1982 JCM800 2205 is driving me nuts

[TassieViking]

The transistors in the Normal and Boost channels are like switches that connects the signal to ground, they are meant to kill the signal by shorting it out.
When the transistor switch on and conduct its like throwing a switch in those circuits.
Replace the BC184 transistor in the the channel that bleeds through and it should fix that problem.
From memory the IC used in later amps in the switching circuit is just full of transistors, nothing else.

The 4 - 1N4007 diodes at the end of the boost channel might be there for clipping, but I have seen diodes in other amps like that and they are used purely to make sure the signal is not to high.
They might be there for a max signal limit and not for clipping, hard to tell unless checking with a CRO.

 

[arthur.lowery]

Yes the schematic is strange! For example, if the lower HT fuse blows, it will remove the negative bias to the output tubes, but the HT will be supplied through the top fuse! Red-plating, for a while anyhow! The transistors as switches are strange too, because they may work as intended for the top half of the cycle, but rely on the CB junction being forward biased back to the switching circuit, which must absorb the negative part of the cycle - but there's a IN914 diode to stop this happening! Maybe that's why V2a has a transistor shorting its grid and another shorting its anode (to short both the negative and positive parts of the waveform). The indicator LED also has no current limiting resistor! The bases of three transistors are also connected together (one pair via IN914s), whereas they should have their own current-limiting (or sharing) resistors, otherwise one transistor might be hard on, and the other slightly on, depending on their temperatures!

As for your problem, it really sounds like high-frequency oscillation; however, the feedback does not look strong enough to cause oscillation (100 k to 4 ohm tap), but maybe the bottom resistor of the tails of the phase splitter is a higher value than usual? (Giving more feedback, which can cause oscillation).

Here's the simulated waveform on the Clean switch output (purple, top end of C1), with the transistor turned on and off. The input is +/- 10 volts at 200 Hz. The switching is not perfect (though the default BJT model in LTspice was used. A BC547 did not make much difference). The leakage is mainly due to the capacitor's impedance.

With a much smaller waveform (200 mV p-p) you don't see the long transients of the DC voltages changing. (Q1 is rectifying a bit, so C1 charges).

 

[Jethro Rocker]

The 4 - 1N4007 diodes at the end of the boost channel might be there for clipping, but I have seen diodes in other amps like that and they are used purely to make sure the signal is not to high.
They might be there for a max signal limit and not for clipping, hard to tell unless checking with a CRO

The split channel 800s have a lot more gain than the single channel series. Without addition of an extra tube, where is this gain coming from if not diode clipping?
I don't know this stiff, so I ask.

 

[Ken Stewart]

Who has experience with working on these? This is an early one without the chip and I’m not to familiar with them and I’m fixing up a friends that sounded like crap cause the tubes were dead. I Cleaned the pots, installed new tubes, cleaned jacks, the normal procedures. Now it roars BUT, the volumes interactions are weird. The boost volume really affects the normal channel even with the boost off. I hear that this is an issue with these and is small potatoes for what I’m really trying to figure out. At certain settings of the volumes and gain, I’m getting a very starved fuzz pedal sound with like a ring modulator gated envelope type sound. When I play the guitar when it’s doing it, you really have to dig in to get a note to pop out. I noticed there are spots depending on how you tweek the gain and normal Chan volume where the volume drops out like there is high frequency oscillation going on. Lead dress is spot on, I did the chopstick poke poke, all components are in spec. I’m unfamiliar with amps using transistors tho, so I’m wondering could it be one of them? I can get great tone and crunch out of it but it just bothers me that I feel something is not right regardless. Oh and voltages seem to be golden. So, is this how these are with the diode clipping? I feel that’s where the problem may be. I usually fix old fender amps so this is a new one for me!
Thanks all!

I have a 2205 which had a similar issue which ended up being caused by poor quality plating on the output jacks. As I am in Perth Western Australia, good amp techs are few and it took 20 years to get it sorted. Pretty sad eh? Anyway the amp is probably my favourite despite contemplating taking to it with a chainsaw on occasion. I would not suggest this is your issue but maybe something to consider. My amp was an early 90s build.

 

[arthur.lowery]

In order to correct the circuit, I would add 1K resistors at the bases of each of the transistors (in series withe the base leads), rather like "grid stoppers". As is, the Base-Emitter junction of one transistor might steal all the current from another base-emitter junction. You can think of these as "base stoppers" but really they help share the drive currents between the bases (current sharing resistors).

A base-emitter junction is like a forward-biased diode, so two in parallel are like two diodes in parallel. It's very difficult to say which diode will take al the current, as they are temperature dependent and highly exponential in current versus voltage.

The LED also needs a series resistor, to limit its current to 10 mA. Try 4K7.

It's a bit of a rookie mistake in the original design!

 

[Pete Farrington]

As is, the Base-Emitter junction of one transistor might steal all the current from another base-emitter junction

Oh yeah, thanks for pointing that out, I'd forgotten the rationale for them

 

[geddy]

The transistor array IC in the later version can cause funny issues. I've experienced this twice but was easily diagnosed with only a DVM. Can't say about the pre IC version though. No idea why anyone thinks a 2205 cct doesn't have diode clipping. It seems to be the main thing people moan about but as pointed out up thread, people then use a pedal with diode clipping. Go figure

 

[Jethro Rocker]

The transistor array IC in the later version can cause funny issues. I've experienced this twice but was easily diagnosed with only a DVM. Can't say about the pre IC version though. No idea why anyone thinks a 2205 cct doesn't have diode clipping. It seems to be the main thing people moan about but as pointed out up thread, people then use a pedal with diode clipping. Go figure

If ya like how it sounds (Jubilee!!) it matters not how it arrives there in the slightest. Try it, play it, enjoy it, whatever it may be!

 

[arthur.lowery]

The transistor array IC in the later version can cause funny issues. I've experienced this twice but was easily diagnosed with only a DVM. Can't say about the pre IC version though. No idea why anyone thinks a 2205 cct doesn't have diode clipping. It seems to be the main thing people moan about but as pointed out up thread, people then use a pedal with diode clipping. Go figure

Hi - I'd be very interested to know what the funny issues were, just to see if they can be explained by the Marshall circuit design. What did the DVM show, roughly?

 

[Pete Farrington]

why anyone thinks a 2205 cct doesn't have diode clipping

With the 1982 version, boost channel selected, if the channel volume is set low and the (type1) master volume set very high, the final clipping stage could be the diode clipper.
But with channel volume high and master low, the final clipping stage might be the LTP.
Whichever, yes, I think the diode clipper will be operating on all but the lowest signal levels.

 

[LyseFar]

After being schooled in this thread about the channel bleed in the 2205 and 10 I decided bring out the big hammer and try to fix it in my 89’ 2210.
When I traded it in I changed all the electrolytics so that part I assume is ok.
First I changed the IC. I added a socket so it will be easier in the future to change it.
Before:


After:

Before the IC change the clean channel was very audible on the boost channel. After IC change a lot better but still there in some degree.

As I have a “few” Philips 344 MKC 0.022uF brick caps in stock I decided to change them all. And also the 0.22uF’s.







And after the changes.

I did not have any .1uF, .47 etc in stock.. otherwise those had been changed also…maybe later.

I measured the old bricks and found readings all over the place and one .22uF completely dead.

If there is any channel bleed left it is surely insignificant. I can’t hear it. So this thing is curable for sure.

Maybe some would say that changing all these components is overkill but the amp sounds really good now. Both channels are enhanced about 20% in soundquality - in my humble opinion. Just better on all parameters.

And thanks to @Purgasound and @Tatzmann for giving me the wakeup call not to just accept the channel bleed as “by design” and make me do something about it.

 

[Purgasound]

Awesome work lysedsr
Awesome work. Glad you got it sorted out. I need to do a repair video on one of these soon. I've got a couple of these split channel amps that need some work. Of course my own projects tend to fall to the wayside when I've got other people's amps waiting too. Plus when so much time passes I tend to forget the "fix" and have to retract my steps all over again.

 

[LyseFar]

Awesome work lysedsr

Awesome work. Glad you got it sorted out. I need to do a repair video on one of these soon. I've got a couple of these split channel amps that need some work. Of course my own projects tend to fall to the wayside when I've got other people's amps waiting too. Plus when so much time passes I tend to forget the "fix" and have to retract my steps all over again.

Thank you!
Excellent idea with an repair video. I will look forward to that.