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Dead Marshall Blues Breaker Mk1

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Dead Marshall Blues Breaker Mk1

Postby BillB » Sun Jul 14, 2019 11:34 pm

Hi folks, I have the above pedal that I bought in the nineties, following a good review in one of the music mags back then. Had in storage for a long time, then lent it to my daughter, who I don’t think ever played it. Over the last few months I have clocked that they became desirable pedals, following John Meyer using one around 2007, and they kind of trace their mojo back to the John Mayall / Eric Clapton BluesBreaker album of the 1960’s. The full story is all over the internet and YouTube if you are interested. Also the pedal has spawned many imitators (and possibly some betters) in the boutique market.

Anyway, thought I should dig it out and see what the magic is that I obviously failed to spot in the 90’s. So plugged in, turned on and - nothing, or nearly so. If I hit a loud power chord, a very nasty snap of distortion gets through and cuts out. Otherwise nothing. Bypass works fine, the LED lights when it should (using Boss adaptor of correct voltage/polarity).

I can take it apart to check for any bad joints, but can anyone say if this sounds like component level issue? Perhaps capacitors or the dual op-amp (JRC072BD or TL072), haven’t checked yet. I could put the op-amp in a socket like this guy:
http://www.tonegeek.com/musicgear/pedals/marshall-bluesbreaker-mods.php
- which would allow me to try other op amps in case it has failed.

Any other suggestions welcome.
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Re: Dead Marshall Blues Breaker Mk1

Postby Folderol » Mon Jul 15, 2019 7:26 am

First (obvious) check is the battery. Is the voltage OK and is it making good connection?
Next, check the voltage across the 100uCap. It should be pretty much half the battery voltage. It's probably the only electrolytic in the unit, and the most likely suspect. If it's one of the dodgy green sleeved ones change it anyway. Contrary to popular opinion, these still turn up occasionally :(

I doubt the IC itself is faulty, but it might not be getting supplies due to a board failure so check the voltage across pins 8 & 4 for battery voltage, then for half voltage across 5 & 4 and 2 & 4.
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Re: Dead Marshall Blues Breaker Mk1

Postby BillB » Mon Jul 15, 2019 7:37 am

Thanks Will. I have never used it with a battery, only DC supply, but obviously your suggestions will equally apply to that, so I'll check them out.
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Re: Dead Marshall Blues Breaker Mk1

Postby Wonks » Mon Jul 15, 2019 7:42 am

Folderol wrote:First (obvious)If it's one of the dodgy green sleeved ones change it anyway.
I'd have thought those Tudor-era caps would be worth a lot by themselves. ;)

It may have been used with an AC adaptor, which could have fried a few chips if there wasn't diode protection. Or a dodgy pot connection, so try some contact cleaner on the pots and also the input/output jacks.

Also worth checking that a blow to the knobs hasn't pushed the back off a pot.
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Re: Dead Marshall Blues Breaker Mk1

Postby Folderol » Mon Jul 15, 2019 7:48 am

Wonks wrote:
Folderol wrote:First (obvious)If it's one of the dodgy green sleeved ones change it anyway.
I'd have thought those Tudor-era caps would be worth a lot by themselves. ;)
:lol:
It may have been used with an AC adaptor, which could have fried a few chips if there wasn't diode protection. Or a dodgy pot connection, so try some contact cleaner on the pots and also the input/output jacks.
It does have diode 'crowbar' polarity protection, and it'd be a chunky PSU that'll pop a 1N400x
Also worth checking that a blow to the knobs hasn't pushed the back off a pot.
Good point!
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Re: Dead Marshall Blues Breaker Mk1

Postby BillB » Mon Jul 15, 2019 9:17 am

Thanks, Wonks. I don't think there is any major physical damage - it's in very good condition. But I now have a checklist to work through. Thank you both.
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Re: Dead Marshall Blues Breaker Mk1

Postby Hugh Robjohns » Mon Jul 15, 2019 2:48 pm

There's a slightly easier to follow circuit diagram here*:

https://1.bp.blogspot.com/-a9F3cPJg3Ic/Wq234jJ9JxI/AAAAAAAAD9I/3rmQH5ljh9AKN4szF8upZRI-rCItSRhGgCLcBGAs/s1600/1366373077_bluesbreaker_sc1.gif

*Apologies; it has to be a link to the source website rather than an image for copyright reasons which I hadn't spotted, but Kwackman did. ;-)

I agree with Will that the decoupling capacitor in the fake mid-point voltage divider is a strong culprit.

I note also that the 'crowbar' diode in the tonegeek schematic is shown as a (far more likely) polarity protection diode in the schematic above. It might be worth checking to see what's actually going on there, if you can!

The other potential physical problem areas after years of non-use worth carefully checks are the pots, as has been suggested. The gain pot sits within the negative feedback loop of the first half of the op-amp, and if that went o/c you'd have a lot of uncontrollable gain. If either of the Tone or Volume pots went o/c you'd only have crosstalk to feed the output, so it would go low level, and very tinny.

the only other obvious possibility is the bypass switch since it grounds the link between the two stages as a mute to kill crosstalk when bypassed. Since the LED is working it's probably fine, but it might be worth just making sure there isn't a short or something across the switch.

If all that pans out it's time to swap the TLO72... but at least it's a cheap and common chip.

H
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Re: Dead Marshall Blues Breaker Mk1

Postby Folderol » Mon Jul 15, 2019 4:18 pm

Interesting - much better having the component ID numbers :)

I've seem both ways of protecting units. The series version in the 'original' drawing has the disadvantage of always wasting at least 0.6V - quite significant when your starting with only 9V with a brand new battery.
The disadvantage of the 'crowbar' method is that it will draw however much the supply can deliver if it's reverse connected.
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Re: Dead Marshall Blues Breaker Mk1

Postby BillB » Mon Jul 15, 2019 4:50 pm

Thank you, Hugh, the checklist is growing. :headbang:
Don't know if I'll have time to look at this tonight, but in Arnie's immortal words "I'll be back".
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Re: Dead Marshall Blues Breaker Mk1

Postby Janneman » Mon Jul 15, 2019 5:42 pm

https://images.app.goo.gl/7PPurVbx9GcEWQVN7


Schematic. A skilled repairman can help you with the repair.
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Re: Dead Marshall Blues Breaker Mk1

Postby BillB » Mon Jul 15, 2019 11:10 pm

Janneman wrote: A skilled repairman can help you with the repair.

Thanks Janneman, but it feels like I have three of them on the case already :ugeek: :ugeek: :ugeek:
Only trouble is, I can’t move as fast as they can decode a circuit diagram. :headbang:
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Re: Dead Marshall Blues Breaker Mk1

Postby BillB » Wed Jul 17, 2019 11:10 pm

Finally got some time for testing. All supply voltages seem fine, including the half rail: 8.36V and 4.18V respectively. Will, you threw me with pins 2&4 - not half voltage, pin 2 is the negative input. 100uF cap looks fine, 25V polarised, with half voltage across it. It is labelled as C6, not C11 as on the circuit diagram Hugh linked to.

Checked the pots, they seem to have correct resistance, although VR15 reads about 13k at 12 o’clock, which suggests it may be Log rather than the Lin indicated on the second circuit diagram. Checked the footswitch terminals, they behave as expected (open/close the circuit for each pole).

Next I deoxited the pots, jacks and foot switch.

Then tried replacing the wall wart, then tried using a battery. No difference between any of the power options but - a slight improvement overall!

Some of the time it sounds like the nasty crackle I reported previously, but some of the time it sounds like a badly-behaved fuzz pedal through a badly-behaved noise gate with the threshold set too high (sound cuts out way before the guitar strings have stopped vibrating). In this second mode, it is at least clear that the Gain knob makes a slight difference to the thickness of the fuzz as gain is increased, and the Tone tone control does alter the tone (more = brighter), and the Volume pot does control overall output volume. But it remains very intermittent and reverts to crackle.

So I am up for changing some components. The capacitors C1,2,3,5,8,9, are odd. As reported on the Tonegeek site, they look a lot like resistors with coloured bands. C4,7,10 look like small fat resistors with no markings. I think I would rather try replacing the TL072, simply because the ‘distinctive’ cap selection may be part of the original’s sound, whereas the TL072 should not colour it one way or the other. Open to suggestions though. Worth changing C6 (100uF)?
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Re: Dead Marshall Blues Breaker Mk1

Postby Folderol » Wed Jul 17, 2019 11:30 pm

2 > 4 should still read at about half V - it's a voltage follower
also 3 > 4 should although that might sag a bit due to the meter loading that 1M resistor.
For the sake of completeness, also 1 > 4, 6 > 4, 7 > 4 should all be the same.

As for the 'dry' caps, the only ones that should have a significant voltage acoss them are the input and output ones, C1 & C10 respectively - about half V, although again, C1 could be rather low due to the high resistances.

P.S.
I'm always reluctant to change chips - it's easy to do a lot of damage in the process :(
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Re: Dead Marshall Blues Breaker Mk1

Postby BillB » Thu Jul 18, 2019 8:48 am

Thanks, Will, I’ll check all of those. For C1/C10, test with a voltmeter across them when circuit is powered?
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Re: Dead Marshall Blues Breaker Mk1

Postby Folderol » Thu Jul 18, 2019 3:34 pm

BillB wrote:Thanks, Will, I’ll check all of those. For C1/C10, test with a voltmeter across them when circuit is powered?
Yes, same for all of them, it's a check for leakage more than shorts, but the actual values you get will also show up O/C resistors or a wonky chip.
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Re: Dead Marshall Blues Breaker Mk1

Postby Wonks » Thu Jul 18, 2019 3:48 pm

Folderol wrote:Yes, same for all of them, it's a check for leakage more than shorts, but the actual values you get will also show up O/C resistors or a wonky chip.

Oi! Leave my chips alone and get your own! ;)
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Re: Dead Marshall Blues Breaker Mk1

Postby ef37a » Thu Jul 18, 2019 4:38 pm

I am with Will, op amps rarely fail and you can easily bugger the print.

An op amp will have its output pin at 1/2 supply within a few mV . If not check the input pin voltages.

You can get a pretty 100% test of an op amp by hanging a meter on the output pin then using a resistor of about 10k (+ or - 100%!) take the inverting and non-inverting pins in turn to either -ve or +9V . The output should move accordingly. I use a wet finger but only do that for battery power!
Not likely any danger from a wall rat but no point in chancing it.

If you are really sure an op amp has failed, chop each pin in turn then remove the bits by heating and banging on the bench. DON'T try to pull them out, that way damage lies. Clean up with solder wick.

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