Two more things to check out meanwhile:
a) is C103 100pF present?
b) C305 - is it really connected between output and ground as the schematic shows?
a) is C103 100pF present?
b) C305 - is it really connected between output and ground as the schematic shows?
No, looks factory but there are no caps installed there
I am pretty bad at reading schematica but one side goes to ground for c305 and the other goes to diode d307 and d306. Bottom rail on the picture.
OK, since one hypothesis of blowing the channel is oscillation then there are two things for you to consider (and discuss with other forum members) that could improve amplifier stabilty.
a) C301 is usually 100pF and even times more (with some vendors and models) - in this amplifier it is 47pF. I would increase it to 100pF if it were my amp.
If C103 were present then this would have been taken care of.
b) C305 is usually in parallel with R326 - perhaps a design mistake?
Bigger area of that schematics:
For example Sony TA-F570S solution looks "traditional":
a) C301 is usually 100pF and even times more (with some vendors and models) - in this amplifier it is 47pF. I would increase it to 100pF if it were my amp.
If C103 were present then this would have been taken care of.
b) C305 is usually in parallel with R326 - perhaps a design mistake?
Bigger area of that schematics:
For example Sony TA-F570S solution looks "traditional":
Is there any reason for me not to populate the output board with the caps in the service manual? I am a bit worried since the circuit on mainboard has some different resistors around the ceramic capacitors that are the same on both sides.
Yes, if such components have been added only for some regions then it means usually that they are good for fulfilling the requirements of all sorts of regional standards and not needed for sound quality (i.e. for listeners in other regions) 🙂
C301 is present in all input scenarios and near the first transistor so if capacitance should be added then that is the one to replace.
Ok thank you for the suggestion. I have read online that oscilations are generally something that kills these amps. I am not sure how accurate that is but if it is true what can I do tho resolve this besides replacing C301? And is there a way to test for these oscilations with just a scope/dmm (I do not have a distortion analyser or other measurement equipment)
I also have a ta-5650 (in the black wega v4810 form)that I got really cheap, it has an issue with the right channel but I believe the v-fets are still ok, I really like the v-fet stuff. But that is something I will look at after I resolve this.
Some parts arrived today so I figured I'd try replacing them to see what happens, I haven't built de new dbt yet but I figured I had enough spares anyway... Anyhow I replaced everything and before I turned it on one peculiarity that I noticed is that the outer ends of the output transistors in the left channel are shorted on the pcb. I believe that this wasn't the case before but I am not 100% certain. I also apparently got replacements for the driver transistors that have a grounded collector pin which was not the case on the originals, since I only wanted to test I just used some plastic to prevent shorting the collector to ground. Despite my better judgement I decided to quickly turn the amplifier on. The left channel doesn't do anything, I think atleast. The right channel however is now causing issues. the emitter resistors get hot as well. Now I have replaced these but I do not think that is the issue, I checked for shorts and stuff. There is also a transformer hum which wasn't there before which makes me think I might have plugged in something incorrectly. I turned it off, checked all the things I could check without taking it fully apart again and besides the weird short in the left channel nothing seemed wrong. I did one more test with only the transformer for the right channel connected and aside from the protection light not coming on it showed the same behaviour of buzzing and getting hot. Still no shorts after this test though. I will be smart now and refrain from testing further until the bulbs arrive and I have revised my dbt. But for now if anyone can see anything obvious incorrect on these pictures let me know.
I also made sure the bias wasn't set to maximum for the right channel, I even tried twisting the potentiometer and nothing really changed though I do think there was a slight effect on the buzzing noise. I did not want to leave it running long enough for me to measure the bias directly.
I also made sure the bias wasn't set to maximum for the right channel, I even tried twisting the potentiometer and nothing really changed though I do think there was a slight effect on the buzzing noise. I did not want to leave it running long enough for me to measure the bias directly.
Sorry, your verbose post does not make any sense.
Try to explain things step by step so it is possible to follow your progress.
Try to explain things step by step so it is possible to follow your progress.
I have this but as per the suggestion made by citizen I am waiting for the parts to make one that I can use with two bulb in parallel. I do not have any old light bulbs or other related equipment laying around so I have to wait for parts. I did not want to use my current dim bulb tester with a 60 watt bulb as I figured that might give me issues and unreliable results.
here is what I said in the previous post summarized.
I replaced the parts that tested as defective.
Before turning on the the amplifier I noticed that on the left channel the emitter and base pins for the output transistors are shorted on the board. I am not sure if this was the case before I replaced the parts, it is not the case on the right channel.
Upon turning it on the left channel is unresponsive while the right channel gets hot and the transformer buzzes noticeably.
I turned the amp off, checked for shorts on both channels, didn't find anything except for the aforementioned short on the left channel which I was already aware of.
I tested the amp with just the transformer for the right channel connected and it showed the same symptoms, I tried adjusting the right channel bias which I believe did affect the buzzing slightly but I do not think it is the issue.
To me it seems reminiscent of something not being connected correctly. I also cannot really see how replacing stuff on the left channel would cause issues on the right channel. I only replaced 2 or 3 resistors on the right channel and the metal emitter resistors which all test fine.
Do you want to say that the right channel is now fried too - to make the transformer buzz (with left channel disconnected)?
As mentioned there are no shorts on the right channel so I do not think the right channel is broken, I did not do anything except replace some resistors so there isn't much that could have gone wrong. But, as I also mentioned in my previous post, it could be caused by something being connected incorrectly, there are a few connectors that can be mixed up. I did reference pictures I took during the assembly process but it can be hard to see. No it does not buzz when disconnected, which makes sense considering the transformers have no damage. I will do some more testing when I have the parts I need to make the new dim bulb tester.
I would advise disconnecting both amplifier boards and verifying the rail and regulated voltages before proceeding.
As in just measuring the transformers directly? Because the transformers plug into mainboard b which goes to mainboard a, or am I misunderstanding?
Nope, rectified and regulated voltages would be good to know - without load and without DBT - as a starting point, to eliminate the obvious.
Don't be angry but give this amplifier to someone who does this kind of work, because you clearly don't know what to do - during repairs there are always further steps to be taken: disconnecting, measuring - you do it very chaotically. This does not bode well for success.
I would prefer to learn. It is not worth it for me to get this repaired. It would be cheaper to sell it as defective and buy a working one instead if it really comes to it.