Rickenbacker TR35B Potentiometer

[dpm309]

Have a Rickenbacker TR35B bass amp that has noisy pots. Tried cleaning with no luck. I am having trouble finding replacement pots since this is an old amp.  These are right angle PCB mount with mounting clips. Have looked at Mouser, AE, Digitech, Small Bear with no luck. The pots are 24mm diameter with a 24mm plastic shaft. There are 2 numbers on the pot: EX5460, and R1378204.
Any ideas?Thanks,Dan

[shooter]

is there enough room to use a chassis mount pot and just run wires to the PCB??

[dpm309]

Yes, that would be an alternative to finding the Rick pot (which is probably not even made anymore). The schematic calls out 100K but does not show if it is audio or linear. Would also have to get one with a long shaft which should be easy to find.Thanks Shooter

[shooter]

if you're shopping
these work good for "PCB connectors"

[Latole]

Did these pots use knob ?

To clean ; do you use good contact cleaner and spray in the access hole ?
And turn at leas 20 times from 0 to 10 ?
Noise can come from a leaky capacitor too.

[Latole]

Pot A is better for volume.
Bass ,treble too
Pot with long metal shaft may work

Always show schematic. ; https://www.rickenbacker.com/wp-content/uploads/2024/01/19342.pdf

[Latole]

https://www.tme.com/ca/en/details/6mmi-100k/carbon-single-turn-potentiometers/piher/t-21ahm0607104a2020/?brutto=0&currency=USD&utm_source=google&utm_medium=cpc&utm_campaign=KANADA%20[PLA]%20EN&gad_source=1&gbraid=0AAAAADyylhJl0JG_1eFdEaPYEmBlDPTkc&gclid=CjwKCAiAk8G9BhA0EiwAOQxmfroQWeHihoSyDR2aJDmeu_iw4_7MT9RG9uHNVqnd0hqythRZXJ0vpRoCrJMQAvD_BwE

[Latole]

I'd buy more “standard” potentiometers and screw them to the chassis.

Wires would connect the potentiometers to the circuit.
Rigid wires may help to keep pot in line with chassis hosle

[dpm309]

I found a 100K, 25mm pot with a long shaft an solder lugs on Mouser. Had to solder bare wires to the pot lugs and then soldered the wires to the PCB. Had to apply caulk to the pot since the originals had mounting tabs that were soldered to the board. Also had to ream out the hole for the pot to accommodate the larger shaft diameter.  Put it all back together, and it is working with no noise when turning the volume control.

[dpm309]

Thought I was done with this amp. After replacing the pot, the amp worked fine. The next day I powered it up and am now getting a weak, distorted signal. Cleaned the old pot and reinstalled it with no luck. Tried a different speaker and am still having the same issue. Rechecked my solder connections and checked the PCB boards for any cold or broken solder joints. Any ideas on where to look next? The schematic is in Doug's schematic library (Rickenbacker TR35B).Thanks,Dan

[shooter]

what do you get at the FX out, clean, distorted??

[dpm309]

I am getting a clean signal from the preamp out into another bass amp (GKRB400).

[shooter]

stick a signal into the FX return, preferably with it's own vol pot, set amp vol pot to "10"  clean, bad??

[Latole]

What I understand: after replacing the potentiometer, the amp was working perfectly well, and the next day, on a second try, the amp was no longer working properly.

 I would think that other parts would have failed, output transistors for example.
Or a very low voltage in power circuit.

Good luck.

[dpm309]

Shooter, what kind of signal should I stick into the return? Bass guitar, signal generator or ?
Latole, I am getting proper voltages from the power supply. I will try to trace the signal with a listening amp device to see where the issue is. I am now suspecting the output transistors, Q9 (PNP) and Q13 (NPN) might be the issue. The schematic lists these as Q9 - 2N5988 (part in amp reads TIP42B) and 2N5991 -Q13 (TIP410C). These are fairly inexpensive parts so I might go ahead and replace them after I check the amp with my listening amp.

[shooter]

"signal level", so output from your phone, MP3 player, guitar with gain pedal, another preamp.......
the point is get the signal "off" the pre/volume/connector and into the PA so you can eliminate 1/2 the circuit.


you could just gatorclip the wiper of the pot to the "input/left side" 100k R feeding the PA, maybe a gatorclip for ground also, since signal ground appears to go through the same pre/PA connector.

[dpm309]

Shooter, still getting a bad signal when I plug in a signal (MP3) to the return. I am suspecting something in the power section. Currently using my listening amp to trace the issue. I am getting a clean signal up to Q4 and 5. Getting a distorted signal before and after Q9 and 13.

[shooter]

what do you have in/out of Q7 Q11??

[dpm309]

Shooter, let me get back to you on that.
Thanks,

[dpm309]

OK, back from doing some home improvements. I am getting clean signal before and after Q11 an Q7. I am thinking of going ahead and replacing the 2 power transistors Q9, and 13 and see if that solves the issue.  What would have caused the power transistors to fail after changing out the pot other than this is a 45 year old amp?

[shooter]

this is a 45 year old amp

I would replace the 2 drivers Q8, Q12, they operate with the Power transistors forming a "Darlington pair" so the drivers "feel" any grief the PA's do


also verify the 1.2K R's either side of the bias pot are real close to = value to each other

[dpm309]

Thanks for your help Shooter. Will put an order into Mouser later this week. They are now charging a 10% tariff!!

[shooter]

 
do cost averaging, you'll feel better


cost + tariff /45 of working

[dpm309]

Ugh, this amp is going to be the death of me. I disconnected the wiring harness and reconnected before I fired the amp up one more time with the chassis out, and everything worked. When I installed it in the cabinet, it blew a fuse. Replaced the fuse and using my lamp limiter, I have a dead short somewhere. Back to the drawing board. Must be something with the connectors. Will try to find out where the short is.

[shooter]

 
welcome to my world
in the early days of complex computer controlled systems, 9 outta 10 problems were cables/connectors, socketed IC's


Data General built a computer with PCBS so close on the buss that you'd scratch IC's on the adjacent boards when you went to pull a board.
the same computer had 6, 40 pin flat cables across the top's of the boards, so you had to unplug all the cables just to get a board out, then you'd spend hours getting the frigg'n thing working, once you fixed the original problem.


I was salaried + OT so I had great money that my wife spent cuz I was either working or sleeping!!!!

[dpm309]

Using my multi meter to check continuity, it looks like Q9 is shorted. Getting continuity between the collector and emitter. Same with Q13. Using the diode setting, I am not seeing a voltage drop on either. Rest of the transistors seem to test OK. I am guessing that this is the cause of the short. Will order a set of power transistors. Will post again after I install the new transistors.

[dpm309]

While waiting on the power transistors, I am trying to find replacements for Q8 (92pu06) and Q12 (92pu56) with no luck. These were apparently discontinued some time ago. The closest substitution I can find, are NTE128P and NTE129P. What other transistors would make a good substitution for these?

[shooter]

my Ref book is buried somewhere in the maze


if you wanna get creative, I've used both these to "upgrade" 50 year old technology
the TIP142 would "replace" both the driver and PA Q8-9 become 1 TIP142, same for the other side

[dpm309]

Installed the new power resistors and when I used the light bulb limiter, it did not light up at all. When I applied power to the amp, got a loud hum. Went to check the voltages and heard a loud snap and then smoke. Q9 is cracked! Back to square 1. Must still be a short somewhere. I am glad that I purchased 20 of power transistors so I have spares.

[shooter]

check these, especially the .27 or 27

[Latole]

Loud hum is more voltage to the speaker .
Coil could be damaged, use a low-end speaker for testing.

[dpm309]

Latole thanks, The speaker is fine. I plugged it into my GK RB400 and there is no apparent damage. 
Shooter - the 2 power resistors are both marked .27 ohms and they tested out OK. Both Q9 and Q13 are bad. Going over the board I noticed the 100uF 25V e-cap (the schematic shows 100uF 40V) was leaking green ooze and had no reading on my meter. I have a couple of extra 100uF, 35V e-caps and will remove and install to see if this might have been the cause of the transistors blowing.

[Latole]

Latole thanks, The speaker is fine. I plugged it into my GK RB400 and there is no apparent damage. 
 

I'm talking about preventing damage during testing.

[dpm309]

I have an old 12" speaker from a dead Crate amp that I will use from now on.

[dpm309]

Latole, Is that the insulator you were talking about located above the power resistor? I might have to do a better job of making sure they are aligned correctly.

[dpm309]

Latole, disregard. I was responding to a post from another forum.

[dpm309]

I reinstalled new power transistors and a leaking e-cap. Powered it up with the light limiter and it did not light up at all. Tried it on my working GK RB400 and it lit up faintly. Not sure where to go from here as I do not want to fry any more transistors with powering it up without the light limiter. I checked the new transistors to make sure they were not making contact with the metal heat shields. Checked several transistors and resistors that Shooter highlighted in a previous post and they all tested good.

[shooter]

with NO power
does C-E Q8 ohm "open" both ways?
does C-E Q12 measure ~~~ the same as Q8?

[dpm309]

Yes, they both read "open" both ways.

[shooter]

NO POWER
ohm bias pot wiper to Q11 C, then wiper to Q7 C, they should be ~~ equal readings


I would tweak the pot to make sure it's "adjustable" and doesn't go open as you slowly rotate
other than that I'm about out of guesses for the month 

[dpm309]

Shooter, yes equal readings and the bias pot is adjustable. Also noticed that I am getting 36VDC where it shows on the schematic and the positive side of the bridge rectifier but am not getting -36VDC on the negative side of the bridge rectifier.

[dpm309]

Checked the power transformer and am getting ~56VAC. The transformer is shown as part No. TS0002. Cannot find any information but am guessing the PT is OK. Could I have a bad/blown bridge rectifier?

[shooter]

but am not getting -36VDC on the negative side

could be, could be something dragging it down.  try and isolate the bridge from the "downstream" load and measure at or close to the bridge

[dpm309]

I finally got the amp working properly. There was an extra 1/4" jack next to the extension output that was flimsily connected to the speaker terminals. Removed it and every thing is working fine. This extra jack was not shown on the schematic. This was probably the cause of all of the issues.Thanks for everyone's help in getting this amp fixed. I did learn a lot about SS amps though.

[Latole]

I finally got the amp working properly. There was an extra 1/4" jack next to the extension output that was flimsily connected to the speaker terminals. Removed it and every thing is working fine. This extra jack was not shown on the schematic. This was probably the cause of all of the issues.Thanks for everyone's help in getting this amp fixed. I did learn a lot about SS amps though.

This output jack short the speaker output ?
A extra jack by itself can be the issue

[shooter]

I did learn a lot about SS amps though.

just in time, since I'm 97% to full time Gardner, landscaper and just plain retired from thinking too much