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Beomaster 2400 - no power up / protection mode repair

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Irata
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Irata posted on Tue, Jan 17 2017 2:24 PM

Hi everybody, first post, so maybe a little introduction as well.

1st off, as of 4 weeks ago, I really knew nothing about electronics, (and for the most part, still don't). I didn't know the difference between a transistor and Mr. Tran's sister.

I've usually been able to figure out most things mechanically, and repair most things on my own. Electronics on the other hand...

At any rate I picked up a great B&O kit off CL. The 2400 in question, plus a Beocord 2400, Beogram 3400 and a pair of S45.2s. Everything is in great shape, but the output on one channel is a bit thin compared to the other. 

Before I go much further, know that I have a cap and lamp kit on order from Martin.

 

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Irata
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Irata replied on Wed, Jan 18 2017 12:12 AM

So the only issue with the 2400 was that the left channel was a little muffled or thin sounding in comparison to the right. Just not as rich.

So I figured I'd open it up and take a look.

Irata
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Irata replied on Wed, Jan 18 2017 12:17 AM

As I kept digging into the guts of it, I wound up taking off the heat sinks. I thought "how hot can they get if I turn it on for a minute.

Turns out pretty hot. Quick puff of smoke and it shut down on me.

You might also notice the non-regulation LEDs on the tone control board. Someone before me has been in here and modified those three lights with a mix of LEDs and resistors. Those will get replaced proper.

Irata
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Irata replied on Wed, Jan 18 2017 12:20 AM

Now I realize I'm a complete idiot, and I've completely hosed up a nearly perfectly good receiver. The sinking feeling in my stomach is full throttle, and I'm upset the rest of the day. That's the feeling of learning from one's mistakes.

 

Irata
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Irata replied on Wed, Jan 18 2017 11:41 AM

I put the heat sinks back on, while noticing that I never needed to take them off. I flip it up so it rests on the heat sinks, and figure I'll see if this thing will fire back up, or if I've really toasted something.

Well it turns on. So I figure okay, not out of the ballgame yet. What happens if I lean over, and monkey around with the balance slider - ZAP! Shuts down again. Okay great. So I've fried something.

Now I'm researching every single post and forum on no power issues with any 2400 or 1900  I can find. Not fully comprehending any of them.

All signs point to Martin. So I skip the forum questions, and go straight for the messaging.

I realize that long distance troubleshooting isn't exactly effective, but I figure maybe this is a common problem, and there's a typical cause/effect and resolution. What I'm really looking for is "replace this part, should fix your problem".

The questions I ask prompt Martin to gently suggest that perhaps I'm not the right person to attempt this repair.

He's absolutely right. But I'm going to try anyway.

In my head I'm starting to think I've got these cracked resistors next to the heat sinks, I bet that's the problem.

Irata
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Irata replied on Wed, Jan 18 2017 2:53 PM

However, after pulling the cracked emittor resistors, the measurements were the same across all four... must be the large Darlington output transistors. When I pulled those, one of them gave an inconsistent reading across the emittor/collector pathway.

Aha! I've found the answer... or so I thought.

Søren Mexico
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Check the small transistors and diodes in that circuit, change all the Darlingtons

Collecting Vintage B&O is not a hobby, its a lifestyle.

Irata
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Irata replied on Wed, Jan 18 2017 6:58 PM

Hey Soren, thank you very much for the input, but this tale has already reached a happy conclusion, it's just taking me a while to get there with my work schedule.

As Soren suggested, I did indeed change all the Darlingtons. And when I tried to start it up, nothing happened. 

So I went back scouring through all the related forum repair threads I could find. Trying to measure different test points, transistor leads, resistors, and whatnot.

Some thread talks about voltages being pulled off TR27. So as I'm trying to get the collector value on TR27...

My probe slips and connects C and B... and boom, it fires up!

Now I'm psyched, I just accidentally hotwired it. It's not dead after all... signs of life.

I shut it down to replicate my results, and it starts up again. Sound comes out of the now attached speakers. Making progress, this will be easy!

 

Back to Martin I went, who again gently suggested that you can't guess your way to fixing an issue like this.

I was pretty bummed out. In my head I'm still wishing there was a simple replace this and you're good, but was slowly realizing that it's not going to happen.

So I took Martin's advice to heart, and kept staring at the schematic. Googling. Symbol definitions. How transistors work. etc, etc.

 

I ordered replacement transistors for the entire power supply and output stages as best as I could determine.

Irata
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Irata replied on Thu, Jan 19 2017 1:40 PM

I was patiently waiting for them to arrive, and as I'm riding the bus to work one day, staring deeply at the sections I highlighted as being the areas to focus on, a lightbulb went on. The schematic no longer looked like gibberish to me.

From day one, the schematic looked like Chinese to me. None of it made any sense. Just a giant bowl of right angled spaghetti. I wanted it to look like the top side of the board, where everything was mapped out as it was attached to the board. Like the backside PCB images in the service manual. that way I thought, I could just match parts up one for one.

In trying to decipher the schematics, I started highlighting the labels that denote the areas they're related to, and circled the PCB numbers to try and orient myself.

So I think I have the output and power supply areas generally figured out so I mark those in pink.

That brings me to the bus where I had my epiphany. I know the majority of you guys will think this is the most basic thing, but for someone who has no idea what they're looking at, this was a monumental insight.

As I'm staring at the schematic from afar, and not at the tiny details, all the blocks took shape. These are drawn almost like city blocks, where each section is in its own little square, more or less. It wasn't until I took a step back that things started to make sense.

Irata
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Irata replied on Thu, Jan 19 2017 2:57 PM

So now rather than replace all the transistors, I figure I'll try to read the schematic a little more closely.

In following one of the troubleshooting threads, I start pulling voltages off the specified connections.

I've got 12V more or less off the mains. Ok.

C92 reads about 22.5V, so I'm still good there.

Test point 16 has nothing, zip, zilch, nada.

Now I zero in on the 15V supply area since that's where everything seems to dead end.

Everything I pull is reading correctly, so it seems all the components outside TR27 are good.

Since TR27 is reading 0V at the collector, that must be it. E and B seem to be reading right, so I'll just pull the transistor, measure against the good one that just came in the mail, swap it out and I'm back in business.

Irata
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Irata replied on Thu, Jan 19 2017 2:59 PM

Wrong.

TR27 that I pull reads almost identical across B/E, B/C, and C/E as the brand new one. I tested against two different ones to make sure.

Great. Now what?

Irata
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Irata replied on Thu, Jan 19 2017 3:03 PM

I trace the line on the PCB this time, I go back up the board from the collector of TR27, since it is not getting what it needs. Based on the schematic, I assume it's being powered from the C92 side of things, but figure since that hasn't panned out, I'll go the opposite direction.

And that leads me to TR31.

Irata
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Irata replied on Thu, Jan 19 2017 3:13 PM

Testing TR31 in circuit gives me pause.

The pathways across B/E and B/C seem fine. C/E however shows 003. To me that's an insignificant number, but I was expecting an open circuit, on my meter it should have read 1.

So I pull this guy, measure it out of circuit and get the same results.

I measure two new transistors, and both read 1 across C/E.

I'm no longer getting my hopes up on any of my discoveries, but it's definitely reading differently than the new ones.

I solder the new TR31 in place.

No speakers, plug the 2400 in. Flip the switch to on. The LED finally lights again. Mildly enthusiastic now, but it's only one step in the right direction.

Tentatively, I touch the pin for FM2.

Everything lights up!

This is the farthest it's gotten in weeks. That's satisfying.

Søren Mexico
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Is TR27 correctly installed (no connection to chassis)

Collecting Vintage B&O is not a hobby, its a lifestyle.

Irata
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Irata replied on Thu, Jan 19 2017 3:21 PM

I check the bias pots, and tweak them just a bit to read -12V while it's cold. Wait 10 minutes, and adjust to 12V now that it's warm.

Hook up the speakers, and with bated breath I turn everything on.

Holy hell, there's music coming out of the speakers!!!

One stinking transistor and everything is powered on and stable.

I came here looking for a simple answer to a complex problem. When that wasn't readily available, I hunkered down and learned what I could to try and solve it.

Big thanks to Martin for putting up with my lengthy messages, a few measuring tips/how tos, and the gentle "encouragement" to either figure it out, or have someone who knows what they're doing fix it.

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