Marshall DSL401 - Fading Out

[Kryzsky]

Thanks for your help in the other thread Enzo, taking your advice and starting a new one. I was off the mark and found the rectifier bridge immediatley on the schematic after reading your post.

Amp started fading out recently, the signal would drift out and all I would hear is quiet hiss. So after reading the other thread and discovering that this is a commom problem with the DSL401 I went hunting for the rectifier diodes, (BR1).

I took a voltage reading across the neighboring resistor, heard some crackling and the amp came back to life. Might have just touched the plastic casing of the rectifier. I've been playing the over an hour now, no more fading out.

But, when the amp was not working, some of the tubes were not on, shown in photo.

Is this looking like a faulty heater/bridge rectifier?

[Enzo]

More likely - in my opinion - is that the solder didn't take to the legs of the bridge very well. But poor solder can still make the connection, and through cycles of thermal expansion, the wire leads can settle to a position that doesn't make good contact. Pushing on most anything will flex the board, and restore the connection. It will be all but impossible to push the thing into the non-contact state. But it is still not fixed.

REmove the bridge amd clean its legs with sandpaper or a fine file or a little Dremel wire wheel, or something. Or just replace it. Either way, we get a fresh solder connection. DOn't just reheat the solder, that doesn't clean the oxidation off the legs. And resolder that resistor anyway.

This is a common problem on all Marshall models with DC heaters. I have a TSL100 sitting two feet from me about to get the same thing done to it for the same reason.

[MarkB]

My 2 cents worth...

Enzo correctly nailed a similar issue I had a couple of months ago on my DSL401. Your preamp tubes will not fire up if your BR1 is faulty or has a bad connection. Replace it ASAP. It's a 3 dollar part. Also look around for a heat sink for BR1. You can use an old CPU heatsink if you have one (you may need to cut it to size).
Also look into a 115AC fan you can mount to the rear of the amp.
As always, make sure you bleed off the high voltage caps before messing around with this amp, and recheck with a meter per Enzo.
MarkB

[Kryzsky]

The amp has been working fine for the past few days, after giving the BR1 a nudge. But I am planning on swapping it out asap.

Any reliable places to order one online?

A heat sink is a great idea, would it have to clip on or would I be able to glue it on with a heat transer compound?

And I have been thinking about installing fan, I just wish there was somewhere for the air to flow. Not much room in there, seems like all the small pc fans out there are need DC, most likely look around for 12V DC and tap in.

[Enzo]

One thing you can do is mount a new one in the air instead of tight against the board. Give it a half inch or so of air space under it. Not only will this allow some air to circulate, but it also allows the lead wires to act as heat radiators.

I don't know about ordering in Canada, but in the USA I use Mouser a lot, and also Allied and Digikey. These companies ship worldwide, but I bet Canada is more of a special case than Europe or somewhere. COntact them and ask. I am sure there are other good suppliers, but from those three I rarely need to look elsewhere.

www.mouser.com
www.alliedelec.com
www.digikey.com

[MarkB]

Br102

http://www.mouser.com/Search/Product...XnmYlBrQ%3D%3D
That will get you a rectifier. Also check with Radio Shack, I swear I saw a BR102 in a drawer a couple of weeks ago.
I haven't done a fan mod yet, but many people have. I did get a Radio Shack 115AC fan model 273-0241. I haven't mounted it yet. I don't know if it is going to be more effective pulling the hot air out, or blowing air in. Also the last time I was in the amp I added output jacks for checking the bias.
http://www.eurotubes.com/index.htm for new tubes if you are not already aware...
MarkB

[Kryzsky]

Can't thank you guys enough. I have enough Tech experience to get the job done, but I definitely needed help identifying the problem and troubleshooting.

I order components through Mouser, DigiKey and Allied all the time at work. Had no idea that a BR1 is such a common part, easy!

mount a new one in the air instead of tight against the board.

Will do!

And thanks for the tubes link Mark, much appreciated.

[Enzo]

Yes, it is just a plain old vanilla 6A 200v bridge.

ANother idea: Instead of sticking the legs through the board and soldering them as they stand - the way it was originally - remove the coating from the copper traces near the pads, and where the wire lead comes through. bend it over and lay it along a trace for a little way, then solder it along the trace instead of just the immediate pad. If you can lay it down on the trace for maybe a quarter inch on each leg, that will give it a LOT more mechanical strength as well as extra dissipation help.

[AdamColliss]

Hi

I have exactly the same problem with my DSL401, I'm from the UK so could someone let me know if this is the correct Bridge Rectifier to replace BR1?

http://www.maplin.co.uk/Search.aspx?...=AR83E&DOY=8m5

(Bridge Rectifier I was looking at is KBPC1002)

Thanks and great forum!
Adam

[Metro]

Yes, it is just a plain old vanilla 6A 200v bridge.

ANother idea: Instead of sticking the legs through the board and soldering them as they stand - the way it was originally - remove the coating from the copper traces near the pads, and where the wire lead comes through. bend it over and lay it along a trace for a little way, then solder it along the trace instead of just the immediate pad. If you can lay it down on the trace for maybe a quarter inch on each leg, that will give it a LOT more mechanical strength as well as extra dissipation help.

Hello, This is a marvelous resource for electron tamers.
This is my first post so Im not sure if this is the right place to post. Im in the process of trying to track down an intermittent fade out in a TSL 601. It sounds similar to this 401 issue, maybe?
Enzo, Good idea with the raised mounting as the Rec is stuck right on the PCB. I thought the BR102 spec is 10A 200v, No?
Anyone have any idea if the BR1 rectifier failing would cause the voltages on the V1-3 anodes to rocket from 230DCVish up to 470DCV. Tubes are all new tested with a b&k and when swapped out for other known goods fault still there.
When the fade out fault develops, after 5-10 mins, V1-3 heaters drop to zero, plates and R60,61,62,64 test up to 470vdc on both legs, then the amp can sometimes stabilize back down to normal after a minute or two, but sometimes not. Ive ordered new BR1 from mouser and thought id replace all the caps in the DC heater section with higher temp ones while im at it. Does that sound reasonable?
Anyhelp greatly appreciated, hope Im not offending anyone by posting here as it seems to relate to my problem.
With respect, Metro

[Enzo]

Yes, a loss of heater will cause plate voltage to soar up to supply voltage - it can;t cinduct current without a heater, so there is no voltage drop across the plate resistor. You will also find the cathode sits at zero volts now.

I don;t know what current your bridge might be, but it has to popwer up how many 12AX7 tubes at .3 amp each? It doesn;t burn out from lack of current whether 6 or 8 or 10 amps. My experience is the legs were not cleaned at the factory and solder never wetted them well. Nice new clean legged bridge soldered in well should work forever.

Metro, welcome to the place. You will get a lot better response starting a new thread for your amp than by tacking onthe end of a thread that has been dead half a year.

[antrecords]

DSL 401

+1 on this problem

10 OCT 2010