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Old 3rd January 2013, 07:13 PM   #651
Salas is offline Salas  Greece
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Having one channel OK and its offset in the mV, your PSUs should be OK too by simple reasoning, because they are common to both channels. Address the proper relay voltage issue first. Start with 2x270 Ohm relay voltage drop resistors, then measure across the diode next to one of them activated relays for a voltage between 3.75V-5V. If not in there, trim the resistors values. Maybe the other channel weakness is a struggling relay contacts issue, I hope you are lucky enough for no 5V relay to have got it bad at 12V. After the above is done, let us know where it stands.
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Old 4th January 2013, 05:38 PM   #652
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Hi Salas,

Well i changed the battery on my multimeter and so did my voltages on my Mez!!
I now have : 4.46 volts at the switch/relay coil (nice!)
0.28v/0.19v DC offset at the output (OK?)
+ 10.63/- 10.09 on the board (acceptable?)
Apart from changing my multimeter battery i did nothing different from my last reading (apart from turn it on!! i will check all joints on the PCB again.
The IFRP240 side gets quite hot, but i have heatsinked it for now, but compared to the other side (which is quite cool) it seems to be working hard but volume at output is the same for each side.
I donít know why this is?
Apart from that i donít know if supplying it with a hexfred rectifier or (more realistically) getting two mundorf M lytics on the inputs would help as well as getting a light speed attenuator when funds allow, for now i will put two 50k series resistor across the 50k Alps pot to keep things in order.

By the way did i say it sounds great!- thanks Mr Salas.
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Old 4th January 2013, 06:33 PM   #653
Salas is offline Salas  Greece
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If its 0.28/0.19 VOLT its wrong. Maybe its something in milliVolt? Check again. Check voltage between gate and source on the MOSFETS. If all work biased open correctly you should see something in the ballpark of 4V on each one.

P.S. What brand/type is your DMM?
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Old 7th January 2013, 06:55 PM   #654
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H I Salas,
My Multimeter is an old Fluke, buts it seem OK.
My offset voltages were incorrect they are actully: 0.028v and 0.018v respectively,
(bit high---still not in the 5 Milivolt ballpark)
The voltages across the r1/r2 set are 1.50v (IFRP240 side) / 1.8v (IFRP 9240 side)
The gate source voltage is 2.65v and 6.28v (IFRP240) the Gate /Drain is 3.60v and 4.02v on this side.
The gate source voltage is 2.36v and 7.20v (IFRP9240) the Gate Drain is 2.36v and 7.20v on this side.
I got the LED sides as symmetrical as possible and attempted to get the gain of the 2 pairs of input transistors as similar as possible, is the bad offset due to a mismatch in the Buffer input pairs of the 2SK740BL? Or is there something more fundamentally wrong on the bias side? (or both?)
Any ideas gratefully received (it still sounded good during initial testing though!)
Cheers all
Johnny
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Old 7th January 2013, 07:05 PM   #655
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Johnny I would try a different Vmeter with mV scale just in case.

Deviations are a PITA when measuring that low voltages in the V range
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Old 7th January 2013, 09:36 PM   #656
Salas is offline Salas  Greece
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Quote:
Originally Posted by JOHNNYCR100 View Post
H I Salas,
My Multimeter is an old Fluke, buts it seem OK.
My offset voltages were incorrect they are actully: 0.028v and 0.018v respectively,
(bit high---still not in the 5 Milivolt ballpark)
The voltages across the r1/r2 set are 1.50v (IFRP240 side) / 1.8v (IFRP 9240 side)
The gate source voltage is 2.65v and 6.28v (IFRP240) the Gate /Drain is 3.60v and 4.02v on this side.
The gate source voltage is 2.36v and 7.20v (IFRP9240) the Gate Drain is 2.36v and 7.20v on this side.
I got the LED sides as symmetrical as possible and attempted to get the gain of the 2 pairs of input transistors as similar as possible, is the bad offset due to a mismatch in the Buffer input pairs of the 2SK740BL? Or is there something more fundamentally wrong on the bias side? (or both?)
Any ideas gratefully received (it still sounded good during initial testing though!)
Cheers all
Johnny
2SK740BL? What is that part? The right JFETs are 2SK170BL. If the DMM is still OK (old Flukes are usually reliable for decades) then the offset is still high also.
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Old 9th January 2013, 01:35 AM   #657
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Default Woops!

HI Salas,

Sorry I ment 2SK170BL (Matched "gang of 4")

Yest sir, i will try another meter, but fluke are quite good?

I am still considering getting the matched transistors from a gent here on the forum to solve this??)

regards

johnny
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Old 9th January 2013, 01:48 AM   #658
Salas is offline Salas  Greece
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Are the PSU voltages on regulated side about +/- 10VDC? Can you check that also? If its really a matter of matching, yes it will solve when all other things are OK. But your data is not very cohesive yet so to tell for sure. Flukes are good and old ones usually stay good as I have mentioned. Try a borrowed meter just to cross check results before doing anything else though. If you need a second cheap DMM to own, I would recommend UNI-T UT61E around $60.
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Old 9th January 2013, 08:03 AM   #659
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Quote:
Originally Posted by JOHNNYCR100 View Post
Yest sir, i will try another meter, but fluke are quite good?
It s not a matter of quality, rather an issue of "by design" expected deviation/error.
It usually is in the range of 1-2% or one digit, whichever is bigger, which makes it more critical for measuring near the bottom end of the scale.
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Old 10th January 2013, 01:42 AM   #660
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Hi GUYS,

Swapped multimeters for another one from work-- on my Mez PCB my voltages on the 3 copper pads are + 10.61/- 10.07 on the board - not really any diffrent from my origional multimeter readings-, so i think my problems in offset are down to matching the four 2SK170BL? (as far as i can tell!)

I will purchase a matched set and let you know what occures.(offset wise)

Thanks as ever

johnny
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