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BM2000 2801 Amp Trimpot change blows left channel fuses

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fkatze
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fkatze Posted: Sun, Jan 12 2020 1:20 PM

Not sure what is wrong - earlier today I removed the 2 old 250ohm trimpots from the amp circuit and replaced with new ones.  Got back to it this evening to start on adjusting it, but got weird readings...right channel gave different values to left.  Took me quite some time to figure it out, but I found the 2 1.6A fuses were done.  Replaced with 1A as that's all I have on hand without going over spec, and they immediately blew. 

I get +/-31ish volts both sides as per the specs, nothing looks wrong or solder bridges that I can see.

 

What could be wrong?

I'll have some follow up questions later on about how to do the adjustment as the instructions are a bit confusing and the diagrams don't seem to match the words, but for now...

Dillen
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Dillen replied on Sun, Jan 12 2020 1:24 PM

The trimmer is set at an extreme.
Always center idle current trimmers BEFORE powering up and adjusting.

Maritn

fkatze
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fkatze replied on Sun, Jan 12 2020 1:31 PM

Ok, I'll try that tomorrow and see what happens, thanks.

fkatze
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fkatze replied on Mon, Jan 13 2020 12:15 AM

Ok did that, instant blow again....Unsure

Something somewhere isn't happy.

 

edit...

Just did a bit of poking around - I get continuity between C&E pins of TIP141 and TIP146 on the left side, but not the right.  I gather that is bad. 

Possibly not related, but an observation from running the stereo previously, the right side heatsink would get a little warm, and there was no perceptible warmth in the left.

Beo_Jean
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Beo_Jean replied on Mon, Jan 13 2020 12:53 AM

Hard to tell at first like this but the mica behind that TIP141 looks really crooked.

A blowing fuse indicates a short or an overload.  Check if you bridged traces accidentally when soldering.

Time to check those output transistors... 

fkatze
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fkatze replied on Mon, Jan 13 2020 1:27 AM

Beo_Jean:
the mica behind that TIP141

 

Is that the white stuff?  I'm guessing that is a type of thermal paste?  It's pretty dry and flaky on all of them.  I do have some modern paste here I was intending to use when finished.

fkatze
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fkatze replied on Mon, Jan 13 2020 6:43 AM

Ok, so I removed TIP141 and it's toast.  And I am pretty sure TIP146 suffered the same fate.  I removed the trimpot and inspected, but all was good.  Installed a new one anyway.

So, next couple of questions are - what is the modern equivalent to the TIP141/6. I can get TIP141s easily enough (most sites list 141G).  TIP147 is available and seems like it might work.  https://au.mouser.com/datasheet/2/308/tip140-d-1196132.pdf

Last question, should I replace the 2 blown or all 4?

Beo_Jean
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Beo_Jean replied on Mon, Jan 13 2020 11:03 AM

TIP 142/147 are fine.

I would replace all four

fkatze
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fkatze replied on Mon, Jan 13 2020 11:46 AM

Beo_Jean:

TIP 142/147 are fine.

I would replace all four

Cool, thanks.  I did find some 141/6s on ebay from HK, about $30 for 10 pairs, ST and ON mix.  Or I can buy 2 sets (4+4) of 142/7s for $20 from local supplier, again ST and ON.  I like the idea of having some spares in case...

Other than that, I couldn't find anything obviously wrong.

Dillen
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Dillen replied on Mon, Jan 13 2020 12:07 PM

I would advice against buying from Asia. The risk of getting counterfeit parts is huge.
Indeed the price you have been quoted is more than suspicious.
Buy from a trusted supplier and expect to pay around 4-5 euros a piece for the real thing.
Personally I would dig through the dungeons for a good used original pair and if that fails I would probably order from Little Diode, from where I have ordered countless times and never received any counterfeit parts.

And why would you replace the two in the unaffected channel?

The trimmer is most likely still fine. But its initial setting was at max idle current, which could easily be the reason for the burned output stage.
Didn't you monitor the idle current at power up? (And replace the trimmer in one channel at a time).

Martin

fkatze
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fkatze replied on Mon, Jan 13 2020 1:43 PM

Dillen:
Didn't you monitor the idle current at power up? (And replace the trimmer in one channel at a time).

 

Actually no.  Did both at the same time. But lesson learnt.

I was following the instructions in the service manual to measure either collector current or voltage across R119/219.  I chose to measure voltage, but I think something in the instructions is wrong or I'm misreading something...actually I've just checked the parts list, R119 should be one of the 2W resistors, but the board seems to be marked elsewhere.  My board shows the 2W resistors as R121/122.   So yeah, it appears my board doesn't match the diagrams in the manual...?  I was probably checking the wrong thing.

 

 

fkatze
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fkatze replied on Thu, Jan 16 2020 12:46 PM

One more question about the transistors...

Where the transistors mate to the heatsink, there is some white powdery goop, and behind that what looks like a clear piece of plastic film.  Should that stay or be removed?  Can I just mount the transistor back in place with a bit of thermal grease behind it or do I need something else?

Thanks!

Dillen
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Dillen replied on Thu, Jan 16 2020 1:03 PM

The mica sheet is important as it electrically insulates the cooling fin of the transistor from the metal of the cooling fin.

Replace the thermal paste. Use the good old white greasy stuff. The modern grey stuff (often sold for use with computer CPUs) is useless.

Martin

fkatze
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fkatze replied on Thu, Jan 16 2020 2:15 PM

Ok, thanks.

Paste I have is silver rated at 3.7W/m.K.  I can probably get the white stuff somewhere..

fkatze
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fkatze replied on Fri, Jan 17 2020 9:26 AM

Beer time!  I got the new transistors installed - got white paste from local electronics shop.  Had to use some generic M3 screws as the new transistor package is a little different, oh well.  Had devil of a time adjusting for 25mA - first the 600mA fuse blew, then I discovered the batteries in my 6 month old MM are swelling and starting to split...replaced those, only had a normal 1A fuse which I know is bad, but meh. 

Happy to say after all that, I got to 22.1mA but couldn't get to exactly 25 - 21, 32, 19, 30, 22, 34...hit 22.1  and decided that is close enough.  I'll do the same to the right side, but not today...

So tested, radio works, I can't tell any difference between sides, so I'd say that's job done.

Thanks always for the help and advice!

Here's a pic:

Craig
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Craig replied on Fri, Jan 17 2020 10:33 AM

May have been easier to set the 25mA with a 25 turn pot.....but a nice tidy job Smile

Craig

fkatze
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fkatze replied on Fri, Jan 17 2020 11:51 AM

Craig:
May have been easier to set the 25mA with a 25 turn pot

 

Well yeah...live and learn Stick out tongue

 

Craig:
but a nice tidy job

Thanks!  I try to do neat and tidy work.

fkatze
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fkatze replied on Tue, Jan 21 2020 9:37 AM

Ok, so I spent a bit more time on it today, and I got both sides to about 24.9mA.  Or rather it starts at about 24.6 and moves up to about 24.8/9mA over about 5-10 minutes.  Definitely close enough.

I also cleaned up right channel transformers and reapplied thermal grease. 

Working nicely :)

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