Marshall JCM 800 - "splatty" Lead channel

[earache]

All

Could someone take a look at the schematic and offer some interpretation of how some parts are being referenced?

Specifically: look at V2a and just below the grid there is what I believe is a polarized capacitor connected to a transistor labeled BC184.

Is it a polarized cap, yes or no?

Can you help me interpret it's value? It also says next to it "now .022" (I think...)

In similar fashion, there is what appears to be a polarized cap just below V1a on the schematic, coming from the middle leg of the Volume pot.

Please help me decipher what it's value should be, as it also says something like "now 1 pf" or something...

This questions arise because, as you all have suggested, I'm trying to trace out this schematic. I'm having some good results but these legends have me stumped.

Thanks in Advance

Eric

[Pedro Vecino]

I'm sorry, but I don't save images from that model.

Regarding these capacitors: they serve to filter the signal through them and make the mute depending on the channel that is activated. They have no other function.
The original indications must be 330uF (an unnecessarily high value). The new in V2 seems to be 22n (0.022).
But: in the most modern version they are originally 220n (0.22) and do not allow full filtering. There is a trace of unfiltered bass that with very high gain / volume settings in the inactive channel appear in the active channel. In the modern version I always install 2.2uF plastic capacitors and with them a perfect filtering is achieved by making a completely independent channel from the other.

The new V1 mute capacitor seems to indicate 100n (0.1uF), of the same value as the cathode (.1uF).

Understanding that the switching circuit is the same but done with transistors, using the value proposed (2.2uF) in all three sites you should have no problem with it.

[earache]

Pedro V

Thanks very much for the quick reply!

They don't need to be polarized caps then?

[Pedro Vecino]

I use them without polarizing, but you can use it polarized keeping the proper orientation.

Note: I see that the second mute string (V2a) is double. A part of it takes the signal from the plate through a 100n (0.1uF) high voltage capacitor. You can leave that capacitor there since its value is adequate.

[g1]

Here's a pic from the web if it's any use.

[earache]

G1

That’s the beast! Thanks!

I wonder why they used polarized caps in those locations if it was not strictly necessary?

[Helmholtz]

I wonder why they used polarized caps in those locations if it was not strictly necessary?

Circuits don't "require" polarized (electrolytic) caps, they just need a certain capacitance. The point is that non-polarized (film/foil) caps are available only up to a few ĩFs. So if the circuit requires a higher capacitance than say 10ĩF, only polarized caps are available. In the range from 1...10ĩF both types are available, but polarized E-caps are typically smaller and cheaper.

[earache]

What voltage rating would be needed here on these capacitors?

[Helmholtz]

What voltage rating would be needed here on these capacitors?

As far as I can see from the schematic, the caps in series with the switching transistors see no DC, only AC. Means that a low voltage rating of 16V should suffice. But a higher voltage rating may increase lifetime. Generally electrolytics would live longer with some DC bias.
If a cap value of 2.2ĩF is sufficient, as Pedro suggested, film/foil caps rated at 50V (or more) are actually the most reliable/durable solution.

[earache]

Hey there you fine peoples

If I were to have to replace the BC 184 NPN transistors on this amp, could I use just about any switching transistors like 2N2222 or 2N3904?

Thanks

[earache]

I’ve been testing the transistors in-circuit with a small multifunction test unit.

I’ve not used the test device before in this manner.
It’s one of those small rechargeable units from China.
Is it a legitimate way to test transistors in circuit?

The first three transistors returned different results. See attached pix

[J M Fahey]

Did you test them with amp powered or unpowered/unplugged?

[earache]

JM Fahey

Tested with the amp unpowered/unplugged

[alexradium]

I have a 4211 on the bench right now, basically it's the 100w version,if you need help I can give you measurements and values.
The boost channel is not the best I've heard on Marshalls,that was one of the first with diodes.

your switching system looks like this one
https://el34world.com/charts/Schemat...v_50w_4210.pdf
you can see the 330u caps used to pull to ground the stages,but there is a correction,in the new one they are all .22u poly type

[J M Fahey]

I’ve been testing the transistors in-circuit with a small multifunction test unit.

I’ve not used the test device before in this manner.
It’s one of those small rechargeable units from China.
Is it a legitimate way to test transistors in circuit?

The first three transistors returned different results. See attached pix

Not very useful here, since transistors are not being used as "transisoirs" , to amplify, with bias, etc. bust as simple switches.

Emitter to ground, collector to signal, if unbiased "switch" is open so signal is unaffected; when biased (some current getting into base) "switch closes" and signal is grounded.
As crude as can be but popular because of cheapness.

Looking at your pictures top to bottom:

1) nice data but irrelevant here. At least it looks like "you have a transistor there"

2) should be same as first one, but it shows only 2 legs
You may have a dead transistor there or simply poor test leads connection, who knows?

3) even worse. Problem is I canīt be certain we have bad parts or simply poor tester connections.

4) weird, it shows Hfe of only 6, while we expect >100 , etc.

I suggest you leave that tester aside for now,and test amp *functionally*

What are transistors expected to do? ... switch channels? ... ok, letīs test that.

We have 3 grounding switches, 3 BC184, Iīll call them Left, Center and Right (I hate unlabelled parts schematics)

Turn amp on, set it to "Clean".

Measure transistor Base voltages.

Left should be 0V ; Center and Right should be about 0.6V .

Does Clean channel work properly?
Test it LOUD, at rehearsal/stage levels, preferrably with a humbucker equipped Guitar.
Does it become "splatty?"

Now swiotch to Distortion/Boost.
Left shoukld show 0.6Vbe ; Center and right should show 0V.
Repeat test as before.

Comment on results.

If on any test splatty sound appears, remove guitar and inject 100mV sinewave at input, scope output ... any weird signal?

Move volume from 10 to 0, is there any weird thing happening to signalbesides clipping at high level?

Splattyness could be completely unrelated to switching, simply instability/distortion which appears at certain levels.

Ok, do the tests and post results.

Notice I am neither shotgunning nor even measuring "parts" expecting to hit the jackpot and by sheer chance find "a bad one".