Help with tracing a Solid State Marshall Hum Please?

[Barnsleyboy]

Understood. So this would be the red (+ve) and blus (-ve) rails that I have marked on the schematic below?

I've found the AC/DC coupling button and depressed it, so now selecting DC. Amp has nothing connected to the input, and as far as I can see the amp does have shorting input jacks (if I'm reading that part of the schematic correctly)
Ready and waiting for the next step!

[Helmholtz]

The main rails only extend between positive rectifier output and R23, resp. the negative rectifier output and R24.

[Tassieviking]

The positive rail goes up to pin 8 of IC1a, but it does not go through and out of pin 3.
The negative rail does not go through C19 to the ground (earth, 0volt),The negative rail goes through R24 and into pin 4 of IC1b.
R23 and R24 drops the supply rails from around 19 volts to around 15 volts or there about, IC1 can not handle +-19 volts.
Pins 4 and 8 on the IC are the power supply pins that drive the IC.
Pins 2 and 3, are the smaller signal input pins that the IC amplifies and changes for the output at pin 1
Do not confuse the + sign at input pins 3 and 5, or the - sign on input pins 2 and 6 for a place for power to enter into the IC, it is just the smaller signal polarities.

I think I recall pinkjimiproton likes to install larger filter capacitors on his 12 watt amps (C18, C19), he even goes up to 10,000uF.
He also likes to change all the resistors over to metal film 1% resistors to reduce the noise in the amps, he loves the 12 watt lead and 12 watt reverb amps.

I build stomp boxes and amps for myself and my family so I am very happy to buy my components from Tayda, If you get resistors I find it best to get them in lots of 50 at a time from Tayda since they are so cheap.
I always get metal film ones for myself, https://www.taydaelectronics.com/res...list_limit=all
If you still have the 2200uF caps you took out, add them in parallel to the new ones you put in and see if it makes a difference in sound.

I also think there is a fault on the schematic from Marshall, TR5 should not go directly to the ground rail, it should be connected to the negative rail. (other side of C19).

[Helmholtz]

I also think there is a fault on the schematic from Marshall, TR5 should not go directly to the ground rail, it should be connected to the negative rail. (other side of C19).

Agreed.

Otherwise I mean scoping main positive and negative power rails (+/- 19V) which end at the right sides of R23/R24.

[Barnsleyboy]

Ok, so the rails look like this:

The faint blue is just the continuation of the rail.

So, ready to go - what should I do next?

[Barnsleyboy]

The positive rail goes up to pin 8 of IC1a, but it does not go through and out of pin 3.
The negative rail does not go through C19 to the ground (earth, 0volt),The negative rail goes through R24 and into pin 4 of IC1b.
R23 and R24 drops the supply rails from around 19 volts to around 15 volts or there about, IC1 can not handle +-19 volts.
Pins 4 and 8 on the IC are the power supply pins that drive the IC.
Pins 2 and 3, are the smaller signal input pins that the IC amplifies and changes for the output at pin 1
Do not confuse the + sign at input pins 3 and 5, or the - sign on input pins 2 and 6 for a place for power to enter into the IC, it is just the smaller signal polarities.

I think I recall pinkjimiproton likes to install larger filter capacitors on his 12 watt amps (C18, C19), he even goes up to 10,000uF.
He also likes to change all the resistors over to metal film 1% resistors to reduce the noise in the amps, he loves the 12 watt lead and 12 watt reverb amps.

I build stomp boxes and amps for myself and my family so I am very happy to buy my components from Tayda, If you get resistors I find it best to get them in lots of 50 at a time from Tayda since they are so cheap.
I always get metal film ones for myself, https://www.taydaelectronics.com/res...list_limit=all
If you still have the 2200uF caps you took out, add them in parallel to the new ones you put in and see if it makes a difference in sound.

I also think there is a fault on the schematic from Marshall, TR5 should not go directly to the ground rail, it should be connected to the negative rail. (other side of C19).

I have both the Lead and Reverb versions as well as this troublesome Bass 12. Great little amps. I'm tempted to increase the cap sizes. Interestingly, my Lead 12 which I believe is the mk I version (used by Billy Gibbons apparently on an early ZZ top track) only has 1000uF caps.

I'm in the good ole' U s of K, so Tayda's cheap components would be lost on me. The way our government are handling the economy at the moment, the exchange rate would put them at about $15 each!!!!

[Helmholtz]

As explained above, main or power amp rails end at R23/24. Scope them with AC coupling.
Left of R23/24 DCV and noise content are lower and harder to scope.

And please post a picture of the PT secondary wires going to the rectifier bridge.

[Tassieviking]

I absolutely agree with Helmholtz that you should scope on the right side of R23, R24 to check the power rails.
Since the large capacitors are really easy to get to you might want to do it there, scope across capacitors C18, and then across C19 and check the waveform.
In case you are interested the transformer should have 14.3v-0v-14.3v ac on the secondaries, I am sure you can use a 30V CT transformer (15v-0v-15v) if you ever needed to change it.
Transformer should be at least 30VA, but 60VA would not hurt at all.
There are circuits where this same power amp is used with 4 ohm speakers and you get 20 watts output, maybe even 30 watts.
The lead 12 watt uses a 16 ohm speaker from memory, 8 ohm for 20 watt and 4 ohm for 30 watt (if I remember right)

[Barnsleyboy]

Top trace is R23
Channel 1 - 2 Volts/div

Bottom Trace is R24
Channel 2 - 20 Volts/div

Time/div set to 2ms

Picture below is the PT secondary wires at the bridge (ignore the red box!)

[Helmholtz]

Why 20V/div on channel 2?
Use 2V/div as with channel 1.

What I'm looking for is some ripple asymmetry (differencies in peak height) indicating a 50Hz content.

Regarding the pic: The 2 red wires should be the PT secondary. Makes sure these wires are well twisted and not longer than necessary

[Barnsleyboy]

If I use 2v/div like channel 1, you can barely see the amplitude. The photo above is set at 0.2v/div.
(I'm assuming that the probes ground clip should be connected to the amp chassis?

[Helmholtz]

If I use 2v/div like channel 1, you can barely see the amplitude. The photo above is set at 0.2v/div.

Why is the channel 1 signal smaller now?

I expect positive and negative rails to have same ripple amplitudes.

Sure you're scoping the right side of R24?

And please change time base to 5ms/div. Makes it easier to spot some 50Hz content.

I'm assuming that the probes ground clip should be connected to the amp chassis?

Yes.

[g1]

If you are using the x5 button on the scope, or the x10 position on scope probe, you must say so. We can only see the setting of the volts/div knob.

[Tassieviking]

Also please make sure the red knob in the middle of the Volts/Division switch is fully clockwise on both channels, otherwise the the Volts/Div is not displayed accurately on the screen.

[Barnsleyboy]

Also please make sure the red knob in the middle of the Volts/Division switch is fully clockwise on both channels, otherwise the the Volts/Div is not displayed accurately on the screen.

The volts/div adjustment was set to the positions shown when I calibrated the probes using the built in 1KHz square wave (set to show 0.2 Vpp as per the scope instructions).
If I turn both fully clockwise, it will knock the probes out of calibration.