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Solid State Amp issues

M

Michaellll

New Member
Sorry for posting on here, if this doesn't belong here or you feel like it should be moved/removed please do so and I apologize in advance. I'm only posting here because the guys on this forum are absolutely the best, and I suck at solid state gear!

I picked up a vintage solid state guitar amp and I'm running into a small issue. The amp powers on and plays just fine, but only sometimes. I usually have to tip the amp forward and power it off and back on, but sometimes it's at random, then right after hitting the power button it comes alive and amplifies the guitar signal like it should. Whenever I turn it on and it doesn't amplify guitar signal the red power light always stays on, however.

I took the amp completely apart twice already trying to locate something out of place, a loose solder joint, connection, but to no avail. Checked the caps, but the board looks almost brand new. I cleaned all the contacts with DeoxIT and yet the issue persists. One interesting thing is, whenever the amp is on and not amplifying input signal from the main input, I can plug in the guitar in the back into an input labeled "PWR AMP IN" and it will amplify the signal at average volume, however none of the tone or volume controls work. That led me to believe that there must have been something wrong with that board so I took it apart, checked all the connections for shorts and everything reads out just fine. Without the inputs board plugged in the amp stays silent and two small red lights light up on the main board.

So I'm not quite sure where else to look, could it be the preamp section? What else can cause such strange issue? I tried wiggling different components see if it makes a difference but it had no effect whatsoever. Only tilting it forward and powering it off and back on seems to bring the amplifier to life, and that works about 70% of the time. I haven't checked the tone controls, as I don't think they could cause such issue, but that's my next move.

Any tips or recommendations from you guys would be much appreciated, I included a schematic just in case.

Schematic:
https://drive.google.com/open?id=1HcSDgUhm-CutspZFWfS1aq-_YW6-s6nH
 
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Check the fuse holder for a snug fit around the fuse.It really sounds like a cold solder connection or loose fitting connector.
 
The fuse holder is fine I did check it. The amp powers on, the power light always turns on regardless whether it is amplifying sound or not. But yeah it does sound like a cold solder, do you perhaps think the tone dials could have something to do with it? That's my next best guess, I really don't want to solder the whole board not knowing if there is a cold solder somewhere and do all that work for nothing
 
Rake and tap on the circuit boards with something non- metallic. Chopstick , drum stick plastic or wooden spoon.
 
Been there done that. Once the amplifier powers on in it's not working state, there is nothing I can do. I tried hitting it as hard as I could. Also, I opened it up, powered, and poked around the main board slightly touching electronic components on the board. (with something non conductive). There was absolutely no change. Unless it starts right up, it doesn't start at all :(
 
Seems like you are dealing with an intermittent connection.

Does not have to be a board or a solder joint. Can be in the wiring. Try gently wiggling some wires and see if it has any effect.

I have to admit finding some bad solder joints can be a time consuming PITA. You know there is one but where. Like finding fasteners that fell onto old pavement and blend in but you need to find it. Sometimes you have to look at them at just the right angle to see them.:mad:

I had a fix and repair Sherwood S-8900A that had extrememely weak output. Identified bad relay contacts not passing signal so new relay got things working again but one channel sometimes worked, sometimes not. I could identify signal was dying in the tone board but took some time to figure out the reason. They used solid wire to run to the tape monitor switch which came out of the tone board. New wires between the board and the switch and it has not had any issues ever since. Apparently one of the solid wires had cracked and sometimes made contact, sometimes not. Physically the wires looked just fine, but one or two had nearly 90 degree bends guessing one of those was where the break was.
 
Last edited:
Michaellll said:
Sorry for posting on here, if this doesn't belong here or you feel like it should be moved/removed please do so and I apologize in advance. I'm only posting here because the guys on this forum are absolutely the best, and I suck at solid state gear!

I picked up a vintage solid state guitar amp and I'm running into a small issue. The amp powers on and plays just fine, but only sometimes. I usually have to tip the amp forward and power it off and back on, but sometimes it's at random, then right after hitting the power button it comes alive and amplifies the guitar signal like it should. Whenever I turn it on and it doesn't amplify guitar signal the red power light always stays on, however.

I took the amp completely apart twice already trying to locate something out of place, a loose solder joint, connection, but to no avail. Checked the caps, but the board looks almost brand new. I cleaned all the contacts with DeoxIT and yet the issue persists. One interesting thing is, whenever the amp is on and not amplifying input signal from the main input, I can plug in the guitar in the back into an input labeled "PWR AMP IN" and it will amplify the signal at average volume, however none of the tone or volume controls work. That led me to believe that there must have been something wrong with that board so I took it apart, checked all the connections for shorts and everything reads out just fine. Without the inputs board plugged in the amp stays silent and two small red lights light up on the main board.

So I'm not quite sure where else to look, could it be the preamp section? What else can cause such strange issue? I tried wiggling different components see if it makes a difference but it had no effect whatsoever. Only tilting it forward and powering it off and back on seems to bring the amplifier to life, and that works about 70% of the time. I haven't checked the tone controls, as I don't think they could cause such issue, but that's my next move.

Any tips or recommendations from you guys would be much appreciated, I included a schematic just in case.

Schematic:
https://drive.google.com/open?id=1HcSDgUhm-CutspZFWfS1aq-_YW6-s6nH
Click to expand...

The PWR AMP input is directly in and bypasses the tone controls so that is normal. The PRE OUT is the output from the preamplifier which includes the tone controls and volume controls. If you have a separate Preamplifier, plug it's OUT into the PWR AMP IN and see if it misbehaves. If it doesn't act up, then your on-board preamp circuitry has a problem. If it still misbehaves, then the fault lies somewhere in the main amplifier, or a connection *after* the preamp. You can also do the reverse if you have another POWER amplifier by hooking up the PRE OUT to the POWER AMP IN of a separate amplifier. But by you "tilting" it to regain sound, it almost sounds like you have a speaker connection problem. Either in the jack (there are two parallel speaker out jacks), or possibly the speaker plug or cable.
Where are you located? Perhaps an AK member is close by and can take a look at it. Sometimes another set of eyes can spot something. :dunno:
 
Is it possible that the issue is coming from the input 1/4 connector. 1/4, both male and female are notorious for dropouts and other issues. This is something I would check first because they are so troublesome.
 
Check +/-16.5Vdc supply for the opamps
Problem with the MUTE function, measure voltage at D43, either side.
 
@Michaellll Guitar amps are used differently than the home audio gear we're used to. What I mean by that is that home audio is set up and left alone for long periods of time. Guitar amps are constantly plugged and unplugged and moved around often. This can lead to broken solder joints and defective jacks, as @KingBubba pointed out. And @audiomaniac2 said, a signal generator and scope are good tools to use. If you lack either (or both) just use something to send a constant signal into the amp while you poke and prod it. Ideally music; a test tone through speakers can get on your nerves in short order!
 
Ray Gianelli said:
@Michaellll Guitar amps are used differently than the home audio gear we're used to. What I mean by that is that home audio is set up and left alone for long periods of time. Guitar amps are constantly plugged and unplugged and moved around often. This can lead to broken solder joints and defective jacks, as @KingBubba pointed out. And @audiomaniac2 said, a signal generator and scope are good tools to use. If you lack either (or both) just use something to send a constant signal into the amp while you poke and prod it. Ideally music; a test tone through speakers can get on your nerves in short order!
Click to expand...

Ideally for me a test tone. I can instantly detect a change in the tone. Whereas with music, you have to press your tongue on your bottom lip, squint your eyes, and tilt your head 4 different ways to try and determine what you heard is a "blip" or not. :p
But yeah, a 1k tone gets old within seconds. I use a more "pleasant" tone somewhere around 600Hz that's not so annoying. :confused:
That's one "bad" thing about an SS amp that is used heavily is solder joints. Pots soldered to boards crack, interconnects crack, grounding screws and other screws loosen, and so on. Tube amps are much more rugged in that regard. But jacks and plugs are the weakest link. By tilting it back and forth to bring it "alive" again, it could simply be the cable from the guitar to the amp has a loose connection.
 
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