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?
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?
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 Turner´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.
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.
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:
More inside pix:
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?
How about a pic of the front panel showing control names.
Quote from: g1 on October 10, 2018, 09:56:32 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.
Also, I bet those are Centralab pots.
Pull one and I bet it says Centralab on the bushing right under the threads.
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.
Quote from: galaxiex on October 10, 2018, 10:08:32 PM
Quote from: g1 on October 10, 2018, 09:56:32 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.
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.
Quote from: that0neguy on October 11, 2018, 11:50:01 AM
Quote from: galaxiex on October 10, 2018, 10:08:32 PM
Quote from: g1 on October 10, 2018, 09:56:32 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.
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?
Not that I could see. Of course that one silver-ish sticker is faded, but it probably wouldn't be on that.
Back
Front:
Have a look at this thread, I think the schematic should be very similar:
https://www.ssguitar.com/index.php?topic=4236.0
Well if it's not a CMI product,
they at least sourced their chassis and PC boards from the same place.
Lots of other design similarities, front panel and knob type to mention 2.
Thanks for the pics. :)
Quote from: galaxiex 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.
Finally took out the chassis again tonight, pulled several pots and found that they are not, in fact, Centralab pots but CTS. So good news / bad news...
- the good: they are (hopefully) still made, or some equivalent
- the bad: I still don't know what their specs are
I thought about just switching the pots from the left channel over to the right channel, but now I'm having second thoughts about that. It's late, I'm tired, and I'll more than likely screw it up or do something else equally stupid.
Quote from: g1 on October 11, 2018, 09:00:51 PM
Have a look at this thread, I think the schematic should be very similar:
https://www.ssguitar.com/index.php?topic=4236.0
I took a better look at the schematic and found one part of the question, as in what type of pot(s) they are. Unfortunately, no indication for the brand.
Also, the other dude that also had a Randall Comander I, where'd he go? I wish he'd stuck around longer.
Quote from: that0neguy on November 11, 2018, 12:01:45 AM
Also, the other dude that also had a Randall Comander I, where'd he go? I wish he'd stuck around longer.
Click on his username and you will see an option to send him an email. Worth a try, he was last here in 2017 so it's not that long ago.
Don't over think it all, Many pots will fit and do the same job. 8)
As the amp has basic controls there are only really two possible pot types,
Linear and Log.
Take them out one at a time and measure them.
Linear pots will read half total resistance at half rotation.
i.e. A 100k pot will read 50k at half rotation while a 100k Log pot will read about 10k~20k at half rotation.
(Meter on the bottom and centre lugs)
So jot down which pots are lin and log then order some 24mm CTS or similar pots. By the pics looks like they are the old 1/4" solid shaft.
Phil.
Here's the deal with similarities...
Standel was sold to Randall Instruments, Inc. in 1972 but for a short while the company also continued its venture with the Standel amps as well. Robert Crooks held his position as president, but was also appointed vice president of engineering at Randall. His son was also an executive in both companies.
The cabinets, chassis, knobs, hardware, etc. of early Randall amps... yes, all those parts were pretty much sourced from Standel's inventory. Others, such as transformers, Don Randall had taken with him when he left Fender. They even kept "Standel-ish" cosmetic styling for a while including the "faux" wood panels. Standel largely designed and produced some of the earliest Randall amps (Alpha and Delta series), which were hybrid designs with 8417 tube -based output stages and solid-state preamps.
I would take a wild guess that these designs largely served as a basis for future Randall amp models that followed.
In 1973 Standel was eventually sold to CMI, or Chicago Musical Instruments (also known as Norlin Industries, which actually was the company that owned CMI). Most prominent company under CMI umbrella was Gibson, and Gibson happily adopted Standel's (solid-state) designs as part of their new "G-series" line of solid-state amplifiers and once again they were largely built using the same Standel parts inventory.
Another side venture of that were the "SG Systems" amps, which were mostly hybrids with guess what... 8417 tube -based output stages and solid-state preamps. Yep, another "Standel design". In difference to Gibson G-series amps these largely relied on FET preamp circuitry, which is actually very, very similar to circuitry you'll encounter in an early Randall amp: Same FETs used in very, very similar circuits and overall circuit architecture. Also, once again they largely relied on existing parts inventory, so hardware is similar in Gibson, Standel and (early) Randall amps.
Eventually, Standel operations were seized completely, Gibson stopped building amps for a while under Gibson trademark (I think because they moved their production plant to another state) and then had a brief side venture with "Lab Series" amps. The SG Systems amps on the other hand simply weren't a very long lasting product.
So, Bob Crooks eventually went to work for Barcus-Berry (where he pioneered the concept of "Exciter" products relying on dynamic phase shifting) and Randall simply hired an interesting character called Gary Sunda to take care of amplifier product design. Gary was a former jet propulsion engine designer from Lewis Labs and did all sorts of freelance electronics design (e.g. he had built the power supplies for world's largest "slot car" race ways) and operated an amplifier and speaker repair business. Since mid 1960's he has produced a series of transistorized "Sunda" guitar amplifiers (manufactured by Lewis Labs), which Randall had found quite impressive. After working a shortwhile as a freelance designer for Randall, Sunda was eventually hired on their payroll and became chief engineer. He is largely credited for tweaking the distortion circuitry of Randall amps, and in that he basically continued from where Standel had left. It is possible that Sunda also co-operated in design of those SG systems amps as well.
Teemuk, thank you. That is a plethora of information I didn't know I needed, but definitely useful and wanted. I've had this amp for years and, until several years ago, never saw another one remotely like it. And to find info about it on the 'net?? It might as well not have even existed. Maybe you should write a book on the early years of Randall... : :cheesy:
And phatt, thank you. You've re-centered my quest to (hopefully) make this thing work.