Repairing a vintage solid-state guitar amplifier

chase_p_a · 2006-02-22T12:01:40+00:00

Hi, I'm attempting to fix a Rickenbacker TR75 GT amplifier, circa 1971.The power amp is dead. The output power transistors have gone CEclosed-circuit and power resistors in the output are open circuit. Ithink the previous owner shorted the output. Other components seem OK,as far as I can test them.Luckily, there's a schematic: -The 2N5988 and 2N5991 transistors are unavailable except at greatexpense from vintage parts suppliers. I'm not gonna pay big bucks forthem, because (a) I might blow them and cry, (b) it would be cheaperjust to buy a ready-made power amp module and use that.I hoped that some cheap BD441/442 transistors might be sufficient forme to find out if the rest of the circuit was OK. They are at leastrated for the voltage (73 volts rail-to-rail). But putting them, andnew 0.27R resistors, in the circuit and powering-up resulted in smoke.I guess their characteristics (higher gain, lower current) are toodifferent, or there's something else wrong with the circuit. At least Ionly blew up a few pence/cents worth of components.Any suggestions about equivalent transistors at sensible price? Anyideas how I can try things out while minimising the chance of morefireworks?I suspect I'll end up with Plan B - replace the whole power amp with amodern one. But it would be nice to fix the original instead.

Feb 22, 2006 21 Replies

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Dave Plowman (News) 20 years ago

2SK135 and 2SJ50 both in stock at RS components.

*Hard work pays off in the future. Laziness pays off now * Dave Plowman dave@davenoise.co.uk London SW To e-mail, change noise into sound.

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Ron(UK) 20 years ago

I dunno then, I dont think kits need CE approval especially if they aren't mains powered, I could be wrong tho.

Some years ago I dismantled an old Studiomaster 1000watt mosfet amplifier with a burnt out mains tranny, and that furnished me with enough power mosfets to make about a dozen 150 watt modules up. I made up some boards of my own with double the number of output devices which proved quite sucessful too.

That same Hitachi[1] based design turns up in a multitude of guitar and pa amps from the 80`s and 90`s virtually anything with the words mosfet on the front. The actual output section rarely gives trouble. (IME)

[1] I believe.

Ron(UK)

Lune Valley Audio Public address system Hire, Sales, Repairs www.lunevalleyaudio.com

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