Mindprint overheating !

Hi guys!

I have a T -Comp which unfortunately  had the infamous PSU overheating problem, which led to 22R and 10R resistors burning.
I have replaced the resistors , the unit is working again, BUT the High voltage transformer ( 15V-> 250V) that drives the tubes is getting freaking hot, above 100 celsius when checked with a thermal camera. Also the replacement resistors that were changed to 1Watt are getting freaking hot too, the same temperature as the transformer.
Is this normal operation , or the HV transformer needs to be replaced?
The unit seems to operate fine now, but  with such a high temperature , I am afraid  to use it.
What temperature is to be expected in such  part?
This problem affects also  mindprint envoice MkI, the Mk2 (and DTC ?)  has a different PSU.
Any ideas/recommendations are welcomed.
thanks!

I know nothing about this device, but from what you describe, there is something in the circuitry that is drawing too much current from the power supply, and that is what causes it to overheat and fail. You can replace the supply endlessly, and the same thing will continue to happen.

It's likely that some sort of component has failed, and this needs to be replaced. Sorry I can't be more specific, but the supply is fine, except for the excessive load caused by the broken circuit.

thanks for the help
Unfortunately the  schematics seem to be  unobtainable and without them  it is difficult for me to understand whats going on.
There have been lots of cases with the same issue , I am wondering if this is a design flaw or a component failure...
Here is a pic of the pcb:

Here  is a thermal pic of the psu

Ouch.....

I'd say some bigger wattage resistors would help them alone but, something is pulling a lot of current....That tiny transformer is some kind of step up deal??? Taking low voltage to higher for the tubes maybe???

Maybe the transformers are just under spec for these things if it's a common issue???

Hopefully someone can help......

I have a question for anyone....Why does the fuse rating go down when running the transformer @  230??? Sorry if that's a dumb question...Just trying to learn....

Why does the fuse rating go down when running the transformer @  230???

Click to expand...

i assume you mean compared to 115V ?
the circuit still draws the same power P . Power equals voltage times current ; P=U*I
in order to obtain the same power , same circuit , when U 230V is used  the current has to be lower .
( ;D old american movies , when they power on some big Frankensteins monster with this big lever and sparks all over the place )

regarding the topic :
where are those resistors , what do they do ? you have to trace it on the pcb. my guess they´re dropping resistors in the HT ?
measure the voltage across those hot resistors --> calculate power
pull the tubes and see with your thermal camera .

scott2000 said:

Ouch.....

I'd say some bigger wattage resistors would help them alone but, something is pulling a lot of current....That tiny transformer is some kind of step up deal??? Taking low voltage to higher for the tubes maybe???

Maybe the transformers are just under spec for these things if it's a common issue???

Click to expand...

The small transformer is indeed a step up, it increases the voltage from 15V to 250VAC and provides power to the tubes.
Thats exactly what I am thinking , the step up transformer  seems to be underrated  (8mA@250V) thats probably why it is getting hot.
Or could it be that some windings got shorted hence the overheating... But then the tubes should not be working....
Without schematics it is difficult for me to figure out whats going on.

nashkato said:

regarding the topic :
where are those resistors , what do they do ? you have to trace it on the pcb. my guess they´re dropping resistors in the HT ?
measure the voltage across those hot resistors --> calculate power
pull the tubes and see with your thermal camera .

Click to expand...

Some resistors are on the output of the main transformer and they are feeding the step up transformer, and the others are on the output of the step up transformer.
Please forgive my ignorance , I should calculate the watts in order  to replace the resistors with appropriate wattage resistors ? ( I have already replaced the 1/4watt burned resistors to 1 watt)
If i pull the tubes I am quite certain that the temperature will fall, because there will be no high current demand.
If there is a component failure then it will still be hot, right ? But if  a faulty component is  demanding so much power, it should be visible with the thermal camera, wouldn't it? 
I have marked the affected resistors in red circles.

yekuku said:

The small transformer is indeed a step up, it increases the voltage from 15V to 250VAC and provides power to the tubes.
Thats exactly what I am thinking , the step up transformer  seems to be underrated  (8mA@250V) thats probably why it is getting hot.

Click to expand...

IDK...8ma seems like it should be more than ok for (2) 12ax7??? Unless that transformer is doing other things??? Hopefully someone can step in  and help more

yekuku said:

Without schematics it is difficult for me to figure out whats going on.

Click to expand...

Yeah

yekuku said:

Please forgive my ignorance , I should calculate the watts in order  to replace the resistors with appropriate wattage resistors ? ( I have already replaced the 1/4watt burned resistors to 1 watt)

Click to expand...

You'd think the original spec would be sufficient..... but who knows.....

I'm still learning the different laws so hopefully someone can help with how you can calculate your voltage drops/current being used .... this will help a lot in figuring out what may be going on......

Looks like some components were upgraded......??? Those F&T caps and maybe some Dale PRO resistors....

nashkato said:

i assume you mean compared to 115V ?
the circuit still draws the same power P . Power equals voltage times current ; P=U*I
in order to obtain the same power , same circuit , when U 230V is used  the current has to be lower .

Click to expand...

Yes!...Thanks! So less current from the wall when using 230v???

Ok here is an interesting update:
I have removed the transformer and used a variac to feed it exactly 15V and I got 254Vac on the output.( off-load)
Then I measured resistance on the primary winding , it was 11 OHm and on the secondary the resistance was fluctuating around 1 MOhm.
Weird or what ?
obviously the secondary has problems but why does it output the correct voltage  off load?
On load the primary gets around 13V and  outputs  aprox 200-220V.
My chances of finding 250V trafo are slim, I am thinking of getting a common trafo with 230V-12V and installing it backwards so that it step ups the voltage, do you think that it will work ?
Any recommendations of finding a replacement transformer?

Looks like Hughes & Kettner used the same transformer in some of  their products........Same deal with powering a couple of tubes in a solid state circuit.....

I see some similarities in components too....Weird....Wonder if there is a connection

Are they still around?? Maybe you can find one but, without me knowing about your transformer measurements, I couldn't say you have a problem there or not.....Doesn't sound right but, I don't know this stuff......

I'm interested in the transformer findings you have.... I'm sure someone with some knowledge will be able to help.....

scott2000 said:

I see some similarities in components too....Weird....Wonder if there is a connection

Click to expand...

they are different brands but the same company
http://www.musicandsales.com/stamer-gruppe.php5

thanks for the help scott2000
Yes i noticed that Hughes & Kettner tubeman and tubefactor used the same trafo, and I had a peek at their schematics.
I had no idea that they were the same company, thanks for clearing this out silent:arts
The manufacturer of the transformer is a German company too.
I ll drop them an email and see if they have any available.  To be honest I am hesitating to install the same trafo with the same rating.

Any other ideas about finding a replacement ?

One thought I had is that the tubes are somehow drawing too much plate current, maybe because they're on the bad end of the production spread. Can you try replacing the tubes with different ones and hoping that the new tubes magically fix the problem? Other than that, you could trace out the tube portion of the circuit and try to find a way to reduce their bias current using the tubes in the device now.

yekuku said:

Here  is a thermal pic of the psu

Click to expand...

how did you do that 

Looks like it was a FLIR camera...... at least the stamp on the pic leads me to that guess.... Fancy

scott2000 said:

Looks like it was a FLIR camera...... at least the stamp on the pic leads me to that guess.... Fancy

Click to expand...

indeed! very tempting to spy on your neighbours with this thing  ;D

You don't live next to mine obviously...lol

I do swear I've seen drones in my back yard late at night.... I'm tempted to string a bunch of fishing line across to snag me one.... ;D

scott2000 said:

You don't live next to mine obviously...lol

I do swear I've seen drones in my back yard late at night.... I'm tempted to string a bunch of fishing line across to snag me one.... ;D

Click to expand...

Hahaha ;D
Lucky i dont have a backyard ;D

Monte McGuire said:

One thought I had is that the tubes are somehow drawing too much plate current, maybe because they're on the bad end of the production spread. Can you try replacing the tubes with different ones and hoping that the new tubes magically fix the problem? Other than that, you could trace out the tube portion of the circuit and try to find a way to reduce their bias current using the tubes in the device now.

Click to expand...

I have ordered a custom trafo and 2 new tubes, hopefully I will have an update by the end of this week.

Concerning the thermal camera , it is a Flir-One  for Iphone.
It can not take high resolution pictures but it is great for inspecting components and finding shorts.
It detects temperature  but it can not 'see through' objects, so I am not quite sure if it is any good for spying

I have uploaded the schematics for the Mindprint T-comp S2 to the library::

https://groupdiy.com/index.php?topic=69370.0

Hope this helps.