Need a bit of help with an amp circuit please

[Markoneswift]

Hello all,

I'm working on my Sharp VZ-3000 system and currently, I'm trying to work out why I am seeing unexpected voltages at the STK463 IC pins..

I see 9.2 volts at the input pins 1 and 16 - which according to the schematic should be at 0 volts with no signal going in. I also see quite a few pins at +31 an -31 volts which seems wrong, along with 9.2 volts at pin 9.

I would like to know whether R417 being out of spec would cause these strange voltages to be present around the IC. When I measure R417 which should be a 100 ohm fusible resistor, I see resistance rapidly rise to open, then it backs down, then goes up again.

I've checked all other resistors in the circuit and they are good, but I have not replaced any caps yet - could some of the filter caps being bad mean the inputs are seeing DC when it should be filtered out ?

If you guys could please have a look at the schematic for the power amp below, I would be very appreciative of any help with this. Cheers

 

[avionic]

When I measure R417 which should be a 100 ohm fusible resistor, I see resistance rapidly rise to open, then it backs down, then goes up again.

Your measuring it in circuit. Right ? Lift one leg and remeasure.

 

[avionic]

STK is probably toast.. What are you seeing at the output pins ?

 

[HiFiThor]

How old is this unit? My experience has shown that the vast majority of problems with vintage equipment is leaky electrolytic caps. I have even taken them out of circuit and tested them, some even in tolerance but when replaced anyway the unit starts working great. Caps are cheap, just replace the electrolytics wholesale. Once done you may have additional problems such as the IC being toast which could have been caused by the leaky caps in the first place. You may be money ahead by just replacing the IC too at the same time. The resistors generally don't change much in value especially in low voltage situations. They are usually are only affected by heat.

 

[Markoneswift]

 

[Markoneswift]

The unit is mid-Eighties I believe. The STK has already been replaced by the guy who had it before me so I have the old unit and the new one in circuit. Is it possible to test the old IC pins to see if it is shorted out ?

Avionic - I will lift a leg of R417 and check again.

 

[Markoneswift]

STK is probably toast.. What are you seeing at the output pins ?

What are the chances of the original STK and the replacement STK both being toast ? I read full rail voltage (same as at pin 9, +32v) on both outputs ( pins 6/7 ( -32v) and 10/11 (-32v) which is the same issues which lead the previous owner to replace the IC. Could DC getting into the IC through the inputs cause it to be on the outputs also ?

Hope someone can help me make sense of this, thanks for the help

 

[avionic]

What are you getting on pin 5 ?

 

[Markoneswift]

What are you getting on pin 5 ?

-32.8v on pin 5

 

[Markoneswift]

I have the old STK in front of me and none of the pins seem shorted in either direction using diode test on my DMM. I cannot get any continuity between pin 9 > 6/7 or 9>10/11 at all. Here's the STK461 schemtic from the SM -

 

[stereoguy70]

What are the chances of the original STK and the replacement STK both being toast ? I read full rail voltage (same as at pin 9, +32v) on both outputs ( pins 6/7 ( -32v) and 10/11 (-32v) which is the same issues which lead the previous owner to replace the IC. Could DC getting into the IC through the inputs cause it to be on the outputs also ?

Hope someone can help me make sense of this, thanks for the help

If there was an issue further back in the stage, the odds that both STK modules being toast is a distinct possibility.

 

[Markoneswift]

If there was an issue further back in the stage, the odds that both STK modules being toast is a distinct possibility.

I'm not sure how much further back the issue could be - the amp board is conmnected to the power supply (which is tested and good) and there is nothing else connected to the amp board other than the IC and speaker outputs.

 

[avionic]

I'm not sure how much further back the issue could be - the amp board is conmnected to the power supply (which is tested and good) and there is nothing else connected to the amp board other than the IC and speaker outputs.

What about the input connector and circuit ?

 

[stereoguy70]

I'm not sure how much further back the issue could be - the amp board is conmnected to the power supply (which is tested and good) and there is nothing else connected to the amp board other than the IC and speaker outputs.

Bad cap, bad resistor, something along those lines causing the STK to short out. I noticed Q401 & Q402 in that truncated schematic you posted. Maybe one of those is shorted? What about the preamp stage? Lots of variables that need to be checked......

 

[stereoguy70]

View attachment 894072 Hello all,

I'm working on my Sharp VZ-3000 system and currently, I'm trying to work out why I am seeing unexpected voltages at the STK463 IC pins..

I see 9.2 volts at the input pins 1 and 16 - which according to the schematic should be at 0 volts with no signal going in. I also see quite a few pins at +31 an -31 volts which seems wrong, along with 9.2 volts at pin 9.

I would like to know whether R417 being out of spec would cause these strange voltages to be present around the IC. When I measure R417 which should be a 100 ohm fusible resistor, I see resistance rapidly rise to open, then it backs down, then goes up again.

I've checked all other resistors in the circuit and they are good, but I have not replaced any caps yet - could some of the filter caps being bad mean the inputs are seeing DC when it should be filtered out ?

If you guys could please have a look at the schematic for the power amp below, I would be very appreciative of any help with this. Cheers

I think I see what the problem may be.........the schematic calls out for a STK461. You stated that what came out was an STK463. They may not be 100% interchangeable......

 

[Markoneswift]

What about the input connector and circuit ?

No inputs connected - purely trying to get a baseline for the amp so there's nothing going in at all. I do get exactly the same problem WITH the input from the preamp attached though.

 

[Markoneswift]

I think I see what the problem may be.........the schematic calls out for a STK461. You stated that what came out was an STK463. They may not be 100% interchangeable......

The previous module was also a 463, but from what I read they are pin compatible and the 463 is just a slightly higher output.

 

[Markoneswift]

Your measuring it in circuit. Right ? Lift one leg and remeasure.

I removed it completely and it measure totally open through all resistance ranges on my DMM, even though it should only be 100 ohm. My question would be, given its location in the circuit, could this be the source of my issue or is it just another part of a bigger issue....Hmmm....

 

[Markoneswift]

It also seems har to find a local supplier of fusible resistors - is there an alternative I could use ? We have Jaycar near by but they don't seem to stock them.

 

[Markoneswift]

I'm wondering whether I could pop in a 100 ohm 1/2 watt metal film resistor instead of the 1/4 watt ceramic that was there ? Alternatively, would a standard 100 ohm resistor work just as well ?