Problem with the STK465 ( B or A ... ? )

"Svein Helge" bravely wrote to "All" (10 Nov 05 18:36:56) --- on the heady topic of "Problem with the STK465 ( B or A ... ? )"

SH> From: "Svein Helge" SH> Xref: core-easynews sci.electronics.repair:348087

SH> I got a problem with the STK465.

SH> The device I tried to replace was marked STK465 with a B on. ( that SH> one is defective. )

SH> The new one is not marked with the B, just STK465. When I measure the SH> voltage at pin 4 and pin 13 with respect to ground, I got between +3.5 SH> to +4 volts.

SH> On the STK465 B I got -1.4 volts, what I did expected to measure SH> according to the internal schematics.

SH> I have tryed with 2 STK IC's, and they both get about the same SH> readings. SH> Output (pin 7 or 11) from the amp. is very distorted. It clips at the SH> negative half wave at low levels. At full power it looks like SH> crossover distortion on the oscilloscope.

SH> Is there any difference between the "old" STK465 B and the new STK465 SH> that I get from the shop today ?

SH> Anyone have some suggestions?

I think the A or B part are similar enough not be totally incompatible. (Often the main difference is more a performance issue.) The only other thing that is common between both channels in the IC, besides supplies and ground, is the ripple filter Pin 8. Perhaps a leaky electro in this external circuit is holding down the bias in the IC which results in the negative clipping even at low levels. There isn't much else except for a low -Vcc that could cause a negative clipping.

A*s*i*m*o*v

... A couple of volts below threshold.

As I wrote in my previously posting, I did put back the "partly" defective STK465 B. Then I measured the "bias" at pin 4 and pin 13 with respect to ground and the reading was -1.4 volt.

"Asimov" skrev i melding news:MSGID_1=3a167=2f133.0 snipped-for-privacy@fidonet.org...

"Svein Helge" bravely wrote to "All" (11 Nov 05 09:55:35) --- on the heady topic of "Re: Problem with the STK465 ( B or A ... ? )"

SH> From: "Svein Helge" SH> Xref: core-easynews sci.electronics.repair:348178

SH> As I wrote in my previously posting, I did put back the "partly" SH> defective STK465 B. SH> Then I measured the "bias" at pin 4 and pin 13 with respect to ground SH> and the reading was -1.4 volt.

That normal looking -1.4 volt might still be indicating the negative leg output transistor(s) is open. The +4 volts in the replacement IC may be indicating that the negative transistor is being turned off hard. I think you have a bias unbalance problem affecting both channels. How much voltage is present on the output terminals? Is it more than about +/- 10mV?

A*s*i*m*o*v

... Back when I was a boy, we carved our own ICs out of wood.

I get about 200 mV.... on then output terminals.

"Asimov" skrev i melding news:MSGID_1=3a167=2f133.0 snipped-for-privacy@fidonet.org...

"Svein Helge" bravely wrote to "All" (12 Nov 05 10:43:35) --- on the heady topic of "Re: Problem with the STK465 ( B or A ... ? )"

SH> From: "Svein Helge" SH> Xref: core-easynews sci.electronics.repair:348308

SH> I get about 200 mV.... on then output terminals.

That is a lot offset but this explains why the negative leg Q's has

+4v at the input and shut off hard. Good news is this shows your new STK might be just fine but you must now find the original problem. Test input biasing resistors values and replace input coupling electros for leakage. The problem is common to both channels so it must be a single external cause.

A*s*i*m*o*v

... A stereo system is the altar to the god of music.

I think you may be missed the important thing I did try to explain.

I think the new STK465 is not "BIAS" compatible with the old SFK465 B. ( May require different bias resistors, in my circuit they are 3k3 + 1k ) The reason for this is that when the SFK465B is put back in the same PCB as the new one, I got the following readings:

a) pin 4 and pin 13: -1.4 volts, ( what I think it should be according to the internal schematics.) b) Outputs is less than 100 mV.

Another difference is that the heatsink is NOT connected to pin 14 on the new STK465. On the old one STK465B it is... ( substrate/pin14 is connected to heatsink. That is according to the internal schematics I have.)

I can't find any information on what the difference between STK465B and STK465 is. ( That is what I am looking for ). ( The new one is STK465 . I have two of these new IC's who i giving the same readings when put in the PCB. )

I think may be the manufacturer has changed something inside the new IC ???? ( and it is not marked with any letter, just STK465)

"Asimov" news:MSGID_1=3a167=2f133.0 snipped-for-privacy@fidonet.org...

"Svein Helge" bravely wrote to "All" (12 Nov 05 18:00:55) --- on the heady topic of "Re: Problem with the STK465 ( B or A ... ? )"

SH> From: "Svein Helge" SH> Xref: core-easynews sci.electronics.repair:348345

SH> I think you may be missed the important thing I did try to explain.

SH> I think the new STK465 is not "BIAS" compatible with the old SFK465 B. SH> ( May require different bias resistors, in my circuit they are 3k3 + SH> 1k ) The reason for this is that when the SFK465B is put back in the SH> same PCB as the new one, I got the following readings:

SH> a) pin 4 and pin 13: -1.4 volts, ( what I think it should be SH> according to the internal schematics.) SH> b) Outputs is less than 100 mV.

I don't think it needs different bias resistors at all. What exactly is wrong with the original broken STK465B? It might give us a clue to what kind of damage might have been done to the bias circuitry.

SH> Another difference is that the heatsink is NOT connected to pin 14 on SH> the new STK465. SH> On the old one STK465B it is... SH> ( substrate/pin14 is connected to heatsink. That is according to SH> the internal schematics I have.)

The plain STK465 has the substrate connected to pin 14. So concerning the substrate, both parts have the same internal pin 14 connection.

SH> I can't find any information on what the difference between STK465B SH> and STK465 is. ( That is what I am looking for ). SH> ( The new one is STK465 . I have two of these new IC's who i giving SH> the same readings when put in the PCB. )

SH> I think may be the manufacturer has changed something inside the new SH> IC ???? ( and it is not marked with any letter, just STK465)

You know, (hypothetically) the letter B might be simply a case measurement that has been changed say from english inches to metric or instead the thickness or shape and it doesn't even have to be something electrical. Because both are rated at 30W +/-28V. I don't think this letter difference has anything to do with the present problem. As far as I can tell the new STK seems to be behaving according to the improper external inputs. Try checking for continuity to ground for both pin 3 and for pin 14. The only other thing I can think of is if the -Vcc is too low, maybe open rectifier.

A*s*i*m*o*v

... A couple of volts below threshold.