STK chip amp troubleshooting help

[gadget73]

I'm working on a Mitsubishi DA-R35 that has no output. As I got it, it came out of protection, but made no sound. Preamp is fine, and gets signal to the amp board with no trouble. I chased the signal with the Oscope and it got as far as the STK3076 voltage amp and stopped. I had no signal output from it, and of course no signal input to the STK1080ii power amps. Voltages seemed to make sense, so I figured the voltage amp was dead. I ordered a replacement STK3076 from Audio Labs of GA that was marked as a "reclaimed" part. I assume this means used working pull, but that may be my mistake.

New part gets here, and its obviously never been installed. The printing looks different than the original, the numbers are slightly off-sized and not spaced evenly. Pins are round instead of flat. Not sure if this marks it as a fake or not.

I installed it, and got some smoke from a resistor in the power supply, and the amp will not come out of protection. Remove the part, and it comes out of protection again, but of course no output. Below are the voltages measured at the voltage amp socket with the 3076 removed. All voltages ref to chassis ground.

1 0v
2 1.1v
3 0v
4 0v
5 0v
6 51.7v
7 -55.1v
8 0v (ground)
9 27.2v
10 -27.0
11 -51.8
12 0v
13 -52.3v
14 1.1v
15 0v

as far as I can tell from datasheets, pin 7 should be the - rail, and pin 9 the + rail. Should those not be the same voltage, but opposite polarity or am I totally missing something here? Solid state amps generally baffle me so any help is much appreciated.


http://pdf1.alldatasheet.com/datasheet-pdf/view/89811/SANYO/STK3152.html

datasheet is for the STK3152, but I'm told thats a cross-reference for the 3076. I cannot find a datasheet specifically for the 3076.

 

[elnaldo]

The STK pins in the datasheet are symmetrical (pin 1=15, 2=14, 3=13, and so on) , so I guess you should have symmetrical voltages at the PCB. Your measured voltages look far from symmetrical. I'd check backwards. To me, something is shorted before that point, and those high voltages are burning your IC.

Pins 7 and 9 are Vcc + and -, pins 1 and 15 are signal + input (R and L), pins 2 and 14 are signal"-" input, and pins 5-6 are L output, and pins 10-11 are R output, so voltages there should be the same for R and L

This is a sample circuit showing the symmetrical connections:
http://html.alldatasheet.com/html-pdf/89811/SANYO/STK3152/388/3/STK3152.html

Aren't those voltages written in the Mitsubishi schematic?

Also, if you can't find the exact replacement, you could check your circuit and replace it with another one of the same series. I recently did that, replacing an STK439 with an STK441. Mine looks as a fake copy too, but it works. I think you need some technology to build those IC, even a fake one.

 

[petehall347]

could you measure voltages with chip removed ?
its looking like a shorted chip to me but might not be ,

 

[ecluser]

First of all, the STK-3076 as nothing to do with the STK-3152. They don't have the same pin configuration.

I don't have the service manual for the Mitsubishi DA-R35, but the one for the DA-R25 is on hifiengine and they use the same voltage amp STK-3076. You will find the internal schematic of the STK-3076 on page 19 of the DA-R25 manual. Please, take note that there is an error on that schematic. Pin 6 is NOT connected to R1, R2, R7... as shown on the schematic, but is connected to D1, R11.... Pin 6 is the high DC positive voltage, so your measurement on pin 6 is good. Pin 9 is connected to a +24V zener, and pin 10 is connected to a -24V zener, on the DA-R25 schematic. It seems that you have 27V zeners on your DA-R35.

Do you have the schematic for the DA-R35? If you don't it would be a good starting point to use the one for the DA-R25.

If you want to debug your amp it would be better to remove the power amp modules (STK-1080ii) and re-install the STK-3076. You will need to add some components between the STK-3076 and the output of the amp, for each channel, in order to close the feedback loop. I can assist you if you are ready to do this. By chance you have a scope, it will help.

 

[EchoWars]

The pins of 'reclaimed' devices are often round because they have been solder-dipped prior to being resold.

 

[gadget73]

First of all, the STK-3076 as nothing to do with the STK-3152. They don't have the same pin configuration.

I don't have the service manual for the Mitsubishi DA-R35, but the one for the DA-R25 is on hifiengine and they use the same voltage amp STK-3076. You will find the internal schematic of the STK-3076 on page 19 of the DA-R25 manual. Please, take note that there is an error on that schematic. Pin 6 is NOT connected to R1, R2, R7... as shown on the schematic, but is connected to D1, R11.... Pin 6 is the high DC positive voltage, so your measurement on pin 6 is good. Pin 9 is connected to a +24V zener, and pin 10 is connected to a -24V zener, on the DA-R25 schematic. It seems that you have 27V zeners on your DA-R35.

Do you have the schematic for the DA-R35? If you don't it would be a good starting point to use the one for the DA-R25.

If you want to debug your amp it would be better to remove the power amp modules (STK-1080ii) and re-install the STK-3076. You will need to add some components between the STK-3076 and the output of the amp, for each channel, in order to close the feedback loop. I can assist you if you are ready to do this. By chance you have a scope, it will help.

Hm, well I guess that was some bad info then. Someone told me the 3076 and 3152 were the same thing.

I do not have the schematic for the DA-R35. I looked for one, but had no luck. I'll check out the DA-R25 to see how close it is. It may be close enough to at least give me some reference if nothing else.

I do have a scope. Thats actually how I found that the original unit was not passing any signal through the 3076. I just hope I haven't killed the replacement.


EW: when I say round, I mean round all the way into the epoxy at the bottom of the package. It looks like solid wire. The original was flat exiting the epoxy, tapering down to an approximately square profile pin like most things seem to use. I can get pictures, but there are lots of cosmetic details that are not the same between these parts.