Kenwood L-07m basket case...

HyKlas

Super Member
Picked one of these up for cheap but I think it needs a lot of work. Whoever owned it before had a blown 10A fuse in the 5A position and a 1A fuse in the other position. A few of the outputs are charred (with missing screws) and the copper ground strap is pulled out of the pcb. Obviously ham-fisted disassembly was attempted here.

I'm on the fence about returning it all together as I'm no whiz with this stuff. But to get me started, what are the current replacements for the outputs? Also, it apparently blows a 10A fuse so something is shorted but what would those 10 amps take out before blowing? That's the magic question..
 
Either find someone who has some Sanken transistors stashed away, or settle for some new On-Semi outputs.

If either the remote turn-on transformer or the large power transformer is cooked, you may have an amp that is beyond help (and with the fuse sizes that have been installed, that may well be the case). Pull the driver board and the output transistors, and see if you can bring it up on a DBT with the proper size fuses. If the remote start fuse pops (the 100mA one), could be the small bridge rectifier...or could be a smoked transformer.
 
If it's just 1 you picked up, I can't see the point, unless you have a few others and want a spare- even then, if the outputs are blown, I'd just keep it for parts- there's nothing quite like the original sankens. Having had 9 of these amps, I can assure you, they simply don't sound the same with anything but the original output transistors.
 
Going to see if I can take it back but I have no clue what I did with the receipt. The main transformer is out. Whoever had their hands in it before really did a number on it..
 
Are you sure the main transformer has a dead primary? Many of the L07s were 120/240v and the switch could be the culprit. Also, check the sub transformer that switches the power relay and supplies power to the primary of the main tx.
 
There's a lot of Canadian model Kenwood gear here in the US, and to meet CSA standards Kenwood was required to install a thermal fuse in the transformers. Not terribly uncommon to have one blow if something dumb is done to the amp (like installing a line fuse twice the required size) or if the unit is having stability issues.
 
Yes, those wretched thermal fuses cause a lot of trouble. I've seen them rated as low as 95 degrees celsius, when I've dug their open circuit a##s out of dead primaries. Horrible things. Usually they aren't too far in- just under the first few layers of paper/insulation, or slipped down from the top below the outer mu-metal shield under the main covers. Sometimes the manufacturers even give you a tapping point terminal (no doubt due to the different regulations) and you can bypass them altogether to save an otherwise perfect transformer that shows no signs of overheating other than yet another premature failed thermal fuse.
We once had a batch of amplifiers that failed brand new at switch on. I personally tested two in a row. Open circuit primary and it turned out to be the thermal fuses- something had happened in manufacture- maybe they baked the transformers, submerged them in laquer, or they were just dodgy to start with. About 10 in all, from three orders. One of our favourite brands too.
 
I tried to dig out the thermal fuse in a L-07MII transformer a few years ago...I couldn't find the sucker. :dunno:

Ended up sending the transformer to Gary Brown in Orono, ME. He fixed it right up.
 
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