Lab Series L5 keeps blowing a fuse.

[stonerbox]

First and foremost I would like to state that I did not know where to post this or if Stompboxes was the right forum to begin with, but anyway here we go!

I got a dear ol' L5 that I use at home from time to time. When I first got it, it was dead, it powered on but with no sound coming out of it. So I opened it up and saw that the problem was as simple as a blown fuse. I replaced it and used the amp without any trouble for a couple of months.. and then one day, it was dead again. Yet again I opened it up and saw that the same fuse (marked blue on picture) was blown again... Hmm..?

Symptoms:
1. The amp tends to get hot after say.. half an hour of playing.
2. Fuses F21 & FF22 are substituted with wires!?! (marked red on the picture)

I also added pictures of all the trimpots I could find, who knows maybe they are set to high/low??

"Power supply and reverb driver"

"The fuse"

"Missing Fuses"

"Power-amp trimpots"

"Pre-amp trimpots"

"Pre-amp"


Schematics:

Board Layout: http://www.netads.com/~meo/Guitar/Amps/Lab/Images/Lab_Series_L5_L7_L9_L11_board_layout.jpeg

Full: http://www.netads.com/~meo/Guitar/Amps/Lab/Images/Lab_Series_L5_L7_L9_L11_scheme.jpeg

Edit: sorry for bad grammar and spelling, super tired today.

[joegagan]

my fuzzy memory tells me these came with circuit breakers, which would be problematic and then be replaced with fuses. does this sound familiar to anyone?

[stonerbox]

Nope, I have not found any circuit breakers in mine.

[Fender3D]

F21/22 will protect your rectifier diodes if any power transistor fails shorted.
Better mount 2 fuses there...

Your "blue" fuse has anything related to pre-amp / reverb supply, or maybe CR205-208, CR203/208 or caps C203/208 C204/206.

[stonerbox]

F21/22 will protect your rectifier diodes if any power transistor fails shorted.
Better mount 2 fuses there...

Your "blue" fuse has anything related to pre-amp / reverb supply, or maybe CR205-208, CR203/208 or caps C203/208 C204/206.

Damn, I managed to mark the wrong fuse. I'll replace the picture with the right one marked blue.
Ok, I better see to that FF22/21 gets replaced with real fuses.. must've been a previous owner who have replaced the broken fuses with wires. 

As stated before the entire amp gets very hot after only a short while, usually that could be a sign of wrong/to little load, right?

Thanks for helping me out, I appreciate it a lot!

[teemuk]

The "power amp trimpots" are DC balance and bias. You want to check that both are set right. If the former owner of the amp was dumb enough to replace fuses with wires I'd assume he has also been dumb enough to make random adjustments to the trimpots. DC balance is adjusted to give lowest DC offset at power amp output. Should be 10mV in maximum. Bias is adjusted so that you read about >3mV across each emitter resistor of the output transistors, basically amp should be set to draw 200mA at idle. If in doubt, refer to schematic, it has instructions.

If this doesn't fix overheating and blowing up fuses, then - as always - start with basic troubleshooting and circuit measurements. The schematic lists reference voltages.

[awe]

I have the same problem with a Lab Series L5. Works fine but after a while on high volume it blows a fuse or two.

When I saw the pictures I suspected Stonerbox was from Sweden and the link in his profile confirmed that. The missing fuses is actually a factory solution to meet the former swedish electrical safety board (SEMKO) regulations, instead they've added the four fuses in separate fuse holders. I think the old rules called for double fuses on the transformer secondary.

Anyway... getting hot is probably not the problem. As you can see, the cooling fins are huge so there's a lot of heat dissipation in this circuit!

The fuse in question (and the next to it) is for the power amp supply so the fault is either in the power amp or in the power supply itself. It could be the offset or bias adjustment but I'm not so sure about that. If you haven't changed any components the factory adjustment should be pretty fine. However, I'll see if I can check it later.
My other suspicition is the filter caps in the power supply. You can see the cans hanging next to the transformer, they are connected to the power supply board with two large phillips screws each. If the the amp been in the closet for 15 years (as mine has) the caps can have dried up.

I'll get back when I know more!

[Paul Marossy]

Maybe the filter caps, but I would think it would also sound bad in that case. But I'm not an expert on SS amps by any means.

What about the wrong speaker load on it? Does the amp have the original speakers in it?  I have recently been told by someone that the original speaker manufacturer for the example shown here (which were in a friend's all original L5), "137", was Chicago Telephone Supply (CTS).



Looks like according to the schematic there should be a pair of 16 ohm speakers in parallel for an 8 ohm load.

Just my thoughts...

[awe]

Probably not the speaker(s). This one has only one speaker left which makes 16 ohms but the fuses also blow when I run it in to a 8 Ohm THD Hot Plate.

Otherwise I agree regarding the filter caps, often (but not always) you get a lot of hum if they're bad.

[Paul Marossy]

Probably not the speaker(s). This one has only one speaker left which makes 16 ohms but the fuses also blow when I run it in to a 8 Ohm THD Hot Plate.

Otherwise I agree regarding the filter caps, often (but not always) you get a lot of hum if they're bad.

There's a whole lot of heat sinking going on in that amp. I wonder if maybe some of those heat sinks have gotten a little loose and aren't dissipating the heat like they should be? Probably wouldn't hurt to check all the physical connections on those heat sinks to make sure they are all solid...

[stonerbox]

Sorry for the late response guys. I really appreciate your help! 

Paul: I checked the load and it is correct, same thing with the wiring from both speakers, no sign of trouble there. AWE: I'll take a look at those filter caps you mentioned, they could very well be faulty, since the amp have some years of experience.


Sound vise this amp has a really nice tone to it. Maybe not when driven by a high output fuzz pedal but when in clean you can get some really nice, smooth tones out of it.
Would be a shame to only use this one as a practice amp and I am playing with the idea of using it for all clean sounds on my bands next album. First things first, it needs to be fixed up and working stable.

[Paul Marossy]

OK, check out those filter caps then. Hope you get it working, they are nice amps when working properly. It's one of the few SS amps that I would recommend to someone.

[awe]

A couple of weeks ago I took out the power amp module to look for obvious faults and measure all the power transistors. Quite easy since they're not soldered in place, 10 transistors in TO-3 casing stuck in sockets. I then reassembled everything but it still blew the fuse.
Then I forgot about it until yesterday when I gave it a try, when I turned it on there was a lot of scrathing going on but it dissappeared after a half a minute. Then I played a little on low volume and the gradually raised the volume, finally I cranked it all the way up to 10/10 and the fuse didn't blow! So I connected the tone generator and the THD Hotplate like I did in the first place thinking that it would push it over the top. But no; the fuse still doesn't blow.

So was it the refitting of the power transistors, grindning out dirt of the sockets? Maybe, but I don't think so since it blow the fuse directly after reassembly. Filter caps starting to soften up? More likely. As you might know, from a electrolytic cap's point of view it's better if the amp is used now and then, leaving the amp in a closet for 25 years makes the caps dry out. So if you find an old tube amp that's been in a closet that long, you can get the caps in shape by applying a voltage that is gradually raised over a couple of days (before trying to turn it on at full voltage!). Of course, you need a variac for that.
So I'd like to think that the caps soften up the hard way. The scratching I mentioned is often a sign of something going on in the filtering. Just hoping it doesn't come back...

Anyway, I'm sursprised how good it sounds! Probably the best solid state amp I've tried!



So Stonerbox, where are you located?

[stonerbox]

I am in Sweden.

[awe]

Yes, I figured that out from the jumpered fuse holders since that was done in Sweden to meet former SEMKO regulations. But where in Sweden?


For foreigners: Before Sweden joined EU in 1990, all mains connected electrical equipment had to be approved by a institute under the swedish government called SEMKO. All equipment confirming to the rules were marked with a decal (or embossment in the plastic) with a S in a circle. The regulations was very hard and a lot of equipment were modified by the importers to comply as you can see in the example above. Seems odd to remove fuses and adding another on the mains... but I'm sure they had a reason for it.

[stonerbox]

aha, another swede here in the forum. I am located in the Swedish highlands, near Jönköping.

[awe]

OK, cool, I'm in Halmstad. Where's near Jönköping?

So what's the deal with the amp, is still blowing fuses?

[Woolly Mother Mammoth]

delete...

[stonerbox]

I'm back!

I will start out with what I have found out and noticed so far.. The amp is very noise (faulty power supply/caps/diodes?). I tried, mainly for fun, to tweak the biasing in the power amp, a very tiny bit, and noticed that when I raised/increased the the trimpot (R311 470ohm) the problematic fuse burned in an instant and the amp went silent again, so I replaced the fuse and rolled back the trimpot to half way and so far no problem. The trimpot is marked red in picture below.

Yes, I KNOW, I should have measured the voltages but I was alone at home and if something would have happen or gone wrong nobody could have "assisted" me, if you know what I'm saying. I what to take baby steps in restoring this amplifier, for safetys sake.

Power Amp Schematic:

Power Amp Board


One problem, since I only have an old analog multimeter I can't measure the caps, what to do?
I have located the caps (c213/c214/c209) and diodes (CR209-212) that is in direct connection to the fuse that keeps blowing. Could it be anyone of these that are faulty? Impossible for you to answer with out measures I guess.. The area is yet again marked red in picture below.

Power Supply


Sorry for any possible bad grammar or misspelling, hell, I'm a sexy swede for christ sake!  

[awe]

OK, first of all, what kind of noise is it? Hum or hiss? Did you notice any change in noise level when tweaking the bias pot?

First of all, try to do a brief bias adjustment, it's describe in one of the service notes above. Just turn the pot CCW Counter Clock Wise (motsols ;-) remove the F21 fuse connect you DMM set to current measuring, let the amp heat up, no signal applied and master volume set to zero. Then adjust bias until you reach 200 mA.
OK, I know your amp has links soldered instead of fuses just like mine, simply cut the wires and install proper fuses (10A).

IF your noise is more like hum, it COULD be in the power supply, i.e. diodes or filter caps (C209, C210). If one or two of the diodes are smoked, you'll probably get a LOT of hum, I mean really annoying amounts. Hum can also appear if you have a fault somewhere in the amp making it draw more current than normally, when the power supply isn't capable of delivering enough power you'll get hum in power supply which then is introduced into the amp and output sound. For that reason, it's a good idea to make the bias adjustment above to see that the amp doesn't draw more than 200 mA.

You can't measure caps with an analog multimeter, you can't even do it on all DMM's (Digital Multi Meter). And even if you have one with the possibility, a measurement will not always tell you about the condition.