Hey hey,
I'm working on a Sony Vfet TA-4650, needs an overhaul. I've completed a cap job on the PS and main amp board. That much was needed at first glance. During testing I found 1-3V of swinging AC on one of the channels. After diode testing a few XS, I had probable cause to dig deeper before doing any more soldering work. Having seen swinging voltages caused by bad caps and transistors before, I took a look to see what the Service Manual has listed for original type XS. This main board is covered in NTE290's (at least 5) and a few other "common" replacement type XS like the 2SC945 (in the Q355 spot). A quick check on cross ref sites for the C634A that I was expecting to find here, doesn't list the C945 as being a no-brain direct swap.
If a only a small percentage of the original equipment are still here, what has happened? In order of importance, this is what I need to see happen:
1. Finish the project quick and cheap.
2. Find replacement XS if need be. I'll do some desoldering to find out what I really need.
3. I opened the Can of Worms when I searched for this amplifier on here and AK. Lots of Mods and comments about design flaws, too much for me to read and make money at the same time. Is there anything I MUST DO before handing the amp back to the client? Thanks.
I'm working on a Sony Vfet TA-4650, needs an overhaul. I've completed a cap job on the PS and main amp board. That much was needed at first glance. During testing I found 1-3V of swinging AC on one of the channels. After diode testing a few XS, I had probable cause to dig deeper before doing any more soldering work. Having seen swinging voltages caused by bad caps and transistors before, I took a look to see what the Service Manual has listed for original type XS. This main board is covered in NTE290's (at least 5) and a few other "common" replacement type XS like the 2SC945 (in the Q355 spot). A quick check on cross ref sites for the C634A that I was expecting to find here, doesn't list the C945 as being a no-brain direct swap.
If a only a small percentage of the original equipment are still here, what has happened? In order of importance, this is what I need to see happen:
1. Finish the project quick and cheap.
2. Find replacement XS if need be. I'll do some desoldering to find out what I really need.
3. I opened the Can of Worms when I searched for this amplifier on here and AK. Lots of Mods and comments about design flaws, too much for me to read and make money at the same time. Is there anything I MUST DO before handing the amp back to the client? Thanks.
Take out the output v-fets and see if they are still alive on curve tracer. If they are ok, advice your client to find a knowledge technician here (ilimzn) or at AK (?) to completely overhaul the amp...they are precious.
Interesting responses. Perhaps they are due to my comment,
I was referring to the gratuitous amount of information and personal opinion dedicated to the topic of modification - the stuff that isn't critical for my client to have a reliable and enjoyable amplifier. I mention my comment about money because I know it is important for me to discern my intentions, knowing work from pleasure. Both are full time occupations for me 😀.
STOCKTRADER200, all of the main amp board caps have been replaced, and all of the power supply caps with the exception of the two largest filter caps. I am scope testing the AC on the output as we speak, and the caps have not solved the rising bias/increase in current consumption over time. So your offer is not viable.
APARATUSONITUS, I don't own a curve tracer. Do I need one for testing v-fets? And why couldn't I use a scope and multimeter to decide if the components are Good/Not Good? It is also important for you to know that I am performing amp repair professionally, and rather than pass off a great opportunity to learn by giving this project to another tech, I would prefer to intern or be mentored in order to grow.
I currently have a post here on DIY "32 year old looking for Mentor in New England Area (I'm Serious)". I'm 34 now, and have successfully, if slower than some, completed many fine projects with an excellent satisfaction rate and return customer base. These projects have been primarily tube amplifiers, but my love of 70's wood panel amplifiers and the Nelson Pass Aleph J have given plenty of reason for me to tap into the pool of Solid-State and HIFI design. Thank You.
I was referring to the gratuitous amount of information and personal opinion dedicated to the topic of modification - the stuff that isn't critical for my client to have a reliable and enjoyable amplifier. I mention my comment about money because I know it is important for me to discern my intentions, knowing work from pleasure. Both are full time occupations for me 😀.
STOCKTRADER200, all of the main amp board caps have been replaced, and all of the power supply caps with the exception of the two largest filter caps. I am scope testing the AC on the output as we speak, and the caps have not solved the rising bias/increase in current consumption over time. So your offer is not viable.
APARATUSONITUS, I don't own a curve tracer. Do I need one for testing v-fets? And why couldn't I use a scope and multimeter to decide if the components are Good/Not Good? It is also important for you to know that I am performing amp repair professionally, and rather than pass off a great opportunity to learn by giving this project to another tech, I would prefer to intern or be mentored in order to grow.
I currently have a post here on DIY "32 year old looking for Mentor in New England Area (I'm Serious)". I'm 34 now, and have successfully, if slower than some, completed many fine projects with an excellent satisfaction rate and return customer base. These projects have been primarily tube amplifiers, but my love of 70's wood panel amplifiers and the Nelson Pass Aleph J have given plenty of reason for me to tap into the pool of Solid-State and HIFI design. Thank You.
I would like to refresh this topic.
APARATUSONITUS, I recently learned of a transistor tester on the market that can test mosfets, jfets, bjts, etc and operate as curve tracer using a USB port and a computer. Is this a fully functioning Curve tracer? No. I have no experience with a stand-alone unit, but I do understand that the max voltage applied to any of the terminals is about 12V, with a max current of somewhere between 10-15mA. The product is the Peak Atlas DCA75 Pro. Does anyone have any thoughts on this device and my intention to use it to check the status of components on this Sony V-fet amp? I will have many other uses for it besides this project. I will call the company this week to learn more.
Here is the link to the manufacturer's product website:
Peak Electronic Design Limited - Atlas DCA Pro - Advanced Semiconductor Analyser - Model DCA75
APARATUSONITUS, I recently learned of a transistor tester on the market that can test mosfets, jfets, bjts, etc and operate as curve tracer using a USB port and a computer. Is this a fully functioning Curve tracer? No. I have no experience with a stand-alone unit, but I do understand that the max voltage applied to any of the terminals is about 12V, with a max current of somewhere between 10-15mA. The product is the Peak Atlas DCA75 Pro. Does anyone have any thoughts on this device and my intention to use it to check the status of components on this Sony V-fet amp? I will have many other uses for it besides this project. I will call the company this week to learn more.
Here is the link to the manufacturer's product website:
Peak Electronic Design Limited - Atlas DCA Pro - Advanced Semiconductor Analyser - Model DCA75
I would also like to mention that I am doing my homework on these V-fets. Nelson Pass wrote a .pdf about them. I understand that the V in the name describes the fabrication and design of the gate.
V-FET or Power Mosfets - Electronic Circuits and Diagrams-Electronic Projects and Design
The Static Induction Transistor (SIT) is either the same device or a close family member.
V-FET or Power Mosfets - Electronic Circuits and Diagrams-Electronic Projects and Design
The Static Induction Transistor (SIT) is either the same device or a close family member.
See VFET testing method and result
Also, do search for ANY post from ilimzn regarding v-fet (sit) transistors or V-fet amps be it from Sony or Yamaha...you can thanks him latter.
Also, do search for ANY post from ilimzn regarding v-fet (sit) transistors or V-fet amps be it from Sony or Yamaha...you can thanks him latter.
Careful:
VFET is euqal to SIT
VFET is NOT equal to VMOS
VFET's are a species of JFET, which operate, in a way, opposite to other semiconductors - other's need to be turned on using a signal on the control terminal. SIT/VFETs are normally completely (*) turned on as it is and need to be turned OFF using a signal on the control lead.
How different is this? If you wanted to use DVM to test one, and are used to bipolar of MOSFET, you will immediately pronounce it dead because it will show a short between Source and Drain. And... I'm afraid to say I've seen it more than once... you may end up scrapping 4, 8 or even 12 of them (at >$50 each if you can even find them!) perfectly working super-rare VFETs.
Also, if you do not understand how the amp works, you might be tempted to try starting it up partially without all of the power supplies present and thus possibly get the VFETs working without proper bias voltage, which will produce a short of the main power supply through them, likely with disasterous consequences. Starting the amp through a variac will do it for the 4650, 5650, 8650 and 5550/8550 amps.
(*) For the usual way they are used in audio, though VFETs also have gate current operating mode for switching purposes.
In other words: if in fact you want to learn amp technology on this amp, the only thing you will likely learn is that it is NOT the amp to first learn on. It requires a LOT of knowledge to begin with and some of it is old and not easy to come by today - VFET/SIT behave very similarly to vacuum triodes, and electrically have similar characteristics and circuit techniques, except for the fact they can be P-type as well. So, this would be for the very advanced course.
VFET is euqal to SIT
VFET is NOT equal to VMOS
VFET's are a species of JFET, which operate, in a way, opposite to other semiconductors - other's need to be turned on using a signal on the control terminal. SIT/VFETs are normally completely (*) turned on as it is and need to be turned OFF using a signal on the control lead.
How different is this? If you wanted to use DVM to test one, and are used to bipolar of MOSFET, you will immediately pronounce it dead because it will show a short between Source and Drain. And... I'm afraid to say I've seen it more than once... you may end up scrapping 4, 8 or even 12 of them (at >$50 each if you can even find them!) perfectly working super-rare VFETs.
Also, if you do not understand how the amp works, you might be tempted to try starting it up partially without all of the power supplies present and thus possibly get the VFETs working without proper bias voltage, which will produce a short of the main power supply through them, likely with disasterous consequences. Starting the amp through a variac will do it for the 4650, 5650, 8650 and 5550/8550 amps.
(*) For the usual way they are used in audio, though VFETs also have gate current operating mode for switching purposes.
In other words: if in fact you want to learn amp technology on this amp, the only thing you will likely learn is that it is NOT the amp to first learn on. It requires a LOT of knowledge to begin with and some of it is old and not easy to come by today - VFET/SIT behave very similarly to vacuum triodes, and electrically have similar characteristics and circuit techniques, except for the fact they can be P-type as well. So, this would be for the very advanced course.
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