole, not on a straight line as on the original. That was a bit of a setback but this is DIY after all and zaibatsu already stated - you might have to get creative
So I bought some vinyl washers with the same diameter as the caps. I then soldered 1.5mm2 solid copper wires to the holes in the positive and negative component leg. I then drilled two holes in the washer corresponding to the positive and negative hole in the PCB. I then bent the copper wire so that it didn't touch any of the other legs and routed the through the washer. I then marked where the four component legs touched the washer. I then drilled another 4 holes in the washer for the legs. I could finally mount the washer to the bottom of my caps, route the wires through the PCB and solder them to the board. The caps are now standing on their legs on the board, fixed by the wires and stabilized by the washer. I think this will work
OK, all electrolytics replaced with new ones and both Tr607 (C458) replaced with two new KSC1815YTA. I put it together started it up, no smoke, great start. Amp coupler switch off, no speaker load, waited 3 minutes to set mid point voltage and bias current.
This is where things got a bit derailed. Mid point voltage on the left amp board a bit high, around 0.7V, managed to bring it down to 0.5V if a turn VR601 all the way. Right side seems OK, does anyone have an idea what could be wrong with the left side?
Bump: Does anyone know what could be the cause of a high midpoint voltage in the CA-600? I have no idea where to start. Could Tr607 cause it? I should have, but I did not check the new pair of KSC1815YTA before I soldered them to the boards. I will order new trimpots and replace the old ones just to rule them out. I was just wondering if there are any other components (that might be the cause of the high midpoint voltage) that I also should get?
Does the relay click on?
Did you take the measurement before you changed parts, or only afterwards?
Have you double checked pinout of any transistors you replaced?
500-700mV is too high for the midpoint (DC offset) - what's the measurement on the good channel? Keep your meter in mV, ideally the value should be <50mV.
It's unlikely the trimpots will be the cause.
There could have been a prexisting problem with TR602 / TR604 if you didn't check this measurement before working on the amp. If you want to check this, you can carefully swap 602/604 with 601/603 and see if the offset migrates to the other channel.
Thanks a lot for your help zaibatsu, the relay click on. I took measurement a while ago, it was quite tough, I got the feeling it was drifting a lot on both sides but blamed my meter (it is auto sensing). I have only replaced two transistors Tr607 (C458) on each Main Amp Board. Pinout should be good but I didn't check the new ones before I soldered them to the board (I wish I would). I will swap 602/604 with 601/603 and check if the offset migrates to the other channel (or would it be easier to buy new transistors and just replace all of the ones on both Main Amp Boards?). Could Tr607 have anything to do with the midpoint voltage?
You didn't mention what the measured offset is on the "good" channel - please share this.
I don't think TR607 should affect that value but I can't say for sure. If you are concerned, you can just swap this transistor with the good channel, but I doubt that's the problem.
I would swap those parts I mentioned first (and check if offset migrates), especially if you are unsure what the midpoint measurements were before you worked on the amp. Otherwise you'd be replacing potentially good parts and potentially creating new problems in the process.
OK, I took new measurements for both right and left side:
Right side midpoint voltage started out at 30mV and slowly dropped to around 13mV after 3 minutes (service manual state 3 to 4 minutes warmup before measurement). The trimpots are really finicky and values jumps around as soon as I touch them but I had no problems bringing the midpoint voltage down to 1.1mV (the service manual state 0 (zero) +/-10mV). I then measured bias current and it was 45.9mV (service manual state 47mV +/-10mV).
Left side midpoint voltage measured 0.588mV after 4 minutes with its trimpot turned all the way down, any additional adjustment brings midpoint voltage up. The bias current on the left side was 47.9mV.
I guess you already got the service manual but attached is the schematics for the board on the adjustment of main amp circuit board page...
Do you mean 588mV?
I'm guessing yes, because 0.588mV would be no problem.
Anyway, it sounds like bias on both channels is good, and DC offset on right channel is good.
I'd swap the 4 transistors I mentioned carefully and see if the offset jumps sides.
If you have a dim bulb tester, I'd leave that hooked up while trying this, and with no speakers / speaker load attached. If not it's fine though, just be super careful and double check your work before turning on.
Yes off course, it should have been: 0.588V
You said "swap 602/604 with 601/603". I can not find any 602/604 in the CA-600 service manual. Main amp boards are the same on both left and right side (only one schematic of "Main Amp Board) in the service manual. The main amp board got a Tr601 and a Tr603. Should I swap the Tr601 and a Tr603 from the right side to the left side and vise versa? I could always build a bulb tester. I probably should have built it before I fired up the CA-600 the first time after I had worked on it...
When you scroll further down in the SM you'll see both L/R boards described separately (see pic), but it's the same as what you said.
IF after swapping 601/603 with 602/604 your problem migrates, then you can replace them:
2SA572 -> KSA992FBTA
RE: bulb tester, you probably don't need it right now (just be careful), but if you plan on doing much of this work I do recommend it, it can save big blowups due to small mistakes.
OMG you are right, I found it on the overall schematics diagram, RTFM...
Thanks you so much. I will order some (and probably some new trimpots too) to be on the safe side. This will take some time since I am going skiing next week. I will post progress as soon as I get back working on it.
To be honest, I always wanted to learn electronics but I always had a tough time doing so. But I take any opportunity to learn and I am a pretty good solderer. I like working on vintage HiFi (even though it is usually woodwork on TT plinths or bringing speakers back to life). I guess a bulb tester wouldn't hurt...
I'd suggest trying the L/R swap first so that you're not replacing good parts, but also in case the problem is elsewhere and you need different parts.
PS. you can use multiturn trimpots if you want finer / less finicky adjustments, it's personal preference though. I like them.
The Bourns blue ones are nice, some versions come with longer legs (check datasheet) which makes retrofitting easier.
The replacements need to be very closely matching Hfe. The closer the match the better.
I have not made the swap yet but I made the following measurements:
Main amp voltage:
Left +B = 36.2V
Left -B = -36.1V
Right +B = 36.2V
Right -B = -36.1V
Left offset = -0.56V
Right offset = -2.6mV
Left D601: cathode = 11.36V, anode = -1.79V
Left D603: cathode = -1.09V, anode = -13.24V
Right D601: cathode = 12.35V, anode = -0.4mV
Right D603: cathode = -0.3mV, anode = -12.40V
Base emitter voltage:
Left Tr601 = 0.596V, changes with pot? No
Left Tr603 = 0.600V, changes with pot? No
Left pot OK? Yes
Right Tr601 = 0.596V
Right Tr603 = 0.600V
What stands out is the difference in zener diode voltage between left and right side, but I don't know what could cause it?
12 volt zeners ---- WZ120 12v 500mw 5%tol. (11.4 v -12.6v)
D601 13.15 v --- just a wee bit out of tolerance
D603 12.15v --- OK
D602 12.39v --- OK
D604 12.37 v --- OK
You can replace it . But I don't think you will gain anything from it.
Your just spinning your wheels. Until you swap out the two long tailed pair (differential pair) transistors.(TR601/603 and TR602/604)
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