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Author Topic: Randall RG100 Looping pop sound  (Read 1433 times)

StiffCookie

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Randall RG100 Looping pop sound
« on: November 05, 2016, 11:06:44 AM »

Hey guys,

I recently acquired a Randall RG100. It kind of wants to work, but sounds like it keeps rebooting (so to speak). Rather than trying to explain the symptoms in detail, I've attached a YouTube video below instead. There is a little more information in the description section on YouTube.

https://www.youtube.com/watch?v=rxfLPj4yJZw&feature=youtu.be

Any ideas?

Thanks guys!
StiffCookie
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phatt

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Re: Randall RG100 Looping pop sound
« Reply #1 on: November 05, 2016, 07:32:07 PM »

Arcing is bad,, yes don't turn it on until the problem is identified and fixed. 8|
A close up picture of the offending section of the PCB might help. :tu:

My guess is a stuffed Zener diode,,look at R6, R58 and R59.
There is a Zener after each of those resistors, D9, D10 (12Volt Zeners) and a 24 Volt Zener after R6.
Also check the capacitors C45, C42 and C43.

Whatever resistor is overheating *That Rail* is where you will find problem component.

That is assuming I'm looking at the right schematic?
Phil.

ed,, oh yeah,, arcing means something is broken,, look for cracks in the solder on the legs of the part.
« Last Edit: November 05, 2016, 09:17:04 PM by phatt »
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StiffCookie

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Re: Randall RG100 Looping pop sound
« Reply #2 on: November 11, 2016, 10:25:33 AM »

Hey Phatt,

Thanks so much for taking the time to check out the schematic and making these suggestions, I appreciate it very much!

I have some free time this weekend so I'll have a look at the those components and take some pics of the PCB.

Cheers!
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StiffCookie

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Re: Randall RG100 Looping pop sound
« Reply #3 on: November 12, 2016, 09:56:26 AM »

Hey again,

I've taken a few pictures of the board.

The first one is a picture of the the overheating resistor (R24). The resistor above is the one that is arcing to the track above it (R41, left side leg). The right side of R41 actually has a solder bridge on the other leg, I don't know if this is by design or if it's a previous owners mod.

The second picture is the PCB number and revision  (maybe helpful).

The 3rd and 4th pictures are of the only zeners that I could find.

The 5th picture is of a 1M resistor mod, again I don't now if this is a factory mod or a user mod, any ideas? I wondered if these were user mods (not sure how I would find out), is it worth undoing them?

Thanks again.












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StiffCookie

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Re: Randall RG100 Looping pop sound
« Reply #4 on: November 12, 2016, 10:05:57 AM »

Oh and by the way, I've discovered there are different versions  of the RG100. This one is a "Valve-Dynamic RG100 G3 (100W Tube/Mosfet Power)".

Plus I also checked the voltages of the PSU (Main board disconnected). I'm getting:

QC1 - 58.70V DC. (QC13 on PSU)
QC5 - 382.0V DC. (QC14 on PSU)
QC6/QC7 - 13.5V AC (The two orange wires directly form the transformer [HTR]).

Full PCB:
« Last Edit: November 12, 2016, 10:32:10 AM by StiffCookie »
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DrGonz78

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« Last Edit: November 13, 2016, 02:20:21 AM by DrGonz78 »
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A person who never made a mistake never tried anything new. -Albert Einstein

phatt

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Re: Randall RG100 Looping pop sound
« Reply #6 on: November 13, 2016, 07:42:13 AM »

R41 looks like a dry solder join which is a common issue with lead free solder,, yuk horrible stuff.
Hit it with a hot iron ,, sometimes it just evaporates.
Add new solder (not Lead free crap)

Other end is a blob ,, maybe someone tried to resolder the join from the top of the PCB in hope it would reflow.

Again Lead free solder is about the hardest stuff to reflow and the new solder is just a blob which may or may not be fully wetted to the PCB.

I'd pull R41 clean the pads and remount that part.

A pic of the underside of PCB might give a better idea as to what happened.

Meantime try and establish if the Schematic that Drgonz has linked to is correct for your Amp then it will by easy to work out how to connect the part.

Hope it helps,,
Phil
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phatt

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Re: Randall RG100 Looping pop sound
« Reply #7 on: November 13, 2016, 08:32:51 AM »

Just found this; Thanks To Drgonz :tu:
Maybe the mosfet is stuffed causing those resistors to overheat.
Phil.
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StiffCookie

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Re: Randall RG100 Looping pop sound
« Reply #8 on: December 07, 2016, 09:05:31 AM »

Hey guys,

I've been so busy lately I haven't had a chance to get the amp out again. Thanks so so much for your replies, I appreciate them very much!

I promise I haven't hit and run this thread, I'll be back with an update soon!

Cheers,
StiffCookie
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StiffCookie

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Re: Randall RG100 Looping pop sound
« Reply #9 on: May 19, 2017, 09:10:55 AM »

Hi guys!

Just thought I'd update you and maybe help people who may have this issue in the future, I fixed it!

I spoke to a friend at work about it and he recommended cutting away a section of the  ground plane above R41 (the resistor that was arcing). So I tried it and the amp now works beautifully! No more arcing!

Cheers for all the help!
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