bass amp hum - cheap to repair?

ecc83

That transformer does not have thru bolts, it has welded lams.

You could still try though with some M6 treaded rod and nuts (often called "studding") . Easier to do with the transformer removed but they seem to have "done cheap" and not used connectors onto the PCB (even tho' the board seems to have legends for them!) .

I used to put the traff in a big Mother of a vice and then tighten the bolts. If you don't want to remove it* you can use a small vice to "nip" each side.

*Personally I would chop the wires at about 1/2 way and refit with "choc-block" terminal strip.

If you bugger about with the mains traff/wiring in any way you must get the amp PAT tested before you sell it..Well, get it done anyway!

Dave.

ICBM

I would think about finding an equivalent-spec toroid, if there's physically room to fit one. If it can't be mounted on the chassis - I'm not sure there's enough room in the footprint of the original - you could maybe bolt it to the end wall of the enclosure, so it's 'upright' relative to the back panel and should fit. You just need to leave slightly longer cables so you can wire it up before attaching the panel - or use a choc-block strip as Dave said,

ecc83

"I would think about finding an equivalent-spec toroid....." Good think there Batman!

If the caps have an 80V rating the rms windings should not exceed 50volts. No idea of the power output but even an RS Comps 50-0-50 500va toroid is only 53quid +VAT and you will certainly find a better price than that.

Small point, you might have to up the input fuse rating to at least the next standard value.

Dave.

ICBM

If this sounds like something @Paul_C would be interested in, I think I have a couple of toroids from similar-powered amps I've scrapped that might do.

Good point about the fuse, the surge current in a toroid is higher.

jpfamps

ecc83 said:

"I would think about finding an equivalent-spec toroid....." Good think there Batman!

If the caps have an 80V rating the rms windings should not exceed 50volts. No idea of the power output but even an RS Comps 50-0-50 500va toroid is only 53quid +VAT and you will certainly find a better price than that.

Small point, you might have to up the input fuse rating to at least the next standard value.

Dave.

Fitting a 500 VA toroid in there could interesting.

Paul_C

I'm going to wait and see what it comes back like, but there look to be a few things to consider if it's still bad - thanks folks !

ICBM

jpfamps said:

Fitting a 500 VA toroid in there could interesting.

Yes. I think I have at least one in the 300VA range which might be enough and which might fit if mounted 'sideways' to the cabinet, though. Came out of an Ashdown 250W amp I think.

ecc83

ICBM said:

jpfamps said:

Fitting a 500 VA toroid in there could interesting.

Yes. I think I have at least one in the 300VA range which might be enough and which might fit if mounted 'sideways' to the cabinet, though. Came out of an Ashdown 250W amp I think.

I chose 500VA to get a "worse case" price. Since I have no idea of the power output of the amplifier (claimed or real!) I could not be more precise.

Dave.

ICBM

Looks like a claimed 250W from the model name, and the size of the original PT would be about right for that. It might not be quite that much in reality, but I don't think the Ashdown is either!

Paul_C

Resurrecting this old thread because:

1) I still have the amp.

It still buzzes.

I occasionally think about either buying a new bass amp or fixing it up, and fixing it is far more likely to be the cheap option, so . . .

If I can fit (or get fitted) a transformer that will replace the rattly one then all should be well.

I'm capable of removing and applying solder and I can follow instructions (including don't do this yourself, if applicable) without doing anything I've not been specifically told to do.

I have both analogue and digital multimeters but nothing more than that so if there is something more I'd need I'd happily pass on the job rather than kill myself or blow up the amp.

I've found a wiring diagram which will hopefully help those with knowledge and save their precious time:


http://music-electronics-forum.com/attachments/31837d1418249947-nc320-sch-final.pdf


So if someone could assist and either point me at or supply a replacement that won't buzz I would be very grateful

ICBM

The schematic isn't a lot more help since there's no voltage spec on the PT outputs - although it's a near certainty that it will be whatever is needed to produce a DC voltage of at least 63V and less than 80V at 253V input. (Since it shouldn't exceed the cap voltage rating, but if it was less than 63V they would use the next lower-rated caps, since they're cheaper.)

I have noticed something though - there appears to be an unused 240V tap on the PT (labeled 2) as well as the 230V one which is in use. It's a long shot but it's worth moving the neutral connection to there and see if it works better - there's a small chance that it will change the magnetic field in the transformer enough to stop it buzzing.

It's hard to see from the pic but it will be one of the white, grey, blue and black wires. It looks like the white wire from the socket neutral goes to the PCB and is then jumpered to the white wire from the PT, with the blue having an unconnected terminal. If so - and make certain that this is the case before you do anything - moving the jumper to the blue will do it.

Paul_C

Thank you very much

I'll have a look later and see what goes where and maybe take some more pictures.

clarkefan

Why not just send it to @ICBM, get a pro to look at it?  You clearly value the amp, why not get it sorted properly once and for all? Buzz can be a nightmare to track down.

I'm in the same boat with my DSL, but he'd probably say no chance

Paul_C

I don't value it particularly, it was given to me for free

If he was close by I would have handed it over ages ago, but we're around 340 miles apart.

Paul_C

The Grey,Black,Blue and white all go to the board as follows:

Grey - to a box marked P1  220-240v -  which links it to a connector marked AC1 and then on to the mains socket, where it's connected to one end of the fuse. The other end of the fuse travels up to the ON/OFF switch and returns to the mains socket and the connector marked L.

Black - to a box marked P2 100-120v -  which links it to a connector marked AC2 and then on to the mains socket, a connector which sits between the fuse connectors and the one marked N.

Blue - to a box marked P3 100,220v  - spade connector marked AC3 - not connected to anything

White - to a box marked P4  120,240v - connected via another link to the ON/OFF switch, and then back to the mains to the  connector marked N.

So Live is connected via the ON/OFF switch to P1 (Grey)
Neutral is connected via the ON/OFF switch to P4 (White)

Blue isn't connected to anything.


So unless I've misunderstood, I might be able to move the connector currently connected to P4 (120,240v) over to P3 (100,220v) and see if that makes a difference ?

ICBM

No - unfortunately it's already wired for 240V, which is not as the schematic shows - the blue must be the 220V tap not the 240.

Paul_C

boo.

Paul_C

So I'm back to your suggestion of finding a suitable replacement (assuming you don't have something you can sell me).

I tried Marshall but they were no help whatsoever.

ICBM

I'll have a look. I definitely don't have anything that shape I don't think, but I might have a couple of big toroids.

You have one major advantage - it's a single winding on the secondary. That simplifies it a lot compared to needing one with more than one, all of which need to be correct.

Paul_C

ICBM said:

I'll have a look. I definitely don't have anything that shape I don't think, but I might have a couple of big toroids.

You have one major advantage - it's a single winding on the secondary. That simplifies it a lot compared to needing one with more than one, all of which need to be correct.

at least I have one thing going in my favour - as long as a replacement doesn't cost the earth I'll have a usable amp for very little cost.