Concert Bass Preamp

[DrewV]

                                             

Loudthud, Here is what I know about this amp at thiis point in time. I don't know what else to check out, but I did find some values that were different from the schematic as well as components that were missing or in a different location on the board than what the schematic shows. I can post piks if that will help. Thanks again...DrewV

RESISTOR READINGS (IN CIRCUIT)

R143 READS 158 OHMS (150 OHM)

R142 READS 11.3K  (18K)
R124 READS 11.4K (18K) 
R106 READS 11.3K (18K)
ALL 18K RESISTORS READ GOOD OUT OF CIRCUIT

R117 READS 10.4K (15K)
R105 READS 10.4K (15K)
R113 READS 10.5K (15K) 
ALL 15K READ GOOD OUT OF CIRCUIT

R310 IS MISSING (BIAS POT @ Q301)
R313 IS A 220K INSTEAD OF 100K (ACCORDING TO SCHEMATIC)
R312 IS A 1K INSTEAD OF 470ohm (ACCORDING TO SCHEMATIC)
R308 IS A 30ohm INSTEAD OF 10ohm/2WATT ACCORDING TO SCHEMATIC)
R325 IS AN 820ohm INSTEAD OF 330ohm (ACCORDING TO SCHEMATIC)

R309 IS BETWEEN R312 & R318. (NOT AT SCHEMATIC LOCATION)
R318 READS 29K (33K) READS 33K OUT OF CIRCUIT
R141 READS 47.5K (100K) READS 102K OUT OF CIRCUIT

R307 READS 466ohm (910ohm)*
R301 READS 466ohm (910ohm)*         ALL 910ohm NEW
R303 READS 464ohm (910ohm)*            (*ALL READ GOOD OUT OF CIRCUIT) 
R304 READS 464ohm (910ohm)*
R324 READS 1K (68K)
I HAVE TESTED EVERY RESISTOR IN THE AMP.

CAPACITORS (IN CIRCUIT)
THE FOLLOWING CAPS READ “OL”: C122 / C121 / C112 / C110 / C111/
C106  / C101.
C120 READS STEADY 2.2Kohms
C114 READS  STEADY 1.2megohm
FROM THE ABOVE LIST, OUT OF CIRCUIT ALL CAPS TEST GOOD.
I HAVE TESTED EVERY CAP IN THE AMP.
C2 IS A 3400UF/75V INSTEAD OF 1000UF/75 (ACCORDING TO SCHEMATIC)

SHORTING TIP TO GROUND AT DISTORTION FOOTSWITCH LEFT ENABLES THE DISTORTION POT. SHORTING TIP TO GROUND AT DISTORTION FOOTSWITCH RIGHT MUTES THE OUTPUT TO A VERY LOW LEVEL AND THE DISTORTION POT FURTHER REDUCES THE LEVEL AS IT IS TURNED UP.

Voltage at R105 is +16VDC on the drain side of Q101 and +30VDC on the other side of the resistor at point (46)

Basically wherever there should be +25vdc I have +30vdc and wherever there should be  –15vdc I have –16vdc.

[loudthud]

It will take me a while to go through all of this. It's not unusual for Sunn amps not to match the schematic. The amp I have I got off ebay as a fixer-upper so I've never turned it on. There are some wires missing on the input jacks and could be a couple of shorted output transistors. The back half of the distort control changes the gain and the front half looks like it adds filtering after the volume control so the tone isn't real buzzy.

There is one problem I found on the schematic where it doesn't match the PCB layout at the distort control. The way it's drawn on the schematic the control is shorted out. I think the original footswitch shorted both jacks at the same time with a double pole switch. Maybe someone who has worked on one of these will chime in.

[DrewV]

The output signal is distorted regardless of where the input signal comes from (front panel 1/4" or poweramp in). I was thinking that the unwanted distortion was coming from the preamp board, but if it's happening with the poweramp in for signal source, wouldn't that bypass the preamp board? Q301 and Q302 both test good as do the five 2N3055's. Could the bias out of whack?

[loudthud]

From your description the distortion is coming from the power amp. I looked at all the Concert schematics that I have and there is nothing on how to adjust the bias pot. The very last schematics have changes to the driver circuit and the bias adjustment is gone.

The one thing I could find says that distortion can come from leaky output transistors and that they should measure greater than 1 meg collector to emitter. I assume that means out of circuit.

[DrewV]

Thanks, I will check the resistance on those and report back. dv

[DrewV]

I found a couple things I previously overlooked, silly me.  Q2 is (wrong) a 2N3056! All four 0.27ohm/10W emitter  resistors are out of spec reading from 0.7 ohm - 1.8ohm.! The parts are on order. I will post the results soon. Thanks dv

[DrewV]

resistors measured out of circuit

[DrewV]

Loudthud, I got the new 0.27 resistors and 2n3055 transistors in and got minimal improvement to the sound. Can you confirm that the wiring is correct?  :
47= black wire to shield dist ftswitch R.
48= black wire to shield poweramp in .
49= red wire to preamp out (note: shield of preamp out is connected to wire #49's shield, but, does not connect at the other end).
50= nothing connected.
51= nothing connected.
52= red wire to tip of dist ftswitch left.
53= clear wire to tip dist ftswitch right
42= nothing connected.
43=red wire to 30 on power amp board.
44= white wire w/yel stripe to 29 on poweramp board .
54= blue wire to 31 on power amp board
45= no connection
46= no connection
thanks Drew

[loudthud]

Loudthud, I got the new 0.27 resistors and 2n3055 transistors in and got minimal improvement to the sound. Can you confirm that the wiring is correct?  :
47= black wire to shield dist ftswitch R.
48= black wire to shield poweramp in .
49= red wire to preamp out (note: shield of preamp out is connected to wire #49's shield, but, does not connect at the other end).
50= nothing connected.
51= nothing connected.
52= red wire to tip of dist ftswitch left.
53= clear wire to tip dist ftswitch right
42= nothing connected.
43=red wire to 30 on power amp board.
44= white wire w/yel stripe to 29 on poweramp board .
54= blue wire to 31 on power amp board
45= no connection
46= no connection
thanks Drew

All those connections look correct. The Power amp board in mine is a little different from the diagram I posted in this thread. I'll see if I can find and scan that info. I've got one shorted 2N3055 and some resistors that need to be replaced on the power board.

[DrewV]

I removed the distortion control from the circuit. It read 28K on the front pot and 25K on the back pot.

Latest observations:
With the guitar plugged into either input the signal sounds clean if picked lightly at low amp volume but goes into massive ugly distortion if amp volume is turned up or guitar is played hard, drawing a good amount of current, thou not enough to burn or heat any components..

With the right distortion footswitch shorted tip to ground/ output level is reduced and the distortion control attenuates the signal as it is turned up with near total attenuation at “10”.  (something backwards?)

With the left distortion footswitch shorted tip to ground/ idle output level is increased by a substantial margin with a buzz/hum like quality. Rotating the distortion control from zero to “10” increases the overall level and causes the amp to go into massive ugly distortion drawing a good amount of current as described above.

With both the left & right distortion footswitches shorted tip to ground/ amp draws very little current and displays the same massive ugly distortion if amp volume is turned up or guitar is played hard. Rotating the distortion control to 10 produces good distortion at very reduced amp output levels, drawing very little current.5/21/2009

[loudthud]

Well I worked on my Concert Bass to see if I could get it going. I couldn't find my stash of 2N3055s so I bought 10 at my local surplus store. Matched up one good set of 4. Just to be on the safe side, I tried to power up the amp with only the driver transistor installed. The floating supply wasn't floating properly. The problem turned out to be a short in the driver transformer between the primary and one of the secondary windings. So it looks like I need a used transformer or will have to get the old one re-wound.

[DrewV]

Loudthud, I started checking the voltages on and around Q104 tracing the circuit path of the -15vdc supply. When I got to R130 I still had the proper voltage. When I got to R128, I had zero vdc. A continuity check showed that the connection between R130 and R128 was open. After a little prodding the end of R128 closest to R130 lifted right off the board. I resoldered it and checking the voltage. I now have C= 14.3vdc / B= 3.4vdc /  E= 2.9vdc. Vbe = 0.6vdc. The amp now works. Current draw appears to be normal. All of the nasty artifacts are gone. The amp tone is overdriven. That may be normal ( I don't think I have heard another concert bass amp). The " left" distortion works (shorted tip to ground), but not the right. How am I doing? Much Thanks Drew

[loudthud]

Those numbers look about right for Q104. I think the right half of the distortion control (J8) is meant to filter the fuzz somewhat. You were saying that it almost killed the signal in a previous post before you got Q104 working. Keep looking.

[DrewV]

Yeah previously to getting the preamp straightened out it was like  dist "R" was working backwards. Now distortion "R" without the dist "L" kicked in is like a tone filter. When the dist control is set lower you get full treble. When it is set closer to ten it rolls of all the highs. With both "L" & "R" dist kicked in The signal stays about the same level as compared to the normal unprocessed signal. If I deactivate dist"R" the volume boost (from dist"L" being set on 10) is massive Where 1 on the volume pot is extremely loud. This seems a little lopsided. I'm wondering if I have the wires from the footswitch jack hooked up correctly. I have the clear wire ("L") going to #53 and the red  wire ("R") going to #52. Could you confirm this? I'll keep checking the circuit. Also: The overdriven tone I was getting was coming from the speaker . I hooked up a Mesa Boogie Black Shadow and got nothing but loud & clean. Thanks Drew