Built up a Maplin copy lateral mosfet amp.
Powered it up monitoring output for DC with no speaker connected.
Looked at scope -10 volts DC on output so turned off quickly.
A careful visual inspection came up with nothing.
So checked all transistors were the right numbers and they were ok.
So checked every resistor and they were fine.
Looked up at scope and it was still at -10 volts dc.
Silly me hadn't zeroed it !
Powered it up monitoring output for DC with no speaker connected.
Looked at scope -10 volts DC on output so turned off quickly.
A careful visual inspection came up with nothing.
So checked all transistors were the right numbers and they were ok.
So checked every resistor and they were fine.
Looked up at scope and it was still at -10 volts dc.
Silly me hadn't zeroed it !
I had setup bias current using multi meter on 200 mA range
Amp sounded fine so removed multi meter from in series with B+.
Amp got red hot very quickly.
Measured bias voltage and it was 3 volts which is way too high.
Couldnt work out why, it got hot but was ok with multi meter in series.
Had a look at schematic and I have wired pot to be min resistance fully clockwise instead of fully anti clockwise as marked on pcb.
So turned pot fully clockwise and amp no longer gets hot.
The multi meter must have been putting a resistor in series with B+ to measure current and so limiting current.
Amp sounded fine so removed multi meter from in series with B+.
Amp got red hot very quickly.
Measured bias voltage and it was 3 volts which is way too high.
Couldnt work out why, it got hot but was ok with multi meter in series.
Had a look at schematic and I have wired pot to be min resistance fully clockwise instead of fully anti clockwise as marked on pcb.
So turned pot fully clockwise and amp no longer gets hot.
The multi meter must have been putting a resistor in series with B+ to measure current and so limiting current.
The last comment reminded me, that when it works all good right away.
You might have hidden gremlins that will show up lately.
The best engineering law is Murphy's: "If anything can go wrong, it will".
You might have hidden gremlins that will show up lately.
The best engineering law is Murphy's: "If anything can go wrong, it will".
I sell quite a lot of systems on pcb's on ebay so I build a lot.
Seen all sorts of problems with new builds.
Had a amplifier that worked on and off, if I twisted the pcb it would stop working.
I had completely missed soldering a resistor leg.
I had a valve amp red plate on one valve. I had completely missed out inserting grid stopper resistor.
Best one was a SMPS before I really understood what i was doing. That had exploding mosfets. I resorted to hiding behind a chair when I turned it on.
That fault was due to badly wound transformer not being properly bifilar on the secondaries and teh transformer drew a lot of current.
Seen all sorts of problems with new builds.
Had a amplifier that worked on and off, if I twisted the pcb it would stop working.
I had completely missed soldering a resistor leg.
I had a valve amp red plate on one valve. I had completely missed out inserting grid stopper resistor.
Best one was a SMPS before I really understood what i was doing. That had exploding mosfets. I resorted to hiding behind a chair when I turned it on.
That fault was due to badly wound transformer not being properly bifilar on the secondaries and teh transformer drew a lot of current.
I have found you have to be a bit of a detective with fault finding.
Also if something goes wrong once dont ignore it as it will likely happen again.
The bias voltage problem also showed itself as the bias current being fairly flat across the range of the pot. The DMM current resistor was flattening out the current curve.
Also, dont give up after fixing first fault as sometimes one component blowing can take others with it.
I have found with power amps removing outputs and taking VAS stage back into ltp usually gives something that doesnt blow fuses. You can then check bias voltage etc.
Also if something goes wrong once dont ignore it as it will likely happen again.
The bias voltage problem also showed itself as the bias current being fairly flat across the range of the pot. The DMM current resistor was flattening out the current curve.
Also, dont give up after fixing first fault as sometimes one component blowing can take others with it.
I have found with power amps removing outputs and taking VAS stage back into ltp usually gives something that doesnt blow fuses. You can then check bias voltage etc.
