Marshall DSL401 filaments

[AtomicMassUnit]

I'm trying to sort out a filament problem with a marshall DSL. There's a website and a couple threads on AGA about a common problem with them and heat/bad solder joints around this bridge rectifier. I've resoldered everything around that area but I'm still trying to figure out how the preamp tube heaters work. I'm not getting any voltage on the first three preamp tubes. Any of you guys understand these or had experience with this problem?

[AtomicMassUnit]

Ok, making progress. The heaters for the EL84s and one 12ax7 are all connected, then there is a bridge rectifier supplying the first three preamp tubes. I don't get any AC voltage between pin 9 and pins 4 and 5 of the 3 preamp tubes. So, are these running on DC?

The amp's actual symptoms were that it worked but after 3-4 minutes the sound would fade away completely, which seems like the standard problem these have with heat and this little rectifier... I resoldered everything around there and now I get no sound at all and can't see any glow from the first 3 preamp tube heaters. Does this sound like the little bridge rectifier is bad? Could I just make one out of diodes to replace it? I have some 1n4007s lying around... I don't know if they could handle the heat. What do you guys think?

[Enzo]

Yes they run on DC, that is what a bridge rectifier does to AC. If the AC gets to the bridge, does DC come out?

Remove the bridge and scrape the legs clean and resolder it to the board. That is why bridges need resoldering in the first place - the bridge legs oxidize and don't wet with solder well enough. Or get a new bridge. You could make one from discrete diodes, but I wouldn't bother unles it was an emergency.

3 12AX7s means about .9 amps, so a 1 amp 1N4007 would be marginal. I'd use 3 amp diodes if I went discrete.

[WMK]

Yes they run on DC, that is what a bridge rectifier does to AC. If the AC gets to the bridge, does DC come out?

Remove the bridge and scrape the legs clean and resolder it to the board. That is why bridges need resoldering in the first place - the bridge legs oxidize and don't wet with solder well enough. Or get a new bridge. You could make one from discrete diodes, but I wouldn't bother unles it was an emergency.

3 12AX7s means about .9 amps, so a 1 amp 1N4007 would be marginal. I'd use 3 amp diodes if I went discrete.

Great advice and in addition to what you
said, I might add that if Marshall would have
installed this part using the full lead length
of the bridge, the problem would never happen.
Failures I've seen (many) are in direct relation
to heat transfer from the device to the joints.

[AtomicMassUnit]

I've taken it, cleaned it and reinstalled it and the amp will work for a good 10 minutes or so, then they shut off again. I may just have to buy another bridge rectifier and use the full lead length like wmk suggested.

[WMK]

I've taken it, cleaned it and reinstalled it and the amp will work for a good 10 minutes or so, then they shut off again. I may just have to buy another bridge rectifier and use the full lead length like wmk suggested.

Just another point to consider based upon
what you've done and the continued trouble:
these are double-sided boards and the problem
may involve an intermittent feed through from
one side of the board to the other at one of
the bridge connections. Using the full lead length
of a new bridge will also allow you to solder both
sides at each of the four leads. (Just a possibility)

[AtomicMassUnit]

Thanks guys for your replies. We fixed it. I ended up just going and buying some big diodes and making a bridge rectifier and I floated it about an inch above the board, really did a neat job, and it works well. I am going to install a fan as well to keep the amp cooler, but I've been running it constantly and it holds up just fine. Thanks!

[Amp Kat]

That's a very common problem with that series and many more Marshalls and they evidently didn't calculate the power consumption right as the rectifiers will get way hotter than they are supposed to and will start turning the PCB dark. Eventually it will either open up are cause a cold solder joint and start messing with the B+ because all of those grounds are tied together and I've seen some freaky things that will even make you think you OT is bad. Changing that rectifier seems to cure the problem only being a quick fix for the time being as it's going to do it again but the higher rated diodes as ENZO suggested are a step in the right direction.

[Thorny]

Disappointed in Marshall

I am convinced EVERY one of these amps fail. They are great sounding amps in my opinion - that is the only good thing that can be said (unlike many of their newer amps). But Marshall has to know about all the failures and the cause - so how could they continue to put something like that out? Fender made modifications to the hotrods after they had multiple failures - they did not continue putting out the same amp that would fail the same way every time. I have seen them (a 401) on the floor go out before they are sold (plus they shipped many amps with that deplorable flyingC/svetlana marshall branded 12AX7 preamp tube that was TERRIBLE - I have had amps where EVERY SINGLE ONE was so bad they were unusable). I can understand cheap, but that really is trashing your customers. The fix isn't difficult and probably could be done for little or no additional cost per unit - they simply must not care. Getting parts from them is not fun either - I am not sure I can any longer directly.

If you have a DSL201 or 401 it will fail (or it already has and someone fixed it already).

I am very dissapointed.

[Kryzsky]

Hi guys, I have a DSL401 which just recently started fading out. Became more frequent until yesterday when it stopped completely. Just normal quiet static on the speaker right now.

Of course I'm suspecting the bridge rectifier diodes after reading through this thread. I have the schematic, trying to locate these diodes right now.

Am I correct in guessing that the bridge is made of 4 diodes? I'm looking at D1, D2, D3, D4, D5, & D6. getting some funny voltage reading off of these.

28V across D1 & D2.
185V across D2 & D3.
0V across D4 & D5.

Any help would be appreciated, thanks!

[Enzo]

Hi Kryzsky. Welcome to the group. FIrst, may I suggest you just start a new thread for your amp instead of bringing back a thread dormant for a year and a half. WE got the room.

If your amp has the problem, then your first couple preamp tubes would not be heating - the two orange glowing spots would be dark in them. If those heaters are still glowing, then this is not what is wrong, and replacing the rectifiers won't help.

However if this IS your problem, then those diodes are not where to look. They seem to be about where they should be by the way. The heater rectifier is a bridge rectifier, not four diodes wired into a bridge. Look on the schematic - BR1. It is located on the main page of the schematic, page 1, and is to the right of the power tubes, just above the OT, and lower right from V3, right under its tone stack. BR1. I forget if this is a square one or a round one. But look for BR1 on the board.

On the schemo, it is drawn with four diode symbols, but note they have no individual numbers, there is a part type indicared in the center of them, and the whole thing is called out as BR1.

[Zouto]

Higher rated diodes?

Thanks guys for your replies. We fixed it. I ended up just going and buying some big diodes and making a bridge rectifier and I floated it about an inch above the board, really did a neat job, and it works well. I am going to install a fan as well to keep the amp cooler, but I've been running it constantly and it holds up just fine. Thanks!

Hi ATOMIC, what type of diodes did you take?A friend's DSL 401 just started fading out.He had put in a fan last year and asked me now to change the bridge rectifier for the heater circuit quits quite often now. To make his amp reliable I'd like to put in four higher rated diodes instead of another BR102.
Zouto

[oc disorder]

The BR102 is 10amps 200v.It should be ample for the job but I often use
the BR108 10amps 800v 'cause I can get them over the counter
Attached part of a spec sheet for the bridges.

[Enzo]

That bridge is more than ample for those couple of tube heaters. The main problem I encounter with them is the wires coming out of the bridge have oxidized surfaces and the solder didn't wet to them well. Polish the legs, or sand them, or scrape them with a razor blade or Xacto. Expose fresh metal on the bridge legs then solder them in well. If the pads are damaged, then bare the traces and lay extra lead length along them and solder down. Or worst case, run wires.

[Zouto]

Thanks

That bridge is more than ample for those couple of tube heaters. The main problem I encounter with them is the wires coming out of the bridge have oxidized surfaces and the solder didn't wet to them well. Polish the legs, or sand them, or scrape them with a razor blade or Xacto. Expose fresh metal on the bridge legs then solder them in well. If the pads are damaged, then bare the traces and lay extra lead length along them and solder down. Or worst case, run wires.

First of all THANK Y'All,
so could I use the old BR102 or should I get another new one with longer "legs" to mount it a bit higher above the board?
Zouto