having some issues with first powerup testing my first output board, dim bulb test goes okay as in there's no short, but after going dim it doesn't come back up to any level of brightness again, and when measuring across R2 as shown I'm getting 0v dc - I rechecked the voltage on the PSU board and that still looks to be correct, and the LED on the output board is lighting up - any ideas what to check here? I set the pots to midpoint before soldering them on, and didn't see any change fiddling with the position of the bias pot
I did go ahead and remove DBT, rechecked Z1 and Z2 and polarities both look correct, I'm seeing 5.3V across Z2 - I believe the orientation on the pots should be correct, the notches match up with the PCB at least - I checked both with a multi to verify they worked & had expected ranges before I soldered them on, they're bourns 3296W-1-502LF
With the dim bulb tester in place, did you set the initial bias and offset as described by 6L6? 'You can, however, set the initial bias with the bulb in place. Start the procedure, turn the pots until something happens, and set, at maximum, 0.1V across the source resistors and zero offset.'
Please don't assume that the pots will work the "correct" way, mine did not!
You may have to move them quite a bit to see a result.
Remember, you are moving both pots, in balance to set bias and offset, don't just try moving one alone.
I suggest one turn on each alternately till you see a change across either of the resistors.
You may have to move them quite a bit to see a result.
Remember, you are moving both pots, in balance to set bias and offset, don't just try moving one alone.
I suggest one turn on each alternately till you see a change across either of the resistors.
Hi Colin,
I did measure the FETs with Peak Atlas DCA75 Pro. Is there an advantage to testing them the way you described?
How much time are you giving to check the voltage after turning the pot?
-The cap takes a bit to charge up. After that, there is a time constant between making adjustments of the pot and seeing the voltage change. Patience is needed.
How many turns have you made?
-It took me quite a number of turns before seeing the voltage rise. I forget how many turns it takes to get thru the pot's range but I seem to recall 25.
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Your results from the Peak are likely fine in showing the jfets are functioning. However, the method Colin described has the advantage that it is pretty much the standard way Idss values are quoted. In the Toshiba datasheets, the Idss values
are given at a constant Vds = 10V with Vgs=0. The Peak (probably because it's a battery powered device with its voltage and current limitations) gives you measurements at low and variable Vds values (In your tests, ~3V for the k170 and 1.78V for the j74).
This did it, thanks for the help guys - I didn't expect that midpoint on the pots would be so far off base as to cause 0v across R2, I was able to set bias and offset after messing with the pots for a while - getting Vgs on mofsets closer to 4v got it a lot closer to a workable starting point
Is there a recommended/standard way of measuring mosfet temperatures? I couldn't find any sort of answer to this and while finalizing bias I'm trying to adhere to the 65c rule that 6L6 stated in the op, is it ok to just measure this with an infrared thermometer? Not sure how hard and fast of a rule this is, but I see some people measuring this with infrared thermometers - when I measure this with mine its obviously reading a lower temp than if I measure the same by putting a multimeter probe on the pin.
Good news! My F6 is fine! Tested all parts and couldn't find anything wrong with them. The smoke I saw might have been excessive heat in combination with fumes from PCB cleaner/ alcohol residue. Was able to put Humpty Dumpty together again! Thanks for all your help guys! 😀👍