Author Topic: Concert Bass Preamp  (Read 16582 times)

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Offline DrewV

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Concert Bass Preamp
« on: April 25, 2009, 03:27:03 pm »
Hi,
I am rebuilding a Sunn Concert Bass head. When I got it most of the wires inside were loose and one of the 22ohm/10w emitter resistors had broken loose. Once reconnected it worked poorly for 5 minutes ( heavy wierd distortion) then blew. It was drawing excessive current right before it blew ( determined by watching my lightbulb limiter). The circuit breaker had been bypassed! After replacing the circuit breaker and various out of spec power resistors on the power amp board , the filter caps all other elctrolytics, the three rectifiers and 2 transistors (2N3055) and resoldering the joints, I got the power amp section working, but the preamp was not working. I used my acoustic electric into the power amp in jack on the back of the amp and got a clear clean loud output at the speaker.
 I replaced all the electrolytics on the preamp board and tested the two transistors (BC107) with a transistor checker. They tested good as did all diodes. Upon powering up (variac) I got motorboating directly proportional to the amount of voltage applied. I could not apply full voltage (120V) as the noise got too loud to bear. I then replaced the three FET's and resoldered a bunch of joints. Powering up I got motorboating, only this time I could apply full voltage as the volume control was controlling the level of the motorboating.
I pulled the preamp and ended up finding some more questionable solder joints which I resoldered. I checked a bunch of resitors and ceramic and mylar caps(all good). Powering up again, the motorboating was gone. Plugging in an electric guitar turning the volume all the way up on the amp, I am now getting a clean but very low level output through the speaker The amp appears to be drawing little or no current. ( the poweramp in w/ acoustic electric is still loud and is drawing current).
 The tone controls and brite switch  are functioning fine, but not the dual distortion pot.
I measure +34VDC at the red wire (on the preamp board /in from the power am board) and –16VDC on the white wire and 0vdc on the blue wire (connected to preamp circuit ground. Q201 reads G= 5.93vdc /  D= 5.93vdc / S= 5.56vdc. Q202 reads G= 0.0 vdc /  D= 11.9vdc (hums through speaker when touching D leg) / S= 1.1vdc. Q203 reads G= 2.6 mvdc (hums through speaker when touching G leg)  / D= 12.3vdc / S= 1.33vdc. The original jfets were 2N4304. I replaced them with 2N5458”s.
  The voltages at the transformer are: Primaries (dark orange pair to rectifier CR1) = 18.9vac   and 16.25vac. Secondaries ( light orange pair to power amp board) 22.7vac and 22.5vac and a third pair (green to power amp board) = 19.8vac and 20.6vac.
 So, the power amp section is working fine. The preamp is barely putting out signal. The “power amp in” works fine with an acoustic/electric guitar. The power amp section is very quiet. (I replaced the 3400uf caps with 3900uf.) There are no pops, clicks or components getting hot. I get hum and static-like noise when I touch the volume shaft with a finger that gets louder as the control is turned up. Tapping on the controls with a chopstick does not reproduce the static. No hum on the rest of the controls or when touching the guitar input jack shield. thanks



Offline loudthud

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Re: Concert Bass Preamp
« Reply #1 on: April 25, 2009, 05:37:00 pm »
Something wrong at Q201. Voltages should be like Q202. The schematic I have calls the first JFET Q101. Drawing D1003. Can't read any dates but there is a big 89 near the title block.

Offline DrewV

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Re: Concert Bass Preamp
« Reply #2 on: April 26, 2009, 01:30:01 pm »
loudthud, Here is a clean preamp schematic. Its for the concert lead head. Except for the reverb circuit it looks the same as the #89 schematic which is hard to read. http://sunn.ampage.org/site/schematics/SunnConcertLeadNewPreamp.GIF. I'll be testing components thanks

Offline loudthud

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Re: Concert Bass Preamp
« Reply #3 on: April 26, 2009, 02:53:07 pm »
I'm looking at my own scanned copy. It needs to be touched up.

Offline DrewV

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Re: Concert Bass Preamp
« Reply #4 on: April 26, 2009, 03:37:33 pm »
I found a solder bridge between the drain and the gate of Q101. I now have volume control working. Hitting a strong signal on the guitar, especially an open G string causes the sound to break up into massive distortion, not the good kind. Flipping the treble boost switch on increases the volume (and breakup) and adds medium to low level 60hz hum to the output. Tapping vigorously on the preamp board does not cause any noise. The tone controls are having a massive effect on the output level. With all controls at 10 amp is loud. With all controls at 0 there is barely any output at all.

Offline DrewV

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Re: Concert Bass Preamp
« Reply #5 on: April 26, 2009, 07:53:07 pm »
Loudthud
Plugging in either input jack breaks contact on the white w/ blue stripe lead wire that goes to the preamp board connecting to the gate of Q102. This causes motorboating with no guitar signal coming through. If I short the open contact (that I just broke by plugging the guitar in) the motorboating stops and the guitar level is very low, but clean. The guitar signal goes via a green wire to the gate of Q101. WHat do you think is going on? thanks DrewV

Offline loudthud

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Re: Concert Bass Preamp
« Reply #6 on: April 27, 2009, 02:55:58 pm »
The purpose of the white/blue wire is to mute the preamp when nothing is plugged in. I would suspect a bad or missing filter cap on the +25V or -15V supply. The absence of filtering is setting up feedback between some preamp stages causing them to oscillate. The mute function kills the oscillation.

Offline DrewV

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Re: Concert Bass Preamp
« Reply #7 on: April 28, 2009, 02:07:53 pm »
I replaced all the caps.. I will double check them and get back with the results..thanks

Offline DrewV

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Re: Concert Bass Preamp
« Reply #8 on: April 29, 2009, 03:40:48 pm »
Loudthud, I found a place (near point "40") on the preamp board that looks like it is missing a component. One side goes to the wiper of the volume pot. (it is the only thing that connects to the wiper) The other side connects to the source of Q101. The space between the mounting holes is about 5/16", which is about right for a small cap. I soldered in a smalll value mylar cap and the volume control stopped buzzing (when touched with a finger). As far as the oscillation goes I found 1 bad 250uf 64v on the preamp board. I'll post when I get the replacement in. thanks DrewV

Offline loudthud

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Re: Concert Bass Preamp
« Reply #9 on: April 29, 2009, 06:18:00 pm »
My 74 Concert Bass has the same missing component near the terminals for the brite switch. The components around the volume pot and brite switch are the same on the Concert Bass and Concert Lead.

Offline DrewV

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Re: Concert Bass Preamp
« Reply #10 on: April 30, 2009, 08:10:07 am »
Loudthud, The 250uf /64v filter cap C129 which connects to the +32v supply through R143 (150ohm /1w) looks like it was installed backwards( it looks original so maybe it was re-installed backwards.) The one on this preamp has the + going to ground and the - connected to the resistor. Also, the power switch bulb is fried. Do you know what the replacement is? It looks like a neon. Thanks again DrewV

Offline loudthud

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Re: Concert Bass Preamp
« Reply #11 on: April 30, 2009, 04:08:14 pm »
This stuff should help. These are not touched up so let me know if there is anything you can't read and I will look at the original.

Keywords: Concert Bass Schematic
« Last Edit: December 31, 2011, 10:28:42 pm by loudthud »

Offline DrewV

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Re: Concert Bass Preamp
« Reply #12 on: May 01, 2009, 12:33:41 pm »
Thanks Loudthud, I'll be ordering parts tomarrow. I'll get back with the results. DrewV

Offline DrewV

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Re: Concert Bass Preamp
« Reply #13 on: May 09, 2009, 07:46:38 pm »
Loudthud, I am making progress! Comparing the parts layout on the preamp board to the diagram I found that C125 was not connected to the ground, but instead connected to the trace that appears under it ( the trace connects directly to C105 ). Once C125 is properly connected (to ground) the "motorboating" ( with guitar plugged in) is gone. Another problem solved!! Here's what is happening now. With the volume on "10" and the bass/mid/treble controls on "0" I get a clean but extremely low speaker output level. If I turn the bass control up I get more speaker output level, but the sound is, for lack of a better term, garbled and dies out quickly like it is being choked . It appears that only the bass control is causing this. With the bass on "0" and the mid and treble controls wide open the signal is clean but very low level. I don't think it is a loose connection ( but I could be wrong) because I can tap the preamp board without getting a noise reaction. I sprayed some deoxit into the bass pot, but it made no difference. The bass control also has a major effect on the volume level. With the bass control at "0" and the volume control on "10" the sound is clean, but very low level. With the bass control on "10" the sound is louder but distorted . Do you think there may be a bad component in the bass circuit? What do you think and Thanks again for your help. I'm not giving up!! DrewV