Hello everyone!
I hope someone reads this and can maybe help with remote advice:
A Holfi NB3 has landed on my bench from a friend who loves this power amp. Although, one channel went "bust" and I don't have a schematic of it. What I know for sure is that NB3 is a pure double mono design having with separate transformers for each side which is an advantage here I think.
The initial visual inspection yielded a few strange things:
Two PCB screws were "travelling" around in the case and on the (failed) PCB which I fixed.
A 330Ohm Resistor in one of the four power supplies was badly burnt which I replaced.
A connection wire of the Standby Switch had somehow "unsoldered" itself from the board which I fixed, so the working side relay clicks, the failed side relay does not.
The Standby LED does not light up which I could not fix yet.
My switch-on test after reassembly showed one of the two mono-amps working OK, the other one (the failed side) showed a 26.5V DC voltage at the Speaker Output, where the working side showed ~0V. My fairly limited repair experience so far only tells me that there may be a broken Power Transistor or a failed power supply component.
I have downloaded the schematics posted here earlier as a rough reference and will see if they're of any use. The power supply with the four rectifications may be separate for line stage and power stage I "guess".
Well, if anyone can shed light on the circuit schematic of the NB3 or hint at a strategy to find the broken component(s), my endeavour to repair the beauty would be much easier!
Thanks to the helpers!
Winfried
I hope someone reads this and can maybe help with remote advice:
A Holfi NB3 has landed on my bench from a friend who loves this power amp. Although, one channel went "bust" and I don't have a schematic of it. What I know for sure is that NB3 is a pure double mono design having with separate transformers for each side which is an advantage here I think.
The initial visual inspection yielded a few strange things:
Two PCB screws were "travelling" around in the case and on the (failed) PCB which I fixed.
A 330Ohm Resistor in one of the four power supplies was badly burnt which I replaced.
A connection wire of the Standby Switch had somehow "unsoldered" itself from the board which I fixed, so the working side relay clicks, the failed side relay does not.
The Standby LED does not light up which I could not fix yet.
My switch-on test after reassembly showed one of the two mono-amps working OK, the other one (the failed side) showed a 26.5V DC voltage at the Speaker Output, where the working side showed ~0V. My fairly limited repair experience so far only tells me that there may be a broken Power Transistor or a failed power supply component.
I have downloaded the schematics posted here earlier as a rough reference and will see if they're of any use. The power supply with the four rectifications may be separate for line stage and power stage I "guess".
Well, if anyone can shed light on the circuit schematic of the NB3 or hint at a strategy to find the broken component(s), my endeavour to repair the beauty would be much easier!
Thanks to the helpers!
Winfried
Hi wgh52,
This is what I made when I reversed mine. https://drive.google.com/file/d/1t8iX8kZtsg4anSZHyC8PqnE1esBw1LJW/view?usp=sharing
Mogens
This is what I made when I reversed mine. https://drive.google.com/file/d/1t8iX8kZtsg4anSZHyC8PqnE1esBw1LJW/view?usp=sharing
Mogens
Hello Mogens,
I have reviewed your reverse schematic by now and found a missing detail just because it is important in my failure case:
The Standby switch on the front panel seems to be missing in the schematic. Not a big issue, just a finding. I will share the detail here when I've identified the connections into the circuitry. Maybe you are interested in completing the NB2 KiCAD schematics
Greetings,
Winfried
I have reviewed your reverse schematic by now and found a missing detail just because it is important in my failure case:
The Standby switch on the front panel seems to be missing in the schematic. Not a big issue, just a finding. I will share the detail here when I've identified the connections into the circuitry. Maybe you are interested in completing the NB2 KiCAD schematics
Greetings,
Winfried
Hello Mogens,
So far as I could "easily" disassemble the PCB to view the copper side, the front panel switch disconnects (i.e. switches off) the two relay windings for "standby" mode and connects them for "on" mode which also turns the red LED off and on respectively.
See the edited picture:
Regards,
Winfried
So far as I could "easily" disassemble the PCB to view the copper side, the front panel switch disconnects (i.e. switches off) the two relay windings for "standby" mode and connects them for "on" mode which also turns the red LED off and on respectively.
See the edited picture:
Regards,
Winfried
Hi Winfried,
Sorry, for some reason I have either missed or not gotten any notification that you have written in this thread.
I actually didn't notice this had been disconnected in my amplifier. I will check next time I take the top cover off.
Did you get the amplifier working again?
Cheers,
Mogens
Sorry, for some reason I have either missed or not gotten any notification that you have written in this thread.
I actually didn't notice this had been disconnected in my amplifier. I will check next time I take the top cover off.
Did you get the amplifier working again?
Cheers,
Mogens
Hi Mogens,
due to some distracting other projects I have not been able to push forward with the repair, but will do so soon. I have also found a HiFi fan who used to be a professional TV and audio electronics repair expert, he's willing to guide me through the repair process. I'll be back!
Greetings,
Winfried
due to some distracting other projects I have not been able to push forward with the repair, but will do so soon. I have also found a HiFi fan who used to be a professional TV and audio electronics repair expert, he's willing to guide me through the repair process. I'll be back!
Greetings,
Winfried
Hi Winfried,
That's fine. I was just curious.
I know the relays has been re-placed in mine. I think they might be a source of fault over time. As I remember Holfi uses the relays to disconnect the voltage for the output transistors. I believe that this put some strain on the relays and the contacts go bad. Just, a thought.
Keep us updated when you have time to look at it again.
Cheers,
Mogens
That's fine. I was just curious.
I know the relays has been re-placed in mine. I think they might be a source of fault over time. As I remember Holfi uses the relays to disconnect the voltage for the output transistors. I believe that this put some strain on the relays and the contacts go bad. Just, a thought.
Keep us updated when you have time to look at it again.
Cheers,
Mogens
Hi Mogens,
I'm now back on the NB3 PwrAmp subject. After completely "extracting" the malfunctioning PCB from the chassis, I've done and continue to do some more measurements. When examining the Supply marked with VCC1 in your schematic, I've found two resistors missing in your NB3 schematic (for whatever reason) and wanted to bring this to attention. On the PCB they are marked R36 nd R35 with 0.22Ohms each and are in the rails for the poweramp section. The supply for the +/-15V stabilizers is connected before R36/R35 respectively. Here's a modified sketch of the Power Supply:
I'm still wrestling with the PwrAmp repair, but gain confidence to repair it eventually.
Regards,
Winfried
I'm now back on the NB3 PwrAmp subject. After completely "extracting" the malfunctioning PCB from the chassis, I've done and continue to do some more measurements. When examining the Supply marked with VCC1 in your schematic, I've found two resistors missing in your NB3 schematic (for whatever reason) and wanted to bring this to attention. On the PCB they are marked R36 nd R35 with 0.22Ohms each and are in the rails for the poweramp section. The supply for the +/-15V stabilizers is connected before R36/R35 respectively. Here's a modified sketch of the Power Supply:
I'm still wrestling with the PwrAmp repair, but gain confidence to repair it eventually.
Regards,
Winfried
You both do a remarkable job, my respect for that.
As long as you are there, could you modify the fixing points of the final transistors (three points instead of two, a central one between the two power transistors and the other two at about 4 cm from the two drivers) that will avoid seeing this bar twisted and these bad support points, in the following, replacing these ugly non-insulated rigid conductors with insulated stranded wire, for safety and from a technical point of view, it would also be good.
Otherwise, congratulations, continue it's really interesting.
As long as you are there, could you modify the fixing points of the final transistors (three points instead of two, a central one between the two power transistors and the other two at about 4 cm from the two drivers) that will avoid seeing this bar twisted and these bad support points, in the following, replacing these ugly non-insulated rigid conductors with insulated stranded wire, for safety and from a technical point of view, it would also be good.
Otherwise, congratulations, continue it's really interesting.
OK Mogens,
maybe, as a coutesy to interested readers and actual Holfi repairers, you'd like to amend your very helpful NB3 document accordingly. It's interesting to see different fitting options of the same identical board, as we have now seen with the Standby switch and the two Resistors in the Power supply.
@huggygood
Thanks for the acknowledgement and your interest! At this moment I feel like "O man! Let me finally find the broken part(s)!" more than to modify the unit. The proposen Mods. would mean to completely unmount the second PCB which was enough of a drag with the first one. I'll very probably rebuild the original hardware as the rigid wires are really rigid and I don't see risk there. I agree, though, that their use can be validly questioned to begin with...
Well, the Amp actually belongs to a HiFi friend wanting it back soon now. What I may still do, though, because it's so simple to measure, straight forward to implement and effective electronically: Add Snubbers to all secondary transformer windings, which may even improve sound (at least slightly).
Greetings,
Winfried
maybe, as a coutesy to interested readers and actual Holfi repairers, you'd like to amend your very helpful NB3 document accordingly. It's interesting to see different fitting options of the same identical board, as we have now seen with the Standby switch and the two Resistors in the Power supply.
@huggygood
Thanks for the acknowledgement and your interest! At this moment I feel like "O man! Let me finally find the broken part(s)!" more than to modify the unit. The proposen Mods. would mean to completely unmount the second PCB which was enough of a drag with the first one. I'll very probably rebuild the original hardware as the rigid wires are really rigid and I don't see risk there. I agree, though, that their use can be validly questioned to begin with...
Well, the Amp actually belongs to a HiFi friend wanting it back soon now. What I may still do, though, because it's so simple to measure, straight forward to implement and effective electronically: Add Snubbers to all secondary transformer windings, which may even improve sound (at least slightly
).Greetings,
Winfried
Hello friends,
the NB3 works again, just Transistor Q3 had died. Actually a hobby friend found the failed Transistor, he's a very experienced repairer (which I'm obviously not ).
Analysing the schematic it was also found out that D7 does not really work as a protection if the Standby switch is used! So 1N4001s were added (anti)parallel to each relay coil. This is a small, but actually a strongly recommended modification for all NB3s!
Hi Mogens,
thanks again for helping out and for documenting the HolFi Amp!
The component R13 actually is (and needs to be!) a 1k2 Resistor, so your schematic should be corrected as it spells out R13 as 22k.
So, this ends my HolFi adventure after long months... At this time the owner listens to his beloved NB3 Amp and I don't know yet if he'll ever be back for recommendable modifications like active rectification and/or transformer snubbing...
Well, so long then for the time being,
Kind greetings
Winfried
the NB3 works again, just Transistor Q3 had died. Actually a hobby friend found the failed Transistor, he's a very experienced repairer (which I'm obviously not
).Analysing the schematic it was also found out that D7 does not really work as a protection if the Standby switch is used! So 1N4001s were added (anti)parallel to each relay coil. This is a small, but actually a strongly recommended modification for all NB3s!
Hi Mogens,
thanks again for helping out and for documenting the HolFi Amp!
The component R13 actually is (and needs to be!) a 1k2 Resistor, so your schematic should be corrected as it spells out R13 as 22k.
So, this ends my HolFi adventure after long months... At this time the owner listens to his beloved NB3 Amp and I don't know yet if he'll ever be back for recommendable modifications like active rectification and/or transformer snubbing...
Well, so long then for the time being,
Kind greetings
Winfried
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Hi Winfred,
That's great news. Well done.
Somehow I missed your previous message. It's a good suggestion to update the schematic. I will do so and post a link to the updated one when I'm done. Then you could perhaps review it so we can make sure I have fixed all missing bits and errors.
Mogens
That's great news. Well done.
Somehow I missed your previous message. It's a good suggestion to update the schematic. I will do so and post a link to the updated one when I'm done. Then you could perhaps review it so we can make sure I have fixed all missing bits and errors.
Mogens
Hi Winfred,
Another question. What pre-amp are your friend using with the NB3? I have been wanting to documenting a Holfi pre-amp. But, haven't found one that was faulty I could pick up cheap. I suspect it might just be a scaled down version of the power amp circuit but it would be interesting to check.
Cheers,
Mogens
Another question. What pre-amp are your friend using with the NB3? I have been wanting to documenting a Holfi pre-amp. But, haven't found one that was faulty I could pick up cheap. I suspect it might just be a scaled down version of the power amp circuit but it would be interesting to check.
Cheers,
Mogens