This is really embarrasing - I put in new diode bridges, and the lamp thing still lighted up.
I then desoldered the PSU bank, and measured across both bridge inputs. One of them showed a short. I couldnt understand that. Then I desolderet the coil in that side, and viola some metal splinters had worked the way through the laquer and made a short. This has probably not been a constant short, because I did have the whole PSU up and running in one point of time.
So far so good - it still does not run yet, but I must have come closer.
Thanks for all your help, I really appriciate that you take your time to help.
Best regards
Arthur.
I then desoldered the PSU bank, and measured across both bridge inputs. One of them showed a short. I couldnt understand that. Then I desolderet the coil in that side, and viola some metal splinters had worked the way through the laquer and made a short. This has probably not been a constant short, because I did have the whole PSU up and running in one point of time.
So far so good - it still does not run yet, but I must have come closer.
Thanks for all your help, I really appriciate that you take your time to help.
Best regards
Arthur.
Short at the transformer....
Hi Arthur,
Your problem happened to me.... I used the mounting hardware from another transformer and I forgot to install the rubber pad, as I fastened the nut to hold the transformer, I made a short with the chassis, blown up diodes, not working at all, etc... It was a lucky test when I realised that one of the secondaries was in short with the chassis, I unscrewed the nut, installed the rubber pad and screwed again but not too tight.... Everything went fine then. That was more than a year ago....and is still working fine.
Regards,
Jorge
Hi Arthur,
Your problem happened to me.... I used the mounting hardware from another transformer and I forgot to install the rubber pad, as I fastened the nut to hold the transformer, I made a short with the chassis, blown up diodes, not working at all, etc... It was a lucky test when I realised that one of the secondaries was in short with the chassis, I unscrewed the nut, installed the rubber pad and screwed again but not too tight.... Everything went fine then. That was more than a year ago....and is still working fine.
Regards,
Jorge
Today I isolated the PSU coil against the chassis and got the PSU assembled, and used my bulb thing of course 
... and the bulb came on? But there were voltages on both sides of the supply. It were because I had loaded the suply lightly with 330 ohms power resistors.
So I proceeded by connecting my Variac from Hell, and the PSU showed almost exactly +/- 23.7v.
Why from hell - it is Ferari red and the mains cable has male connectors in both ends!
I have already testet both amp PCBs earlier with an external PSU with succes for not drawing to much current. I didnt do a signal generator test, but now I have a hard time deciding if I connect one of the amps and do a test, or if I should do the signal generator test first.
But I think I am comming much closer to a working amp.
... and the bulb came on? But there were voltages on both sides of the supply. It were because I had loaded the suply lightly with 330 ohms power resistors.So I proceeded by connecting my Variac from Hell, and the PSU showed almost exactly +/- 23.7v.
Why from hell - it is Ferari red and the mains cable has male connectors in both ends!
I have already testet both amp PCBs earlier with an external PSU with succes for not drawing to much current. I didnt do a signal generator test, but now I have a hard time deciding if I connect one of the amps and do a test, or if I should do the signal generator test first.
But I think I am comming much closer to a working amp.
Normal practice doesn't quite work here.
Take a 500VA transformer at 110V. The steady state maximum current will be 500/110 = 4.5A.
If you fit a standard 5A fuse it may fail.
As the fuse is only there to prevent fire the general consensus is to use a 3 x fuse. This wont protect the circuitry but will blow in the event of transformer failure. Conservatively a 10A fuse would be used.
3times is not close rated fusing.
That is the normal rating required for inductive loads like motors and transformers.
To get them to rupture you need a massive overloading on the transformer. Much more likely that the fuse will not rupture.
Use close rated fusing on your transformer. That virtually always requires a soft start to allow the transformer to start up.
Now when the transformer is overloaded there is a good chance the close rated fuse will blow. Having to get out the screw driver and swap to a fresh fuse will remind the user not to overload that way again.
I think I begin to understand the value of the bulb thing better now.
I connected one side of the amp, with the pots in the off position, and powered the amp up with the bulb thing in line - it lighted up? But only for a certain amount of time, then the bulb went out.
Of course because the PSU caps had to be charged.
Afterwards I were able to adjust the bias, but I only went to 2. something volts, because I suppose the bulb is limiting the current, maybe so much that I could turn the pots up too high. So I turned them down and will start it up again with the variac.
Maybe many of you think: ´All this is obvious´, but probably not for everybody.
So far I kept a 1A slow blow fuse in, kind of to try to protect the circuitry a little bit anyway.
I have now brought it up via the variac, adjusted the bias to 2.6V so far, and I get 0.6xV across. have no current meter on at the moment. Maybe all is ok, but I the power transistors are probably not more than 25 deg. Celsius - isnt that strange ? I use the Toshibas and they should have a lower turn on voltage.
What to do now ?
I connected one side of the amp, with the pots in the off position, and powered the amp up with the bulb thing in line - it lighted up? But only for a certain amount of time, then the bulb went out.
Of course because the PSU caps had to be charged.
Afterwards I were able to adjust the bias, but I only went to 2. something volts, because I suppose the bulb is limiting the current, maybe so much that I could turn the pots up too high. So I turned them down and will start it up again with the variac.
Maybe many of you think: ´All this is obvious´, but probably not for everybody.
So far I kept a 1A slow blow fuse in, kind of to try to protect the circuitry a little bit anyway.
I have now brought it up via the variac, adjusted the bias to 2.6V so far, and I get 0.6xV across. have no current meter on at the moment. Maybe all is ok, but I the power transistors are probably not more than 25 deg. Celsius - isnt that strange ? I use the Toshibas and they should have a lower turn on voltage.
What to do now ?
If the amp is reacting to bias normally do the following -
Remove bulb lead.
Install proper fuse.
Plug it all in, turn on and start increasing the bias;
Set 0.2V across the source resistors and then zero the offset. You will likely loose bias as you do that, this is always a "two steps forward, one step back" process.
Increase to .4v, then zero the offset again,
repeat at .5v,
and finally set full bias - 0.6V across the source resistors with zero bias.
Report back with your results! And we would also love to see some photos...
Remove bulb lead.
Install proper fuse.
Plug it all in, turn on and start increasing the bias;
Set 0.2V across the source resistors and then zero the offset. You will likely loose bias as you do that, this is always a "two steps forward, one step back" process.
Increase to .4v, then zero the offset again,
repeat at .5v,
and finally set full bias - 0.6V across the source resistors with zero bias.
Report back with your results! And we would also love to see some photos...