Audio Amplifier Problem BDT65B / TIP142

[danlay]

I have a stereo amp that needs a new driver transistor for the Right channel.

The amp uses two BDT65B transistors, one for each channel. The bad one blows the fuse for that channel.

To make sure that it is the BDT65B that is causing the fuse to blow, I've tried moving the suspect BDT65B to the Left channel. It does then blow the fuse for the left channel.

Can anyone help me in sourcing a new BDT65B? I can't find one anywhere.
I've heared that a TIP 142 is an equivalent, but when installed, the TIP 142 doesn't give any output?

Any help would be greatly aprecheated!

Dan

 

[Nigel Goodwin]

I have a stereo amp that needs a new driver transistor for the Right channel.

The amp uses two BDT65B transistors, one for each channel. The bad one blows the fuse for that channel.

To make sure that it is the BDT65B that is causing the fuse to blow, I've tried moving the suspect BDT65B to the Left channel. It does then blow the fuse for the left channel.

Can anyone help me in sourcing a new BDT65B? I can't find one anywhere.
I've heared that a TIP 142 is an equivalent, but when installed, the TIP 142 doesn't give any output?

Any help would be greatly aprecheated!

Dan

According to Towers, a BDT65B is an NPN darlington, 100V, 12A, 125W.

A TIP142 is an NPN darlington, 100W, 10A, 125W.

I'd agree it looks a pretty reasonable replacement, perhaps you had a faulty one?.

It's unusual for just one transistor to fail, are you sure everything else in the duff channel is OK?.

 

[danlay]

Thanks Nigel,

Yeah, i'm pretty sure that there isn't anything else wrong with the faulty channel.
At the same time, I put the good transistor in the faulty channel and it works fine.

What about the pinout of the transistor, would the pins be exact do you think?

 

[Nigel Goodwin]

Thanks Nigel,

Yeah, i'm pretty sure that there isn't anything else wrong with the faulty channel.
At the same time, I put the good transistor in the faulty channel and it works fine.

What about the pinout of the transistor, would the pins be exact do you think?

Tower's lists them both as BCE (looking from the pins, with the metal side down).

Presumably you have the TIP142 suitably insulated from the heatsink?, sometimes slightly different encapsulations require slightly different isolating washers and bushes.

 

[danlay]

Yeah, the transistor was insulated from the heatsink. I was pretty careful to make sure that it stayed that way once it had been replaced. I even checked the continuity afterwards to make sure that it hadn't somehow got grounded whilst screwing it down.

The only other thing is that the replacement transistor (TIP142) is in a larger package. I had to drill three new holes in my PCB to mount the new transistor and find a larger insulator, but everythings connected up perfectly. I'm running out of ideas..

Thanks again,

Dan

 

[ChrisP]

I had to drill three new holes in my PCB to mount the new transistor...

Is that PCB a two-sided PCB? If so, your problem may lie in the statement quoted above. If, as is very common, you PCB utilizes traces on both sides of the PCB at that device location, it is likely that by drilling the holes oversized (assuming that you did just that) you drilled out the vias for that device location. If you drilled "new" holes -- i.e. not in the same locations -- and if the board has traces on both sides at that device location, you won't have vias there at all.

What this amounts to is no continuity between the solder side (lower side) of the PCB and the associated traces on the top side (component side) of the board. The vias are there to provide such continuity. Often they are simply made by plating the insides of the drilled holes, but they are also ofte made by inserting a tiny sleeve into the drilled hole, with the sleeve being flanged at one end prior to installation and then being swaged afterwards at the other end to provide a physical connection to the traces and to retain the sleeve in place.

The problem is easily overcome, space allowing, with a two-sided PCB by simply soldering the device on both sides of the board. Be sure to use a heatsink on each lead when soldering, but especially so when soldering above the board, as you are working considerably closer to the component body.

On the other hand, if your PCB is a multi-layer type -- IOW if there are embedded traces sandwiched between phenolic layers -- and if you have in fact drilled out the vias, you've got a much more difficult problem to solve. This is because it's much more difficult to regain connectivity with sandwiched trace layers than with exposed ones.

In cases such as this, when I need to replace a through-hole device with one having larger cross-section leads, I will generally try to avoid the likelihood of developing this problem by using a short length of bare solid hookup wire through the PCB, with those wire leads being soldered to the new device leads. IOW, I avoid drilling the PCB by using wire that will fit the existing holes. It's a little bit more involved, but it sure saves later headaches!

 

[danlay]

Hi Chris,

Thanks for all the info. I probably should have said that this is quite an old amp (Creek Audio CAS 4040 - 1990 ish) and it's just a single sided board. I would have been pretty scared of drilling through anything with more than one layer.

 

[Nigel Goodwin]

Thanks for all the info. I probably should have said that this is quite an old amp (Creek Audio CAS 4040 - 1990 ish) and it's just a single sided board. I would have been pretty scared of drilling through anything with more than one layer.

It would be very unusual for an amp to use more than a single layer board, there's not really much need for it.