I just (almost) finished repairing my BA-F1 but I still Have a problem. Maybe someone can save me some time and searching. It's clipping the negative portion of the output on the right channel even at low power levels. Has anyone else experienced this and can point me in the direction of the issue. The good news now I can justify the scope I bought
Sansui BA-F1 Distortion
- Thread starter Kevin T
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It's with the channel loaded into an 8 ohm 150 watt resistor. Both channels are loaded the same and only the right channel exhibits the distortion so I can't really blame the load. I replaced TR15 the driver transistor on that side since it was heating up drastically and the situation improved but it's still not right.
kevzep
Its all about the Music
I'm not saying the load is at fault, I do not think its the load, but whether the amplifier can swing its normal voltage with the load off is going to give you a clue as to what the problem is.
Standard operating procedure is to make sure the amplifier can operate and swing its intended voltage before connecting a load to it...
You may have a resistor somewhere that is out of spec' and limiting the ability of the amplifier circuit to 'swing' all the voltage it should. The fact that TR15 isn't happy is a telling clue, be suspicious of any left // right differences like that which could lead you to the source of the problem. Start by checking the OP transistor base stopper resistors (4.7Ω), which on this model are on the driver board, then all the resistors around the hot TR15, including R38 (and R37) both 390Ω.
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Thanks for the input. everything around TR15 is in spec. I've done a bit more digging since then but I think it's time for a bit more history on this problem.
I've owned this amp since new, way back in the late 90's it had issues and has sat ever since. Recently I decided it's time to get it running again. I completely re-capped the unit and also replaced all of the output transistors along with the driver transistors on the right channel since the was a bad output transistor on that channel. when I went to set the zero balance and bias the left channel set fine. The right channel wouldn't drop below +42VDC. After much searching and hair pulling I decided it must be the FET so I ordered one and waited fro it to arrive. When I went to change it i discovered that the gate and source leads on the input side of the dual FET were shorted together. Probably when I heat sinked it before changing the cap right there. I separated the leads and the zero balance and the bias set fine.When I went to set the meter levels was when I discovered this problem. Every thing is fine on the left channel but the right has problems only when signal is applied. I've found the DC voltages to be off around the dual FET so I've changed it but haven't tried it yet. I'll keep you posted. Also I've swapped out the output transistors with a good set from the originals and it didn't change a thing.
I've owned this amp since new, way back in the late 90's it had issues and has sat ever since. Recently I decided it's time to get it running again. I completely re-capped the unit and also replaced all of the output transistors along with the driver transistors on the right channel since the was a bad output transistor on that channel. when I went to set the zero balance and bias the left channel set fine. The right channel wouldn't drop below +42VDC. After much searching and hair pulling I decided it must be the FET so I ordered one and waited fro it to arrive. When I went to change it i discovered that the gate and source leads on the input side of the dual FET were shorted together. Probably when I heat sinked it before changing the cap right there. I separated the leads and the zero balance and the bias set fine.When I went to set the meter levels was when I discovered this problem. Every thing is fine on the left channel but the right has problems only when signal is applied. I've found the DC voltages to be off around the dual FET so I've changed it but haven't tried it yet. I'll keep you posted. Also I've swapped out the output transistors with a good set from the originals and it didn't change a thing.
one last Post for today. I'm back to waiting for parts. I decided to do more static testing. Thanks John out of value is an understatement. R704 and 708 are terribly out of spec and would seem to cause the exact problem I'm experiencing. They're at 124K and 3M instead of 0.33 ohm. I had to check like three time to really believe it.
kevzep
Its all about the Music
What transistors did you use to replace the blown output and driver transistors?
petehall347
the brandy coffee man
guessing they are emitter resistors .. whatever their job turn bias down down before replacement .
For the drivers I was fortunate to find NOS Toshiba's. For the outputs I used ISC 2sc2608 and 2sa1117. I expect the chorus to shout oh no Chinese but they seem to be fine so far and they have a decent reputation in Europe. I'll let you know when I get to listen to them.
Yes they are emitter resistors on the OP transistors on the negative side. Being out of spec in the way they are would limit the ability of the OP transistors to fully swing the proper voltage and cause the clipping of the negative portion of the wave form I'm seeing
petehall347
the brandy coffee man
and cause bias to be out of spec . might even be the reason the resistors cooked .hard to say .
kevzep
Its all about the Music
Where are they from though? A reputable supplier, not from China?
Is the channel giving you trouble the one with the new transistors?
We have to be suspicious of these if there is nothing else that makes sense for the problem you are having.
It is part of the reason I asked you to run the amp with no load. You can tell a lot about an amp that can swing voltage no problem with no load, but then wont swing with a load and has issues such as the ones you are having....
It will be interesting to see where it ends up.
I have got one of these amps coming in for restoration next week, I will be replacing the outputs with brand new Sanken's in a TO-3P package, as John did with the one he restored.
Both Channels have the new transistors, ans one channel works fine, so it's had to blame them. Also I swapped in MMA1216's and NMC1216'S that check good on a diode test and there was no change in the problem.
If I had researched further before buying the op transistors I would have gone with TO-3P Sankens also. I these don't work out I'll be going that way.
I have a simulation of the AU-X1, almost the same as BA-F1.
The input stage tail current shows 7.2mA in sim, so measure the V drop across R11? to cal what it is in your unit, measure both channels.
jfet drains run at around 11.5V
I'll post the sim with the DC values, see how they compare to both your channels
The input stage tail current shows 7.2mA in sim, so measure the V drop across R11? to cal what it is in your unit, measure both channels.
jfet drains run at around 11.5V
I'll post the sim with the DC values, see how they compare to both your channels
kevzep
Its all about the Music
Ooooooo Rick, could I please get a copy of your sim?
Thanks, I'm looking forward to see how mine compares.
Well, you can compare your two channels now, and as the BA-F1 is so similar to the AU-X1, the voltage reading details shown in the PDF can be of great use to you?