With setting up an amp you will want two multimeters to watch the behavior. You need to look at 2 critical voltages at the same time as you slowly and carefully bring things to life. One meter connected to monitor the DC offset on the output (we want zero volts), and the other meter across one of your emitter resistors in the output stage to monitor the voltage across and hence the current through, calculated with ohm's law. P1 is the DC offset adjustment and P2 will establish how much current is flowing through the output stage. But go slow and understand what is going on so that you don't burn something (or many something's) when this thing wakes up.
Thanks for the input here guys, so the resistor that has burnt is R29 on the new NP FE board, This is tied to GND normally by the normally closed thermostat and the other end is held at 9.1volts by zener D5 normally. So the standing current through it would be 4 mA for a power dissipation in the resistor of 37mW.
Has anyone here got any clues as to what has caused this or what to look for, to help out the member thanks.
When P2 is turned there is no volts across 1R0 emitter resistors, I am assuming because, the issue with R29 is holding back the drive to the base of Q6.
Schematic is attached.
Has anyone here got any clues as to what has caused this or what to look for, to help out the member thanks.
When P2 is turned there is no volts across 1R0 emitter resistors, I am assuming because, the issue with R29 is holding back the drive to the base of Q6.
Schematic is attached.
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I have never seen a service manual, the only thing I can find is the amp schematic, nothing on the PSU and nothing on the front panel LED power level display.
With a 73V rail, R29 is going to be working hard (73 - 9.1 = 63.9V across that guy). I would put something like an 8.2K, 2W rated resistor there, obviously soldier style mounted.
This would give you a little over 4 mA through the zener and 3.5 mA through the diode string. Should be fine.
This would give you a little over 4 mA through the zener and 3.5 mA through the diode string. Should be fine.
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OK, thanks for your reply, will get the member to try that. I would have thought the zener would keep the voltage at 9.1 V, but then you would have the power loss in R9 from the -73 volt rail. Maybe both resistors should be larger wattage - any thoughts on that?
entire shebang needs some recalculation for so high rails
taking care of voltage nodes (cascodes on input) and also of resistors and other parts dissipation
taking care of voltage nodes (cascodes on input) and also of resistors and other parts dissipation
Oh, that is not so good ZM, might have to get Mr Pass to comment.
Have you any specific thoughts?
Another member here has his S500 running with 80VDC rails with the NP FE board
Have you any specific thoughts?
Another member here has his S500 running with 80VDC rails with the NP FE board
The cascode divider is working extra hard too, without some value adjustments there. I should have caught that myself.
Can you expand on that thanks, I have also just sent Mr Pass an email to see if he can add some thoughts here, that might take a while though.
will think more tomorrow, ZM's time to retire to chambers
you know - my beauty time, self-reflection yadayada
meaning - deep meditation is not good for crunching numbers
The divider.. maybe go R6=10K, R7=4.7K.... I'm just shooting from the hip here with quick back of the napkin doodles..
Yeh, changing to your values for R6 / R7, cuts their power dissipation by half (0.25W for R6 and 0.11W for R7), with original values it is 0.53W and 0,25W. The mid point volts is still 23VDC.
Hi Zen Mod, Mike in the US has sent me a PM where you have mentioned this in the Stasis FE thread on page 43 post #860.
You mention changing a few resistor values in relation to 80 VDC rails in answer to another member.
These were:
R8 on your schematic - R6 on NP schematic changing from 4K7 to 68K
R9 on your schematic - R7 on NP schematic changing from 2K2 to 12K
R13 on your schematic - R29 on NP schematic ( which is the resistor that has failed) changing from 2k2 to 9K1 at least 1watt.
Mike also changed Q1 (Q3 NP) from ZTX450 to ZTX457. Q2 (Q5 NP) from ZTX450 to ZTX457, Q3 (Q4 NP) from ZTX450 to ZTX457, Q6 (Q9 NP) from ZTX450 to ZTX457 and Q4 (Q8 NP) from ZTX 550 to ZTX558, which I believe are higher voltage rated. Q5 and Q7 (Q7, Q6 NP) stay as KSA1220 and KSC2690.
Can you confirm all of this and advise if any other changes would be needed,.
Many thanks for your time in this, I just want to be able to get our member there in the UK sorted as he is up to the stage of bias adjustment with the PSU all good.
Gary..
You mention changing a few resistor values in relation to 80 VDC rails in answer to another member.
These were:
R8 on your schematic - R6 on NP schematic changing from 4K7 to 68K
R9 on your schematic - R7 on NP schematic changing from 2K2 to 12K
R13 on your schematic - R29 on NP schematic ( which is the resistor that has failed) changing from 2k2 to 9K1 at least 1watt.
Mike also changed Q1 (Q3 NP) from ZTX450 to ZTX457. Q2 (Q5 NP) from ZTX450 to ZTX457, Q3 (Q4 NP) from ZTX450 to ZTX457, Q6 (Q9 NP) from ZTX450 to ZTX457 and Q4 (Q8 NP) from ZTX 550 to ZTX558, which I believe are higher voltage rated. Q5 and Q7 (Q7, Q6 NP) stay as KSA1220 and KSC2690.
Can you confirm all of this and advise if any other changes would be needed,.
Many thanks for your time in this, I just want to be able to get our member there in the UK sorted as he is up to the stage of bias adjustment with the PSU all good.
Gary..
With your new R8 and R9 values ZM, that puts the cascode transistor base voltage at 11VDC - where it was 23VDC in the original design with a 73VDc rail voltage - any problems with that?
Thanks ZM 👍🏻will think more tomorrow, ZM's time to retire to chambers
you know - my beauty time, self-reflection yadayada
meaning - deep meditation is not good for crunching numbers
Thanks to all of you , I will patiently wait for the outcome
I'll try to collect some gray/grey cells tonight and do some simple math and compare of active parts V and W figures
in general, I tend to keep parts much less hot than Pa is used to; he's founder of ABFAB, while I'm just kido Pa wanabee
maybe in few decades from now I'll dare to keep my parts same hot