blown OT?

[bakerlite]

Not quite a golden oldie but i have a marshall jcm 2000 tsl 122 here. the combo not the head
basicaly it was run at the wrong impredance - noticed it got hot over the OT and started blowing fuses.
Does anyone know how to test the OT?
Also what follow on damage would a blown OT cause? or once it went belly up is the amp idiot proof designed to blow the fuse to avoid further carnage!
dagnall c3070 is the OT.
thanks

[RicharD]

Here's a video tutorial.

When you're done with that, please enjoy some Iva Bittova

[bakerlite]

and enjoy it i did - that was a lovely song.
Should there be continuity between any pins in an ot transformer?

[phsyconoodler]

Shorted tubes blow fuses.Incorrect impedance causes shorted tubes.
  Replace the bad tubes and fuses and see if it works.

The OT is last on the list.

[RicharD]

Phollow Phsyco's advice.  To answer your question:  with the leads disconnected, you should see a low DC resistance between the brown, blue, and red leads.  You should also see a low DC resistance between whatever secondary leads you might have.  You SHOULD NOT see any DC resistance between primary and secondary leads.

-Richard

[bakerlite]

thanks folks -
its a strange one -
i dont like these pcb type amps for beginners they look more daunting even if t he theory remains the same.

i note that even with the output tubes taken out it blows the mains fuse when switched on - even with the standby in the off position.

[plexi50]

With power tubes removed / remove the secondary high voltage lead from the PT at the bridge rectifier. Does it still blow the mains fuse? This will tell you if your PT is working and not shorted internally.  Also you can measure your voltage at the PT bridge with the OT leads dissconected to the power tubes. All this will rule out the PT as being a possable problem. You can make a 3 amp circuit breaker with leads to clip in place on your fuse terminals. You will save a lot of money on fuses.

[bakerlite]

Sorry to be a dunce - but would that be the purple one heading towards the the strip of diodes on the pcb (presumably some sort of SS recification) - searched for a spec sheet but can only find the OT spec sheet which has useful testing data on it. Also you see the yellow on the transfromer - that looks unusual like it is melted but that could be my lack of expwerience having only held about 6 transfromers in my life! they usuallly look more waxy!

I dont do this for a living though i am confident around the voltages and BASIC theory having a few builds on the go my friends are starting to think i am more useful than i am!

also - each el34 tube there looks to be black bubbling around all pin joints - i guess this must be standard but hadnt to see it on all pins.


thanks again

edit - damn wrong pics, one sec

[bakerlite]

heres that pic

[FYL]

heres that pic

Pretty normal for a volume-built amp.

[jim]

I think boiled solder pads could be problematic--you now have baked plastic goo and rosin on the pads under the solder.  Your amp should not blow fuses with no power tubes installed.  If all is OK intall a new set  Use a "Bias King" type meter or solder in 1 ohm resisitors to set bias. Also check the bias fuses--the cathodes pairs are fused in these amps. Your OT should have adequate protection against failure.  I have repaired a couple of these amps and I know they run hot , hot ,hot even when they operate correctly.  Look for swollen screen resistors and check this page. There are flaws in early versions of this amp.   http://www.lynx.bc.ca/~jc/TSL122.html
 

[bakerlite]

Unfortunately all is not ok!
It does blow a fuse with no tubes installed
It seems the amp had been feeling hot to touch over the PT for a  few practices before it died - shame he didn't mention it sooner....
I got some resistance values for the PT but no spec sheet yet?
all values check out EXCEPT the purple and yellow that are nest to each other in the picture - these should read 14.8 ohms but mine only read 8.7 ohms - i'm not sure this is enough to verify a blown PT?
Note i am referring to lugs on the transformer where the yellow and purple connect to - i am not measuring the actual wires they are all disconnected.
I know the purple wire goes to the collection of diodes on the amp board and the yellow goes to the mini pcb where the mains socket plugs in.
any ideas?

[bakerlite]

I disconnected all wires leading to the main board only leaving those that were hooking up to the mains in board.
It still blew the fuse
So I ordered a new PT
Still though i wonder what would cause a PT to blow - and if it does blow is it likely to take out other components?

[Dave]

Still though i wonder what would cause a PT to blow - and if it does blow is it likely to take out other components?

Not likely. It is shorted internally. That shouldn't send a bolt of bad joojoo on down the pipe.

Dave