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?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.
OK, I took new measurements for both right and left side:You didn't mention what the measured offset is on the "good" channel - please share this.
- Left side midpoint voltage measured 0.588mV after 4 minutes
Yes off course, it should have been: 0.588VDo you mean 588mV?
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...I'd swap the 4 transistors I mentioned carefully and see if the offset jumps sides.
OMG you are right, I found it on the overall schematics diagram, RTFM...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.
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.IF after swapping 601/603 with 602/604 your problem migrates, then you can replace them:
2SA572 -> KSA992FBTA
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...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.
The replacements need to be very closely matching Hfe. The closer the match the better.IF after swapping 601/603 with 602/604 your problem migrates, then you can replace them:
2SA572 -> KSA992FBTA
12 volt zeners ---- WZ120 12v 500mw 5%tol. (11.4 v -12.6v)Zener diodes:
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
You can replace it . But I don't think you will gain anything from it.D601 13.15 v --- just a wee bit out of tolerance
Your just spinning your wheels. Until you swap out the two long tailed pair (differential pair) transistors.(TR601/603 and TR602/604)I have not made the swap yet but I made the following measurements: