Marshall 5275 rev.2 problems

NoiseZone

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Hi.

I have some issues with my 5275, bought used, all looks original (inside),.
I think Its an 88 model (w serial) and today the right (rev.2) schematic arrived from marshall.

Reverb doesnt work, noisy if i turn it up. (tank is ok)

The amp does not sound as good as i remember, got one for cap and pot (repair) job 8 years ago,..since then i always want this beast.

Already replaced all caps and this two 15 volt zener on the power unit.
Got 45 -45, 24 -24 voltage but no 15 -15 Volts, only measure ca. 5 -5 volt there...same on all IC´s only 5 - 6 volts.

Checked the resistors R44 - R47, these are getting very hot, but all still have 270 ohm...

Any ideas why i only get 5 Volts instead of 15 ?

Maybe on IC is broken...i´ll check this tomorrow.

Attached the (readable !) schem
 

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  • 5275-STD-1988.pdf
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NoiseZone

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OK, the IC4, MC1458 (Reverb), was broken.
Replaced it with a RC4558P and everything works...

Sounds awesome.

I´ll check some Burr Browns next week for more mojo... and to lower the overall noise a bit.

This 5275 amps are noisy 🤫
 

FleshOnGear

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Great job! I was going to suggest that an IC got burned out and was dragging down the +/-15V rails. This happened once to my 5215 when I was auditioning different ICs. I put a TLC2262 into the amp, forgetting that the TLC2262 doesn’t handle more than 18V total. It burned out, took down one of the rails of the power supply with it, and the dropping resistor was getting hot.
 

NoiseZone

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The Reverb rails was 30 volts, i changed r44 and r45 from 270 to 150 ohm, now i have 16, 25 and 45 volt´s

Got little buzz on the preamp, bevor return in. If preamp bypassed its gone, maybe ground issue somewhere...

The Reverb sounds....ok, but way too much...who needs so much reverb ?
 
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