Hiwatt bulldog 30r smoking tda 2050

[frank57]

I fixed a wire on a cable assembly going to the audio board from the PCB.
Amp had an intermittent volume problem which seemed to be related to the wires being too tight and getting pulled.
Turned it on, nothing but hum and no controls working.Checked some voltages on the chip and in the front of the amp, accidentally touched pin 4 on chip and smoked it. Is it fried?Turned on today, hum and smoke right away. Also measured -15 or 16 on speaker positive.How do I check anything with the chip smoking?Audio Board has 2 bipolar electrolytic caps and two regular caps. Schematic says they're electrolytic, but they are not.

[glass54]

Frank
Your intermittent problem may have been a fractured wire on PCB end of harness. Seen it before.
Can you post the schematic?
I suspect the TDA is dead! Smoke hum and +15V on speaker gives that away 
Time for a new TDA
Regards
Mirek

[frank57]

I have the TDA2050 that was in there before, but I was getting hum and -15 on the speaker positive with it.
I think I have another one I can try other wise I have to get a real one off EBay.Here's the schematic:

[frank57]

Some voltages. i put the old chip back in and I get a big hum and nothing else. Looks like the connector cable is too loose as well.

[glass54]

Frank
The TDA2050 (and its family of devices) are usually very reliable.
The NTE7169 appears to be an exact replacement.
One very important thing to check is the integrity of C102 and C103 (47uF 63V) in Hiwatt Schematic. C101 and C104 should be fine.
As shown in the data on your other post, ST drawing shows PS decoupling caps, preferably close to the 5pin device.
The above mentioned caps are important for stability.
In my experience, the TDA20xx devices, when blown put DC on speaker line and you hear excessive hum.
Good luck
Mirek

[shooter]

How do I check anything with the chip smoking?

there's nothing left to check once you release the magic smoke 


while you await parts;
follow each legs foil to a more convenient test point, note that TP on schematic, photo, pen n ink...


had a probe in the way back days that helped when leg check'n

[BrownIsound]

there's nothing left to check once you release the magic smoke 

And I’m guessing it must have sounded like the voice of the gods right before the smoke started. 

[frank57]

Looks like I have some improvement.
The voltages are now more even, without the chip in place.
 I think the connectors were not sitting in the header properly.
I drilled 2 holes and moved the heatsink slightly to reduce the strain.
Next  Kevin o'Connor suggested I put a 100 ohm resistor across the speaker connections
before wiring it back to the speaker to see if the speaker output is zero volts.
I have a whole bunch of Chinese tda20150a , so I'll try that.
I also got a Nte7169 in reserve.

[frank57]

Also redrew the pcb with the audio chip board.

[frank57]

Put a 100 ohm resistor hooked across the terminals of the wires going to speaker.China tda2050 in place.Still getting some voltage on speaker pos.Better than before, but not zero.
-.033dc and .003 AC

[frank57]

At times the voltage on the speaker output is in the -.035 range.Other days it goes up to -3 dc.The re also appears to be a steady low hum not affected by any control.This is with the speaker hooked up.I went back to the reisistor across the terminals, because the hum started to get louder.Is it the Chinese chip or something else? Bad diode?

[frank57]

Looks like I fixed it. So the problem was the cheap China tda2050 copy. I put in one of my NTE7169 and the voltages on the speaker right away were -.000 to -.001.The AC was zero. So I think it's as good as it can get.
There's a very slight power hum when I turn it on, maybe from the reverb control, or just normal for this amp.I also redid my wire repair just to make sure.I also used some JB weld steel epoxy to clamp down a loose pad on the audio board.

The volume seems much more consistent from day to day than before. The voltages are in the 21to 22 to -21to -22 on the audio board.So far, so good!