JM, Thanks for finding the main supply on the schematic - I noticed the other supplies but couldn't figure out where the higher voltage supply was located. So where is the center tap located for the transformer? Was I reading the voltage incorectly and should have been measuring from the CT - that would give a reading of around 60VAC.

I guess I'll have to unsolder all these transformer connections and measure each "tap" from the PT to see what's going on?

Do you think this amp is a "boot-strapped" design where you run the low voltage (5V) supply for idle and low power and then bring in the larger supply to drive peak voltage? Class G??

A new transformer is going to coat 200 euro - I assume this amplifier is worth that cost?

THX

George

Yes the schematic is a pig's breakfast - Glockenklang doesn't appear to have any other documentation?? Thanks for pointing out the relay operation - though I'm wondering if the TRIAC could be damaged?? I checked all the output devices with a DVM and I didn't see anything unusual there - though testing active devices this way always leaves me feeling doubtful.

I'm wondering why I see 120VAC on the AC input to the discrete bridge rectifiers for the +-30 V supply ?? Since we are on the secondary of the PT shouldn't this be a much lower voltage? Remember that I'm seeing this condition after the amp "shuts down".

Schematics here:

http://i284.photobucket.com/albums/ll33/bonosurf/soul2.gif
http://i284.photobucket.com/albums/ll33/bonosurf/soul1.gif

The schematic is in pdf format and it's 2 KB over the limit to post??

Power up means press the power button with the amplifier plugged into 120VAC. Unit doesn't blow fuses it activated a relay in the power supplt circuit and shuts down. There is no sound and oh the fans briefly come on briefly too and then shut off. Pictures below.

Good method for testing the toroid would be appreciated.


http://i284.photobucket.com/albums/ll33/bonosurf/DSCF050211.jpg
http://i284.photobucket.com/albums/ll33/bonosurf/DSCF050111.jpg

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 would start by checking if the preamp boards have output. Are you sure there is a signal going into the power amp board? The driver transistor on the PA board is also usually damaged when I get these amps after they have output issues. Also check the large output capacitor on the same chassis that the power transistors are mounted on. Last there is a zobel network on the output speaker jack make sure these components aren't damaged as well.

Anyone?

http://www.irf.com/product-info/audio/classddesign.html

Not sure if the MI world is using DSP to create peak/RMS limiting protection or even a "soft clipper" (though a discrete version of a SC is also a very good solution). Is the issue that the input signal into the amp should not have any "non-linear" processing so one can overdrive the OP section? or is DSP still to costly? ??

I believe Temuk is refering to a "light bulb" limiter used on the mains input to limit current in the case of a fault. The circuit consists of wiring a low wattage bulb in series with the amplifier.

You have a fault to ground somewhere in your amplifier. Does your power cord have a three prong connector? Does the amp have a polarity/ground switch? You risk a potential fatal shock with your amp in it's present condition - it needs to be trouble shot and the problem corrected.

Okay it looks like Q404 isn't turning on with 51V on the base and 46 on the emitter - I've checked all the resistors from R417,418 and 419 and they are okay. The trim pot (R406) also seems to work but it has no effect on the base volatge of Q404. Stumped.

Intersolar is the 3 year old trade show/conferance for everything to do with photovoltaics. I'm tired of working in dead industries (consumer electronics, telecom, instrumentation, etc).

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Back to the 470 - here are some of the node voltages. Almost every semi has been repalced on this board (except Q403/MRF431 - it does test okay with a DMM):

Device   Base   Emitter   Collector   Node Voltage   Comments
   (V)   (V)   (V)   (V)   
Q401   23.8   24.5   0.6      
Q402   23.8      0.6      
Q403   0.6   0.6         
Q404   55.1            
Q405   57.1            
Q406               
Q407   48.2   48.2         
Q408               
Q409               
Q410               
Q411               
Q412      48   56.5      
Q413   48.2   48         
               
V420/18            57.2   node voltage between R420/418
Vsupply            91   PS voltage on board


The differential pair have an issue with the collector voltage - all devices check out okay and all the resistors (R410/11) also measure good with no open contacts/solder joints. Q403 is also not biased correctly. The output from the diff pair should be a current which drives Q403 (VAS). I can only assume that the feedback signal to the base of Q402 is the issue??

Okay, I finally start back on this amplifier. After replacing the transistors in the differential amp, I found that Q404 was damaged and this device was then replaced. The node voltages now appear to be correct but I've run into a problem when a sine wave (1 kHz 150-200mVrms) is applied to the input of the PA board at the 47K resistor.

I have no output at the negative side of C409 - chasing the signal from the input to the output I get correct waveforms until I look at the base node of Q403. Here is the wave form at that node:

http://i284.photobucket.com/albums/ll33/bonosurf/DSCF05211.jpg

I've tried to adjust the R406 ("symmetry") but I don't get any change or lose the signal all together. I haven't chased the signal past Q403. I assumed the output of the differetial amp Q401/2 should be the same as the input with some gain. Is there a feedback loop I'm not seeing - i.e. why is the waveform non-symmetric at the base of Q403?

Also noticed that the node voltages on the differential amp and other points of the board (NOT the 90V supply or the voltages at C415, etc) take almost 15 seconds to stabilize/settle to the correct value.

What's causing the the voltage ramp?