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Author Topic: Old 70's Randall Amp  (Read 1621 times)

that0neguy

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Old 70's Randall Amp
« on: April 02, 2018, 12:02:47 PM »

Good morning and such.  It was by chance that I found this forum, and maybe, hopefully, find some answers too.  I have an 1974 (maybe) Randall RG 90a-115 amp that is mainly a mystery to me.  I can barely find any info about it except for a few other lonely fans that have similar Randall's from that era, which have sustained my hope all these years.

I just recently replaced the 15" speaker with a brand new Eminence (thanks TDPRI, I should get back there and give them an update and thank you), but in doing so, i found that the amp has other problems as well.  The left (1st channel?) sounds dull and muffled, like a wet towel is in front of the speaker.  Ive also noticed that the bass and treble pots on that side don't seem to work as well.  The right (2nd channel?) works, but after the first 2-3 minutes starts the loud popping and snapping, and only seem to stop after I turn both the bass knob off and volume knob down to about 3.  Tremolo works, which my 12 year old son gets a kick out of, and reverb input doesn't, which i'm ok with...I probably need a new RC plug for that side.

If I can get just 1 side working (tremolo side preferably), I would be a reasonably happy man.

Soooo...anyone out there willing to help?
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galaxiex

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Re: Old 70's Randall Amp
« Reply #1 on: April 02, 2018, 08:58:11 PM »

Sure... but a schematic would be a big help.

I realize that may be hard to find.

Can you pull the chassis and take *well lit, in focus, close up pictures* of both sides of the circuit board(s)?

Oh... and what's your experience working with electronics?
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J M Fahey

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Re: Old 70's Randall Amp
« Reply #2 on: April 12, 2018, 06:19:47 AM »

Old Randalls came in two very different flavours.
Earlier ones , which you might have here, were all inspired by "Fender amps" which were king of the hill way back then, so: clean amps, bright (they were used by Tina TurnerĀ“s husband, meaning they were great for Funky guitar) , has Tremolo and Reverb , NO channel switching, Master Volume or any dedicated distortion circuit.
Second generation were "Marshall killers" so two channels: dirty and dirtier, both had Gain and Volume/Master controls, footswitchable, and a very aggressive buzzy sound (they were used by Pantera, that should mean something).
In any case, either squeaky clean or buzzy aggressive, none was "dull"  by any means.
So, which one do you have?
At least post front panel and gut pictures.
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g1

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Re: Old 70's Randall Amp
« Reply #3 on: April 13, 2018, 03:26:05 PM »

As far as I know, that's the orange stripe type, either CommanderI or CommanderII ?
From online pics, going by JM's description it has one master but no gain controls, also trem and reverb in second channel.
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that0neguy

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Re: Old 70's Randall Amp
« Reply #4 on: October 10, 2018, 11:55:11 AM »

Sooo...

I was looking at the amp the other day, and suddenly remembered about this post.  It's only been about 9 months, some vehicle problems, a move, work and the usual distractions that come with life and family.  I apologize to all of you that want to help, or are the least bit curious about this beast

This being the internet - no pics, no proof:
« Last Edit: October 10, 2018, 12:01:45 PM by that0neguy »
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that0neguy

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Re: Old 70's Randall Amp
« Reply #5 on: October 10, 2018, 12:05:05 PM »

More inside pix:
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that0neguy

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Re: Old 70's Randall Amp
« Reply #6 on: October 10, 2018, 12:39:30 PM »

Several things I did notice after taking out the control board (?) / chassis (?) was:
- how much space was used for the circuit board.  If things need to be replaced, this makes it easy
- one of the capacitors has a hairline fracture going through it.  Can that be a problem?  I'm assuming it's ceramic.
- two of the pots on the second channel (reverb and tremelo side) could be problematic.  Specifically the middle tabs of both the treble and mids pots.  Usually there would be numbers on the back specifying what the type of pot it is; these are blank.

When possible, I will upload pix of the front panel.  Right now, I'm having a little trouble with the internet.

Any other details?
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g1

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Re: Old 70's Randall Amp
« Reply #7 on: October 10, 2018, 09:56:32 PM »

How about a pic of the front panel showing control names.
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galaxiex

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Re: Old 70's Randall Amp
« Reply #8 on: October 10, 2018, 10:08:32 PM »

How about a pic of the front panel showing control names.

And the rear panel.

That amp chassis looks suspiciously like a CMI product.

Very similar to some Gibson SS amps from the 70's.

Also, what's up with those 2 loose lock washers just laying there?
Right by the power switch!!!
First pic, second set of pics.

Actually those washers are in all the pics, just more obvious in the close-up.
« Last Edit: October 10, 2018, 10:23:10 PM by galaxiex »
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galaxiex

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Re: Old 70's Randall Amp
« Reply #9 on: October 10, 2018, 10:19:33 PM »

Also, I bet those are Centralab pots.

Pull one and I bet it says Centralab on the bushing right under the threads.
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galaxiex

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Re: Old 70's Randall Amp
« Reply #10 on: October 10, 2018, 10:29:57 PM »

Here's my Gibson G70

Pretty similar... no?

I bet yours is a Gibson G-90, or maybe a G-100 or G-120

just re-branded Randall.

« Last Edit: October 10, 2018, 10:31:31 PM by galaxiex »
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that0neguy

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Re: Old 70's Randall Amp
« Reply #11 on: October 11, 2018, 11:50:01 AM »

How about a pic of the front panel showing control names.

And the rear panel.

That amp chassis looks suspiciously like a CMI product.

Very similar to some Gibson SS amps from the 70's.

Also, what's up with those 2 loose lock washers just laying there?
Right by the power switch!!!
First pic, second set of pics.

Actually those washers are in all the pics, just more obvious in the close-up.

Whoa! After comparing pix of yours to mine... the similarities are too great.  Maybe Randall were making amps for Gibson at that time, and simplified things by using the same design.  I'm speculating, of course.  I was just a tot back then, soiling my Pampers.

I did remove the washers, seeing  how they could cause more problems.
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galaxiex

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Re: Old 70's Randall Amp
« Reply #12 on: October 11, 2018, 12:02:19 PM »

How about a pic of the front panel showing control names.

And the rear panel.

That amp chassis looks suspiciously like a CMI product.

Very similar to some Gibson SS amps from the 70's.

Also, what's up with those 2 loose lock washers just laying there?
Right by the power switch!!!
First pic, second set of pics.

Actually those washers are in all the pics, just more obvious in the close-up.

Whoa! After comparing pix of yours to mine... the similarities are too great.  Maybe Randall were making amps for Gibson at that time, and simplified things by using the same design.  I'm speculating, of course.  I was just a tot back then, soiling my Pampers.

I did remove the washers, seeing  how they could cause more problems.

Actually... I think its more a case of CMI was making amps for a number of brands,
Gibson, Randall, etc.

Does your back panel have CMI on it anywhere?
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that0neguy

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Re: Old 70's Randall Amp
« Reply #13 on: October 11, 2018, 01:06:28 PM »

Not that I could see.  Of course that one silver-ish sticker is faded, but it probably wouldn't be on that.
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« Last Edit: October 11, 2018, 01:09:46 PM by that0neguy »
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that0neguy

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Re: Old 70's Randall Amp
« Reply #14 on: October 11, 2018, 01:11:21 PM »

Front:
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