How I fixed my DSL401 Overheating Problem

[Mars Amp Repair]

My take on why the rectifier needs to be so large

I agree with Enzo, why is such a large rectifier (for such a small job) heating up in the first place?

My take is that most components are not designed to work constantly at a nominal & certainly their max potential. The current for any amp goes up as the volume goes up and at idle draws less. Most components perform differently at a high temp than the nominal one. Transistors for example will have more gain at higher temps & can run-away if the circuit design allows.

With respect to the bridge filament supplies, the demand is relentless EVEN when the amp is on standby. The current draw (albeit not so high) & heat of the bridge never changes.

One other possibility I hadn't considered is if the supply isn't filtered adequately, the bridge ends up conducting for longer during the AC cycle instead of the filter caps holding the charge during most of the cycle...in the latter the bridge only needs to 'bump' the filters at the height of the input ac sine wave (plus side & minus side in the case of a bridge rect) to keep the filters charged.

IOW, if you remove the filters all together, the bridge would need to be conducting for all 360deg of the ac input cycle thereby changing its duty cycle from about 10% to 90%. this would definitely make the bridge overheat.
I'll have to check how well those filters actually filter the filament voltage sometime. If there is appreciable ripple on it, then the answer is inadequate filtering. g

[melvin]

The conduction angle might have something to do with it if the first filter cap is too big.

[Mars Amp Repair]

The conduction angle might have something to do with it if the first filter cap is too big.

Well, a larger input filter would only affect the bridge upon initial start-up (I believe that's what you mean by 'conduction angle)...since this isn't a tube rectifier the input cap isn't nearly as significant at start up, except perhaps for selecting the type of fuse for initial startup surge.

I believe once the filter cap is charged up (after a couple of cycles) the size of that cap wouldn't affect the operation of the supply circuit except to reduce the demand on the bridge rect by holding the charge for a higher duration of the cycle.
The designed capacity of the filter caps is going to be somewhat proportionate to the current demand of the circuit. The more current necessary AND higher duty cycle of the load (in this case 100%), the higher the capacity needs to be. g

[Steve Conner]

It's the other way round. The bigger the filter caps, the smaller the conduction angle, and the hotter the rectifier gets.

(Smaller conduction angle means higher RMS current in the diodes, for a given average DC output current.)

[Mars Amp Repair]

It's the other way round. The bigger the filter caps, the smaller the conduction angle, and the hotter the rectifier gets.

(Smaller conduction angle means higher RMS current in the diodes, for a given average DC output current.)

yeah I hear ya Steve...I'm talking about duty cycle of the bridge after the initial charging of the filter cap. If the load isn't huge, then the filters hold the charge fairly high between cycles & the bridge rect merely needs to 'bump' the top of the sinewave where the filters have just begun to discharge from the load....glen

[solderball]

Math Alert!

All -

I own two of these amps. One I bought new, one I bought used and broken. The broken one had this issue. I took it apart and noticed that a prior repair attempt had added jumpers to the bridge pins and re-flowed the solder. The solder repair had a dull finish, so I removed the bridge and tested each diode. All passed, so I reinstalled the part. Amp was still dead. I then bought some BR102's from Mouser and installed a new one. The amp now works fine.

I can't see how that part heats up at all.
If, for example the tube heater requires 10 mA and two diodes are forward biased at a time (at say 1V), the power dissipation would be .010 x 1 x 2 = 20 mW. I added a piece of "stick on" heat sink that I got at work and have never had another problem with this amp. I gig regularly with it and don't bring a backup.

I think the issue has more to do with the crappy PCB quality of the Marshall circuit. I am an electrical engineer and have designed high reliability communications equipment for 30+ years. I had one of the power supply engineers at work check out the board and he said it was pretty typical two-sided PCB design.

The solder reflow issue seems to be a quality issue rather than a design issue. I solder using a stereo microscope and high quality tools. I have never seen such a weird solder reflow as I did when trying to do the quick fix initially.

Also, every assembly screw seemed to be stripped in the amp that had been repaired. I think these are built as cheaply as possible and this is just a symptom of that

My $.02

Thanks, Mike

[jrfrond]

Usually, upgrading the bridge and elevating it off the board, together with proper soldering, is enough to remedy this, but there are instances when bridges do need heatsinks, and it can't hurt. Anyone who has worked on Soundcraft mixers knows that their smaller CPS power supplies have a nasty habit of scorching the PCB surrounding the bridges. Fortunately, the PCB is fiberglass, but the remedy, plus a heat sink is the same. Fans don't hurt either.

[bnwitt]

Replacing and elevating the bridge rectifier up off the board like Enzo suggested some time back is what I have done to several of these amps that have come through my shop. None of them have returned with the same problem so you're right on that JRfrond.

[oc disorder]

I suggest complete resoldering of the board. I had a couple of these with the rectifier problem and unfortunatly
after reassembly and some hours testing they would cut out again unpredictably.
(I replaced the rectifier elevating it and as Enzo suggested used a bit more length to solder onto the tracks
scraping away some of the tracks insulating coating to help dissapate more heat.)
Looking over the board with very good magnification there were many suspect cracked solder joints .
After resoldering ,reassembly and retesting the amps worked flawlessly.
There's a lot of heat in these and if the soldering process is a bit light on in the first place the obvious conclusion can be drawn.
If you tackle it yourself make sure you make careful note of where everything goes before dissasembly.
A few detailed photos before can be a big help too for reassembly.

Here's an old post showing labeling of leads prior to dismantling.

http://www.piller.at/music/dsl201/

The heat sink and fans should not be necessary but it works for some !

[bnwitt]

I suggest complete resoldering of the board.

Yeah that is getting to be a standard process for me with all wave soldered PC boards. The inconsistent deposition of solder from landing to landing is amazing. Globs like Mount Everest and points where the solder is thinner than the silvering on a mirror. I usually resolder every vacuum tube socket pin, rectifier pins, voltage regulartor pins etc etc. Anything that produces a lot of heat and anything that looks really thin.

[jrfrond]

I suggest complete resoldering of the board. I had a couple of these with the rectifier problem and unfortunatly
after reassembly and some hours testing they would cut out again unpredictably.
(I replaced the rectifier elevating it and as Enzo suggested used a bit more length to solder onto the tracks
scraping away some of the tracks insulating coating to help dissapate more heat.)
Looking over the board with very good magnification there were many suspect cracked solder joints .
After resoldering ,reassembly and retesting the amps worked flawlessly.
There's a lot of heat in these and if the soldering process is a bit light on in the first place the obvious conclusion can be drawn.
If you tackle it yourself make sure you make careful note of where everything goes before dissasembly.
A few detailed photos before can be a big help too for reassembly.

Here's an old post showing labeling of leads prior to dismantling.

piller.at MUSIC

The heat sink and fans should not be necessary but it works for some !

Any experienced tech will check over the general solder conditions of any PCB. It's just the way of the world, and BTW, this goes double for anything made in the UK, which for some reason (no offense to our brothers and sisters across the pond), can't really seem to get their wave-solder act together. Just an observation across many brands from the the region that I have serviced that are riddled with cold solder joints (e.g. Marshall, Ashdown, Allen-Heath, Soundcraft, Soundtracs, etc.). Of course, now with RoHS solder being used, this is a very common issue across the board. I haven't even gone into the nightmares of SMD's.

Wholesale resoldering in many instances is a great idea, but thermally-damaged components and PCB's must be addressed first, and if the addition of a heat sink that costs a couple of bucks will increase the chances of a unit staying in the field, then I am all for it.

[bnwitt]

Wave soldering

Here is the wave soldering in the line 6 unit I'm working on. See the crack and almost no solder present on pin 2. There was a crack on pin 3 as well. In the background you can see the globs of solder on the other power tube socket.

[jrfrond]

Here is the wave soldering in the line 6 unit I'm working on. See the crack and almost no solder present on pin 2. There was a crack on pin 3 as well. In the background you can see the globs of solder on the other power tube socket.

In my opinion, having worked in manufacturing, tube sockets or any other large components that can sink a decent amount of heat should NOT be wave-soldered, but rather hand-soldered. The success of wave-soldering depends on proper board prep, cleanliness and temperature of the solder bath, and proper feed speed. Hand-soldering takes extra time = $$$. Board prep takes time = $$$. Solder bath maintenance requires down time = $$$. Proper feed speed (i.e. not too fast) takes extra time = $$$.

Get it? It's all about money!

[Jazz P Bass]

Large components should take a longer preheat cycle.
Ideally.

[steven13]

how much should a tech charge for this fix... I have a 1999 model and would like a ball park estimate to get it fixed