Trouble Shooting a Solid State Guitar Amp

[goatshred]

I have just recently got my hands on an 80s solid state amp. I love the tone of the amp, but it has a hum. I found an amp guy (who tends to stick with valve amps) who changed the filter caps for me, but no joy. I found that plugging in a pair of headphones into the FX loop Send or the Line out resulted in the hum going down, but I need to use the boost, and max out the volume to get to sensible volumes. I've got its schematic, and checked the connections going to earth and cleaned them with some contact cleaner but no joy.

In summary, looking for somewhere to start to start trying to work out what's going on with it - I'm handy with a soldering iron, but have no idea how to troubleshoot an amp.

 

[Nigel Goodwin]

Without the schematic it's hard to help - but generally plugging a jack plug in the FX loop splits preamp and power amp - so does it stop humming if you do that?.

Along with the schematic, pictures of the inside would be helpful - a picture of the outside doesn't help at all.

 

[goatshred]

Without the schematic it's hard to help - but generally plugging a jack plug in the FX loop splits preamp and power amp - so does it stop humming if you do that?.

Along with the schematic, pictures of the inside would be helpful - a picture of the outside doesn't help at all.

Thanks Nigel. It doesn't stop all the humming, but does improve it (similarly putting a noise gate in the fx loop helps, but doesn't completely get rid of the problem. Also, at some point, someone replaced the reverb tank with an incorrectly specced one; it should be an 8FB2E1, but the fitted one is 8BB2A1, so aside from the impedance being 10% of what it should be, I also understand to mean it's expecting to be grounded from the outer channel, but there's no ground connection in the amp for it, as it's expecting one without an outer channel). Also, the mounting plane of the tank is incorrect (it's a B, which means open side down, like in some Fender amps, but it's mounted vertically like an E or F). The reverb tank is introducing some hiss, but I don't believe it to be the root of the issue, as I've disconnected it, and the main hum still occurs.

In the attached image,
1. The filter caps were changed
2. The connection where all of the ground connections are connected
3. There was some corrosion under this board, so I removed the nuts from the jacks so I could get underneath it to clean with contact cleaner.

 

[Nigel Goodwin]

C1 and C2 are the smoothers on the preamp supply - created by the two zeners Z1 and Z2, and the two 620 ohm wirewound resistors.

So you might check the voltages on C1 and C2, and if you have a scope, check for mains hum on them.

 

[rjenkinsgb]

Can you provide a photo of the schematic?

 

[goatshred]

C1 and C2 are the smoothers on the preamp supply - created by the two zeners Z1 and Z2, and the two 620 ohm wirewound resistors.

So you might check the voltages on C1 and C2, and if you have a scope, check for mains hum on them.

Thanks - I don't have a scope, but will check out C1 and C2. Would it be fair to assume that at 50hz, and any smoothing my multimeter may do, would mean that it's not possible to spot mains hum without a scope?

 

[Nigel Goodwin]

Thanks - I don't have a scope, but will check out C1 and C2. Would it be fair to assume that at 50hz, and any smoothing my multimeter may do, would mean that it's not possible to spot mains hum without a scope?

Exactly, a meter is useless for checking hum.

 

[rjenkinsgb]

Try your multimeter on AC volts across each of the main smoothing caps?
A lot of meters (but not all) will ignore the DC component and allow you to see any AC ripple.

Also try across C1 and C2, the 15V smoothing?

It does look to be more a missing ground somewhere though, as connecting to the effects send should not do anything much, just reduce gain slightly.

With power off, try a jack plug in each socket one at a time & check continuity from the plug body/screen contact back to the 0V (white) on the two main caps?

Also check any separate metalwork such as the springline case - everything should show a dead short, or less than one or two ohms at least.

 

[rjenkinsgb]

Another thought - if you can get any ripple reading on the main caps, and your meter has a frequency setting, see if it is 100Hz or 50Hz (or 120/60) ??
A faulty bridge rec or a single blown fuse could possibly allow it to keep working but with rather higher ripple and hum than it should have?

It would show as mains frequency ripple, rather than double that if everything is intact.

 

[Nigel Goodwin]

Try your multimeter on AC volts across each of the main smoothing caps?
A lot of meters (but not all) will ignore the DC component and allow you to see any AC ripple.

I would very much disagree, it's more a tiny number of meters will ignore DC, but very few - most happily read DC on the AC ranges, and give completely wrong values.

 

[goatshred]

Try your multimeter on AC volts across each of the main smoothing caps?
A lot of meters (but not all) will ignore the DC component and allow you to see any AC ripple.

Also try across C1 and C2, the 15V smoothing?

It does look to be more a missing ground somewhere though, as connecting to the effects send should not do anything much, just reduce gain slightly.

With power off, try a jack plug in each socket one at a time & check continuity from the plug body/screen contact back to the 0V (white) on the two main caps?

Also check any separate metalwork such as the springline case - everything should show a dead short, or less than one or two ohms at least.

Unfortunately when I bought my meter, I didn't foresee the need for a True RMS one, so even though it's a 4000 count autoranging meter, I don't think it will ignore the DC component.

 

[Nigel Goodwin]

Unfortunately when I bought my meter, I didn't foresee the need for a True RMS one, so even though it's a 4000 count autoranging meter, I don't think it will ignore the DC component.

Just put it on AC, and connect it across a battery - it should read zero on a battery if it ignores DC.

I don't think true RMS has anything to do with it, and probably quite the opposite - as a true RMS meter would need to read both AC and DC components to give a true RMS reading.

Assuming your meter does read DC on AC ranges?, just connect it via a capacitor (which will block the DC, and is all that a meter that blocks DC uses internally), a 0.1uF should be fine (it's reactance is insignificant compared with the 10Meg of the meter).

 

[goatshred]

Just put it on AC, and connect it across a battery - it should read zero on a battery if it ignores DC.

I don't think true RMS has anything to do with it, and probably quite the opposite - as a true RMS meter would need to read both AC and DC components to give a true RMS reading.

Assuming your meter does read DC on AC ranges?, just connect it via a capacitor (which will block the DC, and is all that a meter that blocks DC uses internally), a 0.1uF should be fine (it's reactance is insignificant compared with the 10Meg of the meter).

Unfortunately you're right - the battery was reading 8.9v but in AC it just jumps all over the place. I'll have a look in my box of tricks to see if I've got a cap I can use.

 

[rjenkinsgb]

That looks to the the autoranging getting confused?
Try using the range button to fix it in a range?

 

[rjenkinsgb]

Try bridging from the main 0V junction (white wires on the large power supply caps) to C1 / C2 where both wires are close together on the lower board in the photo?

As it's a single-sides PCB, I'm wondering if pin 26 has a cracked connection on the underside of the board? It has some rather large wires hanging on it, so could be susceptible to vibration, over time.

 

[goatshred]

That looks to the the autoranging getting confused?
Try using the range button to fix it in a range?

I think you were right - it starts off thinking there's a couple of V, but then in a second goes down to 0v.

 

[goatshred]

Try bridging from the main 0V junction (white wires on the large power supply caps) to C1 / C2 where both wires are close together on the lower board in the photo?

As it's a single-sides PCB, I'm wondering if pin 26 has a cracked connection on the underside of the board? It has some rather large wires hanging on it, so could be susceptible to vibration, over time.

I'll double check the pin - also, probably asking the super obvious, but using the positive probe on the caps, and the negative / ground to the ground connection.

 

[rjenkinsgb]

I'll double check the pin - also, probably asking the super obvious, but using the positive probe on the caps, and the negative / ground to the ground connection.

Sorry, change of topic from meter tests - bridge as in just directly connect the two by holding a piece of wire between them, and see if the hum goes away when you do that?

 

[shokjok]

Could the amp have a loose transformer mount causing the hum? Perhaps a rubber gasket at its base or an external wall-wart could resolve this issue.

 

[Lightium]

I wonder if you could record 4 seconds of audio?