Symptoms of an aging amplifier circuit? - Page 1
Forum section: electronics / projects

author: e100
author_rank: Frequent Contributor
timestamp: January 10, 2018, 09:12:10 am
content: I have a 30 year old transistor guitar amp that sounds a bit muffled with not much top end.Is that a typical problem with aging amp circuits? Will simply replacing all the electrolytics bring it back to life again?The link to the circuit diagram is herehttp://www.tremolo.pl/Firmowe/LANEY/L30R.pdfA photo showing part of the PCB is attached.

author: danadak
author_rank: Super Contributor
timestamp: January 10, 2018, 12:30:03 pm
content: Its complicated.Electrolytics usually the first culprit, followed by carbon comp resistors whichabsorb moisture over time changing their values..Many of the problems discussed here -https://antiqueradio.org/FirstStepsInRestoration.htmhttps://www.everythingradio.com/Regards, Dana.

author: ogden
author_rank: Super Contributor
timestamp: January 10, 2018, 12:42:54 pm
content: Quote from: e100 on January 10, 2018, 09:12:10 amHow does it sound if you connect some signal source (mp3 player?) into "reverb return" or "D.I."? Could be capacitors to blame, but could be treble/middle/bass pot failure as well.

author: e100
author_rank: Frequent Contributor
timestamp: January 10, 2018, 01:14:19 pm
content: Quote from: ogden on January 10, 2018, 12:42:54 pmThere's an DI out from the preamp, but no return input. I could chop the pcb trace linking the pre and power amp sections and inject a signal into the power amp circuit.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: January 10, 2018, 01:16:21 pm
content: Just how old is it? As transistors age their gain decreases. This could mean that it's bandwidth is no longer as high as it used to be, resulting in a muffled sound with little detail in the highs.This in addition to the other points raised here such as increasing electrolyic ESR and such.

author: oldway
author_rank: Super Contributor
timestamp: January 10, 2018, 02:21:32 pm
content: Could you not use a measuring instrument a little more accurate than the ear?Already, it looks like you're not sure at all that there is a problem .....Injecting a 100Hz, 1000Hz, and 10Khz square wave and checking the output on the oscilloscope would give you a more credible and unbiased result.No need to buy an expensive frequency generator, a simple mos TLC555 can do this for cheap.Generally, if there are electrolytic capacitors that are bad, you will see it , they are swollen or they leak.In an amplifier, a faulty electrolytic capacitor generally causes a lack of bass, hum and a decrease in output power.Measure the value of the 2 capacitors of 2.200 ?F of the power supply (C29 and C30)

author: flynwill
author_rank: Regular Contributor
timestamp: January 10, 2018, 02:40:41 pm
content: Does the "Guitar Amp" have a built in speaker? Old speakers with rotted-out surrounds can have a "muffled" sound.

author: cvanc
author_rank: Frequent Contributor
timestamp: January 10, 2018, 02:51:49 pm
content: Quote from: TheUnnamedNewbie on January 10, 2018, 01:16:21 pmExcuse me? Can't say I've ever heard that before, care to cite a reference?

author: Zero999
author_rank: Super Contributor
timestamp: January 10, 2018, 02:56:38 pm
content: Quote from: oldway on January 10, 2018, 02:21:32 pmI agree. It's more likely one's ears have aged, more than the amplifier.Always use measurements, when testing something like an amplifier. Never one's senses which are objective and can be fooled.

author: e100
author_rank: Frequent Contributor
timestamp: January 10, 2018, 03:17:56 pm
content: Thanks everyone, I'll substitute a different speaker, and see how a square wave looks at various points in the signal chain.

author: Zero999
author_rank: Super Contributor
timestamp: January 10, 2018, 05:51:45 pm
content: Quote from: cvanc on January 10, 2018, 02:51:49 pmThere you go:http://www.dfrsoft.com/DfRSoft%20-%20Transistor%20Aging.pdfIn BJTs the Hfe degrades over time, which won't make any difference to the transconductance, but will cause bias currents to increase and the input impedance decease. This might reduce the over all voltage gain of the amplifier, due to loading effects. If the amplifier is well-designed this shouldn't matter, as the closed loop gain should be lower, than the open loop gain.

author: ogden
author_rank: Super Contributor
timestamp: January 10, 2018, 10:33:57 pm
content: Quote from: Hero999 on January 10, 2018, 02:56:38 pmI doubt that particular amp had flat 0-20KHz frequency response when new, so comparison to some reference using ears could be not that bad measurement at all (for given hardware)

author: richnormand
author_rank: Unknown
timestamp: January 10, 2018, 10:53:44 pm
content: Don't forget to check if the speaker system has a tweeter and crossover. If it uses a non-polar capacitor to the tweeter it could have lost value.Don't know your age but if you have not used it in years your earring could have deteriorated in the high end.Although true that electrolytics are main suspects they usually cause power supply type of issues but in your case have a particular look at the coupling caps between stages ( C2 C8 C17 C19) and the various inputs if they are DC blocked.Try the phone jack and see if the sound has the same issue. If not that would isolate it to the speaker system.Reverb switched out of circuit? They also have transducers that can go bad.You did not specify how you did the test. Was it with the guitar? Could it be an issue with the guitar pickup?Best way would be to feed a low volume test signal and follow its progression in the amp and then repeat at higher frequency to check if the various stage gains are comparable (taking into account the bass and mids boosts)Last would be to check that the power supply and rails on each stages is withing specs compared to your schematic.Best of luck with it.