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January 25, 2022, 05:44:04 PM

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Orange has released the Orange Super Crush 100.  They have this as a head as well as a combo unit.  According to designer Ade Emsley, this amp doesn't use opamp circuitry but rather single ended jfet transistors.  With two circuits, reverb, and independent eq sections for clean and overdrive, this unit really rocks.  It has what I consider to be (from the video clips) the best sounding solid state overdrive sound I've heard.  This is worth a listen, check out Ty Tabor's demo here:


MSRP is $499 USD for the head, $699 for the combo.
Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by g1 on January 02, 2022, 10:36:26 AM »
So you have found out that a continuity 'beep' is not the same as a short.  That is a good thing to know, as it leads to many errors for novice users.  Different brands have different ranges that will cause the beep, some anything below 40 ohms, some as high as 200 ohms!
If you know the spec for your meter, continuity function can be useful.  But usually for things like looking for an automotive wire that is shorting to the chassis or something.
When reporting resistance always use resistance range.

The 7 ohms you measure at the headphone jack will be the speaker in parallel.  And the common way to disconnect the speaker for headphone use is to disconnect the ground side of the speaker via a switch contact in the phones jack. (with attention to what Phil said about current feedback resistor)
If you have an empty TRS plug to put in the phones jack, it might help with the resistance readings and wiring tracing.
Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by phatt on January 02, 2022, 03:08:49 AM »
 @*Will* thanks good call.
@ *Cin*
As the DC voltages read normal and if the Caps are ok then you are back to tracking down what seems like a grounding issue.
(Yes Elcap means Electrolytic Capacitor)

Other possible causes,
Check that there is continuity all they way from the Earth pin on the power plug through to the chassis and onto the sleeve contact at the input socket.
If ok then try another power point in the house just in case that power point is faulty.
(I recently got caught with an intermittent failing power point at my place which had me chasing my tail for a few weeks)

There is still the possibility the reverb is not right.
Although you checked the wiring, if the internal ground connection of the tank has broken it can induce hum.

It looks like the reverb utilizes one dual opamp for Drive and Pickup so you could remove that chip which (As I see it?) will render the reverb out of the circuit. A sure way to find out if the reverb is causing an issue.
The pic with light behind was helpful except there is a pot over the reverb section, so hard to tell.

As to the Headphone socket, I can't see clearly but looks like Tip and sleeve are tied so you get mono sound through both ears when TRS phone plug is used, While disconnecting the internal speaker.
As to the cap/resistor from socket to pot. Hum??  :-\ well maybe that is a poor attempt at what could be a Zobel network. (google it if you're not sure what that means)
From my reading A zobel network should be as close to the output of the LM1875 power chip as possible.

And yes if you don't use the phone plug you can bypass it but that Zobel setup would need to be rewired.

Be aware that the Neg speaker node may not be at ground potential as a lot of these amps use what is called current Feedback. That is likely the large Cement 3Watt resistor near the Lm1875, can't read it but likely 0.22R or 0.27R
Sorry although the pictures help without more info it's hard to trace where the wiring goes.

Regards to the single/dual supply Q;
If you have a circuit that runs on a 30VDC single supply then Obviously zero Volt is common and often Ground/Chassis. So you have 30volts potential to swing the audio signal.
If you have a dual +15/0/-15VDC supply then all you have done is move *Common* to the middle of  what is still a 30Volt potential.  The Audio cares little as it's just floating on a DC potential.

Whenever you look at Amp schematics like yours you are actually looking at 2 circuits intertwined.
 There is the DC parts and then there are the AC parts.
You setup the DC bias points through each stage so as to pass the best Audio.
The Newcomer's Forum / Re: Am I reading this correctly and does this look suspicious?
« Last post by DrGonz78 on January 01, 2022, 02:38:50 PM »
I am still curious, like Enzo was, about what make and model amp we are looking at. Perhaps there is a schematic to look at to figure out what could have burnt up that resistor in the first place.
The Newcomer's Forum / Re: Am I reading this correctly and does this look suspicious?
« Last post by Den. on December 31, 2021, 06:30:54 PM »
You have a good board?  Stick your ohm meter probes on the good one and see what it measures.
I was trying to avoid having to disassemble it again. But I did. That resistor measured 182K. Replaced the resistor and the cap and now the board functions. Thanks.
Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by cin on December 31, 2021, 02:04:19 PM »
That's a great trick, Will. I went through all the ElCaps with my good MM on AC V and there's no AC voltage across any of them. I checked the film capacitor next to the phones jack (or is it a paper cap?) and it leaks about 30mV, but then I'm not sure those caps are polarized, so that might be normal.

I may be wrong, again... about the phones jack. So all pins on the phones jack have continuity to ground (duh), but the signal pins are at -4mV compared to ground, and test with 7.1 Ohm of resistance to ground, so I don't think they are "grounded". I was expecting the ground pin of the phones jack to have very little resistance to ground (to be actually "grounded"), but it actually measures at 10 Ohm. And.. looking again the datasheet for that jack, the pairs of signal pins are connected, I believe, so then the second wire is probably not for switching purpose but actually the signal coming in to the jack, then leaving for the speaker.

Would bypassing the phones jack, and cap/resistor to ground assembly, be a worthwhile test?
Or, should I be following the buzz upstream on the signal path?
Or, should I be looking for something that's connected to ground that shouldn't be?

I'm probably thinking about this wrong, so yeah, your help is greatly appreciated hahah.

Cheers, and a happy new year to you all!

EDIT: Ok, so when I said the "ground pin" of the phones jack I meant the sleeve pin. Turns out that's probably not ground or it would be connected to the reverb knob chassis right beside it... I don't yet understand the use of negative voltage in DC circuits, outside AC for signal and such. Can you tell?  8)
Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by willpirkle on December 31, 2021, 09:13:54 AM »
For the caps, set the DVM on AC volts (preferably AC mV) and read the AC voltage *across* the caps, from one plate to the other with power on. The AC voltage should be 0.0 since caps pass AC. If the AC voltage is higher than a few mV then the cap is leaky and should be replaced. You will need a DVM that can measure relatively low voltages (i.e. not the cheapo ones). The absolute DC voltages on one plate won’t tell the whole story. There is not a particular voltage threshold above which the cap is considered bad (thus the vague “a few mV”) but if it is in the 100’s of mV or in the volt range, then it’s definitely bad.

The nice thing about the AC voltage test is that you can set the DVM and then very quickly test every cap on the PCB. My old boss would blindly test every cap (even the little 0.1uF bypass caps on op-amps and especially logic chips) as soon as he opened a unit, and very often had the device fixed within a few minutes since this is such a common failure point. For the big electrolytics in the PS, you can usually hear the 60 or 120Hz noise from the speaker as well.

Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by cin on December 31, 2021, 01:14:40 AM »
So there actually was some user error with the oscope... There's a tiny bit of noise (<10mV) on OUT1 of the IC closest to the power end of things, while there's no such noise on the other OUT pins. Still, I'm not sure that's the noise I'm looking for, at under 10mV on the out of a 12V fed opamp, it wouldn't be that loud? But again, I really know nothing. Making assumptions.

I put my oscope on the phone jack, knowing that it was all grounded, and it showed what I think is probably the buzz I'm looking for. 60Hz 157mV peak to peak. Which makes sense since I can hear it loud in the phone jack... captain obvious here.

I made sure the square capacitor and resistor between the phone jack and the reverb pot were good, they check out. And there's no continuity across the square cap, so that's not how that path gets grounded.

My guess right now is that neither of the 2 white wires on the phone jack should be grounded, the black wire should be the ground. I'm assuming one of those 2 wires carries the signal, the other one acts as a switch to turn off the speaker when headphones are connected, but not sure how. One of those 2 white wires goes to the speaker terminal, and that confuses me... I don't really understand the signal path here. I did check that there was nothing stuck in the phones jack connecting it all together.

I may be getting ahead of myself... getting late, I will investigate further tomorrow.


Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by cin on December 30, 2021, 11:25:55 PM »
I'm assuming ElCap is short for "electrolytic capacitor" and not another specific type of cap, or brand.

One of those 2 ElCaps has its anode to ground, the other its cathode to ground. Comparing to chassis ground, the ElCap that has its anode connected to ground reads -12.36V at the cathode, the other reads 12.34V at the anode. At the Zeners I find 12.32V and -12.35V. The resistors by the Zeners read 12.34V and -12.38V on one side and 25.6V -25.6V on the other. All looks normal to my tourist eyes so far.

Going down the line, both opamp ICs are supplied with -12.xV at pin 4, and 12.xV at pin 8. I've reseated them to be sure. No change.

I put the 1OUT and 2OUT pins of both opamps ICs on the oscope and found nothing (with no guitar plugged in and no signal), though I'm new to using this oscope so it could be user error.

Looking for another good place to put my oscope ground probe using my MM, I found that all 5 prongs on the phone jack are grounded (when there's nothing plugged into it).

That surely can't be right, can it?

The phone jack is one of those I believe: https://www.switchcraft.com/Drawings/ra49b_Series_cd.pdf

It's setup like this <see photo>

Thanks for guiding me through this... I'm learning a lot, and it's tons of fun.

Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by phatt on December 29, 2021, 11:25:35 PM »
Well as the output is stable then read the DC voltages that supply the preamp.
Those 2 rails are regulated via 2 Zener diodes and filtered via 2 ElCaps.

Using the first picture posted they are top left corner next to the mounting post at the edge of the PCB. The 2 small Blue Elcaps are just near the Silver Zeners.

Use your meter to check you have both rail voltages +12v & -12v. they will likely be 12 Volt but could be 15V rails, both rails should be very close to the same.
likely easy to check voltages at the pig tails on the Blue ElCaps rather than the Zeners.

If a Zener is blown then the voltage will be almost equal to the main voltage which can be read on the bottom side of those 470R resistors below the zeners.
Also the Capacitors could be dying causing hum.
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