Final stages of sony ta-5650 restoration, weird voltages?

[smurfer77]

Here is the post where the Poster used two halogen lamps in series with the PS to help protect the V-fets .

he then only installed one pair at a time as not to destroy all v-fets if something was wrong.

http://www.diyaudio.com/forums/soli...a-v4810-solid-state-heaven-2.html#post1918056

And here on audio karma which I'll post here:




I highlighted the part in the post about one pair at a time in RED

nacho

Nice link. Only thing that worries me is that PTC limiters are usually relatively slow (compared to the time it takes to damage a transistor) since they need to heat up before the resistance goes up, right? (but I'm more familiar with conventional PTC limters, rather than gas lamps). Another idea (I had suggested to me) was to temporarily replace the slow fuses in the power supply with quick-acting fuses of amperage just big enough rating to survive switch on, so maybe they will blow quickly if the FETs are full open.

 

[music2]

So I've had a chance to have a look at the amp today. I replaced the small value ceramic caps on the amplifier board, i'm still waiting on the film caps to arrive to do the rest, all of these are most likely fine but i am doing them just in case, that said the ceramic 5pf caps were reading 10pf and the 10pf caps were reading 20pf, the new ones test no more then 1pf off their rating. @smurfer77 regarding AC on the gate with the pre and amp disconnected my meter has a fit and keeps jumping all over the place going 40v, 1v, 0v O.L repeating really fast until it settles on 1.3v. With the inputs shorted it goes straight to 6v ac. I tried the extra capacitor on the c306/356, i didn't solder these in, only held them against the solder tags to see if they would have any effect, with the just capacitor the 49v rises to around 51v however, if my arm touches the top of the transformer it instantly goes to 77v, take my arm off the transformer and its back to 51v. The transformer is painted and therefor insulated so the magnetic field must have some sort of effect on the circuit causing it to stabilize. As requested, the location of c306 and c356

 

[smurfer77]

So I've had a chance to have a look at the amp today. I replaced the small value ceramic caps on the amplifier board, i'm still waiting on the film caps to arrive to do the rest, all of these are most likely fine but i am doing them just in case, that said the ceramic 5pf caps were reading 10pf and the 10pf caps were reading 20pf, the new ones test no more then 1pf off their rating. @smurfer77 regarding AC on the gate with the pre and amp disconnected my meter has a fit and keeps jumping all over the place going 40v, 1v, 0v O.L repeating really fast until it settles on 1.3v. With the inputs shorted it goes straight to 6v ac. I tried the extra capacitor on the c306/356, i didn't solder these in, only held them against the solder tags to see if they would have any effect, with the just capacitor the 49v rises to around 51v however, if my arm touches the top of the transformer it instantly goes to 77v, take my arm off the transformer and its back to 51v. The transformer is painted and therefor insulated so the magnetic field must have some sort of effect on the circuit causing it to stabilize. As requested, the location of c306 and c356

Mine values were jumping around like that until i sorted out the power supply. Could you double check B2 (the ~+/-77V input to power amp board) is stable during all of these observations?

 

[music2]

B2 voltages follow the same trend as the gate voltages in all situations so when the gates are reading correctly with the inputs disconnected the 77v reads correctly however when the inputs are shorted the voltages go down as well.

 

[smurfer77]

B2 voltages follow the same trend as the gate voltages in all situations so when the gates are reading correctly with the inputs disconnected the 77v reads correctly however when the inputs are shorted the voltages go down as well.

Ok. My B2 was stable after recapping power supply and power amp boards, with or eithout preamp connected. I would forget gates (or installing vfets) and focus on B2 stability at first.

 

[music2]

@smurfer77 Bringing this thread back to life, Have had a very busy last 2 months and unfortunately working on the ta-5650 had to take the back burner. I've pulled it back off the shelf and started having a look at it again, regarding the B2 voltages although they do go down when the inputs are shorted the voltage they drop to is more appropriate. Eg with inputs separated they are sitting at 87v however inputs shorted drops them to 77v leading me to think 87v is an unloaded voltage, when the inputs are shorted creating the oscillation this in effect loads the supply dropping it to the correct 77v. I am starting to think more and more this amp is giving the same problems as yours did and that i am chasing a fault that doesn't exist, still the idea of putting the v-fets in is a scary one. I have 8 working vfets all of the same rank that i could kill in an instant. The chances of finding a set like that, well... i don't think i ever would.

 

[Nashou66]

@smurfer77 Bringing this thread back to life, Have had a very busy last 2 months and unfortunately working on the ta-5650 had to take the back burner. I've pulled it back off the shelf and started having a look at it again, regarding the B2 voltages although they do go down when the inputs are shorted the voltage they drop to is more appropriate. Eg with inputs separated they are sitting at 87v however inputs shorted drops them to 77v leading me to think 87v is an unloaded voltage, when the inputs are shorted creating the oscillation this in effect loads the supply dropping it to the correct 77v. I am starting to think more and more this amp is giving the same problems as yours did and that i am chasing a fault that doesn't exist, still the idea of putting the v-fets in is a scary one. I have 8 working vfets all of the same rank that i could kill in an instant. The chances of finding a set like that, well... i don't think i ever would.

I think it was member Helmut that used only 2 V-Fets on one channel to test if everything was ok incase it did have a fault so only one set of V-fets were at stake.

The amp was the Black WEGA version of this amp. I think the 4810?
Let me find the thread and I'll link it here.

http://www.audiokarma.org/forums/in...dstate-heaven-by-helmuth.244646/#post-2943183

He also uses two 500 watt halogen work light lamps in series with the power supply to protect the v-fetts if something is wrong. That step is a couple posts back from this one i linked.

Hope this helps.


Nashou

 

[smurfer77]

@smurfer77 Bringing this thread back to life, Have had a very busy last 2 months and unfortunately working on the ta-5650 had to take the back burner. I've pulled it back off the shelf and started having a look at it again, regarding the B2 voltages although they do go down when the inputs are shorted the voltage they drop to is more appropriate. Eg with inputs separated they are sitting at 87v however inputs shorted drops them to 77v leading me to think 87v is an unloaded voltage, when the inputs are shorted creating the oscillation this in effect loads the supply dropping it to the correct 77v. I am starting to think more and more this amp is giving the same problems as yours did and that i am chasing a fault that doesn't exist, still the idea of putting the v-fets in is a scary one. I have 8 working vfets all of the same rank that i could kill in an instant. The chances of finding a set like that, well... i don't think i ever would.

You are right to be scared. Sorry, I dont wish to encourahe you further . As Nashou says you can use 1 pair per channel to reduce the risk, but even replacing a single unit is a hard thing to do so you really want to be sure you don't have issues elsewhere.

 

[music2]

I still worry that perhaps using one pair may still allow the amp to oscillate, i do know that although one of the vfets was damaged the amp was working when i got it and had this issue immediately after i had removed the vfets, had the issue existed while they were still installed then the amp could not have worked so it helps give me a little more confidence. I still want to check as many things as possible though, however i am running out of things to test. Just about every single transistor has been pulled and tested, every single capacitor both film and electrolytic has been replaced with much higher quality items then original, many resistors have been replaced etc. I have seen that post on the wega at some point, i do remember some members expressing concern that the halogens could still allow to much current to conduct but i will have to have a read over it again. By memory that particular unit ended up blowing up again a year or so later but the op suspected the replacement vfet's were to blame :/

 

[Binkman]

What you might be testing and how.. i.e. rail v's at idle versus powering up and the rails should see more power. Pretty common Sony method of class A at idle and switching to Class B with more 'gain' via volume pot. doesn't take much gain to switch but the measurements to the vfets circuits can tested without installing them. I'd not touch the VR's for anything until you sort out the circuits, the settings worked and no need to touch. even if you change some caps and rails look good etc..

bink

 

[smurfer77]

I still worry that perhaps using one pair may still allow the amp to oscillate,

I don't know about one pair, but my TA-4650 was stable with two pairs installed in a one-pair per channel config.

 

[music2]

I'm going to give the unit one final going over and if i still cant find a problem i think i am going to have to just bite the bullet and hook up the halogens as the other member did with the wega unit and hope they do their job if required. Problem with this unit is i don't think any single part is faulty. I think its either a case of ta-5650's are prone to oscillation without the vfets installed or a bunch of components have drifted slightly causing the amp to oscillate without the vfets installed.

 

[Nashou66]

I'm going to give the unit one final going over and if i still cant find a problem i think i am going to have to just bite the bullet and hook up the halogens as the other member did with the wega unit and hope they do their job if required. Problem with this unit is i don't think any single part is faulty. I think its either a case of ta-5650's are prone to oscillation without the vfets installed or a bunch of components have drifted slightly causing the amp to oscillate without the vfets installed.

I think its the latter. Just do one complimentary pair and adjust the bias as Helmut did. then install the other set and moveon to the other channel.

nashou

 

[music2]

I replaced the input differentials to lower the dc bias, i got the 54mv on the left channel and the 23mv on the right channel down to 6.2mv and 5.8mv. I did a basic hfe test on the original pulled 2sa705's, now i don't know how accurate my meter is but they both tested significantly lower then the minimum on their data sheet and were no longer closely matched. I then decided to test the remaining 2sa705's in q301 and q351 and these met and slightly exceeded the minimum gain on the data sheet. Given the poor health of the input differentials i suspect maybe some of the other semi's on the amplifier board may have aged also. When i pulled and test them i only did a diode check so i am going to pull them again to better check their health. In the event some are not so healthy i'd like to put together a list of alternative transistors for all of the transistors on the amp board. A list has been made before for the entire TA-5650 however the substitutes mentioned are mostly all obsolete now as well. Easy to obtain substitutes preferred, hopefully we can get a complete list going to help myself and other members in the future.

2sc1663
2sa835
2sa705- Alternative bc556b, gain may be a little on the high side?. A few members have used them for input differentials including me and they seem to work fine.
2sa677
2sa678
2sa639
2sc926a
2sc1363

 

[Nashou66]

DC bias for these amps is about 50mV so you dont want to get it down . Now DC offset is another story. is that what you meant?

 

[music2]

Yes I meant dc offset sorry, I was running late when I wrote that so I was rushing. I can't adjust the bias until the vfets are installed. It does generate correct bias voltages though of around 28v with the trimmers set for minimum bias. The offset voltages are nice and low now

 

[Nashou66]

I think your ready. Just do one pair on one channel and see if you can set bias to 25mv then add the other pair and go to 50mv . I think you'll be fine as i think the voltages will be off till you get the V-fets in.
The 4650 I think is a little different in my case. But I was repairing a A960 phono section and the jets were blowing. So i had them out while trying to get voltages right. But they never got to where they should be. Weeks went tracing it and chasing it till my tech who was helping me said well just install the j-fet and monitor the voltage at the gate while powering up, if it goes positive shut it down ( these need negative gate voltage) . Did it and all voltages stabilized once in.

Might be the same issue for you.

good luck

Nashou

 

[music2]

Thanks for your replies, I've just got to work out just how helmut installed the limiting halogens as his description isn't that clear then I will give this a go. As far as I'm aware if the 70v on the vfets still drops to 40v when the pre amp is connected once the vfets are installed they will Conduct full current across them and will be toast

 

[music2]

@Nashou66 @smurfer77 @ilimzn Ok so i've got some more positive progress starting to happen now. I installed 68 ohm resistors on the 45v rails to help protect the vfets and installed them and turned the unit on cautiously. Everything went fine, set the bias to a low level of 10mv. Then i connected the shorting plugs and again nothing bad happened so i connected the pre amp jumpers and again nothing bad happened until i turned the volume dial above 9 o'clock with no load and bam gate voltages went down to 30v however the 45v source voltage dropped to a massively low 16v, this being due to the 68 ohm resistors restricting current, no damage done. Turning the volume dial a little more and the voltages shot back up to where they should be and stayed there, listened to the amp monitoring my meters carefully for about half an hour no problem. Let the amp cool down for a few hours and tried again and was able to replicate the same issue again very briefly, it seems to occur around the 9-10 o'clock range where the volume pot is a little scratchy. I'm suspecting a resistor or transistor is operating out of spec until it gets some heat in it because once the amp warms up it is rock solid, that or the scratchiness of the pot commands more current then the resistor will allow causing source voltages to drop to low making the whole circuit become instable, this section of the volume dial also produces a slight amount of hiss. Will investigate further and update.

 

[Nashou66]

Might just need to clean the volume pot well.

Nashou