OK...I've been attempting to resurrect a couple of JLH 80W monoblocks!
I've got to the part where I'm setting up the rails of the PSU.
The construction manual states that:
" Occasionally the heavy current 55V supply rails, particularly the +ve one, will not setup without a load. If this happens, use two 1K5 1W resistors supplied in series across the line to earth rail to load it with a few milliamps"
Really need 'baby steps' here!!!
The 'other' monoblock setup fine....both 50v and 55v rails... without this step BTW!!
Please advise...and thanks!! 🙂
I've got to the part where I'm setting up the rails of the PSU.
The construction manual states that:
" Occasionally the heavy current 55V supply rails, particularly the +ve one, will not setup without a load. If this happens, use two 1K5 1W resistors supplied in series across the line to earth rail to load it with a few milliamps"
Really need 'baby steps' here!!!
- It's the -55v rail that is misbehaving for me.
- I've got 3 x 1k(3W) resistors which i can connect in series...got that bit!
- Then...connect the 55v output to ground (case) via the resistor chain???
- Next step struggling....do I adjust rail 'under this load'....or remove load and go back to setting up rails as before???
The 'other' monoblock setup fine....both 50v and 55v rails... without this step BTW!!
Please advise...and thanks!! 🙂
Got some advice, and tried to make the adjustment to the -55V rail 'under load', and problem still persists. Rail can't be setup and looks like I've got another problem altogether... 🙁
looks like you will have to find the fault.
check all soldering and connections
check components for correct orientation
no luck? start pulling out actives/diodes for that rail and testing them
also check the preset pot
discharge the main caps first before starting the above.
check all soldering and connections
check components for correct orientation
no luck? start pulling out actives/diodes for that rail and testing them
also check the preset pot
discharge the main caps first before starting the above.
Thought I'd revisit this one!
I've attached a copy of the PSU circuit if that helps!!
With some other help, was initially directed towards Q20 and C8
Found that a 3n0 capacitor (C8) in the regulator circuit had gone awol...and on replacing this, regained control of the -55v rail!! 🙂
THEN....noticed that...when PSU was switched OFF then back ON again .....volt age would either : return immediately to -55v...OR....creep slowly, and settle around -27.5v.
So...on first switching on (cold!)...goes to -55v....but after short time, switching OFF/ON, rail only reaches -27.5v.
Using a bit of a scatter-gun approach...I've replaced some diodes (all 1N4148's) in the area (Why not...I thought!)
...to no effect!
I've tested Q20 with a DMM...and it looks OK.
Just don't know what to try next.....anyone!!???
I've attached a copy of the PSU circuit if that helps!!
With some other help, was initially directed towards Q20 and C8
Found that a 3n0 capacitor (C8) in the regulator circuit had gone awol...and on replacing this, regained control of the -55v rail!! 🙂
THEN....noticed that...when PSU was switched OFF then back ON again .....volt age would either : return immediately to -55v...OR....creep slowly, and settle around -27.5v.
So...on first switching on (cold!)...goes to -55v....but after short time, switching OFF/ON, rail only reaches -27.5v.
Using a bit of a scatter-gun approach...I've replaced some diodes (all 1N4148's) in the area (Why not...I thought!)
...to no effect!
I've tested Q20 with a DMM...and it looks OK.
Just don't know what to try next.....anyone!!???
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Having built the same in the 80,s I came across this fault that manifested itself due to a faulty component activating the safety control part of the circuit .
I do not remember which component it was but I persevered by testing all the components in that bottom rhs section including the gate components ,the mosfet itself like the others was pretty robust and stood up to quite a bit of abuse unlike the previous bjt pass transistors which would blow under many fault conditions .
I agree with the above posters you will have to take this a step at a time to locate the fault.
I do not remember which component it was but I persevered by testing all the components in that bottom rhs section including the gate components ,the mosfet itself like the others was pretty robust and stood up to quite a bit of abuse unlike the previous bjt pass transistors which would blow under many fault conditions .
I agree with the above posters you will have to take this a step at a time to locate the fault.
Thanks for venturing your thoughts duncan2...much appreciated 🙂
Having now removed/tested/replaced transistors in that 'bottom rhs' area, (BTW...what is the 'gate'?) find myself wondering if it's possible for a semiconductor to test OK on a DMM, but actually be faulty in circuit???? (Thinking of Q20 here )
Have re-flown solder joints, and checked for continuity all over the PCB.
Notice that Q19/Q20 are BC449/BC450, and these are considered 'obsolete'.
Any knowledge please on current equivalents for these...should I need 'em??
Found another complimentary pair of BC450/BC449 on the board, and swapped them for Q20/Q19.......same problem, so appears that Q20 is fine!(?)
Have re-flown lots of solder on the PCB.....hasn't changed anything.
With a 2K load on each 55v rail....these go to +55v and -55v when first powered ON ......THEN.....these settle at +12v and -7.5v when the unit is switched OFF...and back ON again.
I noticed that when I switch PSU OFF...from it's 55v rail 'stuck' at 12v stage, the same peculiar (to me) thing happens.....DMM voltage falls to 0.4 volts, then display 'stutters' and climbs instantly to around 3.5 volts, before then falling to 0v. Only at this stage do the 55v rails return to full potential when unit is switched back to ON.
Because this behaviour is so predictable....it's hard to believe that the problem is due to anything other than a component!....Does anyone think that a 'dry joint' or crack in the PCB track could behave so consistently?
Any thoughts please??
Have re-flown lots of solder on the PCB.....hasn't changed anything.
With a 2K load on each 55v rail....these go to +55v and -55v when first powered ON ......THEN.....these settle at +12v and -7.5v when the unit is switched OFF...and back ON again.
I noticed that when I switch PSU OFF...from it's 55v rail 'stuck' at 12v stage, the same peculiar (to me) thing happens.....DMM voltage falls to 0.4 volts, then display 'stutters' and climbs instantly to around 3.5 volts, before then falling to 0v. Only at this stage do the 55v rails return to full potential when unit is switched back to ON.
Because this behaviour is so predictable....it's hard to believe that the problem is due to anything other than a component!....Does anyone think that a 'dry joint' or crack in the PCB track could behave so consistently?
Any thoughts please??
The "gate " is the mosfet equivalent of the base in a bjt (bipolar junction transistor ) --ie- the signal control part of the active component or grid as in a valve (tube -triode) .
I now see in your later posts that both rails drop to low values not just the minus 55V rail ,while still pointing to faulty component it could also be a fault in the safety control components .
Of course ,as you say it could be a dry joint ,if you did not build it yourself that's possible and down to bad soldering flow techniques, a crack in the PCB would mean testing with a low reading ohmmeter or the non technical way of powering it up and with one finger on a bare section of the board pressing it up and down with a test meter connected to the rails .
Checkout the gate components on both pass mosfets, one part of this design that's goes faulty can affect the other rail due to the safety design to limit fault conditions affecting the output and therebye causing component failure in the actual power amp, John was very safety conscious in this regard.
Read the pre build articles by John in Electronics/WW for power supply design philosophy.
I now see in your later posts that both rails drop to low values not just the minus 55V rail ,while still pointing to faulty component it could also be a fault in the safety control components .
Of course ,as you say it could be a dry joint ,if you did not build it yourself that's possible and down to bad soldering flow techniques, a crack in the PCB would mean testing with a low reading ohmmeter or the non technical way of powering it up and with one finger on a bare section of the board pressing it up and down with a test meter connected to the rails .
Checkout the gate components on both pass mosfets, one part of this design that's goes faulty can affect the other rail due to the safety design to limit fault conditions affecting the output and therebye causing component failure in the actual power amp, John was very safety conscious in this regard.
Read the pre build articles by John in Electronics/WW for power supply design philosophy.
I would be looking around the power supply and see if volts are what is expected and where the volts start to go wrong.
Zeners quite often fail or go dodgy.
Zeners quite often fail or go dodgy.
Thanks for commenting Nigel! Wouldn't know where to start with this.....but I'd like to!! 🙂
As it happens...have a supply of diodes/transistors heading this way....but looks like I'll be waiting until after the bank holiday for them now🙁
Update....After waiting 9 days for a selection of Zener diodes (will be sticking to next day delivery with RS-online where I can, from now on) was able to attempt a scatter-gun approach to problem.
Zener diodes can misbehave (apparently)...so I replaced ALL of them with 'new'. Also renewed Q13/Q14 (part of clamping circuit) for good measure!
(took opportunity to check each item removed with a DMM and couldn't find any fault!
)
Powering on...seems this has had no effect...and symptoms I referred to in my post #10 continue un-changed.
Perhaps my scattergun approach should've been wider!! ?
Zener diodes can misbehave (apparently)...so I replaced ALL of them with 'new'. Also renewed Q13/Q14 (part of clamping circuit) for good measure!
(took opportunity to check each item removed with a DMM and couldn't find any fault!
Powering on...seems this has had no effect...and symptoms I referred to in my post #10 continue un-changed.
Perhaps my scattergun approach should've been wider!! ?
The only logical-practical actions in fault finding in any branch of engineering is to pursue a step by step approach and while a DMM will find some faults for obscure faults more test equipment is required even if you have to borrow it.
Because of the high impedance and sensitivity of DMM,s they are liable to pick up in long lines giving erroneous readings due to RF/induced signals which was apparent while working for a telephone company.
It might seem slow but it gets there quicker than a haphazard approach.
A very basic introduction to fault finding in audio equipment when starting off is given here-
Troubleshooting Audio Electronics Without Tears
Because of the high impedance and sensitivity of DMM,s they are liable to pick up in long lines giving erroneous readings due to RF/induced signals which was apparent while working for a telephone company.
It might seem slow but it gets there quicker than a haphazard approach.
A very basic introduction to fault finding in audio equipment when starting off is given here-
Troubleshooting Audio Electronics Without Tears
Thanks duncan2...a nice collection of general wisdoms there! Perhaps the "Hey, I got no fancy test equipment, what about me?" section, and the idea of troubleshooting a working unit (The other Monobloc is fine, at least PSU-wise!) is a possible way to go for me.
Can't help thinking that this is a complex unit for me to be cutting my 'Janet & John' diagnostic level teeth on!
Still....onwards..and downwards🙂
Like your sense of humour Tonewheelkev you sometimes need it when fault finding --best of luck !
Got there in the end!!!
Thought I'd update folks here!! 🙂
While reading through the 'JLH 80w mosfet power amplifier - modifying it' thread, I came across a contribution from vac231....who had also removed/disabled the components for the 15v bar-graph power supply. It emerged that it was also necessary that 3 x wire links also be removed to achieve correct function.
So...finally got there, and now have 2 fully working PSU's 😀😀😀
Love this forum...and thanks vac231
Thought I'd update folks here!! 🙂
While reading through the 'JLH 80w mosfet power amplifier - modifying it' thread, I came across a contribution from vac231....who had also removed/disabled the components for the 15v bar-graph power supply. It emerged that it was also necessary that 3 x wire links also be removed to achieve correct function.
So...finally got there, and now have 2 fully working PSU's 😀😀😀
Love this forum...and thanks vac231