Well I was just setting up my newly minted F5 and a wayward DMM probe let the smoke out of one channel. I have built the stripped down version with no protection so it's probably a resistor or Q1, Q2, Q3, or Q4. I haven't pulled it apart yet but I'm speculating that it's one of the semiconductors. Resistor tend to be pretty hardy. The only clue I have so far is that I'm getting 17VDC 😱 at the speakers terminals.
Thoughts anyone?
Regards,
Dan
Thoughts anyone?
Regards,
Dan
Sounds like you have not put the pots down to zero or you have adjusted one side too much before opening up for the other.
If you let the smoke out, you have probably blown one of the O/P devices.
With the PSU discharged, check Q3 for a short circuit across drain source.
If Q3 is OK, start again with the bias procedure, P1 & P2 at minimum resistance, begin biasing with P2, slowly adjust counting the number of turns until it starts to conduct, if it doesn't, Q4 is open circuit - no voltage across R12.
With the PSU discharged, check Q3 for a short circuit across drain source.
If Q3 is OK, start again with the bias procedure, P1 & P2 at minimum resistance, begin biasing with P2, slowly adjust counting the number of turns until it starts to conduct, if it doesn't, Q4 is open circuit - no voltage across R12.
In my experience, there is also a lot smoke trapped inside the source resistors. And if your amp is oscillating, the feedback resistors may start smoking.
Look for burned components and smoke residual on pcb and tell us about it.
Look for burned components and smoke residual on pcb and tell us about it.
Yeah, the source resistors get cooked, rise in value or go open and it puts the bias right off. Check those before the fets.
Mother's Day here so not much time. Will pull apart and post more info in the next couple days. Thanks for the suggestions.
Regards,
Dan 🙂
My bet is the either of the JFETS...I blew a 2sk170 and it immediately put rail voltage to the speakers.
This is my circuit. So far all I've found is R11 open. Does that explain the 17VDC at the output?
Regards,
Dan 😕
Regards,
Dan 😕
Do R5-8 get really really hot?
When my 2sk170 blew they did to the point of the solder got soft.
A blown source resistor is obviously not good...but I'm not sure that would put DC on the outputs.
When my 2sk170 blew they did to the point of the solder got soft.
A blown source resistor is obviously not good...but I'm not sure that would put DC on the outputs.
I have spare 2S's but no spare resistors or MOSFETs. Alittle more continuity checks on the weekend.
Regards,
Dan
Regards,
Dan
Have you checked the pots? I cannot stress enough how easily you can let out smoke if they are set in the wrong position.
An open R11 may give you 17V, but most likely -17V and not +17...
Can you take a picture of your populated board and show us? Do you have the same error on both channels?
I have checked the pots and have not 'figured out' the readings I'm getting yet. I have one spare pot so I have options.
I wasn't paying attention to the polarity of my meter leads so it could easily be -17VDC.
Only smoked one channel. No burn marks on populated board so likely a misadjustment of one pot.
Right now I'm trying to create a list of parts to replace in an 'educated guess' to get back on track. Referring to the schematic in the previous post my list so far is R11, Q1, and P1.
Regards,
Dan
Put your DMM on R3 and try turning the pot in both directions - you may need to change the polarity of the DMM to get it right (the active devices are playing tricks). And then do the same for R4's pot.
Br,
Christian
It didnt end well...lost a CHR-70...
Sad part is that I was using a crappy pair of test speakers and ran for a while with no issues...Went to crank it up a little and POOF!
The dual toshibas seem to be holding their own for now...I have them hooked up to the chr-70s in Fonkens and they seem to be getting along nicely.
Looks to me like a blown JFET will allow full DC voltage to the outputs through R5-8 if the drain and source are "closed"...at least thats my summation. R5-8 wont drop a significant amount of voltage...
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