Randall Modern to Orange Rockerverb - Sorted

[Mattallica]

(FYI This topic has now been solved)

Hi all,

I bought a Modern and it turned out to be faulty. I got it very cheap so I wasn't worried and figured i'd use it as a doner module for a home project.

I traced out the board and discovered there was a break in the trace between V2 cathode 1 and it's pin connection. I put a jumper wire across believing it would solve the problem. In hindsight I should have checked if this would fix the issue before proceeding with the rest of the mod. Too late now!

I believe I have done an almost exact copy of the pre-amp of the Rockerverb schematic posted on orangefieldguide. The only change is replacing the VR4-A with resistors to get rid of the stereo pot.

Once I finished, I reassembled the module and ran it in the amp. To no surprise, it didn't work.

When I play a note it makes a clipping like buzz sound and dissipates quickly. I think I can also hear a very faint clean sound coming through.

The main board is Brit module A, Rev 2.0 2-27-02

The tube board is Brit tube Rev 2.0 2-27-02

Here is a list of the changes I made:

.R9 to 1M
.R15 to 68k
.Removed R14
.C6 to 10uF
.R16 to 1k5
.C7 to 10uF
.R17 to 1k
.R21 to 220k
.R12 to 220k (after R21, grounded to R7)
.R7 to 470pF
.C13 to 10uF
.R29 to 2k2
.R30 to 1k5
.R27 to 220k (with 1M+470k grounded to C18)(pot replacement)
.R31 to 39k
.C14 to 560pF
.R36 to 220k
.R34 to 470k

The major change i made was breaking the trace between V2 plate 2 and V2 grid 1. I added a 470pF and a 470k in series between V2 plate 2 and V2 grid 1. I added a 220k between the 470k and V2 grid 1 and grounded this to R14.

The only issue i can see is that the 470pF cap i used has a small hole in it and it's exposing the metal inside... could this be the problem? i have gone through all my soldered joints and they are fine.

If i have missed some fundamental rule for modding the Randall modules please tell me! any help will be appreciated! (...silently hoping for Rob to chime in..)

 

[withmittens]

While I have no ideas, I'm following the thread to see how it turns out.

hopefully it's something fixable and you can use the mod. you could always send it to Rob, asking that he only fixes the problem, changing as little as possible....

good luck

 

[Mattallica]

Thanks,

I'm not located in the US, so shipping is an expensive exercise. Also i'm not quite ready to admit defeat just yet!

If i get it working i will update the thread.

 

[Jaded Faith]

I'm terribly visual, so unfortunately the parts and positions do little for me. I often repair DIY work and almost always the issue has to do with damage to the module from a heavy solder hand or something done that takes out the switching circuitry. I'd love to help, but I'm not sure what can be done with the info provided.

 

[Mattallica]

Hi Rob!

Very reasonable response, thank you. I don't believe i've overcooked it, but it is entirely possible. The switching still functions correctly. I will drop you an email with some photos.

 

[Mattallica]

I've had another go at making this work before emailing Rob.

I discovered that when I back the volume almost off with the guitar, the signal actually makes it through the module. It sounded terrible, but it's there.

I tried to balance the guitar volume with the module gain and then the module volume. It was still cutting out.

So I came to the conclusion that the input stage is being overloaded.

I dropped the grid resistor on V1A down to 10k from 68k.

It has now stopped cutting out, even with the guitar volume at 10. So that's a small win.

The problem now is that it's very quiet and stills sounds rubbish. Like a crappy distorted fuzz pedal.

I'm going to try tweak the resistors between the other gain stages to see if it brings the volume back up, and hopefully sound better.

 

[Mattallica]

I'm now revisiting this since learning about the initial gain stage before the module.

My error was not accounting for this when modding the module. No wonder the circuit was overloading.

So as a quick fix to see if this was the problem, i simply bypassed the first stage on the module with a jumper wire.

It worked, so now i have a nominal level of volume and all the knobs seem to do something. It sounds ok, but not great.

So more work needs to be done. I'm still in the process of understanding how this all works.

I'm going to add the first stage back in but with a lower value grid leak resistor to attenuate the signal. I don't know how much this will make the module deviate from the Orange sound, but there isn't much choice.

Bypassing the amp V1 would be the easiest option but that's not a practical approach in the wider picture. I think there is another resistor before the opto coupler i'm not accounting for as well...

Noob ramble over.

 

[galao]

Don´t forget that V2B of the Marshall Brit style PCB is a cathode follower. The Rockerverb has no CF in this position. Instead the tonestack must be connected to the anode of the last GS. As you have a spare GS on the module you could cut traces to remove the CF and connect the tonestack with V2A.

 

[Mattallica]

Thanks Paul, you're spot on. I've rewired the module since my last post and v2b is indeed bypassed with the trace cut.

It's all as per the schematic and it's working great! Well, except for the stereo gain pot. Vr2b is a fixed resistor to mimic the pot being wide open. I'm going to add a mini 1m vr here instead to give more control.

Now that I understand the PCBs better I've made some more Marshall based variants: ironheart, klipp and aor.

The OR with the baxandall eq and inductor boost is proving much trickier to achieve. I'm doing this with a blackface module.

I really want to make a hiwatt module now but I didn't order enough 100k pots...

Thanks again to all who commented, I'll close this post out now as it has been solved.

 

[withmittens]

sounds great. thanks for posting. I tried CrazyNuts' Dirty Sanchez with pretty good results. This post reads like it may be reproducible.

 

[Mattallica]

sounds great. thanks for posting. I tried CrazyNuts' Dirty Sanchez with pretty good results. This post reads like it may be reproducible.

I will have to edit the OP with the correct changes before anyone tries to reproduce what i've done. This thread should read as someone progressing from no idea to some idea of what needs to be done. The biggest part is understanding which stages are which on the PCB.

 

[withmittens]

Wondering if you've taken this anywhere beyond whats posted here. Still sounds interesting.

 

[Mattallica]

Not really unfortunately. Other things have been getting in the way. Not having the means readily available to finish the chassis is the biggest issue. So i'm getting a bunch of modules done to a point where i can do the chassis work on all of them at the same time. I keep changing my mind on the switch options too.

I have finished the faceplate design however.

 

[withmittens]

Still following in case you finish