[Solved; kinf of] 300W Inter-M Audio amplifier repair, stuck in "Protection"

[Francenthusiast]

Hi !

First I want to thank everyone here because I just registered but it's half a couple of years that I read topics on this forum that helped me learning electronics ! Principally Dave ! Double 
And by the way, I'm french so I hope I'll not make you cry with my bad english.

I just purchased for 10 euros an old and broken 300W amplifier branded Inter-M R300 and I need help to repair it (I think it's possible).
Here is the schematics:

https://fr.scribd.com/doc/118053738/Inter-m-r150-r300-r500-Power-Amplifier-Service-Manual
Here are photos of it:

The symptoms are: When plugged, it automatically lights up the "Prot" LEDs. I guess the PSU stage is OK but I still checked. This kind of amplifier goes into protection mode in case of overheat, short circuit on output or if you put DC voltage on output. By the way: I don't have any 4 ohms 150W speakers, to test it, I used 4.7 ohms resistors on the output and no input. Normally, even without any input or output, it shouldn't go into protection mode.

Here is a list of what I have already checked.
Live:
Secondaries on the transformer: +/-40V AC on CN602 connector and  +/-24V AC on CN601 connector

Disconnected from mains:
Mains fuse F1 : good
MOV C1: high impedance
Bridge rectifier D600: 0.5V on each internal diode
PTC thermistor P0: 338 ohms (@room temperature 24°C), looks weird (R300's documentation description says "P0 is POSISTOR P42 T70 330 B04 200MM", I guess that it's 33 ohms @ambient temperature but I did the reading without desoldering the PTC) (Positor PTC datasheet https://www.jameco.com/Jameco/Products/ProdDS/1875280.pdf)
High power transistors (KTA1659 and KTA4370A): all forward voltages at 0.44V and 0.60V, seams good
Desoldered C613 and C614 (big 10000uF capacitors): 8920uF / 0.06ohms / Vloss=0.1% and 9200uF / 0.05ohms / Vloss=0.2% seams good (measured with home made


Any ideas ?
Thank you very much in advance for the help !

Francenthusiast

[TheMG]

Looking at the schematic diagram it should be pretty simple to figure out what's triggering the protection.

The way it operates, it looks like the protection circuit simply opens the speaker output relay when triggered.

First thing I would do is power up the amplifier, and using a multimeter on DC volts, with no signal going into the amplifier, measure the DC voltage at the output of each of the amplifier boards. The DC voltage present should be quite small, less than 100mV is typical for most audio amplifiers. If it is much more than that, you have a problem with the amplifier board.

If the DC offset voltage seems ok, connect a signal source to the amplifier, 1kHz sine for instance, and check output of the amplifier boards with a scope if you have one.

If all that checks out, then start looking at the thermal detection circuit.

BTW, it does not look like an overload would trigger the protect LED. The amplifier is protected by means of a current limiting circuit in the amplifier circuits themselves, but I don't see any feedback going to the protection circuit so nothing to trigger the protect LED or shut off the amplifier. It looks like a short circuit fault in the amplifier circuits would likely just end up blowing the mains fuse (if the fault doesn't blow itself open first).

So you're looking at either a high DC voltage on the output, or a faulty temperature sensor. I'd check the DC levels on the output first as a number of components failing in the amplifier circuits can cause a high DC voltage to be present on the output.

[Francenthusiast]

Hi TheMG !

Thank you very much for your answer, I've checked and I have no DC voltage on the output so I think it's definitely a faulty temperature sensor.
Sadly, I've been very busy and I couldn't find time to answer you earlier nor I can open the amplifier again to change the temp sensor now... I'm sorry for that but thank you very much again !

[Francenthusiast]

Hi !

Just an update about the amplifier.

I didn't saw that one coming !! After spending couple hours trying to find out where could be the problem I just abandoned the idea of repairing it and started ripping it off in order to convert the harvested pieces into a power supply (powerful transformer, low noise opamps, just needs a hole for the display and voilà !)
But while desoldering component I saw that:

So now I'm half and half 

A good lesson for next repairs: even if you think you checked all the obvious, you've always missed something ! Continue checking !

Anyway, I have another question now I've started ripping it off: how to test maximum output power of the transformer ? I have a rough idea (300W ?) but is it possible to refine that guess without loading it ?
The input fuse is 3.15A on 240V AC EU.

Output voltage is 81.6V AC with a center tap (22AWG wire) on true RMS fluke, the other output is 45 V AC with a center tap for the opamps but I think is negligible.

22AWG can deliver 7A continuous @60°C; fuse says: 240V*3.15A=756W max input power; 300W/81.6V=3.67A ??

An idea ?

[CJay]

That crack extends across the mounting hole and through the track to the right of it as well.

If I were in the market for an audio amplifier I'd definitely be patching that up with some tinned copper wire.

[oldway]

To make a repair, it takes patience and perseverance.

Unfortunately Francenthusiast is a beginner with no experience and did not know it.

Result: he destroyed a perfectly serviceable 300W amplifier to recover a single transformer.

There had to be a high DC component on one of the two amplifiers, but he said there was no dc voltage at the output.

Why has he made such mistake?

He measured the output voltage, then he should do it directly on the output transistors because there is a protection relay which disconnects the output when the amplifier is in protection.

The voltage measured at the output is therefore always zero even if there is a fault in the output stage.

Fortunately he has paid this amplifier only 10 euros.

NB: To prevent this crack extends, one must drill a hole 2 or 3mm at the end of the crack.

[CJay]

Makes no difference, it was 10 euros, he's learned a lot and made a few mistakes which are the best lessons you can learn.

So it's worth every Euro and he now has a nice pile of parts and a transformer to continue learning from.

[Francenthusiast]

I agree with you oldway, I have been impatient (even if a spend literally a day on it, at least 24 hours, not strait but you know what I mean) but in the end, I learned a lot, it could be seen as a 10 euros electronics course, I learned about troubleshooting transistors, what are MOVs, capacitors ESR and how to measure it (I've bought a Chinese gadget kit for that and learned how to program an ATMEGA chip that only has serial port, I learned how bad it is to short big capacitor (

), I learned how to calculate the DC voltage of a rectified transformer output (the root mean squared stuff) and many many more... And like CJay said, I just looked at ebay and other buying places and a 750VA transformer is way more than 10€ 

Speaking of that transformer, I will be hard to reuse it as I wanted (building a regulated power supply) because of its output voltage: there is  no"classic" low dropout voltage regulator IC that can handle 55V DC on their input...
On an old "salvage components box" that I grabbed in a garage sale, I have two LM723, la huge amount of LM317, an LM338, an L200, but none of them can handle the 55V...

The aim is to build a regulated power supply that can exploit the transformer capabilities (81.6VAC 6A), like 30, 40 or 50V 6A with a full wave rectifier, or even better would be a 100V 3A when using full wave bridge rectifier !

Any suggestion ?

[cheeseit]

Perhaps an TL783?

[singapol]

I agree with you oldway, I have been impatient (even if a spend literally a day on it, at least 24 hours, not strait but you know what I mean) but in the end, I learned a lot, it could be seen as a 10 euros electronics course, I learned about troubleshooting transistors, what are MOVs, capacitors ESR and how to measure it (I've bought a Chinese gadget kit for that and learned how to program an ATMEGA chip that only has serial port, I learned how bad it is to short big capacitor (

), I learned how to calculate the DC voltage of a rectified transformer output (the root mean squared stuff) and many many more... And like CJay said, I just looked at ebay and other buying places and a 750VA transformer is way more than 10€ 

Speaking of that transformer, I will be hard to reuse it as I wanted (building a regulated power supply) because of its output voltage: there is  no"classic" low dropout voltage regulator IC that can handle 55V DC on their input...
On an old "salvage components box" that I grabbed in a garage sale, I have two LM723, la huge amount of LM317, an LM338, an L200, but none of them can handle the 55V...

The aim is to build a regulated power supply that can exploit the transformer capabilities (81.6VAC 6A), like 30, 40 or 50V 6A with a full wave rectifier, or even better would be a 100V 3A when using full wave bridge rectifier !

Any suggestion ?

The linear regulator should be in the error correction path and not directly regulating 55V. See the links below:

http://www.acopian.com/high-voltage-power-supplies.html

This book although old list most of NS' ic regulators then.The principles of voltage regulators are the same today--
www.smcelectronics.com/DOWNLOADS/1980-VOLTREG.PDF

http://www.repeater-builder.com/astron/pdf/understanding-and-using-the-723-vr-hr-89-march.pdf

A circuit to test your power supply:
http://www.ko4bb.com/Test_Equipment/DynamicLoad/

[oldway]

When you have something to repair, you can also look for help in a "repair-café" nearby your home.

http://repaircafe.org/fr/visiter/

[Francenthusiast]

Wow ! Thanks a lot everyone !

@oldway Thank you, I did heard about that a while ago, didn't remember... There is one 60km from me in La Seyne sur Mer, worth give a try !

@singapol Thank you very much for the documentation, I will apply the driving principle, good to know the basics. About the dynamic load, It might be a good small project to test it when it'll be built.
thank you again for the applied example of how to use the LM723, very clear and simple, I saw another example here about how to use it (french article on elektor december 1982) but it wasn't that simple for a beginner ! This one is nice too but doesn't fit the transformer.

@cheeseit Oh ! Thank you ! I've never heard of TL783 before, and the datasheet seems very similar to the LM317 on how to use it ! I'm thinking about that design that would fit the transformer (it can give around 110V DC in full wave bridge rectifier mode to feed the power transistor)

After that, I would need to look at adding current limiting to the circuit.

Or if I use the LM723, can I modify this design like this :

Of course, resistors values has to be changed as well as the driving transistor. I have a BUT12AF that would be good as power transistor I guess.



What do you think ?
(Sorry for the crude windows paint editing)

[oldway]

At 2V 5A, power dissipation in series power transistor will be :

Filtered dc voltage :  50V
Power dissipation: (50V - 2V) x 5A = 240W
Ptot BUT12AF: 23W....!!!!

You need a pre-regulator.
See schematics of HP power supply 6296A for example.

[Francenthusiast]

I'm currently looking at the schematics of the 6296A and adding a pre-regulator would add "SCR diodes" (I'm discovering them... and don't have them in stock  ), a SCR controller circuit (page 26), and add too much complexity for me
I'm sure that's a clever idea and the state of the art but way too much expensive (intellectually and time investment).
Why not changing the BUT12AF by two or three 2SC3856 (in parallel) I gathered in that very amplifier ?
The amplifier had a 300W 12 inches 4 pounds heatsink... Could be usefull !