need help with TA-503 switching power supply repairing
 

Hello! I need some help. Maybe here are any audio electronics engineers. I need help with type of impulse power supply driver circuit type. Impulse power supply doesn't work, I checked by scope, that there is not driving signal on swiching mosfets transistors.

There is small circuit (8-pins DIP) generating impulse signal for controlling power supply and there is any marker on it. Can anybody help me? I need datasheet and pinout of this circuit, but I don't know what is it :(

Sorry for my english :) I'm writting from Poland.

Well!
 

Sadly there is very little information about Markbass power supplies
It's all their own design and they don't seem to want anybody repairing them as they would rather just sell you a whole new one but only via their authorised repair centres. Sorry for your loss.:bassist:

I cannot help you with the TA503 but I do have the schematic for the LMII. Perhaps the power supplies share some commonalities that might help. PM me with a proper email address and I'll send it over to you.

Maybe in your heads there is this circuit marked...? If someone of 503 owners can open TA-503 head and look at small 8-pin circuit marker near to power transformer..? :) Many thanks!

I can attach photo what I have on mind...

It's most likely a IR21531DPBF. That's in the Little Mark II and will scale up from there. There is a 'service' manual online for the LMII but the bridge chip is not labeled. The circuit is so simple just shotgun all the semis on the power supply.

If you are making measurements on the primary side, I hope you are using an isolation transformer.

Several external things can cause the supply to have no drive pulses on the gates, even though it is good.

1) The control chip must get some power to start operating. Often this small current comes through high value resistors from the rectified high voltage DC. If those resistors are bad, or if the capacitor which filters the local power supply is leaky/shorted, the supply will not start up, because it does not get enough voltage.

2) some other signal may be controlled by a protection circuit, and is not reaching the control chip. Sometimes the internal feedback signal (often called "comp" on the data sheet) is used, by shorting the "comp" pin to ground in case of a fault. Whatever part does that may be bad.

3) The control chip may not get a "start" signal. Most chips only require voltage to start, but some also require an "enable" signal. Sometimes the "enable" is used as part of the protection circuit. If there is an "enable" required, it may not be getting a good signal. However 8 pin control chips do not usually have this.

4) bad IGBT may have damaged the gate drive. There is usually visible evidence of this, so I assume it is not the problem, but I have to mention it "for completeness".

These are just a few possible reasons for the control chip not working because of some external problem.




Yes indeed ..... +1 on that.....

But some oscilloscopes have isolated inputs...... very very very nice to use for this.

Hello! I don't believe.. After week of waiting markbass replied to me with... schematics :) But it's schematics of Little MarkII, but I hope it has similar power supply. Didn't look yet at it. Jerrold, I have good digital oscilloscope.

One thing is strange, cause after voltage filtering and DC converting I have about.. 320V (symmetrical, so 2x160+-) on big capacitors (behind the first transformer). I know, that 220V~ after Graetz bridge and capacitor gives 1.41*220=310 I think, that this transformer should make voltage lower :| so I guess it's damaged. I heard, that other electronic engineer had it exchanged already. I disconnected power supply from the rest of amplifier circuit. I will look at it today, so I will be in touch with forum if have any problems with damage identification.

Yes, I looked at pin connections and I'm almost sure it's IR21531DPBF chip. In schematics it's not marked too..

Errm OP I sent those to you not Mark Bass! :p

Yes, sorry! :) I thought that was Markbass :D