Hello everyone,
some of you may remember my thread, about the repair of a completely destroyed Teac BX-500. I can't find this thread anymore for some reason, so I'm starting a new one. In a nutshell: I found a Teac BX-500 on the scrap yard, that was completely destroyed. The heatsink and PCB had several drilled holes, some transistors were broken, and many tracks and pads were torn off. Most of the internal wiring was cut.
I repaired the tracks with wire and replaced all defective semiconductors, a list of the parts I replaced is at the end.
After I did all this I had the problem that I had a huge DC offset at the output of both channels (-44V!). After intensive troubleshooting I found the cause, a capacitor between supply voltage and ground turned into a resistor and with a resistance value of about 1.2kOhm and raised the ground potential to about 11V, which of course made the input stage go completely crazy.
After that is also repaired, at least the protection circuit relay clicks again, so not too much seems to be defective anymore.
On both channels the DC offset can be set to 0V.
On the left channel you can also set a quiescent current, but it drifts continuously higher. I set it to 26mV according to the service manual, but it rises continuously, about 0.5mV per second. When the quiescent current reached 120mV, I switched off the amplifier. The problem definitely has to do with the temperature, if I blow the VBE multiplier transistor slightly, the quiescent current drops significantly. The thermal coupling in this amplifier is not really good, the TO-92 quiescent current transistor is just placed next to the heat sink, without any heat conductive connection. Would it make sense to replace the transistor e.g. with a BD139 in a TO-126 package, and screw it directly onto the heatsink? In the original BX-500 and similar Teac BX-330 this is not the case, the proximity to the heatsink seems to be sufficient, but I'm not really satisfied...
On the right channel there is no quiescent current at all, but there I still suspect the VBE multiplier transistor or its solder joints and pads, these pads are in a very bad condition, almost torn off.
I would be happy if someone has another idea for the right channel, what I should look for and if someone has an idea how I should deal with the quiescent current drift of the left channel.
The service manual with circuit diagram can be found here (TEAC BX-500 SERVICE MANUAL Pdf Download | ManualsLib). For troubleshooting I also made an LTSpice model of the power amplifier part of the schematic, but for the input JFET and the two double transistors I chose alternative components, because I couldn't find an LTSpice model for them. I will attach the spice model to this post.
Thanks a lot for your help 🙂
Regards
Lukas
PS: I almost forgot the list of semiconductors, here it is:
Q110,210: 2SA970 --> KSA916
Q111,211: 2SA1815 --> KSC1815
Q112,212: 2SA949 --> KSA916
Q113,213: 2SA2229 --> KSC2316
Q114,214: 2SC2235 --> 2SD669A
Q115,215: 2SA965 --> 2SB649A
Q116,216: 2SD718 --> TTC5200
Q117,217: 2SB688 --> TTA1943
Q118,218: 2SC2240 --> 2SC2240
some of you may remember my thread, about the repair of a completely destroyed Teac BX-500. I can't find this thread anymore for some reason, so I'm starting a new one. In a nutshell: I found a Teac BX-500 on the scrap yard, that was completely destroyed. The heatsink and PCB had several drilled holes, some transistors were broken, and many tracks and pads were torn off. Most of the internal wiring was cut.
I repaired the tracks with wire and replaced all defective semiconductors, a list of the parts I replaced is at the end.
After I did all this I had the problem that I had a huge DC offset at the output of both channels (-44V!). After intensive troubleshooting I found the cause, a capacitor between supply voltage and ground turned into a resistor and with a resistance value of about 1.2kOhm and raised the ground potential to about 11V, which of course made the input stage go completely crazy.
After that is also repaired, at least the protection circuit relay clicks again, so not too much seems to be defective anymore.
On both channels the DC offset can be set to 0V.
On the left channel you can also set a quiescent current, but it drifts continuously higher. I set it to 26mV according to the service manual, but it rises continuously, about 0.5mV per second. When the quiescent current reached 120mV, I switched off the amplifier. The problem definitely has to do with the temperature, if I blow the VBE multiplier transistor slightly, the quiescent current drops significantly. The thermal coupling in this amplifier is not really good, the TO-92 quiescent current transistor is just placed next to the heat sink, without any heat conductive connection. Would it make sense to replace the transistor e.g. with a BD139 in a TO-126 package, and screw it directly onto the heatsink? In the original BX-500 and similar Teac BX-330 this is not the case, the proximity to the heatsink seems to be sufficient, but I'm not really satisfied...
On the right channel there is no quiescent current at all, but there I still suspect the VBE multiplier transistor or its solder joints and pads, these pads are in a very bad condition, almost torn off.
I would be happy if someone has another idea for the right channel, what I should look for and if someone has an idea how I should deal with the quiescent current drift of the left channel.
The service manual with circuit diagram can be found here (TEAC BX-500 SERVICE MANUAL Pdf Download | ManualsLib). For troubleshooting I also made an LTSpice model of the power amplifier part of the schematic, but for the input JFET and the two double transistors I chose alternative components, because I couldn't find an LTSpice model for them. I will attach the spice model to this post.
Thanks a lot for your help 🙂
Regards
Lukas
PS: I almost forgot the list of semiconductors, here it is:
Q110,210: 2SA970 --> KSA916
Q111,211: 2SA1815 --> KSC1815
Q112,212: 2SA949 --> KSA916
Q113,213: 2SA2229 --> KSC2316
Q114,214: 2SC2235 --> 2SD669A
Q115,215: 2SA965 --> 2SB649A
Q116,216: 2SD718 --> TTC5200
Q117,217: 2SB688 --> TTA1943
Q118,218: 2SC2240 --> 2SC2240
Hi Lukas,
I have a BX-550 that I am working on, which looks to be very similar to the BX-500.
Here is a link to Rod Elliot's general troubleshooting amplifier guide:
Troubleshooting & Repair Guide
Colin
I have a BX-550 that I am working on, which looks to be very similar to the BX-500.
Here is a link to Rod Elliot's general troubleshooting amplifier guide:
Troubleshooting & Repair Guide
Colin