Hiwatt bulldog 30r Solidstate chip problems

[nick900]

The amp works off a TDA 2050 which ended up going up in smoke when I accidentally shorted it.
The problem was the sound seemed to be intermittent.
I think because the connections to the audio board were being strained.
There was also -15 volts dc on the speaker positive and a huge hum.
Since then I put a 100 ohm resistor across the speaker terminals, put in a chinese tda2050 for now.
The voltages on the speaker seem to be very inconsistent.
I hooked the speaker back up and there was a persistent low level hum not affected by the controls. It sounded like it was getting louder so I shut it off and put the resistor back in place.
At times the voltage on the speaker seems to be in the -.030-040 dc range.
Then if I restart the amp it seems to be fluctuating closer to -1 volt dc, but I can't get a steady reading.
The voltages on the board seem to be fairly consistent.
I do have 2 nte 7169's I can use, but I don't want to blow them up.
The pcb quality as a whole is not great.
Is there an issue with the chip?

 

[Nigel Goodwin]

There's little else that could be damaged by you shorting it out and killing the chip, I would suggest you try a proper quality TDA2050, it's quite likely the Chinese ones are fakes - there are huge numbers of fake semiconductors coming out of China.

I bought ten LM35's from Banggood (just for stock) and when I came to use one I was really struggling, so I tried another - still didn't work. So I ordered some from RS - worked perfectly.

I tried all ten on one of my component testers, some were bipolar transistors, some were FET's, some were SCR's - all had presumably been removed from reject bins, had the numbers removed, and relabelled.

A friend of mine, who still does TV repairs, sees it all the time - he won't fit anything from China without testing it first.

 

[nick900]

I'll give it a go.
Could it also be related to wire 5 from the audio board to the pcb?
I had to repair it. Maybe my repair wasn't good? Although the resistance seems good when I check it.
The connectors have been a problem.
They were being pulled towards the audio board and weren't sitting right on the pcb.
There was a strange thing that was done years ago when I bought it and they did a warranty repair. They connected a wire from the cathodes of diode 5 to 7 on the pcb. I could never figure out why they did it, because the connections there were good, so I removed it a few years ago. The only other oddity is that the insulation for the black wire from the transformer going to the power receptacle looks very slightly cut.

 

[nick900]

There's little else that could be damaged by you shorting it out and killing the chip, I would suggest you try a proper quality TDA2050, it's quite likely the Chinese ones are fakes - there are huge numbers of fake semiconductors coming out of China.

I bought ten LM35's from Banggood (just for stock) and when I came to use one I was really struggling, so I tried another - still didn't work. So I ordered some from RS - worked perfectly.

I tried all ten on one of my component testers, some were bipolar transistors, some were FET's, some were SCR's - all had presumably been removed from reject bins, had the numbers removed, and relabelled.

A friend of mine, who still does TV repairs, sees it all the time - he won't fit anything from China without testing it first.

Looks like you nailed it.
I put in the NTE7169 and the voltage on the speaker was down to -.000 to-.001 and stayed there consistently. Voltages going to the audio board are around 22 and -22.

I removed the resistor and hooked up the speaker.
At first it was very noisy but this turned out to be a bad guitar cable.
So far everything is working well and the amp seems louder.
One of the problems before was the inconsistency of volume from day to day.
Hopefully that's over. My guess is because the audio wire cable going to the audio board was being stretched and not sitting well.

The only slight issue is a very low level hum from the reverb control which was there before.
When I turn up the knob it goes away, but then comes back as we go all the way up.
I suspect it has to do with the layout of the wires running from the audio board to the pcb.
They pass close to the transformer and the reverb wire on the pcb.
I don't think there's much I can do about it unless something else is causing it.