Mindprint overheating !

The fact that you are getting the step up voltages shows that it does work that way.

yekuku said:

HI scott, it is a simple 3VA 230V-15V trafo, just a single secondary.
Model number is Thai Lin TL 38s-150-213  (213mA@15V)
here is the manufacturer link :
http://www.thailin.com.hk/?page_id=1645
As it looks like , using an ordinary trafo in opposite configuration is not possible.
Anyways , I am still waiting for the custom trafo .....

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I'm guessing it's because of the small transformer specs....maybe smaller windings.......???.... Probably has something to do with why it's not stepping up as much......

the original was rated higher from what I remember.....

The original was a 2VA 15V-250V trafo.
Obviously it has sth to do with the  windings , maybe wire thickness or different number of turns, I really dont know.

yekuku said:

The original was a 2VA 15V-250V trafo.
Obviously it has sth to do it the  windings , maybe wire thickness or different number of turns, I really dont know.

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hmmm....... I'm sure there must be some explanation.... I may post a question in another thread later.... there are a few here I'm certain would have some insight.....

it would be nice to know the theory behind this.

It's interesting to do this for a couple of tubes...... I can see it being useful if there aren't too many catches.....

Using a normal 230 / 110 V transformer backwards was common practice in the beginning phase of this newsgroup because over a decade ago the availability of parts was really bad compared to today. Transformers for tube circuits wer really hard to get. For example Gyraf's projects (like the G9 http://www.gyraf.dk/gy_pd/g9/g9_sch.gif) used that concept. Quite some people here will have experience with that, those projects were (and probably still are) very popular.  Looking at the schematic of the G9 seems to support your experience. He uses a 2 x 110V to 2 x 12 V transformer backwards with the 12V windings paralleled (essentially 220 to 12V) - being fed with 15V. So the backwards ratio seems to be lower than just the numbers reversed...

Maybe someone with better transformer knowledge can chime in and shed a light on that?

In your test you did measure the transformer output without load, didn't you?

Michael

Michael Tibes said:

In your test you did measure the transformer output without load, didn't you?

Michael

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The rectified voltage??? I forgot to mention this........... :-[

yekuku said:

Quick update:
I bought a generic 3VA 230V-15V trafo, and I tried to use it "backwards"
I fed 16V on the secondary, and I got 190 V on the primary.
Theoretically I should get near 240 V, I have no idea why this is happening. 

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That would be an important point. The AC voltage without load will be a touch higher than with the intended load. If the load gets too big, it will break down. If you measured in the circuit with load and there is something else broken which draws too much current, it will pull down your transformer voltage - and burn out the transformer in the long run because it will run too hot. Your original transformer being broken indicates  either a circuit fault, bad part quality or bad luck ;-)

If you measure the dc voltage after the rectifier and smoothing cap it should be about 1.4 times higher than the incoming ac voltage. Without knowing what you measured exactly it is hard to estimate what is going on.

Michael

The 3VA, 230v -15V transformer will deliver about 200ma. The output voltage is about 6.5% of the input voltage. Running it backwards with 16 volts input you would expect about 245 volts. You get 190 volts and this is 77.6% low. The 16 volts input is really about 12.4 volts on the input due to the IR and core losses. Small wire on the primary, secondary and core loss make up the IR drop and the turn’s ratio remains unchanged.
Getting a larger VA rating will give you better output voltage as the internal IR losses will be much lower. I would guess that 20-30VA would work.
Duke

Audio1Man said:

The 3VA, 230v -15V transformer will deliver about 200ma. The output voltage is about 6.5% of the input voltage. Running it backwards with 16 volts input you would expect about 245 volts. You get 190 volts and this is 77.6% low. The 16 volts input is really about 12.4 volts on the input due to the IR and core losses. Small wire on the primary, secondary and core loss make up the IR drop and the turn’s ratio remains unchanged.
Getting a larger VA rating will give you better output voltage as the internal IR losses will be much lower. I would guess that 20-30VA would work.
Duke

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This is great info...Thanks!

So the fact that it's a lower rated transformer in regards to VA, it is actually dropping the input voltage due to the losses? And a higher VA rating will give more predictable results based on the input to output voltages???
I'm getting clearer....

The original transformer was a 15v to 250v step up transformer I guess??? it was only 8ma.....

I wonder what the VA of this was and, were the designers not too concerned because it had enough current for their purpose and the voltages after being rectified were ok?????

Or maybe this is a cause for concern with the apparent failures of these from what the op says???

I hope I'm being clear...Just interested in this....

Hi to everyone!

First of all, thanks for all the useful info I found in this post.
I have the same problem, bought a Tcomp with a broken step up transformer and burnt resistors (R142, R162, R166, R191, R167, R192). I managed to get a new one, and replaced the resistors with 2w new ones, with long legs far from the PCB trying to minimize the heat problem. All the burnt traces repaired as good as I could...

It has been working just ok for a year or so, but recently I opened it to check and the resistors going to the heaters are almost burnt again (R166, R191, R167, R192), just looking ugly brown. Clearly there's something not wright, too much current, but I cannot figure why... All the voltage readings in the schematic pic attached by yekuku seem to be ok (300v, 22v, 15v, -22v, -15), and the unit is working just fine, but even with 2w resistors is burning them...

Is it safe to use 3w, ??? I'd be gad to hear some ideas about what could be happening, or what measurement I could take to trace the problem reason.

Thanks in advance and best regards!

Jose

Might it not be an idea to, i dunno, see WHY those resistors get cooked, as opposed to just slapping band-aids on the symptoms?

Khron said:

Might it not be an idea to, i dunno, see WHY those resistors get cooked, as opposed to just slapping band-aids on the symptoms?

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Hi Khron, thanks for your response!

Yes, that´s exactly what I was talking about. My electronic knowledge is limited and i´m just trying to learn. I made the repair reading a lot on this forum and most of all the info in this post. I just replaced the resistors for 2w in the first place hoping that the main issue was only a bad transformer, that it would have been solved with the new one, and that the change in resistors wattage would help in a possibly undersized design... Now I see the problem may be somewhere else, and the transformer has been a victim of it, but I don´t know where to beguin.

As I said, something it's clearly not right, but all the voltage readings seem to be ok (going out the transformer and into the tubes), the unit is working fine and I don´t know how to beguin to trace the problem. I don´t want to put 3w resistors in a circuit that´s burning 2w resistors, designed for 1w resistors. I just could use some help to trace the reason of the problem, as I said, and try to solve it for good. Really, any idea about how to tell what is happening and why would be highly appreciated. I´m happy to post any photo or give any info about it.

This post is from 2018, so I don´t know if the original repair was ever made, i´m just trying to continue in a similar way because I thinks my problem and the original problem may be related. Please, let me know if I should open a new post.

Thanks again!

Well, you could begin by measuring the voltages before and after those dropper resistors, to thus compute what sort of currents the circuits are drawing.

The main transformer, at least on the schematic, seems to be spec'd at 25VA, which is not particularly high power, but the secondary voltage might be a bit on the high side.

Two ECC83 tubes will be drawing at least 150mA each @ 12.6v. If the AC voltage coming out of the transformer is indeed around 16v, that's about 3.5v needing to be dropped, with 300mA minimum, which works out to 1.225W in total across R166/167/191/192. So in theory, assuming mains voltage isn't much higher than 230v, the stock resistors should have been spec'd just fine.

New tubes perhaps?

Hi! I'll be measuring those voltages on the dropper resistors ASAP, thanks for the advice. Meanwhile I've been reading about dc filament powering and the 12ax7, and found something interesting about the inrush current problem with cold filaments:

https://www.diyaudio.com/community/...n-expect-to-power-a-12ax7.382814/post-6933543
https://audioxpress.com/article/con...end-the-life-of-your-amplifier-s-vacuum-tubes
Maybe a high current demand because of the cold filaments is putting too much current on the resistors too? Just from the post above:

The cold resistance of a 12AX7 filament is approximately . . .

12 Ohms from pin 4 to pin 5 (series filament for 12.6V).

6 Ohms from pin 4 to pin 9; and 6 Ohms from pin 5 to pin 9 (parallel filament for 6.3V, is only about 3 Ohms cold).

When you first turn the power on . . .
You get about 1 Amp inrush for series filaments,
And you get about 2 Amps inrush for parallel filaments.
The 12AX7 data sheet does not tell you this.

That is a lot more than the 150mA for hot series filaments,
and 300mA for hot parallel filaments.

Do not use either super narrow super thin PCB traces, or super low current sockets, to power Cold filaments (unless you are using some kind of very good soft start circuit on the filaments).

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I remember measuring B+ going over 450v on powering up the compresor, before it stabilizates to 300v...

I've also red about using a thermistor as a "soft start inrush current limiter" to fix this:

This $1 inrush current limiting NTC thermistor is used in modern Fender amplifiers to give the rectifier and power supply components a "soft start". It limits the the cold start inrush current as the rectifier charges the filter (reservoir) caps. It is placed between the power cord's neutral line and power transformer primary. It's rated for both 120 and 240 volt mains at 220 ohms (cold resistance) and 2 amps. When cold the thermistor's 220 ohms of resistance limits inrush current to 0.6 amps with 125v mains and 1.2 amps for 240 volt mains. As the thermistor warms up its resistance drops.

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Maybe I'm shooting in the dark, but may be related to the overheat problem at all?

I'll check those resistors as soon as I can.

Thanks again!!

I doubt inrush current is the issue - if those resistors get hot enough to fail, in time. How long does it take for the filaments to heat up anyway, 10-15-30 seconds?

If they burnt up on power-up, then yes, you could start suspecting inrush current, but it sounds to me like that's not the case, but a long-term heat dissipation / overcurrent issue.

Well, I just finally measured the voltage drop on the resistors, and here are the numbers. Sorry for the delay, a lot of work lately...

All measures at 230v mains voltage. As Khron pointed, the main secondary is on the high side (17,5v AC) and the resistors are dropping about 5v, but I think everything is within specs...

The tubes were brand new when I changed the resistor and the transformer.

Maybe it's just a heat problem?? The resistors are just betwin the main transformer and the tubes, but I don´t know if it's enough to turn the resistors brown like it did. It´s a closed unit, no ventilation, but I took the rear plate where the digital card goes and put a grill in order to get some hot air out.

Thanks again!

Are those readings on the AC range, or DC range, or..?

Khron said:

Are those readings on the AC range, or DC range, or..?

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Sorry, corrected in this picture: