Hi, amp enthusiasts!

I have an old LAB Series L5 (100W), bought in Sweden 1980. It was a reliable work horse during the 80s, but I quit playing in the 90s and kept it stashed in the closet for some 30 years. A few weeks ago I brought it out and plugged in my guitar, but unfortunately it was out of order. Something obviously happened to it during all these years.

I have let two technicians have a look at it. Both found that two fuses are blown out as soon as the power is turned on. One of them came to the conclusion that the transformer has to be the problem. The other, who I believe is the most reliable, concluded that the transformer seems to be transmitting at least reasonable voltages (although he couldn't find out from the scheme or board layout what the exact output should be). According to his investigation, the problem rather seems to be the card "Power supply and reverb driver printer circuit board", somewhere on the part that supplies power to the power amp. When he disconnects everything else but this card, the fuses still blow when turning on the power. He can't see anything that looks burned, but something seems to short circuit, for example a capacitor.

My questions then are:

1) Are these symptoms familiar from this amp?
2) Is it still possible to get spare parts to such an old amp? Is such a card still available somewhere or could it be manufactured at a reasonable cost?

Usually, the problem is electrolytic caps. They are generally available but usually smaller than they were forty some years ago. This means you may have to improvise how the caps are mounted.

Spare circuit boards will only be available from some amp that has been cannibalized, they will likely have the same problem your amp has.

See if you can find a tech who knows what a "Light Bulb Limiter" is and knows how to use it.

Thanks a lot! I have shared your reply with my tech guy (a former TV repair man) and he wonders if you have any ideas about how to identify the broken electrolytic cap. According to him, there are two really big ones on that card, but they are not the only ones. Are there any usual suspect that he should try to disconnect in order to measure the voltage? And why does he need a light bulb limiter? To avoid blowing more fuses during troubleshooting?

With a DVM, first check a cap in circuit for a short or very low resistance. Then, remove from circuit and check for a short again. Normally when you check a cap for resistance, at first it will seem like it's a low resistance but as the cap charges up, the resistance reading will increase until the DVM indicates over-range. If the DVM can measure capacitance, try that to see if you can get a reading close to the value and tolerance of the cap.

Verify that any rectifier(s) are good with the DVM's diode check feature.

Check for shorts around the power transistors.

A light bulb limiter will usually prevent fuses from blowing. A Variac is a better option, but should have a way to measure current to avoid blowing fuses. Sometimes you can use the Variac to apply a very low Voltage to the amp and find which circuit or part of a circuit is drawing excessive current. This is a procedure for a qualified and experienced technician. 

You should not connect a speaker to the amp until it can operate without blowing fuses and it is verified that there is no DC on the speaker output.

All parts should be available except the CA3080 IC and the CA3094 IC, most people either find some old ones in some shop or they substitute them with a LM13700 IC.
One example of how to substitute the LM13700 can be seen at Aion FX L5 pedal build, look in the Build Documents.
https://aionfx.com/project/l5-preamp/

All the capacitors should be measured before buying new ones to make sure they fit, new modern capacitors are usually smaller so with the Lab series there should be no problem at all. The Lab series of amps had all axial electrolytic capacitors from memory with leads on each end of the capacitor, if you want to get new capacitors with a similar size as original then go for a higher voltage capacitor. It will cost more though.

You bought the amp in Sweden but you did not mention the country you are in now, if you mention where you are now there might be someone here who knows the best place for you to get components in your country.
There might even be someone here who lives near you who would be able to help you, you never know.

Here is how a light bulb limiter works:https://www.ssguitar.com/index.php?topic=2093.0
It is just a light bulb connected halfway along the active wire on the way to he amp, the neutral goes straight to the amp.
If you have a fault the light bulb lights up bright but you don't destroy anything in the amplifier.

Here is a copy of the schematic in case you can't read yours too well.

Thanks a lot! I'll pass this info to my technician.

By the way I still live in Sweden. If anyone knows where components could be bought in Sweden I'd like to know. I got the amp from Hagström music store (they also had their own guitar brand), but it is no longer in business.

Tjenare, welcome to the SSGuitar forum as well, I was born in Sweden but moved to Australia in 1973 so I have no idea where you go for components in Sweden.
I buy most of my components from online stores like Tayda, Element 14, Mouser, Digikey etc.
You can most likely use most of them too.
https://www.taydaelectronics.com/
https://au.element14.com/?langId=46&setLangId=true
https://www.mouser.se/
https://www.digikey.se/

There are many more I use and some local ones in Australia as well.
I changed the URL's to the Swedish sites for you.
Cheers
Michael