JCM 2000 Blown PT troubleshooting

[bakerlite]

hey folks -
I really need some help -
In an old thread here i mentioned a marshall tsl122 with blown transformer -

http://www.lydian.ca/Marshall_DSL_files/jcm2-60-02-iss5_TubeBoard100w.pdf

my friend gave it to me along with a new transformer - asked me to fit it. sure
that new transformer burnt out in 2 hours playing amp was red hot he said.

Out of guilt for not diagnosing the problem like i should i have purchased a replacement -
It was a foolish error by myself and i have learned it the hard and luckily a safe way.

What would you do in this situation regarding hooking up the 'replacement' replacement?


i was thinking in steps:
 hook up all primaries
hook up the heaters
hook up 22-0-22
hook up HT


the pcb board in this amp shows no visable failure like so many of this model has suffered.

is there anyway to diagnose a blown transformer? so i can learn where it failed?

thanks folks

[RicharD]

I take it that the blown transformer is the power transformer?  First off, take DC resistance measurements of the new transformer.  Now do the same for the old transformer(s) and compare them.  If you have a blown secondary, it will at least tell you if the problem lies within HV side or the filament side.  More than likely it's in the HV side but you never know.

[tubeswell]

Sometimes you can't tell with a blown pt until its loaded. Better to use a (incandescent light bulb) limiter to light the 'dodgy' PT up. If there is a short in the PT the bulb will glow brightly.

[bakerlite]

OK folks -

Could do with a dig out!

Installed new power transformer and one set secondaries at a time with light bulb limiter -
looks ok - dim after brief flash coming out of standby.

I just dont know what smoked the last 2 transformers and not sure where to go next?

I have a new set of output tubes ready to go in - thing is i tried the light bulb limiter with the old tubes in and it seems fine too  - regarding causing a short anyway.

any ideas?

because i think its gonna be the same scenario again of seeming and sounding fine - biasing up fine - then played for 2 hours and then bam, mains fuse blows , hot and dead transformer.....

thanks

[bakerlite]

Ok - Complete dunce moment.

WITH NO TUBES INSTALLED THERE IS A LIGHT GLOW FROM THE LIGHT BULB LIMITER!!!!

How do i begin to look for a short?

[bakerlite]

And with ALL secondaries disconnected i still have a slight glow from the bulb?

[sluckey]

WITH NO TUBES INSTALLED THERE IS A LIGHT GLOW FROM THE LIGHT BULB LIMITER!!!!

And with ALL secondaries disconnected i still have a slight glow from the bulb?

That's OK.

[PRR]

The power transformer will pull a small load even with no external load connected. The primary inductance is low enough to suck inductive current, the transformer iron core sucks power too.

What size PT? What size lamp? A 100W PT sure would glow a 25W nightlight. A 60W Champ PT should not make real glow in a 100W lamp.

MUST be incandescent lamp! These pig-tail "bulbs" work totally different, and not even all the same-different.

[bakerlite]

standard bulb 100 watts.
100watt amp.

apparently its perfectly normal to see a dim glow even with all secondaries disconnected.


such a shame, i thought the culprit was in sight.

So i am back to the drawing board as to what can roast a power transformer before destroying  any tubes .  i am thinking filter caps or bias drift but would have expected tubes to suffer from the later?

does anyone know what that horrid VVR circuityry does on these jcm 2000's?

[jojokeo]

I'm sorry but there's a lot of info missing. You're not even saying what transformer's you're using, what ratings, what voltages you are or aren't getting...I feel like you're asking us to find a noodle in a bowl of spaghetti? You need to be more specific on a lot of things you have or havn't done and what's a VVR got to do w/ anything? 

[tubeswell]

The VAC on the secondaries are the next thing to measure. (Theses should all be alright with a fresh PT) Without any tubes loading the PT down, they will all measure a little higher than spec.

Then filter caps are the next thing to check. With the PT powered up measure the B+ rail DC voltages at each filter cap node. They should all be about the same (approximate B+ voltage) with no tubes installed. If one of them was completely shorted, the voltage would be 0 at that node (and the load would be going to ground at that point).

Then check the bias supply voltage. It should be a few tens of minus-volts

[bakerlite]

Thank you so much for your advice.

AC transformer readings no tubes:

530v - 510v - 3v - 3v - 21v - 21v

B+ No Tubes

The 2 main filter caps c38 c49 in series are 330uf 315v Cans.
With no tubes installed the intersection +- is 244v and the final  + is 490v

C34 and C35 are 2 100uf caps in series exactly the same 244v intersection 490v +

the three 22uf caps all read 490v on the +

C1 and C3 (both 10v 3300uf caps)  again in series C1 shoes -4.3v on the negative side and 0v on the +- intersection then 4.8v on the + of c3

C2 and C4 both measure -4.3v on the negative and 4.8v on the positive

schem
http://www.lydian.ca/Marshall_DSL_files/jcm2-60-02-iss5_TubeBoard100w.pdf


too much info maybe but worth a try!

[bakerlite]

Been going over the main board with a fine tooth comb looking for any signs of stress at all , everything looks brand new apart from the 100k cathode resistor on V3, it measures in spec but does show some heat discoloration.

I'm sorry to drag this one out, i appreciate i'm in a little over my head but i feel responsible and would love to rectify the problem.

if there is any information at all i can give to help give you guys a clue please let me know!

Those b+ test i did on the caps doesnt completely disqualify them from being the culprit does it? ssome fail only under load? somethign about small holes in the dielectric or something?

[jojokeo]

Your voltages appear to be okay on B+, the one I worked on recently measured 466v w/ all tubes in and bias way down. It slightly lowered after correcting bias to -39mA. You for sure have to properly spec'd PT right?

Have you carefully checked all of the contacts on all of the boards especially the main board w/ all of the tubes' sockets? I've found many suspect and outright bad contacts that all needed to be re-flowed. I resolder all tubes' sockets whether they need it or not because they mostly all do regardless how they look to you. Those boards are so cheaply made and manufactured. But that's what helps keep techs in business. Remove and check the IEC/B+ board. I found hideous cracks on solder joints there too. While you're at it, solder a 220k/1W or higher "soft start" resistor on that board to the wires that put it across the standby switch. (see pic)

Test the 100K discolored resistor and replace if necessary, maybe w/ a 1w if needed? Get the amp up and running and correctly biased. Then check your B+ voltages again w/ the chassis on a cradle or I'll turn it upside down on the head enclosure. Don't forget to put a load or speaker on the output jack (sorry, but had to say it). I'd leave it running checking for signs of excessive heat anywhere and testing what I could. Then set the chassis on a speaker cab and play it for an hour or more while monitoring everything. Sometimes it's not obvious and certain parts begin to break down after heating up more while under load. Re-check the bias afterwards also to make sure it isn't drifting.

(oops , forgot pic)

[chabby-ao]

Been going over the main board with a fine tooth comb looking for any signs of stress at all , everything looks brand new apart from the 100k cathode resistor on V3, it measures in spec but does show some heat discoloration.

I'm sorry to drag this one out, i appreciate i'm in a little over my head but i feel responsible and would love to rectify the problem.

if there is any information at all i can give to help give you guys a clue please let me know!

Those b+ test i did on the caps doesnt completely disqualify them from being the culprit does it? ssome fail only under load? somethign about small holes in the dielectric or something?

Good luck with this series of the PCB board from hell.
These amps have cause good techs to lose hair. But I wish you the best. Could be a ghost you're chasing in those damn things where it only shows up when heat is applied, or not. I just ripped one apart and used it as a donor to build a hand wired PTP JCM800. I could add about 20 things that haven't  yet been said to look for, but you've gotten some good direction. All the obvious things have been mentioned in a systematic way as any tech would look down the lines. But with those beasts it can go way beyond the norms, especially the idea of finding ghosts that appear and disappear. Problem is one thing leads to another in those. Many people just buy brand new boards and be done with it because they've been frustrated by these before, or lost their A$$. But maybe you'll be lucky. One thing certain: at some point even if you do get it running beyond a few hours, it will fail again, possibly with fatal blows. I hate to be a downer and not just try and help you, but it'spretty tough, to impossible with those.