Sunn Concert Bass Troubleshoot

[DrewV]

Hello, I am restoring a Sunn Concert Bass amp. I have tested replaced, as necessary all resistors and capacitors including all five 2N3055 transistors, all electrolytics, all three rectifiers, and all emitter resistors.

Voltage readings on the power transformer (in circuit) are:
Lt Orange pair = 22.5vac & 22.7vac
Green pair = 19.9vac & 20.7vac
Dk Orange pair = 16.1vac & 18.5vac

Speaker terminal: acv = 0/  Dcv = 6.4mv
Collector to ground resistance:
In circuit Q1= reads like a capacitor / Q2= 0.5ohm/ Q3=465ohm/ Q4=465ohm/ Q5= 7ohm.  Q2 & Q5 pass continuity test. The 910ohm resistors associated with Q3 & Q4 also read 465ohms. With the transistors mounted but not electrically connected all collectors read OL.

The + rail voltage is 5 volts too positive (reads 30v when it should read 25v). The – rail voltage is 1volt too negative (reads –16v when it should read–15v).
               
Here are the symptoms:         
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.
Thanks in advance
DrewV

[J M Fahey]

Hi DrewV.
Can you please post the schematic, or a least a working link to it?
Thanka.

[Enzo]

DOn't worry about the voltages.

STart with making sure power amp in jack si OK.  Patch a cord from preamp out to power amp in and see if it helps.

Put a signal into the power amp in jack.  Is that OK, or does it react the same way?

I'd be betting your output stage is not running on all cylinderrs.

[J M Fahey]

Excuse me Friends but you are speaking about absolutely different amps o schematics.
DrewV, please either post your own version or refer to what Enzo posted so kindly, if applicable to your amp.
Some (heavy) doubts: I see no 910 ohms resistor, Q2 and Q3´s collectors are tied together and should give the same reading to ground, the same goes to Q4 and Q5, the power transformes has only one center-tapped winding , a grand total of three wires, not 6, etc., supply voltages should be quite symmetrical and around 35V .....
The only healthy reading is the almost 0 dc volts on the output jack.
Good luck.

[DrewV]

  I've been working on this amp since March so please forgive any confusion on my part. Put a signal into the power amp in jack. Plugging an A/E guitar into the poweramp in produces an acceptable loud and fairly clean output at the speaker. Plugging in to the normal input with a regular electric guitar produces the symptoms outlined in my initial post. Taking the preamp out and plugging it into a mixer also produces the same symptoms. The problem appears to be coming from the preamp. ( Patching from the preamp out to the poweramp in has no effect) As with the power amp board, all preamp board components have been tested and replaced as necessary. Preamp transistors read good on a transistor checker. I'm thinking that the problem is connected to the dual distortion circuit, but I can't find any bad components, solder bridges or broken traces.   I removed the dual distort pot from the board, cleaned it with Deoxit and checked the resistance on each pot which reads 25k and 28k. I have the component layout document  and have checked the board against it. Here's a link to the Concert Bass schematic. There are a few differences between the actual amp and the schematic. The amp I'm working on is a 1973 model. There are six wires (three pairs) coming out of the transformer. One pair going to rectifier (CR1) with the two other pair going to CR306 & CR307. The sunnampage.org server is down temporairly. Loudthud has posted a touched up schematic and parts layout there. Until the site is back up the following link shows the schematic. http://www.schematicheaven.com/newamps/sunn_concertbass.pdf thanks DrewV

[phatt]

Hi DrewV, sounds like your getting closer, :tu:

Um I don't believe you can test FET's in situ? Just wondering how you tested Q1,2,3?  (head scratch?)

My guess is ,,One of the first 3 FET's has died and gone to Electro Land.
The schem is blured but Data  for a 2N4304 says;
general purpose 30 volt JFet.
I'd just grab a couple or three of something similar and replace them (one at a time)
Maybe try 2SK30?
Or if you want keep going backwards with you detective skill,,,just keep working through each stage.
Break the signal path between each stage and just inject an audio signal untill you find which stage is stuffed.
Hope it pans out,, Phil.

[DrewV]

Phatt I had already substituted the 4304's with new 2N5458's and got no improvement so I put the original JFET's back in. I went back and rechecked today also swapping the 4304's between the different sockets with no improvement. Then I substituted a new BC107 in the three places it is found and still got no improvement. I also put BC141 (Q302-power brd) on a transistor checker and it read good.
Touching Q104 with a pair of  needle nose pliers produced a substantial increase in static like noise. I then broke the circuit by removing Q104.At first the results were: 1) The "avalance" type distortion detailed in my orig post was completely gone. 2) the amp sounded like it was putting out about half power ( and wasn't drawing much current). 3) The sound was nicely overdriven. ( distort control not affecting the signal). I then powered the variac down and off to type the results. Powering back up the I found output level had dropped dramatically and the amp started drawing massive current as the channel volume was turned up. (My setup for testing is: amp into lightbulb limiter into variac.) Putting Q104 back in the circuit the amp output level is still very low and the amp is drawing massive current as the channel volume is turned up. Even the poweramp in (A/E guitar) is producing the same effect. I think i fried something on the power brd! I'm going to recheck everything. I will post when I got some new information Thanks Drewv

[Enzo]

I would suspect those two little low value electrolytics on either side of the preamp out transistor Q105.  Looks like 2uf/50v.

Apply a sine wave to the input and follow it through the preamp with a scope or signal tracer.  Don;t have those?  At least monitor DC voltages throughout looking for some skewed funny.  Make sure both +25 and -15 rails are available at the preamp stages, not just at the supply.

[J M Fahey]

Hi DrewV.
The power amp*is* weird, to say the least, but if it works, leave it alone.
Rather than pulling parts and measuring/testing them at random, please measure and post all drain and source voltages, referring them to the published schematic (I've  downloaded the Schematic Heaven you suggest), as to everybody talk about the same. Thanks.

[DrewV]

The output level drop and the  current draw was caused by the speaker wires touching on the 1/4" phono plug. Using the poweramp in the amp works and draws what I would call normal current. The sound has a definite buzzy edge to it especially noticable as the sound decays. It sounds as if a nice distortion box was on, mixed way back. The normal preamp input is still breaking up, esp. at higher volume.
I have +35.6vdc on pin 43 on the preamp board and -16vdc on pin 44. C122 and C124 have been replaced. Q104 has 29.6vdc on the collector and Q105 has 12.2vdc on its' collector. Q301 has 11.9vdc on its' collector. I have attached

[DrewV]

Sorry, I have posted a cleaner schematic and the preamp board layout. I will check the voltages and get back. Thanks

[DrewV]

 The following are the most recent measurements. (Ignore the source and drain voltages on the schematic (in red)) Q101: D= 15.6vdc / S= 0.4vdc.  Q102: D= 15.8vdc/ S= -19mvdc. Q103: D= +15.7vdc /S=106mvdc.  The measured source voltage is off. Enzo I have no scope! thanks

[phatt]

I'm not the best expert here but to the best of my Sims Knowledge,,

With Pos = +25VDC and a Neg rail =  -15VDC
Then Drain sounds about right,,, around +15VDC

My Sims says; Source should be about -3VDC
Therefore Gate should hang around -2VDC

I'm only using the jfet model so won't be perfect but should give you an idea of what to expect.
Long time since I played with fets but generally the gate voltage should be close to Source voltage.
Hope it helps,, Phil.

[J M Fahey]

Hi DrewV. As I was about to post I was informed of Phil´s fresh post. I agree fully with it. I do not Sim (maybe I should learn how to do it some day) but I´ve built (hardwired) a Ton of circuits, including many Fet types, which I love.
As Phil does, I find your Drain voltages perfect. The Gate should be at "0" volts and the source slightly negative relative to it. The slight deviation compared to theory may be due to the fact that Sources are "force-fed" through a resistor from the -15V supply.
The distortion you hear, which gets worse at low level and almost acceptable at stage levels, is crossover distortion, due to the *very* poor biasing of the output transistors. There´s no easy practical way for you to get rid of it.
Now to the problem I do see: Q104 is wrong: It should have around 3V in its Emitter, around 3.7V in its base and around 12VDC (+/-3V) in its collector, and *not* the almost 30 Volts you measure, which indicate an open or unbiased transistor (or wrong pinout if replaced, or broken tracks somewhere around it). Measure the VBE (Red probe to Base, Black to Emitter) voltage, you *must* have around 0.65 to .7 Volts there. If not so, pull the transistor, you must have "0" volts on the emitter pad, around 10V on the base pad and almost 30V on the collector pad. If you don´t, you have a bias problem, check resistors, tracks, cold solders, etc. , still without the transistor . If you get what I suggest, either the transistor is bad or its pins are misplaced. After that the amp should work properly. Good Luck.

[DrewV]

  J M Fahey & Phil, Enzo, It's sinking in! 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