I need some extra eyes (brain cells would help too) looking at this problem.....

What "we" know - the amp will power on and not blow fuse(s) - you can chase a signal from Q1 (NPN) to Q2 (PNP) but there is no signal at the collector of Q2. When checking the supply voltages I get a +15/+9 and -15/-3.5 - checking the 9V zener diode (ZD5) and it is okay. When I lift R63 on the -9V supply I get -9V at ZD5 - someting is loading down the supply - but what? I checked Q2 through Q5 with a DMM in circuit and found no issues. D1 and D2 (are these bias diodes helping the BJT turn on??) also test good and the ESR on all the bypass caps (C8,12,20) all test out okay. Check the voltages on Q2 - Q5 but they have to be wrong since the -9V supply line is pulled down. I hate to start removing parts without a strategy.......

Here is the link to the schematic - http://www.drtube.com/schematics/marshall/2099u.gif

The story starts with a simple power cord change since the original cord needed to be replaced. During the R&R of the power cord, a (base) lead broke off the TO-3 socket on the heatsink, and was soldered back - this was the PNP output darlington BJT MJ2501. When the amp was powered up with a variac I noticed that as the line voltage was increased excessive current would start to flow. The amp was working without issue before the mains cable was changed. GGrrrrrrrr.

If the PNP output transistor is removed from the circuit the mains current does not increase when the line voltage is increased. When a new MJ2501 is put into the circuit the over current issue comes back. I even removed the socket from the heatsink and when the MJ2501 is off the heatsink BUT the attachment screw (connects TO-3 case to collector tab on socket) over current is observed with increasing mains voltage. I even tried a new socket but still have the issue. I went back and looked at the driver transistors in the circuit - BC184 and MPSA06 - even though they tested out okay with a DMM I replaced them with no change to over current situation. So to recap ...new output transistors, socket and correct wiring but still over current when PNP (MJ2501) is in circuit - no issue when it is removed or no connection to the collector of the MJ2501.

Finally, I have looked at the mains cable that was replaced and can find no issue with the cable. I noticed on the schematic that the "neutral" side of the the mains is switched and fused (black colored wires) while the brown (hot) side is not in series with the fuse. Would swapping out the mains cable leads, so the hot side of the mains is also in series with the fuse, cause an issue with the 120/240 transformer??

I know I am missing something very subtle and simple but what is it?

Any historians out there who can comment on  MI amp designs that FIRST used DSP?  Also interested in comments on the early DSP chip suppliers.

I've run into a hum issue (60Hz) on this amp. After fixing the blown output section and replacing the leaky electrolytics this amp is working again. I upgraded the line cable to a grounded type and when the ground is attached to the chassis I get a very noticeable increase in 60 Hz at the output.

The ripple on the voltage rails is .025V but I get almost .3V on the output terminals with the chassis grounded. Ground loop?

http://www.acousticbassusa.com/

Has anyone checked out this site? One of the principals of the company posts on the unofficial acoustic site (cheap advertising).
http://unofficialacousticcontrolcorpmessageboard.yuku.com/directory

Not sure what to make of the design other than it looks like a make over of the 360/1 for $5K.

Acoustic 470 which seems to have several issues. First off, the vibrato is bleeding though even with the intensity turned down and footswitch used to turn the feature off. You can just barely hear it audibly and it shows up on a scope modulating the input signal. I believe the feature is always "on" and the footswitch turns it off? 
Next problem and it's possibly related to the above issue, is that there is a residual distortion that can be heard especially after playing single notes.  Sounds like very mild crossover distortion.  The distortion pot is switched all the way off – switch engaged.  Sinewave input doesn't show crossover distortion at the output.
The preamp boards are a real pain to probe in the chassis since there are a few dozen wires soldered on the edge terminations and they like to break when moving the boards. Looking from the input onto the 170043 board, it looks like I shouldn't see a change in the signal comparing the outputs of Q102 and Q104 – that's my first measurement.  Any other ideas?

I brought this amp back from being "air locked" but it seems to be very noisy. Not the 60/120 Hz kind of noise but more "white" or 1/f noise associated with semiconductors or resistors. All the active devices were replaced on the pre-amp board along with adding sockets. Power amp board also has new active devices as well. I haven't really been able to isolate the noise from either preamp or power amp since the power amp out is located before the CMOS limiter section and the preamp out "sounds" okay - hard to hear over the fan noise. Were these old (80's) Sunn amps just noisy?

I continually read that soild state amplifiers don't not have the low frequency response of tube designs. WTF is this about? Urban legend? Is this just an issue with certain legacey designs? I guess it's time to fire up the old HP 8903B and make some measurements.

I finally have this PA working again but have a question about the AGC circuit. Are Q1/2 some sort of detector used in the AGC loop? How does the AGC work in this design?

This amplifier just about puts out 18Vrms (@2.7 ohms) before the positive part of the input signal starts to degrade. I'm also only getting 60 V out of the secondary at 120 V on the mains. I'm going to assume this was typical ACC production control in 1977.

I have a Sunn 300 Coliseum (circa mid 80's). I replaced the blown 10A/32V fuse  but after power up with the VARAC it would draw excessive current. I disconnected the secondary and connected a DVM. Using the VARAC - I can turn up the mains voltage (fan runs) no excessive mains current but the secondary only reads 30mV across the secondary leads.

DC resistance of secondary is .2 ohms. I'm assuming the secondary is damaged  Also assuming this PT has a 100 V secondary (+ or - 50V rails). Anyone know where to find a replacement?

This is the Fender/Sunn solid state version not the tube design. Anyone work on one and is this a design with a switching PS? Also could use a schematic if someone has one.

Can you not attach/post gif.s?

I have three Sunn Concert series amps that are showing various degrees of second order harmonic distortion in the preamp section. For example, I have a Concert Bass which doesn't shown any 2nd order harmonics at Q101 but it shows up at Q102, Q103, etc. I have the distortion feature turned off but I'm wondering if there is a ground path that I don't see

If R112 or R126 were shorts then the distortion would be "on"? I'm assuming the distortion feature works by adding the RC feedback networks into the output of Q102 and the input of Q104 via the ganged 25K pot ?

http://sunn.ampage.org/site/schematics/concertbass.gif

http://i284.photobucket.com/albums/ll33/bonosurf/conLeadPA.jpg

The issue is Q3 is getting very hot at turn on with no signal input or load. R303/304 get really hot (+200F) in about 30 sec.

I don't measure 1/2 VCC at emitter - only around 4.6V with the base at 5V. Supply voltages correct and all new transistors in the output but only Q3 gets hot at idle (yes I swapped another device and it still gets hot). Bias trim pot is measuring correct resistance but has on change on the issue. Something is wrong with the bias but I can not see it - changed or inspected all components on this board  :grr

This poor amp was a pig's breakfast when I received it - someone pulled off a stack or dropped it and then some tweeker tried to "fix" it.

Working on a very early version of this bass amp. The amp will shut down on power up (doesn't blow the fuse). I noticed that the wires from the PTprimary connected to the IEC mains socket (neutral connection) show a slight heat discoloring. The later version of this amp has a PTC thermistor wired into the primary side of the PT. I hope this PT isn't fried - it looks like an expensive toroid. Anyone work on this product before?

I have one of these on the bench and would love to have a schematic for reference. Thank you Thank you

Anyone have a schematic for the Model 123 from Acoustic? Circa 1983 1x12" 100W solidsate combo amplifier. Thanks in advance....

I'm having problems with the bias on a 470 PA board. schematic link below

http://i284.photobucket.com/albums/ll33/bonosurf/470PA.jpg

This is the 170045 version of the board since it is a 470.

The issue is I can not get the 46V bias at various points on the board. The 92V rail is fine. I have replaced Q401/2 Q412/13 tested all other devices and they pass.

The signal ground and PS ground are connected.

I have tested almost every resistor on the board and replaced all the caps but C404.

Needless to say the board does not pass a signal.

Q403 has .85V on the base and .25 volts at the emitter.

I get 63V at the node of R418/R417 but only 4V on the other side of R418 - and R418 is in spec???

If I didn't know better I would say there is a ghost in this machine.

I'm toying with a portable (i.e. battery powered) amp design and would like folks opinions on low power ICs for this type of application. Think Micro Cube only better.

I have three reverb tanks from Accutronics that were in 1) Acoustic 150 and 2) Acoustic 470 amplifiers. The input and output on the "type 4" units are 160-175 ohms for both the 150 tanks and the 470 appears to have a very high impedance on on of the ports. I noticed that the Accutronics site has the type 4 tanks specified with different input and output impedances.

Are these good units? Is there a way to bench test these tanks with an signal generator/scope?

THX

I'm starting to believe this is the amp from hell. I spent way too much time trouble shooting and replacing the A/B channel lamps. During that process, a bit of lead triming accidentally shorted the 90V rail to the zener in the BJT regulator on the PA board. I had to replace the zener in the regulator and all those voltages are now passing to the other PCBs just fine.

Here is a link to another forum where you can find the PA schematic - http://music-electronics-forum.com/showthread.php?t=10360

The current problem is no output on the PA board - signal from the other two PCBs are loud and clear but right after the 47K reaistor on the PA input I get 12Vdc on top of the input signal. By the time the signal reaches the base of Q401 (differential pair) it's almost non-existent and noisey. Checked all the active devices in the circuit and they are passing.

The sad thing is that this board was working fine before the regulator ckt was damaged. Maybe someone else can see something I've missed.

BTW I've modeled this board in TINA and I get agreement on all the posted factory diagnostic nodal voltages.