JCM 2000 - Breaking my heart

[bakerlite]

Hey folks -

Sorry to start of with a 'help me please!" but....

I am at the end of my wits here - I've got a jcm2000 tsl100 that melted the power transformer. Alot of the mainboard was worse for wear so i managed to get a new replacement board .

So - with new tubes, new mainboard, new Power transformer..... I still have problems....

She does pass the lightbulb limiter test - but; she (the bulb) seems a tad brighter than i expect not much but just a bit more.

The problem is this - with every dial down when you turn on the amp out of standby there as a low level hum always present - the same sound as mismatched power tubes except the tubes are perfectly matched and biased at 80ma a pair.

no dials effect this hum.

voltage of the PT are good both connected and disconnected from the mainboard.

Also the new PT heats up slowly but she does get hot - after 30mins i have to shut down - its heating up on the primary side especially.

The amp does however sound good - just she has that constant hum and heats up so is pretty unusable as is - it seems whatever killed the last PT is still present.

I have reflowed the IEC board which made no difference, i ave checked the DI/output board for cracked solder / bad grounds to no avail. I have checked the input jack for same.

I am out of my knowledge depth and would greatfuly appreciate any help if it could be offered!

thanks folks

[km6xz]

How did you determine the cathode current was 80ma per pair? Was there a way to verify that? What frequency is the hum, 60 hz, 120 hz or buzz that has high frequency harmonics? What amplitude is the hum in AC volts when no signal is present?
Does the amp get hot with no signal or just when played hard? Does the temperature rise occur at the same or similar rate if you under bias the tubes a lot, turning the bias pot for minimum current? That will tell you whether there is a general circuit problem or specific to the output section. What about temperature rise with the power output tubes removed?
What test gear do you have available? A scope, AC current meter,signal generator, Variac with current meter?

[bakerlite]

thanks for your help,

How did you determine the cathode current was 80ma per pair? Was there a way to verify that?

well first at the standard test points then i have one of those weber bias rites that gave the same figures give or take an ma.

What frequency is the hum, 60 hz, 120 hz or buzz that has high frequency harmonics?

its at 100z - it is not a lound hum but it is there - it is also the same mechanical chattering /hum coming from the power transformer.

What amplitude is the hum in AC volts when no signal is present?

not sure how to take this reading - sounds simple but from where is the test point?

Does the amp get hot with no signal or just when played hard?

gets hot with no signal

Does the temperature rise occur at the same or similar rate if you under bias the tubes a lot, turning the bias pot for minimum current? That will tell you whether there is a general circuit problem or specific to the output section.

What about temperature rise with the power output tubes removed?

yes temp rises with the output tubes removed - this is gonna sound funny but its the mounting bolts that get real hot first! then from there the endbell heats up.

What test gear do you have available? A scope, AC current meter,signal generator, Variac with current meter?

unfortunately I have no variac or signal gen -
My metre does test for AC current - And i picked up a scope a couple ofweeks ago but have never used one bfore...


thanks!

[km6xz]

This is interesting, it appears that excess current is being pulled from the mains and related to the transformer or circuit connected to the secondary of the transformer. The 100 is specified at 90mv across the 1 ohm cathode resistor so if anything the outputs are running slightly cooler than normal.
Is there any heating if the amp stays in standby mode? If there is, the transform will need careful scrutiny. If there is no noticeable temp rise check the filter capacitors(C38&C39) and rectifier diodes(D3, 4,5 and 6). If the transformer still get hot in standby the transformer is likely the problem. Standby on a DSL100 opens the HT line from the transformer before the rectifiers.

[bakerlite]

I don't beleive it (and that I didn't check it first!)
You are right - even in standby she still heats up
and from an unused transformer
This must be pretty rare for a new PT to be gone.
Strange she didn't fire up the bulb on the light bulb limiter test.
Thank you so much - you've no idea how much i appreciate your help at this stage!

As a last precautionary check is there anything on the IEC board that could cause this?

[km6xz]

There are a number of parts on the IEC board but none would cause those symptoms. The only other potential is on the heater secodndary but being high current low voltage, the heaters would have been low and you would not have gotten 80ma from heater starved power tubes.
One thing about trouble shooting is often overlooked is that the defect is the one that fits ALL, not just some of the symptoms so it is often a matter of taking a few relevant measurements and then sitting back and thinking it through, before even trying to replace parts. Any parts replacement interferes with that process of deduction because it introduced uncontrolled variables. Why? Because the two times in a component's life is it most likely to be bad, when first put into service and when its design life has been exceeded, assuming it has not been stressed as a symptom of some root cause. Actually most replacement parts used by shops are wasted and LOWER reliability since they are parts which are at their worst failure odds period, and they were not needed. The customer loses out twice, paying more for a less reliable unit. Quite often unit failures are single source root causes that are not as complicated as people assume.
So, to answer your question, it is entirely possible you have a bad transformer with shorted turns. You can further isolate the problem by disconnecting all the secondary wires and see if it still heats. You can also measure the windings looking for one that is lower than expected meaning a seconddary shorted turn problem. Or, is ALL the secondary windings are high, shorted primary would be expected for that symptom. Good luck!

[J M Fahey]

Agree and add.
It looks like a transformer with shorted turns.
So far so good.
The turns may be *internal* (shorted copper wire internals) or *external*.
You *may* be mounting it in some way that the metallic endcaps plus the metallic bolts form a shorted turn.
I say this because you speak of heating mounting bolts, and then of heating endbells.
I would lift and tape all secondaries, pull all mounting bolts, and put the (now floating) amp over a piece of wood.
And then connect only the primary winding to the power line.
If it still overheats, the transformer is shot.
If not, post it here and we'll continue the investigation.
Good luck.

[bakerlite]

Sorry I only saw this reply just now - and thank you - you are definitely on to something.

I followed your advice and with just primary connected she didnt overheat.

I checked all secondary voltages - spot on

i connected them one at a time and left amp run each time - no problems no heat

With floating PT with no mounting bolts no end bell the PT is no longer heating up and obviously transmitting less vibration into the amp.

I been playing 20mins now and she's fine.

I am kind of out of my depth here as to your theory on a mounting bolt completing a shorted run?

Any more help greatly appreciated!

[bakerlite]

And to add with the transformer floating as J M Fahey suggested there is low level voltage buildup on the sides of the transfromer in between the laminations - if you run the tip of the multimeter along the sides of the PT (like playing an old washboard except not as fast!) you get the sound of crackling through the speaker.

So it may be after all the transformer is leaking voltage to ground when mounted properly thus and heating up?

[Tom Phillips]

I would next make resistance readings from each winding to the case (mountings bolts) of the power transformer. The resistance readings from each to the case should be infinite. Otherwise it indicates that a winding is shorted to the case. Let us know what you find.

[Tom Phillips]

Having just read your post #9 after I posted #10 the evidence is pointing to a winding shorted to the case. You can find out which winding is shorted to the case by making the resistance readings I suggested.

[bakerlite]

thanks -
all secondaries read infinite to ground

edit - oops! one of the 21v bias taps read continuity to certain points between the laminations on the PT

sorry tom you reply quicker than i can update! thanks dude

[Tom Phillips]

thanks -
all secondaries read infinite to ground

OK. Measure the resistance of the primary to ground too and the primary to each secondary.

[epis]

Hi bakerlite, who is manufacturer of new transformer ?
I'm asking because of similar experience, actually I have rewound Hammond transformer secondaries to my specs(it was low voltage transformer), all voltages measured fine, but it was overheating even at idle. It didn't pull any extra current from mains.
Finally, I replaced all lamination, and solved my problem.

[epis]

Double post, sorry