NAD - Jansen Bassman

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Lyle
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Re: NAD - Jansen Bassman

Post by Lyle »

https://streamable.com/np5txg

Quick clip of some buzzing here.

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Re: NAD - Jansen Bassman

Post by Lyle »

rickenbackerkid wrote: Tue Apr 05, 2022 12:07 pm I particularly like the new undersized filter caps, wedged in place with bits of mains wire!
That was my highlight too!

I think it looks a bit worse than is actually. I need to plan out the easiest/safest way to clean it up.

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Re: NAD - Jansen Bassman

Post by jeremyb »

I'm heading off for a tetanus shot after looking at that!!! :lol:
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Re: NAD - Jansen Bassman

Post by Lyle »

jeremyb wrote: Tue Apr 05, 2022 12:38 pm I'm heading off for a tetanus shot after looking at that!!! :lol:
I should have taken a picture of all the mouse poo I had clear out of the head cabinet.

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Re: NAD - Jansen Bassman

Post by RectifiedAmps »

That’s definitely an early model Bassman- you can tell by the rectifier arrangement. Strange that someone replaced the first two filter caps in the power supply but not the rest. The remaining two older caps could be partly to blame for hum.

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Re: NAD - Jansen Bassman

Post by Jay »

What's not to like?

Vertically mounted soldering strips, 25W rail heater, few dry joints, ancient diodes, dried mouse piss and a parts layout that looks worse than a traffic jam in Mumbai...

Love it :D
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Re: NAD - Jansen Bassman

Post by MikeC »

Jay wrote: Tue Apr 05, 2022 2:27 pm What's not to like?

Vertically mounted soldering strips, 25W rail heater, few dry joints, ancient diodes, dried mouse piss and a parts layout that looks worse than a traffic jam in Mumbai...

Love it :D
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Re: NAD - Jansen Bassman

Post by sizzlingbadger »

I wonder what that little signal diode is in there is for.
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Re: NAD - Jansen Bassman

Post by murky »

What do they sound like? Big and clean? What about when dimed?

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Re: NAD - Jansen Bassman

Post by Lyle »

murky wrote: Tue Apr 05, 2022 8:22 pm What do they sound like? Big and clean? What about when dimed?
Yeah big and clean. I tried to crank it up again this morning and some smoke escaped... it's still working but I'll have to do some more testing, I suspect it's one of the ancient looking filter caps.

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Re: NAD - Jansen Bassman

Post by Anvil Amps Alan »

sizzlingbadger wrote: Tue Apr 05, 2022 6:22 pm I wonder what that little signal diode is in there is for.
Bias rectifier
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Re: NAD - Jansen Bassman

Post by RectifiedAmps »

Lyle wrote: Wed Apr 06, 2022 9:37 am
murky wrote: Tue Apr 05, 2022 8:22 pm What do they sound like? Big and clean? What about when dimed?
Yeah big and clean. I tried to crank it up again this morning and some smoke escaped... it's still working but I'll have to do some more testing, I suspect it's one of the ancient looking filter caps.
In your last photo of the guts you can see a big resistor (10k?) that looks charred- it’s next to one of the old 47uf capacitors. Something is pulling too much current through that resistor so it may be the source of the smoke. Could be the 47uf cap is shorting a bit, or something else, but I’d start looking there.

Also, if you check the voltages at the various nodes along the power supply, you should find a spot where it’s much lower. That’d be a good indication as well.

And please be careful! There could be >500v of DC inside that amp, so watch out if you’re testing on a live circuit.

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Re: NAD - Jansen Bassman

Post by Lyle »

RectifiedAmps wrote: Wed Apr 06, 2022 9:58 am
And please be careful! There could be >500v of DC inside that amp, so watch out if you’re testing on a live circuit.
Thanks!

I've read up enough to know when things are getting dangerous. I'll get my friend who knows what he's doing to come show me the ropes before I mess with a live circuit.

*edit* I'm also aware a dead circuit can still be very dangerous!

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Re: NAD - Jansen Bassman

Post by Lyle »

RectifiedAmps wrote: Tue Apr 05, 2022 11:39 am A hum is pretty standard on many Bassmans due to their horrific grounding scheme.

If it helps, here’s a schematic (attached).
Going through the schematics it looks like a few of the mustard caps are the wrong value (or in the wrong place??), and the voltage readings seem to be a lot higher than shown on the drawings.

More testing to do!

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Re: NAD - Jansen Bassman

Post by riffer »

sizzlingbadger wrote: Tue Apr 05, 2022 6:22 pm I wonder what that little signal diode is in there is for.
That signal diode is part of the bias section. It's to rectify the 50V secondary tap from the power transformer to DC voltage. It's smoothed by that cap, and then reduced using the two resistors in there. From there it goes to the Control grids on the power tubes.

I have a fully restored one of these and I'm currently converting a Silverface Bassman 50 to original PTP handwired spec, so happy to help if you have any questions. As I'm an amp tech I can even fully restore this amp for you too.

Cheers,
Simon
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