Power Amp IC Replacement on Sharp VZ-3000 - Page 7 - UK Vintage Radio Repair and Restoration Discussion Forum

BillyTheKid · 4th Feb 2017, 9:47 pm

Thanks Clyde.
I got a DIY scope which took about 5 hours to make.
Also picked up a signal generator which also seems to work.
Here's both working together:

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I put back together the amp after testing everything on the board. I put the signal generator on the left input pin. For some reason I tried to tap the whole in to pin 10 & 11 together to see the output on that channel. Didn't read anything. I was then going to try a reading from the speaker out and accidentally shorted pin 9 and 10 with some pretty catastrophic results!!! The resistor r417 went up in flames! I totally understand why the thing should be disabled now

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The hum came back, I've re replaced the resistor but still humming. Only on one channel this time. If the stk wasn't damaged in pretty sure it is now!

I'll test a few things and start again soon. Ive ordered a replacement modern Ic.

There seems to be two power inputs on the original and only one on the new device. Do you guys reckon I can use just over of them?

Thanks
Billy

Lloyd 1985 · 5th Feb 2017, 12:22 am

Oh no... I did exactly the same thing yesterday! Been trying to repair the 12V charger for my Dyson, and slipped on checking voltages on the PWM controller chip, shorted the output pin to ground, and killed it instantly... It looks like you shorted 25V to the output of the right channel, so that'll have toasted that! Do you still have the original STK? It might be worth trying it again, as you changed it before you found the open circuit resistor. And, all that smoke damage is why it should be a flameproof resistor!

I like your little scope!

The real question though, did you get any output from your signal injection before you blew it to bits?!

I did think of something else that could have been checked before, there are 2 transistors connected between ground and each input, Q401 and Q402, they look to me like some sort of muting arrangement, and I wonder if they could have been responsible for the silence? Well, maybe not the transistors themselves, but whatever is biasing them, could possibly be holding them in 'mute'. From the circuit diagram it looks as though both the +25v and -25v are connected via R416, R419, R420, R421 and PTH401, the thermistor that should be bolted to the heatsink. Check to make sure all of that is in order.

Regards,
Lloyd.

Clydeuk · 5th Feb 2017, 12:38 am

As mentioned, if it were me I would put the original STK back in and replace R417 then see if you get any results. Without reading back I'm sure you mentioned early in the thread that you had sound output over the hum with the original i.c.

I think it's worth a try before you go the route of substituting the whole amplifier, which may throw up other issues like suitable power, how to mount it, etc. Of course this assumes you kept the old device and it is intact. I never throw away undamaged removed components until I know they are faulty, confirmed by successful replacement.

Clydeuk · 5th Feb 2017, 12:39 am

Oh and I'm sure we've all had an accident with probes. Only last month I blew a PSU in an LCD TV when the probe slipped.

BillyTheKid · 5th Feb 2017, 1:02 pm

Hi guys,
I thought I chucked out the old IC but found it yesterday. The problem here is that in removing it I clipped the pins and bent the crap out of them. Id need to clip the pins on the current IC and solder what's left of both sets of pins together. Many points if failure in that but I've nothing to lose as I'm pretty sure the new IC is a gonner.

I'll check out the transistors to see if they're muting. I rechecked the input cables last night with my ground to the chassis and red probe of the in put pins. Each of the three read as 13 volts and not connected to the pcb.. Bit strange but maybe normal?

Thanks !

Lloyd 1985 · 5th Feb 2017, 1:30 pm

That sounds a bit odd, I'd have expected pin 2 of CNP401 - CNS401 to read 0V as it is ground! And if the transistors were stuck muting then pins 1 and 3 would also be pretty much 0V too. Check that again with your black probe to pin 2 of CNP401, should be reading close to 0V.

Regards,
Lloyd.

WaveyDipole · 5th Feb 2017, 7:06 pm

I didn't at first understand why a short between pin 9/10 would have caused R417 to blow with such ferocity, but looking at the photo, pin 1 is on the right and counting from the double line, pins 7,8 and 9 are charred, so I'm wondering whether, in fact, 7 and 8 were shorted? That would explain the current surge across R417. Of course, the end result is very unfortunate!

Looking at the diagram of the STK461 in the spec sheet, a short between 9/10 or 7/8 would have put the collector and emitter of the output transistors at the same potential. The net result - no current. However, 6/7 and 10/11 are co-joined and +25v into either 6 or 11 seems rather likely to cause damage to the IC. On the other hand, the speaker would have formed a low resistance path to ground so the voltage would not have risen significantly unless the protection fuse (either F401 or F402) or the speaker also blew in which case 6/7 and 10/11 immediately rise to +25v. All this would have happened in a split second of course and its a matter of chance as to what has blown and what has survived.

Since the resistor has been replaced and the hum persists, it would seem that the IC has failed in some way, but I would check those fuses and the speakers as well.

BillyTheKid · 6th Feb 2017, 6:26 pm

Quote:

Originally Posted by Lloyd 1985

That sounds a bit odd, I'd have expected pin 2 of CNP401 - CNS401 to read 0V as it is ground! And if the transistors were stuck muting then pins 1 and 3 would also be pretty much 0V too. Check that again with your black probe to pin 2 of CNP401, should be reading close to 0V.

So I remeasured all pins on CNP401 and now see 0. Think the latest DMM I have is also a bit of a dud.

Tonight, I'm going to tackle putting back on the old IC with lots n lots of solder and

... Not sure how long the thing will last as it's pretty mangled!. I'd love to get this thing working again. I also ready do need to off load the other 2 VZ 3500s I have.

dseymo1 · 6th Feb 2017, 6:43 pm

Those kit scopes are good value and look fun to build, but the bandwidth is very limited. If the amp is suffering from ultrasonic oscillation, there's a good chance that you won't see it.

BillyTheKid · 7th Feb 2017, 2:10 pm

Hi Guys, I tried my best to get the old IC back onto the board but just couldn't join the gaps and mangled pins. Maybe I'll make up some wires and joing them with wires instead.

Refugee · 7th Feb 2017, 2:26 pm

Flexible wires will help stop it getting dry joints in the long run.

Clydeuk · 7th Feb 2017, 10:51 pm

Just use individual wires, it should work fine if you have enough pin to solder to. It won't look pretty but that's not important, you can always replace with a new IC later if you feel that way inclined.

I recommend soldering one wire at a time, pin 1 on board/pin 1 on IC....etc. That way it avoids any errors.

4th Feb 2017, 9:47 pm	#121
BillyTheKid Retired Dormant Member Join Date: Jan 2017 Location: Tooting Broadway, London, UK. Posts: 43	Re: Power Amp IC Replacement on Sharp VZ-3000 Thanks Clyde. I got a DIY scope which took about 5 hours to make. Also picked up a signal generator which also seems to work. Here's both working together: I put back together the amp after testing everything on the board. I put the signal generator on the left input pin. For some reason I tried to tap the whole in to pin 10 & 11 together to see the output on that channel. Didn't read anything. I was then going to try a reading from the speaker out and accidentally shorted pin 9 and 10 with some pretty catastrophic results!!! The resistor r417 went up in flames! I totally understand why the thing should be disabled now The hum came back, I've re replaced the resistor but still humming. Only on one channel this time. If the stk wasn't damaged in pretty sure it is now! I'll test a few things and start again soon. Ive ordered a replacement modern Ic. There seems to be two power inputs on the original and only one on the new device. Do you guys reckon I can use just over of them? Thanks Billy

5th Feb 2017, 12:22 am	#122
Lloyd 1985 Nonode Join Date: Jun 2008 Location: Coningsby, Lincolnshire, UK. Posts: 2,819	Re: Power Amp IC Replacement on Sharp VZ-3000 Oh no... I did exactly the same thing yesterday! Been trying to repair the 12V charger for my Dyson, and slipped on checking voltages on the PWM controller chip, shorted the output pin to ground, and killed it instantly... It looks like you shorted 25V to the output of the right channel, so that'll have toasted that! Do you still have the original STK? It might be worth trying it again, as you changed it before you found the open circuit resistor. And, all that smoke damage is why it should be a flameproof resistor! I like your little scope! The real question though, did you get any output from your signal injection before you blew it to bits?! I did think of something else that could have been checked before, there are 2 transistors connected between ground and each input, Q401 and Q402, they look to me like some sort of muting arrangement, and I wonder if they could have been responsible for the silence? Well, maybe not the transistors themselves, but whatever is biasing them, could possibly be holding them in 'mute'. From the circuit diagram it looks as though both the +25v and -25v are connected via R416, R419, R420, R421 and PTH401, the thermistor that should be bolted to the heatsink. Check to make sure all of that is in order. Regards, Lloyd.

5th Feb 2017, 12:38 am	#123
Clydeuk Hexode Join Date: Mar 2012 Location: Liverpool, Merseyside, UK. Posts: 453	Re: Power Amp IC Replacement on Sharp VZ-3000 As mentioned, if it were me I would put the original STK back in and replace R417 then see if you get any results. Without reading back I'm sure you mentioned early in the thread that you had sound output over the hum with the original i.c. I think it's worth a try before you go the route of substituting the whole amplifier, which may throw up other issues like suitable power, how to mount it, etc. Of course this assumes you kept the old device and it is intact. I never throw away undamaged removed components until I know they are faulty, confirmed by successful replacement.

5th Feb 2017, 12:39 am	#124
Clydeuk Hexode Join Date: Mar 2012 Location: Liverpool, Merseyside, UK. Posts: 453	Re: Power Amp IC Replacement on Sharp VZ-3000 Oh and I'm sure we've all had an accident with probes. Only last month I blew a PSU in an LCD TV when the probe slipped.

5th Feb 2017, 1:02 pm	#125
BillyTheKid Retired Dormant Member Join Date: Jan 2017 Location: Tooting Broadway, London, UK. Posts: 43	Re: Power Amp IC Replacement on Sharp VZ-3000 Hi guys, I thought I chucked out the old IC but found it yesterday. The problem here is that in removing it I clipped the pins and bent the crap out of them. Id need to clip the pins on the current IC and solder what's left of both sets of pins together. Many points if failure in that but I've nothing to lose as I'm pretty sure the new IC is a gonner. I'll check out the transistors to see if they're muting. I rechecked the input cables last night with my ground to the chassis and red probe of the in put pins. Each of the three read as 13 volts and not connected to the pcb.. Bit strange but maybe normal? Thanks !

5th Feb 2017, 1:30 pm	#126
Lloyd 1985 Nonode Join Date: Jun 2008 Location: Coningsby, Lincolnshire, UK. Posts: 2,819	Re: Power Amp IC Replacement on Sharp VZ-3000 That sounds a bit odd, I'd have expected pin 2 of CNP401 - CNS401 to read 0V as it is ground! And if the transistors were stuck muting then pins 1 and 3 would also be pretty much 0V too. Check that again with your black probe to pin 2 of CNP401, should be reading close to 0V. Regards, Lloyd.

5th Feb 2017, 7:06 pm	#127
WaveyDipole Nonode Join Date: Mar 2013 Location: Leicester, UK Posts: 2,535	Re: Power Amp IC Replacement on Sharp VZ-3000 I didn't at first understand why a short between pin 9/10 would have caused R417 to blow with such ferocity, but looking at the photo, pin 1 is on the right and counting from the double line, pins 7,8 and 9 are charred, so I'm wondering whether, in fact, 7 and 8 were shorted? That would explain the current surge across R417. Of course, the end result is very unfortunate! Looking at the diagram of the STK461 in the spec sheet, a short between 9/10 or 7/8 would have put the collector and emitter of the output transistors at the same potential. The net result - no current. However, 6/7 and 10/11 are co-joined and +25v into either 6 or 11 seems rather likely to cause damage to the IC. On the other hand, the speaker would have formed a low resistance path to ground so the voltage would not have risen significantly unless the protection fuse (either F401 or F402) or the speaker also blew in which case 6/7 and 10/11 immediately rise to +25v. All this would have happened in a split second of course and its a matter of chance as to what has blown and what has survived. Since the resistor has been replaced and the hum persists, it would seem that the IC has failed in some way, but I would check those fuses and the speakers as well.

6th Feb 2017, 6:43 pm	#129
dseymo1 Retired Dormant Member Join Date: Dec 2007 Location: Shropshire, UK. Posts: 3,051	Re: Power Amp IC Replacement on Sharp VZ-3000 Those kit scopes are good value and look fun to build, but the bandwidth is very limited. If the amp is suffering from ultrasonic oscillation, there's a good chance that you won't see it.

7th Feb 2017, 2:10 pm	#130
BillyTheKid Retired Dormant Member Join Date: Jan 2017 Location: Tooting Broadway, London, UK. Posts: 43	Re: Power Amp IC Replacement on Sharp VZ-3000 Hi Guys, I tried my best to get the old IC back onto the board but just couldn't join the gaps and mangled pins. Maybe I'll make up some wires and joing them with wires instead.

7th Feb 2017, 2:26 pm	#131
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: Power Amp IC Replacement on Sharp VZ-3000 Flexible wires will help stop it getting dry joints in the long run.

7th Feb 2017, 10:51 pm	#132
Clydeuk Hexode Join Date: Mar 2012 Location: Liverpool, Merseyside, UK. Posts: 453	Re: Power Amp IC Replacement on Sharp VZ-3000 Just use individual wires, it should work fine if you have enough pin to solder to. It won't look pretty but that's not important, you can always replace with a new IC later if you feel that way inclined. I recommend soldering one wire at a time, pin 1 on board/pin 1 on IC....etc. That way it avoids any errors.