Marlboro - Quadra Sound Blender QBS  [schematic]

Discussion regarding early stompbox technology: 1960-1975 Please keep discussion focused and contribute what info you have...
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Blitz Krieg
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Post by Blitz Krieg »

Intripped wrote:Am I wrong if I say that you had better take those two 20n capacitors off?
I mean C1 and C2, aren't they two "death caps"?
Bernardduur wrote:Yep, and already done!

They were also part of the noise in my unit
just remove them and it's already better?

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Bernardduur
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Post by Bernardduur »

Blitz Krieg wrote:
Intripped wrote:Am I wrong if I say that you had better take those two 20n capacitors off?
I mean C1 and C2, aren't they two "death caps"?
Bernardduur wrote:Yep, and already done!

They were also part of the noise in my unit
just remove them and it's already better?
Yep!
'No more....... loud music.......'

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Bernardduur
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Post by Bernardduur »

Mattyr wrote:I've used a qsbII for many years and I love it. Recently my reverb broke and upon inspection I found that the small wires that hold the spring had broken. The wires have what appears to be a small magnet that sits between the two bars connected to the transducer. How can I repair this and where would I get parts, including a new spring as I would like to alter the effect slightly. I have no education in electronics but I'm not stupid so I think with some direction I should be able to do this. Any input would be greatly appreciated.
I have searched for the same reverb but to no avail. You can have mine as I replaced mine with a Accutronics reverb tank (much better reverb now)
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Mattyr
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Post by Mattyr »

If you're offering me your qsb reverb tano I enthusiastically accept! Let me know and we can figure out a way for me to pay the shipping fees

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Bernardduur
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Post by Bernardduur »

Mattyr wrote:If you're offering me your qsb reverb tano I enthusiastically accept! Let me know and we can figure out a way for me to pay the shipping fees
Of course, you have a PB
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abogicevic
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Post by abogicevic »

Bernardduur wrote:Schematic time:
Preamp.JPG
Reverb.JPG
Tremolo.JPG
Thanks for schematic, really appreciate it :thumbsup

I also have Marlboro G-50R.
It's wonderfull sounding amp, but it's a bit noisy when I turn it on.
Is it problem with old capacitors, maybe amp needs recap? Do you solved that problem with your amp?

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Bernardduur
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Post by Bernardduur »

My amp became a lot more quiet with new caps. I also added a zobel network
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Post by abogicevic »

Bernardduur wrote:My amp became a lot more quiet with new caps. I also added a zobel network
Thanks! I'll recap mine too. And, ialso I'm going to change reverb tank.

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Post by pablomago »

What reverb tank did you put in your QSB II? I have a few small tanks around.

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Bernardduur
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Post by Bernardduur »

pablomago wrote:What reverb tank did you put in your QSB II? I have a few small tanks around.
I'll check when I'm home!
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Bernardduur
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Post by Bernardduur »

Bernardduur wrote:
pablomago wrote:What reverb tank did you put in your QSB II? I have a few small tanks around.
I'll check when I'm home!
It is a P-RMOD-8AB2A1B
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Post by RingyDaDing »

I have the QSB II and it seems to have lost it effects almost completely. Signal is going through ok accept when I click on to the echo setting the volume decreases a fair amount. I have a few questions. Advanced warning I am a novice. 1) Would it be the "Tropical Fish" capacitor I have a close-up of here causing this? It appears to be cracked open. I'm think maybe so. I have some Mullards of the same on the way. 2) I am thinking about replacing the tank, maybe with the one mentioned on here. (Accutronics - P-RMOD-8AB2A1B) If I do this is there anything special I need to do & do I reconnect the same ceramic disc tone capacitors on the exit, if so are they the best for the job? 3) Anything else I can do to improve the unit?

Thanks in advance!
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Zokk
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Post by Zokk »

Hello
the general rule with old electronics is to change first all the electrolytics capacitors, so you can have a steady power supply and make measures of the rest of the circuit clearly.
Those "tropical fishes" capacitors are prone to cracking, if you want to avoid further problems I'd suggest to replace it with a modern film capacitor.
Apart from being nice and colored, they are not better for audio than other film capacitors. If you want an upgrade use a paper in oil cap, or styroflex caps... but you probably won't hear any difference in this circuit.

Then if you still don't have any reverb effect, you may want to check the signal patch with an oscilloscope or an audio probe ( http://diy-fever.com/misc/audio-probe/).
I'm not sure about the wiring of the tank, it seems only two pins are connected to the tank, the middle pin seems unconnected and is used as support for the output caps.
Yes you can replace both ceramic cap by film caps.
You need to measure the tank input and output impedance to get a proper replacement tank, it's not just a matter of spring size.

Also this effect creates the "echo" trick by chopping the reverb tail with a square wave tremolo, funny but not really an echo.
With new caps the LFO should have a better shape and you may have less volume drop with this setting.

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redcapacci
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Post by redcapacci »

I have this amp and think is really great!
i would like to lower the depth of the tremolo because it's too choppy for my taste, What do i have to change ?
i know to solder but i'm learning how to read schematics and how to mod them, any advices would be great, thank you so much!

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Post by rocket512 »

@Bernardduur: did you notice any drop in thr reverb volume with the new medium output impedance (B) of the tank?
At least the G50R has a rather high impedance at the reverb tank output ~1800 Ohm DC --> ~ C in accutronics terms.
(I am planning to upgrade the reverb tank in mine and have to decide between a 9AB and a 9AC type)
Also there is a small mistake in your preamp schematic: C2 goes from the "mellow" input to ground, not to C3.

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Post by rocket512 »

redcapacci wrote: 05 Apr 2021, 14:15 I have this amp and think is really great!
i would like to lower the depth of the tremolo because it's too choppy for my taste, What do i have to change ?
i know to solder but i'm learning how to read schematics and how to mod them, any advices would be great, thank you so much!
You can put a potentiometer in the Quadra 1 connection, where the tremolo circuit is connced to the preamp.

You can use the potentionmeter at the back (which is P1 in the reverb circuit) I think its useless for the reverb.
I am refering to Bernardduur's schematics.

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