Aw poop! PL400 fried!

[Gepetto]

Replaced PS fuses today, went to do a DBT startup but pushed the switch the wrong way, so it went into a full current startup.

The odor of ozone and burned components came afterward...

So... Where do I go from here?

The protection circuit fried too Mark. The protection circuit can only do its job when the output transistors work. The reason you took out the emitter resistor was that you get shoot through when you have a shorted transistor, no way to turn that short off circuit wise.

 

[orange]

I've only welded one or two voice coils, not intentionally and I replaced both promptly. The one I do know for sure was an Altec Model One and it was because the Sansui integrated lost the right channel and took it out.

 

[Gepetto]

And this is just a guess Mark, but I have done the same thing when not checking to see if the fuse "went nuklar"....always a bad sign....

Agree with Lee. The fuse pop was telling you that you had a problem.

 

[MarkWComer]

The protection circuit fried too Mark. The protection circuit can only do its job when the output transistors work. The reason you took out the emitter resistor was that you get shoot through when you have a shorted transistor, no way to turn that short off circuit wise.

So the DC protection circuit fried- I halfway expected that. Since the transistors have to be pulled to get to the backplane I'll check all those as well. Since the meter circuit connects directly to the outputs I expect that my left channel cylon eye could be blind. Ha- a cycloptic cylon!

Not dead- just needs a transplant...

 

[FredR]

May Gort guide and protect your work.

 

[laatsch55]

So the DC protection circuit fried- I halfway expected that. Since the transistors have to be pulled to get to the backplane I'll check all those as well. Since the meter circuit connects directly to the outputs I expect that my left channel cylon eye could be blind. Ha- a cycloptic cylon!

Not dead- just needs a transplant...

The DC protection circuit exists only on Don's board. The SOA protection circuit on the WO driver board is designed to protect the output section by cutting drive current to the outputs.

 

[MarkWComer]

The DC protection circuit exists only on Don's board. The SOA protection circuit on the WO driver board is designed to protect the output section by cutting drive current to the outputs.

Yes- He said DC not SOA, so Don's board is assumed to be fried. So the SOA may have protected the meter?

Upper six transistors of the left channel, both PNP and NPN, are showing shorts, tested them in circuit- not removed yet. Burned emitter resistor shows no conductivity, in fact, all the emitter resistors on the PNP side show no conductivity yet have no obvious toasting. Six fried transistors indicate a bigger issue (if indeed I'm testing them correctly), transistors on the right channel side don't exhibit the same issue, nor do the bottom two of the left channel.

So... DAMN! Should have put some weenies on top and roasted them...

As far as the control board- should I concern myself with the components that are obviously toasted alone? Given that the burn occurred in an isolated area, are other parts suspect?

 

[laatsch55]

Mark, if one output shows a short , all in that row will show a short until you take them out. Check the 651's and 751's too, and all associated resistors and diodes...

 

[MarkWComer]

Check the 651's and 751's too, and all associated resistors and diodes...

I see no reference to 651 or 751. I have the rev D board- I think you're looking at rev E. I guess you mean the 2N1305 (Q9) and the 2N5416 (Q7).

...And yes, found 2 more suspect capacitors.

Further transistor check (out of circuit) shows 3 PNPs bad, 1 NPN bad. I can only do a diode check at present, my meter does have an hFE position, but has a pin header for TO92/TO18/TO218 type trans, have to hook up a jumper for the TO3 type (going to RadioSlap for some alligator clips tomorrow).

Thanks to Joe for the BOM! I have all the part numbers!

 

[laatsch55]

Yep, when you have a catastrophic failure like that on a "D" board Q7,Q8. Q9, Q10 are all suspect...

 

[Gepetto]

Yep, when you have a catastrophic failure like that on a "D" board Q7,Q8. Q9, Q10 are all suspect...

and the fuse element was probably vaporized as opposed to just the lazy open if you push the amp to hard

 

[laatsch55]

Yep...the "nuklar" option....

 

[MarkWComer]

Yep, when you have a catastrophic failure like that on a "D" board Q7,Q8. Q9, Q10 are all suspect...

Q8 and Q9 were the 2N1304/2N1305 optional germanium transistors. From what I understand, they aren't necessary. If they pass testing I'll leave them in, if not will take them out. As I understand, they're obsolete/unavailable.

EDIT: Spoke too soon. Can still get them from American Microsemiconductor.
http://store.americanmicrosemiconductor.com/2n1304.html?gclid=CJ6IhLfs8M8CFYJkhgod3mgI0Q
http://store.americanmicrosemiconductor.com/2n1305.html?gclid=CMjX25bs8M8CFUMkhgodlNYL5A

and the fuse element was probably vaporized as opposed to just the lazy open if you push the amp to hard

Yes- metal vapor deposited inside the glass tube, missing center section.

 

[Northwinds]

Damn, that sucks Mark but it looks like you know what your doing and will have her back up running soon!

 

[Gepetto]

Q8 and Q9 were the 2N1304/2N1305 optional germanium transistors. From what I understand, they aren't necessary. If they pass testing I'll leave them in, if not will take them out. As I understand, they're obsolete/unavailable.

EDIT: Spoke too soon. Can still get them from American Microsemiconductor.
http://store.americanmicrosemiconductor.com/2n1304.html?gclid=CJ6IhLfs8M8CFYJkhgod3mgI0Q
http://store.americanmicrosemiconductor.com/2n1305.html?gclid=CMjX25bs8M8CFUMkhgodlNYL5A



Yes- metal vapor deposited inside the glass tube, missing center section.

That vaporization is a dead give away Mark.

Lee and I both have some 1304/5 devices that we have kept for spares.

 

[laatsch55]

Yep, lot of A's, C's and D boards out there yet....

 

[MarkWComer]

Damn, that sucks Mark but it looks like you know what your doing and will have her back up running soon!

Know what I'm doing? Yeah- if you mean to ask someone who knows their stuff!
"Smart" isn't always knowing the answer, but knowing who to ask...

What sucks most is having three new unplayed records, two more to come.

The upside: Tested the speakers (with a woefully underpowered amp) and they're undamaged.
Another item to add to the Mouser order- SPEAKER FUSES! (And SilPads- I don't think they should be reused.)

 

[laatsch55]

Don's board did it's job.....Yep , new sil-pads...

 

[MarkWComer]

A little update, also I'd like to thank everyone for your suggestions and help:

Building was so much easier than repairing...

Visibly, the only damage to the backplane board was R44, but will check others as well as diodes. Funny: ALL the emitter resistors measured .7Ω- even the unused extra that was included with the original backplane kit package- and they're clearly marked .33Ω (checked them with two different DMMs...).

No visible damage to the right channel section of the control board or the right channel backplane, cautiously assuming that no damage was done there.

Q9 on the control board seemed iffy, but it's one of the germanium transistors and they're a bit leaky to begin with.

hFE testing of the left channel transistors confirmed what I already discovered, so that exercise was moot. I'll have to pull all the transistors anyway in order to do the step-by-step startup testing, so the right channel output transistors will be checked as well.

Nothing to do now except wait for the box from Mouser. Everything ordered except for the germanium transistors, and I should get some 1A fuses for the startup tests. Maybe a foolish hope, but I'm putting off ordering the DC protection board for now.

So, this just seems to be a spontaneous failure of a transistor that cascaded to other transistors causing the blowout on the control board- any thoughts on that theory?

 

[laatsch55]

Yep.....shit happens.....thank god for Dons board....