What in Thorens TD 320 could cause a Transformer To Burn??!

Bought this TT from Germany off the ebay a month ago, and from the day one had issues. First, it did burn out its original PSU of 16V - 160mA, just after five minutes of working!!
Then I tried another PSU I had at hand, AC/AC but with 18V - 200mA - Thorens did kill this one too!?!
Power at home (new building, new wires) is always stable 230V, and I have all my devices attached into the power conditioner. Never a single issue with any of those; amps., preamps., players, other turntables!

After reading about similar issues with this model on other forums, I've decided to replace crucial OP AMP - LM324 op-amp IC, but two others on the board as well; TL084, and TL071.
In the meantime also bought the ultimate PSU by Thorens TPN 2000, and... with it TT works but slow on both speeds!?! Also TPN PSU is getting warm too soon.
Found another AC/AC transformer, of 16V 600mA, switched in, turned on, and it was playing great, stable speed for about five minutes... and PSU DIED! Again!?
While working on the PCB board I found nothing suspicious, all caps, and resistors seems fine, nothing burned. Why is this happening is beyond my understanding. I tried to find solution with the guy who sold me this crap (although turntable looks great, neat and well preserved), and he proposed to send it back to him. However, I would like to find a solution, and make it work if possible.
Anyone knows or have an idea what the hell is going on here!? Thanks!

 

My best guess (and it is only a guess) is motor/bearing lubrication, and/or motor failure.

 

The highest current draw of this TT at running 33.3 RPM is 148mA. This is not enough to kill even the stock wall wart PSU. If it killed a 600mA PSU something is way off. I would suspect a shorting transistor and it would be wise to verify all transistors integrity, but first I'd install a 200mA fast blow fuse before the rectification (after N1) to avoid further butchery of any future PSU's. Page 8 of the service manual displays a table of voltages and currents at various speeds with various test points. I'd verify all of them. It could be a motor stress or failure but I would trim the voltages to their spec'd values and then monitor current draw before suspecting the motor.

 

Same thing happened to my TD 318 after I cleaned and re-lubed the platter bearing. The turntable ran for about an hour and then quit. The PS was very hot to the touch. I figured the oil I used was too heavy and caused the motor to work too hard. Haven't done anything with it yet.

 

Thank you guys for feedbacks!
@tnsilver, I will try to do all what you're suggesting. But also think that the main problem is the motor itself?
@maiden, I did clean the old oil from platter bearing, and oiled again with very fine oil for precise bearings and wheels. Platter turns smooth, so I don't think there could be an issue.

 

Look at the boards inside the turntable. Replace any caps that say "Frako" on them. They go bad and short out. Sometimes they explode.

 

Thanks, but none of these four has that label. Could you point it out which ones do you mean? All look fine, although I can read values with V-meter on main-blue cap only!?

 

I can't read the manufacturer's name on any of those caps in the photo. If none say Frako, you should just replace all the electrolytic caps on the board. That is most likely the problem but I don't think you will be able to measure the excess leakage with your meter. You would need a capacitor tester that tests them at the working voltage and measures leakage. In my mind if you don't have one, and don't anticipate doing a lot of electronic repairs it isn't worth it to buy one, just replace the caps. That should fix it. If there is a bad semiconductor it would probably not work at all. One other thing it could be is a shorted rectifier diode. There is a bridge rectifier, probably near the large blue cap, GR101. Test it with your meter on the diode range.

 

I would first replace GR101 (the bridge rectifier) and C101 (the 1000 µF main filter cap) even if these parts tests statically good. Also add a 250 mA fuse in series with the low voltage AC input (N1) to avoid burning out another transformer during your tests. I don't believe it's a motor issue, or the R103 (22 ohms) protection resistor would have burned out. Check if there is any sign of severe overheating of this resistor, if yes you may suspect the motor or one of the 3 motor drive output stages (complementary transistor pairs). Also check (when the TT is operational) if when you start spinning the platter backwards by hand before turning the power on it doesn't continue to rotate backwards when power is applied. This is usually a sign of a missing phase or a defective motor. I can't see any other decoupling caps on the +18V line and you have already replaced all the opamp chips, thus there are not much parts remaining who can overload and burn out the transformer. Though it may seems obvious, also check the AC power input connector on the pc board for a possible intermittent short due to a mechanical damage, these parts are not of the highest quality and can fail.

Edit: I can see R103 in your picture and as far as I can tell it doesn't show any visible sign of overheating. But I read 12 ohms and the schematics says 22 ohms. Maybe a design update/change ?

 

OK, thanks a lot dr*audio and Tubologic! Appreciate much all suggestions and hope it will be a success at the end. Upon checking the Service manual again I made the list of all required components. Not cheap at the end, but found them at one place, bg-electronics.de, so have to wait now. That GR101 B80C800D is particularly hard to find, but they have it!

 

The bridge rectifier type is not critical and you can use any small plastic encapsulated bridge rated at min. 40V/1A with long wire connections which can be formed to fit the PC imprint. Should not cost more than 50c.

 

I understand, thanks! Just wanted to go as close to the originals as possible. Neither the B80C800DM is expensive - 1€, but then all other things together + taxes + shipping. Not bad actually, and I've ordered some double and spare stuff, just in case if I burn accidently something ...

 

UPDATE!
Well, I've replaced two electrolytic caps: C105, C109 - 10 uF/ 35 V (those values were on the board), with two new 10 uF / 25 V, although schematic says; C109 is 10 uF /6 V, and C105 is 10 uF /25 V.
New; GR101 - the bridge rectifier diode, and C101 - the 1000 µF/25V main filter cap. However, both new and old rectifier diodes read both ways around 550, which should be good as it shows that the diode is open, so not the cause of the problem, right!?
(edit) hmm ... forgot to mention that one way (on the new diode) reads like ''INFINITY'' numbers in thousand, is that right to be good?.
If pushed by hand, the platter does not keep turning counterclockwise when the power is switched on, so the motor should be fine too!? Speed is also stable, no fluctuations here.
Nothing is heating up on the board while testing, neither the R103 protection resistor, all looks fine there.
Yet, any transformer attached, either of 15 V, or 18 V, is heating up very quickly, even with the fuse there I don't want to risk to keep it on for too long, and I'm switching all off after 10-15 minutes.
The AC power input connector on the chassis was replaced some time ago by the seller, still I'm not sure why the metal around it looks so bad and scratched? What's happened there? However, AC power that enters there is OK, and the DC current flowing into the turntable on N1 N2 points both on soldering points and on the pins reads ''0'', as it should be so??

All in all - replacing electrolytic caps, rectifier diode and all three OP AMP chips didn't cure the thing so far!

What's left, forgotten.... I still didn't check the integrity of all transistors neither all other seven diodes, and that's a job. If I understand well, to check all diodes at least one end has to be desoldered from the board?
argh


btw. hope that fuse of 200 mV that I've placed right on the AC input is OK there? Anyway, two of transformers tested for about 10-15 minutes are well and alive. The last one, prior these mods, died just after 5 minutes.
Still, why this transformer heating

 

Dare I ask if you have successfully tried any other wall-wart powered kit via the same socket, or powered your turntable from a different domestic circuit? Shouldn't make any difference, but.............

 

Thanks for your input Graceman! It does not really matter, and yes I did try various scenarios; few different PSUs, including that Thorens monster TPN 2000, nothing helps, all are just heating up very quickly!
I bought another Thorens TT, in the meantime, beautiful TD 325, and that one works like a champ, no matter what AC/AC wall-wart PSU kit is attached on - no heating on those transformers at all! TD 320 also works nicely, stable speed, lift/switch off at the end, but just heats up transformers beyond normality.
I sold one TD 147 a month ago, this one had no issues of that sort, so no - different circuit, or sockets, through the Power filter or just in-the-wall sockets really does not make any difference, neither is important.

 

Well... this is still mysterious. A transformer can only overheat (and fail) if it's overloaded by a load current exceeding his maximum ratings by a considerable margin (or a dead short).
With the 200 mA fuse you added in the AC secondary line the transformer is protected and if this fuse didn't failed the current never went over 200 mA, this is a definite fact. Now I'm starting to consider that maybe your problem is not related to the turntable. Could you clarify the following points:

1) How much is the max. secondary current ratings (mA) and voltage (V) of the last transformer you used ? (after you replaced the parts)

2) Could you measure the actual transformer t° after 15 to 20 minutes with the turntable connected and running. Small wall transformers have poor efficiency and high copper losses, thus a significant t° rise is normal. (but burning out isn't of course)

3) Repeat the same test as above but with your protection fuse removed. Leave the cable and connector connected to the turntable to exclude any possible short or leakage in the connector ahead of your fuse. (very unlikely but possible)

4) As above, but with the cable completely removed from the turntable but the transformer still connected to the mains. Check the t° after 15-20 minutes.

Report your findings here.

 

230/15V 140 mA EDIT: (NOT 133mA, my bad)
It is one of those encapsulated, and fixed in the box of the previous (burned out) transformer, which was open type, btw.

Temperature exceeds the body temperature, and goes well beyond 40°C or 104° F, or figuratively - it is too hot to keep the finger on it for too long!

If I do this it is very likely that transformer will burn out within minutes!! It doesn't make sense

It stays simply cool.

 

Use a good quality transformer rated for 500 mA and your problem will be solved. I don't think there is any issue with the turntable if it works correctly and no parts show a sign of overheating.

 

The one that burned out was 230V / 600 mA (before mods)
The other one - Thorens TPN 2000 - 240V / 700 mA is heating up easily within 5-10 minutes as well (after these modifications!) and it shouldn't at all! (there is the image of the beast attached, before putting it in the box)
As I said, my latest Thorens 325 works with any of these AC transformers, and all stay cool, no matter how long are attached.
The reason is in turntable, that's for sure. But what??

edit: the link to the transformer is correct - it is of 140 mV, I made a mistake, it's not 133 mV
and Thorens official info says that Power requirements are 17V max. 140 mV (Manual)

 

I would suggest you get hold of a low cost infra-red thermometer (like this:
http://www.ebay.co.uk/itm/Handheld-...030500?hash=item542734e3e4:g:1D4AAOSwnNBXW0De )
and use it to try to identify whether any part of the circuit board is unexpectedly showing signs of overheating.
I would be especially interested in the 4 pairs of output devices that drive the motor.
If one or two pairs connected to the same phase of the motor shows signs of getting warmer than the others
then it is possible that you have a shorted turn in the motor windings.
I can't think of anything else at the moment, but good luck; I'm sure you will find the problem in the end.