On 07/01/2021 18:11, pinnerite wrote:
On Wed, 6 Jan 2021 20:37:27 +0000
Fredxx wrote:

On 06/01/2021 20:16, pinnerite wrote:
Ten years ago I bult a low-noise pre-amp based on a 1970's article in Elektor magazine.

It used a TBA231 dual op-amp IC.

It didn't work but I did not have time to test all the copper runs etc before I wa over-whelmed with 'work'.

This week I took it out and started fault-finding.There was one open connection, poorly soldered. I then checked the power links throughout the board and they were fine.

Next i connected in a little waveform generator to the input and one of those timy LCD screened chinese oscilloscopes to the output. Nothing.

So with no shortcircuits, I concluded that the capacitors were OK whch only leaves the IC.

I do not have another TBA231 or equyivalent but have no idea how to test the chip.

I am a bit out of date with electronics since my heydays

So any suggestions will be welcome.

I suggest you measure all the voltages on each pin and come back here
for analysis.

For Info the datasheet:
http://www.bg-electronics.de/datenbl...ise/TBA231.pdf

Although the example test circuit doesn't include the use of Output Lag.
This circuit does:

https://www.vinylengine.com/turntabl...719d&mode=view


Thank you. That focussed the one or two brain cells that I have left.

The DC voltages on the chip with both no external connections (audio oscillator and oscilloscope) were virtually the same:

Pins Voltage Pins Voltage
1 14.3 14 23.6
2 22.5 13 0.43
3 21.0 12 22.9
4 21.1 11 21.0
5 0 10 21.0
6 0 9 0.9-1.6
7 0 8 0.9

Thanks too for the links. I downloaded the spec sheet.

OK. Looks like the frequency compensation is plausible.
Assuming that the thing is being used as a stereo amp the two channels
should be very similar on corresponding nodes and they are not.

1-6 . Output is hard against the positive rail
inputs are equal voltage and zero which is plausible. In many OPAMP
designs there is a virtual earth at the feedback summing junction.

8-13. Output is close to OV but not exactly
inputs are somewhat away from OV but similar?
Pin 9 seems to be flapping around so odd that the output doesn't.

You have (at least) two independent faults one in each channel and they
are different. My suggestion would be make yourself a simple
multivibrator signal injector and apply to the input. A wet finger and a
crystal earpiece might do if any of the gain stages are working.

Big questions a

Why is output pin 1 so high
Why isn't output pin 13 closer to 0V
Why are input pins 8 & 9 not nearer 0V.

I think it must be the components around the chip or a solder bridge
somewhere that is preventing it from working. May have damaged the chip
itself if you are unlucky.

--
Regards,
Martin Brown