I didn't say it was working. It's been sitting here in the shop for over a year. The owner decided the VFETs needed cleaning and put them in the wrong location when he reinstalled them, 3 out of 4 are shot. He's supposed to be looking for new ones as I refuse to take the chance on getting fakes. Other than that it works and is useful a pre-amp at the moment.
I really haven't had the interest in it until the VFETs arrive.
Craig
I really haven't had the interest in it until the VFETs arrive.
Craig
ahess
Member
So I'm still without any theories on why positive (or proper) voltages from the power supply lead to inverted (+ to -) voltages and burnt components on the amp board. Before I buy new parts and plug them into the same spots, I'd like to at least think that I've done something about this. 
Also, once I've figured out what was wrong, how far should I go to check the components? Do I check all the caps, resistors, transistors, etc--or can I work the borders of the burnt items on the schematic until I hit working parts, the way I did with the speaker protection circuit?
Also, once I've figured out what was wrong, how far should I go to check the components? Do I check all the caps, resistors, transistors, etc--or can I work the borders of the burnt items on the schematic until I hit working parts, the way I did with the speaker protection circuit?
ahess
Member
More results!
I started testing the PS transistors (ran out of things to do) and Q409 and Q410 are good from base to collector but shorted from base-emmiter (don't hold me to the leads--I'm still learning). The other trans seem okay.
SO! What would this do to the functioning of the PS? This goes waay beyond my pay grade.
I started testing the PS transistors (ran out of things to do) and Q409 and Q410 are good from base to collector but shorted from base-emmiter (don't hold me to the leads--I'm still learning). The other trans seem okay.
SO! What would this do to the functioning of the PS? This goes waay beyond my pay grade.
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MiamiBoy
Active Member
I know there are four resistors that need to be replaced to a higher value (R313, R314, R363, R364) as per the service bulletin for the 4650 but who said that ALL resistors needed to be replaced? If the amp is functioning well do all the resistors really need to replaced or this is only because you had to pull them anyway looking for a cause to your problem?
ahess
Member
Resistors: That's a reasonable question. There were a few things that led me to replace all of them:
1 The problem I found was a burnt/open resistor. Then I found several that were way off their value--e.g., measuring over 30k instead of 22k. So I was wary of the resistors in general.
2 I wanted to upgrade the amp anyway, and was planning on doing the caps.
3 I read a post or two where they upgraded the resistors.
4 I tend to go all-in on projects like this.
1 The problem I found was a burnt/open resistor. Then I found several that were way off their value--e.g., measuring over 30k instead of 22k. So I was wary of the resistors in general.
2 I wanted to upgrade the amp anyway, and was planning on doing the caps.
3 I read a post or two where they upgraded the resistors.
4 I tend to go all-in on projects like this.
ahess
Member
Since they've already been replace, I'd say yes, it is justified.
I don't suppose any of you have a handy list of equivalent transistors for the Sony ta-4650?
Oh, and I just found the post that got me ordering a 1K pot instead of the 2.2K pots:
Helmuth wrote: "I advise a 1K 10 turn potentiometer to replace the 2k5 one turn."
in this thread.
I don't suppose any of you have a handy list of equivalent transistors for the Sony ta-4650?
Oh, and I just found the post that got me ordering a 1K pot instead of the 2.2K pots:
Helmuth wrote: "I advise a 1K 10 turn potentiometer to replace the 2k5 one turn."
in this thread.
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Looked at the scat again. The 1K is probably OK, as long as you are able to reach the final setting for the bias current. Need not sweat that ATM.
I replace resistors when needed...if one is burnt up or if the enamel is peeling. Otherwise, too easy to make mistakes.
As far as finding transistor subs:
1. Find the specs for the original - http://www.audiokarma.org/forums/in...ransistor-specifications.149509/#post-1655960
2. ID the case style - http://www.audiokarma.org/forums/index.php?threads/common-transistor-case-styles.116021/
Supply that info, and assistance will be a lot easier. But for a pass regulator like Q402, I just toss in big bad On-Semi TO-220's, like the MJE15032G (NPN) and the MJE15033G (PNP). Note that assuming the pinout of any replacement transistor is the same as the original is a recipe for disaster. Check the data sheet, or spring for an Atlas DCA 55. And note that a simple diode test, or even a test with a device like the Atlas, is good for finding perhaps 60-70% of failures, at best. But the Atlas is great for lead identification.
The only irreplaceable transistors in the amp are the VFET's (although the 2SK63A FET's in the preamp might be a challenge as well).
I replace resistors when needed...if one is burnt up or if the enamel is peeling. Otherwise, too easy to make mistakes.
As far as finding transistor subs:
1. Find the specs for the original - http://www.audiokarma.org/forums/in...ransistor-specifications.149509/#post-1655960
2. ID the case style - http://www.audiokarma.org/forums/index.php?threads/common-transistor-case-styles.116021/
Supply that info, and assistance will be a lot easier. But for a pass regulator like Q402, I just toss in big bad On-Semi TO-220's, like the MJE15032G (NPN) and the MJE15033G (PNP). Note that assuming the pinout of any replacement transistor is the same as the original is a recipe for disaster. Check the data sheet, or spring for an Atlas DCA 55. And note that a simple diode test, or even a test with a device like the Atlas, is good for finding perhaps 60-70% of failures, at best. But the Atlas is great for lead identification.
The only irreplaceable transistors in the amp are the VFET's (although the 2SK63A FET's in the preamp might be a challenge as well).
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ahess
Member
I’ve checked all the transistors and diodes, taking them out of circuit when I got ambiguous readings or checking the readings with its pair on the other side of the amp board. Diodes are all good. Here are the transistors that need replacing, with the specs furnished by EW. I also got links to recommended replacements from help at Mouser--but I am not sure I trust their recs, and I'm out of my depth here.
Can you either confirm that these are good replacements or recommend others?
Identification Type Vceo,V Ic,А Pc,W B Ft
Can you either confirm that these are good replacements or recommend others?
Identification Type Vceo,V Ic,А Pc,W B Ft
For PS Q409 & 410:
2SC926A n-p-n 210 0,030 0,320 100 160
Mouser Rec or This
For Q404/406:
2SC634A n-p-n 50 0,200 0,250 400 140
Mouser Rec
For Q405:
2SA678 p-n-p 50 0,200 0,320 250 140
Mouser Rec
2SC926A n-p-n 210 0,030 0,320 100 160
Mouser Rec or This
For Q404/406:
2SC634A n-p-n 50 0,200 0,250 400 140
Mouser Rec
For Q405:
2SA678 p-n-p 50 0,200 0,320 250 140
Mouser Rec
The wiper (center) and one side are shorted together at the PCB. Sounds like it is turned all the way in one direction. So, it sounds right except obviously you want to be very careful about which direction it is turned toward. I'm not going to venture a guess as to the correct direction in this application, since I've had one of the strong local brews already.
Either would be fine. Kinda like the KSC1845, as it's likely to be quieter.
These are being used as switches. I typically use the Zetex ZTX694B (NPN) and the Zetex ZTX795A (PNP) for applications like this.
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ahess
Member
Wow! I just discovered something really weird. Looking over my pictures of the amp before I replaced the caps, I saw that caps c420 and C 421 were 50v/47uF. C421 also has a red marker slash across the top . On the mounting diagram they are listed as 100v/100 uF!
I compare the mounting diagram to the schematic, and saw that while the schematic is listed as former type the mounting diagram is not. The parts list at the end of the service manual also differentiates between the values of some caps for the former model. I suspect I installed the wrong values in some of these locations. I need to look over the board tonight.
Could this have been the reason I blew the speaker protection transistors and these caps? Could this have been responsible for inverting the voltages?
I compare the mounting diagram to the schematic, and saw that while the schematic is listed as former type the mounting diagram is not. The parts list at the end of the service manual also differentiates between the values of some caps for the former model. I suspect I installed the wrong values in some of these locations. I need to look over the board tonight.
Could this have been the reason I blew the speaker protection transistors and these caps? Could this have been responsible for inverting the voltages?
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