Sears Solid State Guitar Amp LA-500
- Thread starter CJ
- Start date
Help Support GroupDIY Audio Forum:
this power board is mounted on the bottom of the amp in the back, there is a 3 prong connector from the control section seen on the front,
not much info online, however there is a similar chassis in Aspen Pittman's book,
we have a cooked resistor and a wire disconnected from one of the pwr transistors,
looks like somebody has been poking around this chassis,
not much info online, however there is a similar chassis in Aspen Pittman's book,
we have a cooked resistor and a wire disconnected from one of the pwr transistors,
looks like somebody has been poking around this chassis,
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Common faiilure mode for power transistors is short collector to emitter. The fried resistor may be the emitter degeneration resistor (low ohms to stabilize output current etc).
If the power devices are shorted you may need to replace driver transistors too.
JR
If the power devices are shorted you may need to replace driver transistors too.
JR
also looks like a trace got fried, so a wire is soldered over it, going to get some perf board or G-10 and just breadboard it,
> schemo for the 257.14232000
That's got a driver transformer. You don't.
> the Pittman schemo
I swear that looks JUST like the Fishers I used to get reject from the factory. Gain bumped-up a bit. Lacks the SOA protection of later Fishers. (Which is why it's sick and dumping DC in your speakers!)
It's an unexceptional Hi-Fi amp when it works. When it don't work, it's good training in debugging logic. But we are both too old for that.
You got a PT. Be sure it outputs on both wires.
Assume the Main Caps are fixing to die (if not already deceased). Old, and IMHO those waist-clamps encouraged early death.
Get a LM3886 amp-kit off eBay. You want the chip, the PCB, and at least some big caps. (If not 2 * 5000uFd onboard, also score some snap-caps, they're cheap.) Bugger the gain-set network on the '3886 to approximate the Sears' gain (33K 25uFd 330r will probably work with '3886).
Re-create the dropping/filtering for the power feed to the control panel amp.
It won't be the most toneful soulful amp on the planet, but it never was. There's other candidates for flavorful cooking. Just get this back "playable" and someone can use it.
And the '3886 will NEVER coff-out its guts like the original did.
And 3-pin power cord!!
That's got a driver transformer. You don't.
> the Pittman schemo
I swear that looks JUST like the Fishers I used to get reject from the factory. Gain bumped-up a bit. Lacks the SOA protection of later Fishers. (Which is why it's sick and dumping DC in your speakers!)
It's an unexceptional Hi-Fi amp when it works. When it don't work, it's good training in debugging logic. But we are both too old for that.
You got a PT. Be sure it outputs on both wires.
Assume the Main Caps are fixing to die (if not already deceased). Old, and IMHO those waist-clamps encouraged early death.
Get a LM3886 amp-kit off eBay. You want the chip, the PCB, and at least some big caps. (If not 2 * 5000uFd onboard, also score some snap-caps, they're cheap.) Bugger the gain-set network on the '3886 to approximate the Sears' gain (33K 25uFd 330r will probably work with '3886).
Re-create the dropping/filtering for the power feed to the control panel amp.
It won't be the most toneful soulful amp on the planet, but it never was. There's other candidates for flavorful cooking. Just get this back "playable" and someone can use it.
And the '3886 will NEVER coff-out its guts like the original did.
And 3-pin power cord!!
on the way from China, free shipping, 23 bucks for a pwr board and two 3886 modules,
they have the same thing at chipamps.com, quicker shipping but 26 bucks for 1 module only, probably better quality, but i can run 1 speaker off each module and turn the wattage down a bit for dependable service, i hope,
there are 403 listings for the 3886 on evilbay, all from China, due here sometime in August or September,
thanks for the tip!
they have the same thing at chipamps.com, quicker shipping but 26 bucks for 1 module only, probably better quality, but i can run 1 speaker off each module and turn the wattage down a bit for dependable service, i hope,
there are 403 listings for the 3886 on evilbay, all from China, due here sometime in August or September,
thanks for the tip!
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CJ
A friend had a solid state SEARS amp it was a cool sounding little amp. Had a compressed sound for lack of a better description.
I understand PPR's point but sometimes the older simple solid state amp stuff is cool
Have you traced the preamp section?
Think about the modules you are ordering and what you are using them for. How will they sound when you clip them like often happens with guitars and bass, it might sound bad when the protection circuit works.
The good/bad thing with some older amps is the lack of protection circuit(s)
How hard could it be to fix?
A friend had a solid state SEARS amp it was a cool sounding little amp. Had a compressed sound for lack of a better description.
I understand PPR's point but sometimes the older simple solid state amp stuff is cool
Have you traced the preamp section?
Think about the modules you are ordering and what you are using them for. How will they sound when you clip them like often happens with guitars and bass, it might sound bad when the protection circuit works.
The good/bad thing with some older amps is the lack of protection circuit(s)
How hard could it be to fix?
thanks for the link Gus!
he has some good stuff on counterfeit parts also, which saved me from getting those Sanken pwr transistors for the Acoustic 120 amp from across the ocean,
if we get this old board working, we can do a side by side comparison with the chip amps,
he has some good stuff on counterfeit parts also, which saved me from getting those Sanken pwr transistors for the Acoustic 120 amp from across the ocean,
if we get this old board working, we can do a side by side comparison with the chip amps,
