Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: freegan72 on March 14, 2020, 05:20:15 pm
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I have been troubleshooting my Sears Silvertone Solid State 100 guitar amp, model 1464. Schematic is here: https://imgur.com/a/15npj
It has this low-frequency hum that sucks most of the power. Otherwise, the input controls, reverb, tremolo work. I removed the big PS filter caps (C26, C27), disconnected everything from the bridge rectifier out, and used a lab DC power supply for the +26 V and -26 V. I started lifting components to remove different parts of the circuit (reverb, tremolo, channel 2, channel 1, in that order). Nothing changed. I replaced all the e-caps (many of which were bulging and/or a little burnt-looking). That just made the hum brighter and more powerful-sounding.
If I disconnect everything before Q5 (removing Q4), the hum goes away. I can plug a guitar into the base of Q5, and it sounds great, super loud. I checked continuity on the leads of Q3 and Q4, nothing is shorted. They are marked 67467296. Any suggestions on replacements? Anything else I should consider?
Thanks!
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I'm just glancing at this because my puter is slow n your page wasn't loading, n it's early!
https://el34world.com/charts/Schematics/files/Silvertone/Silvertone_1423.pdf
is the power stable now with new caps?
without a scope it's kinda hard doing signal path, but a meter should get ALL DCvolts where they should be.
hum is typically bad grounds, bad filtering, bad coupling caps.
you can usually isolate to a stage once DCV is correct by using a listening amp. Inject a signal in, sample with test amp after each transistor stage.
unknown spec on a transistor can be "guessed" by what's the DC it's operating at, ballpark calculate the Ic using the collector and emitter resistors and DC volts across them.
example Rc = 1000 ohms, Re = 100 ohms vdc 16 so Ic ~~~ 16/1100
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> low-frequency hum
Power line pitch? Or something else??