Rotel RA-1412 tone control troubleshooting

AngrySailor

Veteran and General Yakker
Joined
Oct 15, 2014
Messages
2,244
Tagline
---not quite right
#1
Hey guys, I just had a look at this Rotel and thought I had any easy fix on my hands. The bass tone control had NO effect whatsoever. It is not a potentiometer, but a multi position rotary switch which selects resistors. The driveshaft between the control knob and switches was not in place. I corrected that and fired it up. Now the turnover switch works, and the tone control does have effect on things but it's not right. At 0dB and up, it does nothing. At -2dB it adds bass and reduces as you turn it to -10dB.

heres a schematic and some nudies!
 

Attachments

Northwinds

Veteran and General Yakker
Joined
Mar 18, 2013
Messages
7,519
Location
Coventry, CT
Tagline
Fondler errrr... fan of all Nav's avatars
#3
Those are a major PITA, good luck sincerely

You could probably get away with fabbing in a Fender TBX type stacked pot that will work just as well and eliminate all that other stuff if all else fails
 

Northwinds

Veteran and General Yakker
Joined
Mar 18, 2013
Messages
7,519
Location
Coventry, CT
Tagline
Fondler errrr... fan of all Nav's avatars
#4
"A lot of people think of the TBX tone control as a treble boost, but that’s not quite accurate. The TBX control actually consists of a custom dual-ganged pot (aka a “stacked” or “stereo” pot), a resistor, and a capacitor that cuts the bass and treble out of the circuit, depending on which way you turn the knob. This can add some new dimension to your solo parts, especially if you are going for those bright, crystal clear Jeff Beck tones.

The basic configuration of the TBX control (Fender part no. 0992052000) changed several times over the years. Fender used several different values for the two pots, the capacitor, and the resistor. The first few versions also lacked a center detent function. The current version consists of a detented 250k/1 Meg stacked pot, a 0.022uF standard film capacitor, and an 82k-ohm carbon-film resistor. In a nutshell, the TBX tone control is a special pot that cuts either treble or bass instead of a normal tone pot, which cuts only treble. This is done with the dual-ganged pot, which is wired to work as a low-pass filter in one direction and a high-pass filter in the other. The center detent in the middle is provided for the off or “flat” position.

The dual-ganged pot is cleverly designed, meaning you can’t substitute a normal stereo pot to make your own budget TBX control. How does it work? The bottom pot (with the shaft up) is pot B on our drawing and is the normal tone control we all know. It’s a standard 250k audio pot with a range from 0 to 5 on the knob. At the detent (middle) position, it goes open and acts like a no-load tone pot, remaining out of the circuit from 5 to 10 on the knob.

The engineering behind this is actually very clever. Normally, the resistive material ring inside of the pot is a band of carbon-containing gunk that is printed onto the phenolic wafer. On the lower TBX pot, only half of the ring is conductive, as the other 50 percent is made out of a non-conductive material. So we can say it is a no-load tone control pot, but instead of going open at approximately 98 percent of its rotation, it goes out of the circuit at exactly 50 percent.

The other pot, which is labeled A, acts in the opposite direction. It also has a split resistive material ring inside, but instead of non-conductive material, metal is used for one half of the ring. This means that between 0 and 5 on the knob, its resistance is at maximum. After the detent position, the normal function takes place from 5 to 10 on the knob. This 1 Meg linear pot comes into the circuit in series with the resistor after the detent position. Because of the high resistance (1 Meg ohm), the load added to the passive guitar circuit is very low"


There is a zillion ways to build these to fit most any application

 
Last edited:

AngrySailor

Veteran and General Yakker
Joined
Oct 15, 2014
Messages
2,244
Tagline
---not quite right
#5
Yes, I do have both the owners and service manual. After stepping back, driving a cheeseburger into the old cheeseburger locker, then washing that down with some "speaker lubricant", I'm happy to say I got it working! After a few checks with the meter, things weren't makin any sense. It turned out that the switches which are tied together with a small shaft (that has two flats on it 180* apart) had come out as I said earlier, but was 180* out of phase compared to the knob! Switched that around and good to go. I will be looking into the power switch next, it says on all the time, but other than that I can't find anything else wrong. The power amp works great, overload works, meters work, speaker relays, everything else seems good... I just gave her a cleaning at all the switches and selectors and they are all quiet. I'm setting it up as my pre amp right now. I wish I had some big 8 ohm resistors so I could drive the power amp just to get them old school VU meters swinging! This thing came from the dump, aside from being full of pet fur, there's no water damage or anything. There are a few very minor nicks on the edge of the face plate, but it's more than presentable.
 

laatsch55

Administrator,
Staff member
Joined
Jan 14, 2011
Messages
68,746
Location
Gillette, Wyo.
Tagline
Halfbiass...Electron Herder and Backass Woof
#6
All the amps at my house don't need a load to make the meters Bop, just an input....
 

AngrySailor

Veteran and General Yakker
Joined
Oct 15, 2014
Messages
2,244
Tagline
---not quite right
#7
All the amps at my house don't need a load to make the meters Bop, just an input....
If I throw the switch to join the pre amp and amp they work, but at that I'm sending signal to the power amp with no load connected. The manual says not to do this? I was just worried about hurting it?
 

AngrySailor

Veteran and General Yakker
Joined
Oct 15, 2014
Messages
2,244
Tagline
---not quite right
#8
Lee, just checked my dc offset and I'm getting -10mv on the left channel and +4mv on the right. The amp is cold but that seems wonky no?
 

laatsch55

Administrator,
Staff member
Joined
Jan 14, 2011
Messages
68,746
Location
Gillette, Wyo.
Tagline
Halfbiass...Electron Herder and Backass Woof
#10
If I throw the switch to join the pre amp and amp they work, but at that I'm sending signal to the power amp with no load connected. The manual says not to do this? I was just worried about hurting it?
The manual says not to run the amp section unloaded? Or not to run the amp section without the pre-amp section driving it?
 

AngrySailor

Veteran and General Yakker
Joined
Oct 15, 2014
Messages
2,244
Tagline
---not quite right
#11
It says not to turn the unit on without speakers hooked up. Right now the the power amp is on, but not receiving any input as I've thrown the separation switch. I'm using the pre out to drive the QSC stuff right now.
 

AngrySailor

Veteran and General Yakker
Joined
Oct 15, 2014
Messages
2,244
Tagline
---not quite right
#13
Yeah, I'm trying to get my head around electronics, it's not something I've dabbled with too much so bear with me... With no speakers hooked up that's infinite resistance, even if the amp receives signal that would just allow there to be potential at the outputs but no current flow. How could this damage the amplifier? I realize that's a simplified view of it but...????
 

AngrySailor

Veteran and General Yakker
Joined
Oct 15, 2014
Messages
2,244
Tagline
---not quite right
#15
Horry fack! There is a bunch of replies that didn't show up till now.

Thanks everyone!

Northwinds, this tone control is not a potentiometer, but a two pole, ten position rotary switch which just stacks resistors in series. I was so set in diagnosing an electrical problem that this mechanical problem didn't jump out at me. I can imagine the nightmare if there was physical damage though, this is a "unique" switch...
 

Northwinds

Veteran and General Yakker
Joined
Mar 18, 2013
Messages
7,519
Location
Coventry, CT
Tagline
Fondler errrr... fan of all Nav's avatars
#16
That's weird, I know tube amps need to be hooked to a load or they will be damaged but I never heard of a SS that requires it at power up
 

orange

Veteran and General Yakker
Joined
Jul 6, 2010
Messages
17,704
Tagline
Broken beyond repair but highly affable
#17
I seriously doubt that myself or there wouldn't be an "OFF" function for the speaker switching.

You would normally want to leave the outputs off while changing RCA cables to prevent that lovely loud hum from being temporarily ungrounded :puke:

Likewise you either turn the set off or the output when changing/adding speakers to avoid messing the amp up.


We sure pick some esoteric stuff to screw up, don't we all? :toothy5:
 
Top