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Author Topic: B200S behaving badly - blown fuses  (Read 1484 times)

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MikeB

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B200S behaving badly - blown fuses
« on: Time Format »

Hi all,
I lost a channel on my B200S a couple of days ago.  Yesterday I popped it open and found that one of the PCB mounted fuses had blown.  It was an 8A 32V fuse.  I replaced it today and when I fired it up, with both volume pots on zero, it made a loud hum and then another fuse blew. 

Bit of history: This unit was originally owned by @rnolan.  Back when it was his, something went wrong (richard can fill in these details) and he had a friend rebuild it for him.  While he was doing that he filled it full of fuses. 

There is one in the wire that goes from the transformer to a thing that is screwed to the chassis.  Sorry I can't be more technical than that.  It is the second one that blew.  It is on the same channel as the original fuse.

Bit more history:  In 1997, just before I was about to head overseas for what turned out to be six years, this same unit went dead on me.  I left it with a friend and wrote out a big long explanation of what had happened and asked him to take it to a tech to be repaired.  Anyway, so when I returned six years later, he gave me back the amp and the two pages of notes I had written for the tech.  He had done nothing.  I took it to be repaired and the tech replaced the transformer.  It has been fine since then.  All I have done since then is replace all the jacks and the pots (within the last year or so).

Can anyone give me an idea of where I might start looking for problems.  There is a guy nearby that has just set up shop repairing analogue gear so I'll probably take it to him if no one has any suggestions.

Many thanks.
Mike.

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MarshallJMP

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Re: B200S behaving badly - blown fuses
« Reply #1 on: Time Format »

That 8A fuse is between the out of the amp and the speaker out. So if that blows it's usually that you have DC on the output which  means you have blown one of the output transistors (or a cascade of one of the driver transistors).

Can you take a pic of that other fuse?
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rnolan

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Re: B200S behaving badly - blown fuses
« Reply #2 on: Time Format »

The original "something went wrong" way back was a speaker died and took out one channel (the cone was ripped from a guitar peg head and I used some cotton and sowed it up but over time the coil bobbin got a bit off centre  :facepalm: ).  Replaced the output transistors (can't remember what else, driver transistors sounds right ??) and, as Mike said, tech put a bunch more fuses in it so it wouldn't happen again (ie blow the transistors due to speaker short). It's also a very early B200s, my later one is allegedly improved  :dunno: .  However, this time the speakers are fine (thankfully as it's my spare split stack wired stereo Mike was using).  BTW in case it matters, the split stack has 75w 16 ohm celestions and was connected in stereo when it stopped working, so the channel was into a 16 ohm load.
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MikeB

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Re: B200S behaving badly - blown fuses
« Reply #3 on: Time Format »

Here are the pics.  The inline fuse that blew is actually from the "thing" on the chassis to the pcb - on the dark wire.  The other photo is of the original fuse that went.  I replaced it with a 7.5A 150V fuse.  I hope thats alright.  The inline one that went is a 2D250V5A.  Its tricky to get a decent angle on any of this stuff.  Let me know if you want me to redo.
Thanks
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MarshallJMP

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Re: B200S behaving badly - blown fuses
« Reply #4 on: Time Format »

Ok I see they put an extra fuse between the secondary's of the tranny and the bridge rectifier. Good idea.But still I think the problem is a blown output transitor (one of those big blocks with 3 pins,2SA1494, or 2SC3858).
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MikeB

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Re: B200S behaving badly - blown fuses
« Reply #5 on: Time Format »

Thanks.  My electronics enabled mate is coming over tonight, so I'll get him to have a look.  In case we don't get a chance to look at it, How would I test them myself?

Thanks for all your help. :waving-banana-smiley-emoticon
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MarshallJMP

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Re: B200S behaving badly - blown fuses
« Reply #6 on: Time Format »

Do a search on YT, lot's of vids about testing a transistor with a diode checker on a multimeter.
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MikeB

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Re: B200S behaving badly - blown fuses
« Reply #7 on: Time Format »

OK.  So, I have pulled all the transistors along the heatsink on the busted channel.  The 2SA1494, the 2SC3858 anf teh TIP32 are all open.  The TIP31 and the MPS8099 are OK.  There is a lot of blackening on the PCB below the big white resistors.  Is that normal? (see picture)
Is there anything else I should check while I've got it all apart and am about to order parts?
Cheers,
Mike.
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MarshallJMP

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Re: B200S behaving badly - blown fuses
« Reply #8 on: Time Format »

Are you sure they are open? usually they short out causing the fuse to blow. Best is to remove the solder (on the pcb) from these big resistors and reflow them again.
If you have a bad contact (they can desolder themselfs) you can blow up the transistors. This was a big problem with the first gen of marshall valvestates.
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MikeB

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Re: B200S behaving badly - blown fuses
« Reply #9 on: Time Format »

I'm new to this transistor testing stuff and am easily confused :facepalm:.
I have retested and here are my results:

2SA1494 (PNP) -ve Base, +ve emitter and collector
B-E = 0
B-C = 0
C-E = 0
I assume this means that it is shorted?

2SC3858 (NPN) +ve Base, -ve emitter and collector
B-E=0
B-C=0
C-E=0
Shorted?

TIP32 (PNP)
B-E=OL
B-C=0
C-E=OL (both directions)
??


TIP31 (NPN)
B-E=0.471
B-C=0.471
C-E = 0 (both directions)
??


MPS8099 (PNP)
B-E=OL
B-C=OL
C-E=OL (both directions)
Just out of interest I reversed the polarities (+ve B and -ve E,C) and got:
B-E=0.675
B-C=0.675 
So what does that mean??


So, should I just be replacing these components and resoldering the resistors?  Anything else I should check?

Thanks,
Mike.
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MarshallJMP

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Re: B200S behaving badly - blown fuses
« Reply #10 on: Time Format »

if it's zero then they are shorted out. Here's a link on how to do it. BTW you need a diode test function on your meter for this.

Here's a link to a good explanation on how to test transistors.

https://www.wonkeedonkeetools.co.uk/mitre-saw-protractors/mutimeters/how-to-test-diodes-and-transistors-with-a-multimeter/

Or you can get one of those cheap component testers.They work quite well for quick and dirty testing.They also check other stuff like mosfets, cap ,resistors,IGBT's.Hell it can even test remote controls.

https://www.ebay.com.au/itm/LCR-TC1-3-5inch-Colorful-Display-Multifunctional-TFT-Backlight-Transistor-T-T8C3/292297056665?epid=2272519590&hash=item440e432199:g:AXkAAOSw0UBZ5iyC

Now if I have this, I just replace all the transistors, because if one is still bad it will blow the entire amp again. It's all DC coupled so if one goes they all go.(on one channel)
« Last Edit: Time Format by MarshallJMP »
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MikeB

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Re: B200S behaving badly - blown fuses
« Reply #11 on: Time Format »

Apparently the MPS8099 obsolete.  There seem to be some around but them are a little pricier than I expected. 
Is there an equivalent?  I've tried to find out but I get freaked out by all the numbers and I don't know how to compare them with confidence.  :dunno: Can you help?
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MarshallJMP

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Re: B200S behaving badly - blown fuses
« Reply #12 on: Time Format »

Well a BC538 should work if you can find these. If not let me know.
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MikeB

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Re: B200S behaving badly - blown fuses
« Reply #13 on: Time Format »

Bloody hell!!  I think I'm over this vintage gear trip!
I can get the two TIP transistors locally but the  two big ones are coming from the US with the shipping more than doubling the cost.  I found a BC538 in the UK.  The seller is away until the 14th and then it will cost 3 times as much to ship as the price of the transistor, but luckily it is a fairly cheap unit.
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rnolan

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Re: B200S behaving badly - blown fuses
« Reply #14 on: Time Format »

Hey Mike, does that mean you are using the TS100 on Sunday ?
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