I just did a stupid mistake performing the functional test of the 2nd mono block. I will try to explain in details what I think caused it. As you can see from the picture the 15R GBR resistor is burned (it measures about 300R now if I measure in-circuit).
I used a battery driven 1000 Hz generator which has two outputs (RCA). One went to the input of amp and the other to the scope. Speaker of amp was connected to a 8R load resistor. 2nd scope channel connected to load resistor (if I just did it as with 1st mono block everything had been perfect). But as you know a scope has common ground for the two channels and I am quite sure that LS+ got connected to +IN via the common ground. The 8R load resistor got very hot and after a while a bad smell. And I did not see any signal from generator. Changing the 15R GBR is relative easy from top but what other damage could we expect?
I am thinking a bit on the input JFET. If it gets 24V at +IN?
Also if my theory is correct then we can expect a voltage gab over the 15R?
Just 5V will cause it to overheat.
I used a battery driven 1000 Hz generator which has two outputs (RCA). One went to the input of amp and the other to the scope. Speaker of amp was connected to a 8R load resistor. 2nd scope channel connected to load resistor (if I just did it as with 1st mono block everything had been perfect). But as you know a scope has common ground for the two channels and I am quite sure that LS+ got connected to +IN via the common ground. The 8R load resistor got very hot and after a while a bad smell. And I did not see any signal from generator. Changing the 15R GBR is relative easy from top but what other damage could we expect?
I am thinking a bit on the input JFET. If it gets 24V at +IN?
Also if my theory is correct then we can expect a voltage gab over the 15R?
Just 5V will cause it to overheat.
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I can’t figure out what caused the 15 GBR to smoke. What you say you did is to connect two grounds with a 15 ohm resistor. Shouldn’t cause a fire
If you make a connection from speaker out (LS+ at schematic) to +IN at schematic the amp will probably clip and you get 24VDC at output?
That explains why the 8R load resistor got very hot.
Then the small battery driven test generator was connected to +IN and GND (not PGND). Who knows what happens inside the small generator as it also gets +24V at its output. Maybe this would cause its GND to raise and create a voltage gab from GND to PGND?
That explains why the 8R load resistor got very hot.
Then the small battery driven test generator was connected to +IN and GND (not PGND). Who knows what happens inside the small generator as it also gets +24V at its output. Maybe this would cause its GND to raise and create a voltage gab from GND to PGND?
replace GBR and try it again, checking both Iq and DC Offset as first time; with some luck that would be it
I can't fathom what happened in your setup exactly
one thing is always possible - swapping scope probe polarity by mistake, so .......
anyhow, that's the reason why I have my scope floating, powered through isolation xformer; paid the price of my own stupidity, had it for years simply floating and then through isoxf.
I can't fathom what happened in your setup exactly
one thing is always possible - swapping scope probe polarity by mistake, so .......
anyhow, that's the reason why I have my scope floating, powered through isolation xformer; paid the price of my own stupidity, had it for years simply floating and then through isoxf.
I will find a new 15R and replace it during the weekend. Then I start it up slowly using my vario transformer.
Regarding the scope it is a good idea to always have Gnd to Gnd in circuit for both channels. I got one channel reversed by a stupid mistake.
If it is only the 15R......it was a relative "cheap" mistake and the 15R sits in a position that is easy to reach.
Regarding the scope it is a good idea to always have Gnd to Gnd in circuit for both channels. I got one channel reversed by a stupid mistake.
If it is only the 15R......it was a relative "cheap" mistake and the 15R sits in a position that is easy to reach.
you can put there (GBR) anything between 0R and 47R, and you'll not hear any difference vs. other channel
just in case, visit your components on offended board with diode test; compare with good board if needed
if there are not obvious shorts, high chance that you got lucky
just in case, visit your components on offended board with diode test; compare with good board if needed
if there are not obvious shorts, high chance that you got lucky
I put in a new 15R GBR. It seems it bias up fine but offset is not good. It stays at about 0.2 - 0,3 V (I did not try to adjust as it was adjusted before the fault).
So probably the JFET or both are gone?
I measured without power in-circuit between the two outer pins on jfet (D and S) and the diode test showed 0.02V. But maybe that is not an indication as it is always on? .....it there a better place to measure without power?
So probably the JFET or both are gone?
I measured without power in-circuit between the two outer pins on jfet (D and S) and the diode test showed 0.02V. But maybe that is not an indication as it is always on? .....it there a better place to measure without power?
Between D and G 0.12V and G and S 0.1 V on input jfet. Both in-circuit without power (DMM set to diode).
On BC556 if I have +probe at E and -probe at B I measure 0.67V (a diode as expected). If I reverse the probes it measures 1.12V (don't know what to conclude here). RCA-in was shorted during the measurement.
If I need to exchange SJ74 which Idss to go for?
I have some SJ74. Most are probably in a pair or quad or octet. In these cases I think Idss is indicated.
If I cut the legs I can pull it out and if I am able to suck out the solder I can insert a new one without unmounting the PCB.
I have some SJ74. Most are probably in a pair or quad or octet. In these cases I think Idss is indicated.
If I cut the legs I can pull it out and if I am able to suck out the solder I can insert a new one without unmounting the PCB.
Ok, 6.5 mA mentioned in #8. Will see what I have when I am home from work in the evening.
With DIY-audio you don't time to go to work.......
A question:
If 25 VDC is put at input RCA would you expect the SJ74 to break?
My guess is that this was the case. Else I have no idea what caused it to break (if it is broken but that is the assumption).
With DIY-audio you don't time to go to work.......
A question:
If 25 VDC is put at input RCA would you expect the SJ74 to break?
My guess is that this was the case. Else I have no idea what caused it to break (if it is broken but that is the assumption).
most likely you did swap scope probes on output side
having other scope channel (thus scope GND too) on input, you directly routed output hot to input, and shortest path to amp gnd was through GBR
was there some damage to JFet, quite possibly
it'll be best to replace both channel JFets, with matched pair
remedy, resulted form negligence or stupidity, must be painful ......... been there, done that ........
having other scope channel (thus scope GND too) on input, you directly routed output hot to input, and shortest path to amp gnd was through GBR
was there some damage to JFet, quite possibly
it'll be best to replace both channel JFets, with matched pair
remedy, resulted form negligence or stupidity, must be painful ......... been there, done that ........
Ok, if not matched but Idss still around 6-7 mA will it affect the gain so I will have different gain in the two amps?
At least I think all my SJ74 are matched so I will remember which amp board got the new jfet so I can change the other channel also either "now" or in the future.
Yes......very painful.......but at least I expect to learn something from it 🙂
At least I think all my SJ74 are matched so I will remember which amp board got the new jfet so I can change the other channel also either "now" or in the future.
Yes......very painful.......but at least I expect to learn something from it 🙂
It usually goes that when in a hurry. A good thing with getting older is realizing that DIY is a hobby that shall be enjoyed and the outcome is the bonus. Stressing things is usually not enjoying them. Take time, step away, have a coffee or tea, get some air etc. Check and recheck what you are about to do before doing them.
I’m sounding like your usual tedious dad, but it actually gets more enjoyable doing so.
We all had our share of sparks and smoke 🙂
I’m sounding like your usual tedious dad, but it actually gets more enjoyable doing so.
We all had our share of sparks and smoke 🙂