ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
This started out as a completely routine repair of a BM4400, with one of the Left Channel TIP146's shorted.
My plan was to replace the transistors and finish the job, just like many before.
But here I came across an obstacle I have never seen before.
After replacement of the transistors - Brand-new identical job-lot TIP142's and 147's Full-case Plastic, I inserted my headphones at Zero volume, and heard buzzing and scratching, and saw the Bias Current voltage, jumping around from 3-20mV.
Yes, I know that Bias must be adjusted with no speakers, but here I connect the headphones initially to ensure that there are no additional faults.
Increasing the volume, I heard normal music above the background of the buzzing.
My first thought was that I had shorted something, or that there is a piece of scrap solder floating around.
I went back and cleaned the PCB again, no improvement.
I then started feeling/pushing each component in the output amp to see if the noise changed, indicating a bad solder joint. Nothing on the PCB itself.
I then started pushing on the Output Transistors on the rear panel (even though they were secured correctly).
Aha! Pushing on IC100, the noise changed, and the Bias voltage changed.
I then released, and retightened the bolt securing the IC to the rear panel, and so, saw a similar reaction. With each slight tightening of the bolt, there was a change in the noise and Bias voltage.
I then assumed that for some reason, the original Mica isolators were defective, and that there is stray capacitance, causing this problem. This is the first time that I have seen this kind of problem – I have never had any problem reusing mica isolators when replacing transistors.
I removed them, but could not see any scratches or rips in the mica.
I have some spare grey mica isolators (see photo), and I installed them – the noise and buzzing is gone now, but now there is a new issue:
Above about 9mV and below 3mV, there is a hum. The quietest setting is about 6.5mV, nowhere near the manual spec of 10-15mV.
Could this be that the different thicker grey isolators cause a requirement for a different Bias setting?
Or is it an issue that the new Transistors are FullPack – no metal exposed?
Anyway, I will order new transparent isolators, just like the original, but in the meantime, does someone have any light to shed on this?
Thanks
Menahem
If the TIPs are of the "light"-versions with complete plastic encapsulation, also at the back with no metal exposed for cooling, they won't be any good for this purpose.The idle current adjustment is depending greatly on the temp. feedback and a good thermal contact.(It looks as if you are using both silicone insulators and heat sink compound, I suggest you use either the silicone alone or mica with paste on both sides - I recommend the latter).And of course, there is always the risk of getting counterfeit transistors - even from well-established suppliers with good reputation.Sadly, counterfeits are now by far more plentiful than originals. Their specs are often everwhere and nowhere and they often show the strangest behavior.Some are not even completely flat at the back and I have seen some with metal flash sticking out, easily enough to go through the mica when bolted up.Most fakes are easily spotted if you are familiar with semiconductors and brands.
I suggest you use TO-218 as the original (or TO-247, which I am not particularly fond of but f.e. ON have discontinued TO-218 devices andnow only do them).Buy NOS if possible - or even used original parts, rather than new.Replacing output stage darlingtons because of a fault in the circuit - like in this case - check everything else in the output stage.Components with internal scars can do odd things.
Martin
Thanks Martin
These particular TO-247's are EXPOSED at the rear.
I had some suspicions about their suitability with the temperature issue, but they were the only ones available from Mouser.
But I will source some "bare" units instead.
What about using these grey insulators? Are they OK, or should I definitely use original Transparent Mica?
Some people like them, but I was never a fan of the grey silicone sheets.Perhaps because I've seen devices working themselves lose - because of the flexibility of the material and the changes in temperature, I'd suspect.
I never saw a mica sheet fail (unless caused by something else).The thermal compound will dry over the years, but it will easily last 25+ years if you use the good quality white stuff, and someof the stuff B&O used in the 1970s is still fine today.The grey thermal paste sold on Ebay in large numbers for computer CPUs etc. is useless in my opinion.I tried two or three different brands and threw it all away. It hardens in weeks at room temp..
Looking at your test probes it appears your only across one emitter resistor, if so then 5.5mV will be fine....11mV if you measure across both resistors.
Regards Craig
Craig,
Very observant!
However the physical mounting of those two bias resistors is "opposite", in order to provide measurement across BOTH of them, with the probes mounted as you see them.
Clever engineers - those B&O chaps. That's called real user-friendly engineering.
Ahh.......I see, I have looked long and hard at these in the past as I inserted one the wrong way around on one occasion which resulted in me not being able to measure the 5,5mV.....without digging out the manual that suggests that if both are the wrong way around I would have been fine and able to measure the 11mV.
OK, I have now ordered some new mica sheets.
Regarding the TIP's.
I agree that it is completely frustrating to not be able to trust suppliers anymore, in order to get genuine parts.
Mouser offers ST TIP142 and TIP147's in bare TO-220 packaging.
https://www.mouser.com/Semiconductors/Discrete-Semiconductors/Transistors/Darlington-Transistors/_/N-ax1sb?P=1z0y1znZ1z0y48c&Keyword=tip142&FS=True&Ns=Pricing|0
They have the same 10A rating as the TO-247.
What do you think about that? Would that be a viable substitute?
Glad to see you again! Do you mind if I'm sharing a little bit of my experience working on a BM4400?
- If you are looking at the BM4400 restoration projects posted in here, many, many if not all had failed output transistors. And also many had nothing wrong with the preamp stage. My feeling is that the original Texas Instruments TIPs are aging badly where they just fail OR the modular heatsink does not provide enough cooling to them. The heatsinks themselves are made of aluminum extrusion and does not sit flat on the unit’s back panel, that’s a fact!
- TO 218 vs TO 247? Yes TO 218 look more the same as the original but if you pay attention to construction of both TO 247 provides a better spread of the tightening force across the chip surface compared to TO 218. This is an important aspect because you want to maximize the heat transfer of your chip to the cooling surface and having a thicker isolation subtract will result into less effective heat dissipation.
- Mica vs Silicon: Like Martin said, one or the other both not both. Myself, I prefer the “pre-cut” silicon pads because they are cleaner to work with and easier to install. Yes, silicon pads were less efficient back then but now they are as good as mica / thermal paste combo but, like anything else, you have good and bad stuff and cheap stuff may behave like what Martin said. I’m using the Laird TGARD 210
Good luck!
Hi Jean
All replies are very welcome!
So, I'll continue when I have the new mica's.
Stay tuned!