. So I soldered in the wire on r21. No luck so far, SIT does not seem to warm up so I measured PSU, which is ok. Then I measured only 5mV across resistor in line with negative rail (0R1 resistor). I did not change the trimpot so would be expecting 160mv. Where can I start troubleshooting? 
I would start by disconnecting the SIT and measuring resistances between the three terminals - D-S, D-G, and S-G. What do you get ? Then we could move on to the IRFP and the biasing mechanism.
Regards, Claas
Regards, Claas
disconnect SIT from circuit
it is most important part anyways, so best to start there
test it by testing method I did post before, somewhere...... everywhere
tried to find it in my PC, but no luck in this moment
it is most important part anyways, so best to start there
test it by testing method I did post before, somewhere...... everywhere
tried to find it in my PC, but no luck in this moment
Thanks Claas,
Below R channel measurements:
D-S: 1.6R (including 0.8R offset due to innacuracy of multimeter)
D-G: nil
D-G nil
I will try to find the testing method, I have seen it before😉
Below R channel measurements:
D-S: 1.6R (including 0.8R offset due to innacuracy of multimeter)
D-G: nil
D-G nil
I will try to find the testing method, I have seen it before😉
yup!
no need to move SIT outa case- use existing positive rail of amp, just disconnect it from channel pcb
no need to move SIT outa case- use existing positive rail of amp, just disconnect it from channel pcb
This is going to take a while as I need to source a proper second dmm, one just failed on me (and a 9v battery). In the meantime, is there anything else I can check with the SIT removed?
first thing first
you can check mosfet for shorts
considering that you don't need matching for that, and that every Greedy Boy owe to have handful of N and P mosfets in drawer, you can just replace it
you can check mosfet for shorts
considering that you don't need matching for that, and that every Greedy Boy owe to have handful of N and P mosfets in drawer, you can just replace it
Test setup with SLB PSU is ready. I've read the notes on the measurement circuit drawing. Just for my understanding, starting at -9 volts at gate of SIT means trimpots fully open, so lowest resistance, right?
Can I try the setup with bulb in place (on positive) just for safety sake? As I turn the pot the bulb should glow increasingly, right?
After previous errors I'd made I am slinging the bulb in where possible😅
dunno about pot twiddling orientation - can't remember so I don't care - multimeter is thing I'm trusting always, in memory never
so - without SIT drain connected, feel free to check neg value on SIT gate, ref to gnd, and set it to lowest down (as much negative it goes)
bulb in line with SIT drain will work it's job only at start of testing/setting, while Iq is still low
clever thing is to include 3A15 fuse in positive rail; when everything done - you can leave it in, or move it out
so - without SIT drain connected, feel free to check neg value on SIT gate, ref to gnd, and set it to lowest down (as much negative it goes)
bulb in line with SIT drain will work it's job only at start of testing/setting, while Iq is still low
clever thing is to include 3A15 fuse in positive rail; when everything done - you can leave it in, or move it out
good
for future - invest at least 20 Greenies in each multimeter; my fave ( ZM Cheapskate) is Uni-T brand - you're always getting more than you paid for, and they're sturdy and long lasting
extremely cheapo ones are fiddly, you can't trust them ...... with time I decided that is better not using DMM at all, than using fiddly one
for future - invest at least 20 Greenies in each multimeter; my fave ( ZM Cheapskate) is Uni-T brand - you're always getting more than you paid for, and they're sturdy and long lasting
extremely cheapo ones are fiddly, you can't trust them ...... with time I decided that is better not using DMM at all, than using fiddly one
Second SIT seems fine to, luckily,😅. Like you mentioned, they are tough. I mist admit that I am not a proper greedy boy yet, having no spare IRFP9240..
I will follow your advice and replace the MM's. These date from when I started with the ACA a few years ago, not knowing how addictive the hobby would become!
Anyway, I cant seem to trace back what I did wrong. I thought I only changed the transformers. I can try to follow you sequence of biasing again, or should I replace the IRFP9240 first?
replace IRFP, prior to any further testing/setting work .... just in case
better safe than sorry ..... it is wise not exactly hurrying when you know Nirvana awaits
if you're not exactly savvy dealing with clumsy big pins and pads of IRFP, just cut the pins and deal with one at a time, to remove them and clean pad holes (with solder sucker) in the end
they're cheap (mosfets) anyway
better safe than sorry ..... it is wise not exactly hurrying when you know Nirvana awaits
if you're not exactly savvy dealing with clumsy big pins and pads of IRFP, just cut the pins and deal with one at a time, to remove them and clean pad holes (with solder sucker) in the end
they're cheap (mosfets) anyway
Thanks again so far, will do that. It will go with my next mouser order. Indeed, I know what it sounds like and it's def worth the wait🙂
I have made good progress! Biasing up as we speak, slowly increasing to 160mv on negative rails. I did notice something strange. I shorted the input with solder wire. If the short is not proper, or if the input is not shorted, It lowers by approx. 80mv on the negative rails (to 80/90mv). If i short the input properly, it goes back to 160mv. Is this expected behaviour?
If so, how does it bias if you have the inputs not shorted after you have soldered wires to your rca connectors?
If so, how does it bias if you have the inputs not shorted after you have soldered wires to your rca connectors?
it is not expected behavior
there is no DC connection between input and output stage ( see that 10-22uF SIlmic) and input (voltage) state shouldn't influence output biasing in any way
is that the case with both channels?
it would be extremely handy that you have a scope, to check for any oscillations on output
no logic in that exactly, but Gremlins are above our Mortal Logic
though - be sure to recheck setting of input buffer - Iq (20mV across 1R) and DC Offset, rails voltage ( after cascodes)
there is no DC connection between input and output stage ( see that 10-22uF SIlmic) and input (voltage) state shouldn't influence output biasing in any way
is that the case with both channels?
it would be extremely handy that you have a scope, to check for any oscillations on output
no logic in that exactly, but Gremlins are above our Mortal Logic
though - be sure to recheck setting of input buffer - Iq (20mV across 1R) and DC Offset, rails voltage ( after cascodes)