Positive and negative rails switched?

[Bert Beukema]

Hi guys,

today on my bench is an otherwise not very interesting amplifier, the Sony AX410. It's part of a system (early 80's) and it's owner wants it repaired. It was easy to see that one of the STK modules was broken, so far so good.

There is however a much stranger thing that i've actually not seen before. On this machine, the negative and positive rails seem to have switched. So, everywhere a + voltage is supposed to be, you will get a - voltage. And the other way round. This affects the entire main board. Oddly enough, the voltages are correct, just not the right polarity.

I've done some preliminary testing on the machine but the usual suspects all check out fine. So before I spent hours playing unpaid Sherlock Holmes i'd like to hear some thoughts from you guys.

Thanks!

Bert

 

[petehall347]

check your meter .and be sure negative probe to chassis .

 

[mbz]

OK, you've got my interest but I'm something of a newb, probably better wait for more sensible suggestions.

A grounding issue/shift in the ground would not cause it, eg, add + "rails voltage" to everything give one rail at twice normal, the other at zero and GND in between.

Connecting the main rectifier diodes wrong way around was my second guess however I would expect that would upset the main filter caps since the voltages
are reversed.

 

[mbz]

yep, pete makes sense again...

 

[petehall347]

if they were really switched over there would be some smoke for sure .

 

[Bert Beukema]

Exactly, smoke and boom would happen. The problem originated from my measuring system, it had to be recalibrated and was behaving intermittently. Oddly enough that system had been recalibrated last week as part of the general maintenance so i'll schedule it for a health test.

 

[Bert Beukema]

Ps: if you're working on these 'newish' amps don't bother with the STK modules. You can find them but they are uniformly crap. In this machine I will install two complete new TDA modules. It will also make the amplifier sound better. I will add an additional heatsink layer because the heatsinks generally are part of the problem of why these STK modules fail in the first place: overheating.

 

[gadget73]

TDA module ? I have one piece of gear that runs an STK module that has gotten nearly impossible to find. A practical alternative appeals to me but space and cost are a consideration.

 

[Bert Beukema]

There are many TDA boards, most popular is the TDA 7293. You can also try LM4702. These boards have a complete solution but to incorporate them you would need the schematics and figure some changes out. Usually they are small (like removing resistors that are no longer necessary) but it really depends on the amplifier / receiver. In this case, the Sony already has the STK modules on separate boards. We can simply remove those and install a new TDA board & rewire.

 

[gadget73]

Oh, so its not a pin and package compatible drop-in? Won't work in the goofy amp I have unfortunately. Its a car amp, and the STK modules sit under the board. No room to fit anything else in the case unfortunately. The other option is just to replace the amp with something else entirely, and since that can be done for reasonable cost its honestly not worth spending a large amount of effort or money on.

 

[audiotemp]

On this machine, the negative and positive rails seem to have switched.

Switched compared to what? Schematics are sometimes wrong.

 

[Northwinds]

Could be the Earth's magnetic poles flipped and f'ed the amp up

 

[Bert Beukema]

No, it's just that my equipment had a calibration error. Sometimes you trust your gear too much, especially when it's been calibrated and serviced just a few days back.
Hence my puzzlement seeing that on all accounts this shouldn't be possible. Or that indeed the schema was wrong, which it wasn't.
Lesson learned: always triple check your equipment ;-).

Switched compared to what? Schematics are sometimes wrong.

 

[Bert Beukema]

Nope. That would be impossible. These boards give you an entire power amplifier. They only need power input and signal input plus some wiring for ground etc. It depends on the quality of the board and the options you're wanting.
Car amplifiers are notorious for being built to fail, especially with these STK modules in them. The boards i'm using won't fit in those flat amplifiers, they need some space.

That being said, I think it is possible to find TDA boards where you can bend the IC from standing upwards to lying down. It won't matter for the performance. Some years back I used one cheap mono TDA board that was only 4 x 4 centimeters wide and roughly 2.5cm high. I used this board with a simple heatsink in such a flat orientation to create a very simple package I could drop into a small speaker case. That thing is just an audio test device for really quick and dirty things, but it works fine (but only 5w).

Oh, so its not a pin and package compatible drop-in? Won't work in the goofy amp I have unfortunately. Its a car amp, and the STK modules sit under the board. No room to fit anything else in the case unfortunately. The other option is just to replace the amp with something else entirely, and since that can be done for reasonable cost its honestly not worth spending a large amount of effort or money on.

 

[Hyperion]

Rails swapped! - that's a hell of a 'calibration error' if you don't mind me saying.

 

[Bert Beukema]

No I don't mind, it's an old system but up to now reliable just as long as it's serviced as any old equipment should be. Hence the puzzlement. Hence the slap your face moment when you realize that it is impossible unless the schema is wrong (it wasn't) or your measuring system is wonky (it was). Internal routing and some wiring seems to be the problem but i've replaced it for now with a new machine.

 

[Lavane]

No I don't mind, it's an old system but up to now reliable just as long as it's serviced as any old ediscretet should be. Hence the puzzlement. Hence the slap your face moment when you realize that it is impossible unless the schema is wrong (it wasn't) or your measuring system is wonky (it was). Internal routing and some wiring seems to be the problem but i've replaced it for now with a new machine.

Do post some pictures of the TDA's your going to use and pictures of them mounted. Some Pioneer owners with blown STK's have been building new boards with TO-3 type outputs. If the TDA's could fit without much modding that would be pretty cool. I have a nice working SX-780 with working original STK's. The TDA's may be a cheaper and easier insall if they were to fail.

 

[petehall347]

Do post some pictures of the TDA's your going to use and pictures of them mounted. Some Pioneer owners with blown STK's have been building new boards with TO-3 type outputs. If the TDA's could fit without much modding that would be pretty cool. I have a nice working SX-780 with working original STK's. The TDA's may be a cheaper and easier insall if they were to fail.

TO-3P is more likely ...

 

[Lavane]

Looking at some of these boards they come in stereo, mono, DC and AC powered and lots of different packaging. I could see finding something that could be made to fit in older STK powered amps. A few I saw had pretty wide ranges of power output depending on the operating voltages used.

 

[Bert Beukema]

Anything based on TO-3P would mean lots of work unless you have a prefab pcb. There are literally hundreds of boards available using TDA, LM, MOT etc. The difference is that you can simply buy the package (either assembled or a DIY board) that fits what you need for a single iteration. Replacing an STK with another IC is almost always going to be easier seeing that most STK implementations are endpoints. TDA & LM IC based boards generally follow the same approach as an STK and can be mounted on the heatsink in generally the same way. And most amplifiers / receivers using an STK will follow the basic implementation sample shown in datasheets of the corresponding module. Remember, these are all-in-one packages made for easy implementation and assembly.

You can find many samples online and I know there are many videos on Youtube showing implementation. Remember, although generally easy it is not 'plug and play'. You do need to understand what you are working with. You do need to understand the machine you are altering before you start making those alterations. Either find a datasheet or analyze the main board, voltages and circuit.