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
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.
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.
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.
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.
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.
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.
Check the small transistors and diodes in that circuit, change all the Darlingtons
Collecting Vintage B&O is not a hobby, its a lifestyle.
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.
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.
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.
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?
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.
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.
Is TR27 correctly installed (no connection to chassis)
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.
The caps and lamps have now arrived, and I'm in process of updating those aged components, and fixing that damn tone control light board!!
All the new lamps are in, and the tone control board has been restored back to its original specifications. Gone are the LEDs and weird mix of resistors.
LEDs before in post above.
Wiring before:
Back to normal:
Since the previous owner had switched out to LEDs, they also threw out the lamp retaining clips. I fashioned some out of an aluminum can I had laying around. Not perfect, but it gets the job done.
All the caps have been replaced with Martin's kit. The odd sounding speaker out of one channel still exists.
My next move is to replace the idle trimmers and the large caps.
I suspect the trimmers won't have effect as I had just calibrated the existing without effect. I hope that the new large caps should do the trick.
I also picked up another pair of S45.2s, however these have the dreaded unstuck magnet/stuck voice coil issue. One of the magnets and pole piece was just rolling around in the cabinet when I got it, so that's the next fun project to tackle!