Hello, is anyone here familiar with this amplifier, I am trying to get one back together that has been in pieces for a long time...
There are no service manuals available that I am aware of...I only have a schematic that i found via google that is very hard to read.
I have the amplifier 95 % functional now but have a problem with not knowing where two wires are to go, the lead from the base and emitter of transistor Q307 which is next to fan relay...
Puzzled at the moment, the amplifier was stuck in protection when i got it. I have replaced all capacitors , some resistors and checked all transistor.
the amplifier has very minimal gain, I need to work out where these wires go..
I hope someone can help, thank you.
There are no service manuals available that I am aware of...I only have a schematic that i found via google that is very hard to read.
I have the amplifier 95 % functional now but have a problem with not knowing where two wires are to go, the lead from the base and emitter of transistor Q307 which is next to fan relay...
Puzzled at the moment, the amplifier was stuck in protection when i got it. I have replaced all capacitors , some resistors and checked all transistor.
the amplifier has very minimal gain, I need to work out where these wires go..
I hope someone can help, thank you.
Without photos and without even a poor schematic, it is difficult from here to say where wires might go. Might you share?
If they are involved in fan control, my first instinct would be to some sort of heat sensor on the heat sink.
If they are involved in fan control, my first instinct would be to some sort of heat sensor on the heat sink.
I could share, but it is pretty easy to google it for the schematic, well the picture of the schematic, that is all i have to work from..
bit frustrated on this one, I was looking for contact information for soundtech, but all emails i can see just return demon via email..was considering asking if maybe some of you American guys could chase it up via phone..
though the photo of the schematic maybe makes more sense to an experienced tech
if you plug Soundtech A-1000 power amplifier into google Enzo, you will get the rough picture i have to work from..I am sure it must make sense to an experienced tech..the circuit must be common
Same amplifier.
http://www.audiokarma.org/forums/index.php?threads/help-with-a-power-amp.683768/#post-9125915
http://www.audiokarma.org/forums/index.php?threads/help-with-a-power-amp.683768/#post-9125915
An externally hosted image should be here but it was not working when we last tested it.
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Thanks zeonrider. The fan switch transistor emitter and base go on the schematic to a box to the left off board. As the Thermister in the bias network is shown convntionally as a resistor with a name and value, I would suggest this box is probably a snap action thermostat on the heat sink. These are usually round about 1.5 cm diameter with two flag terminals on top, with the big round bottom glued to the heat sink.
The minimal gain may have something to do with the clipping circuit on the daughter board to the left. If not use an analog VOM with a 2 vAC scale (or a scope if you're a pro) and inject 1 VAC music into the input with a transistor radio or signal generator. Trace the signal from the input through the op amp then Q7? then Q4? to see if the voltage boosts normally.
The minimal gain may have something to do with the clipping circuit on the daughter board to the left. If not use an analog VOM with a 2 vAC scale (or a scope if you're a pro) and inject 1 VAC music into the input with a transistor radio or signal generator. Trace the signal from the input through the op amp then Q7? then Q4? to see if the voltage boosts normally.
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Yes indianajo, correct in what you are saying, that thermister and the two transistors and the 10k ohm resistor as pictured in the box on the schematic are stitched to a very small pcb that sandwiches the lot to the heat sink.. the pcb has tails that lead back into main amplifier board marked, B+ D+.
Having a beer now so i wont go probing that op amp..but i would think if there was a problem with it i would have no output signal?
Having a beer now so i wont go probing that op amp..but i would think if there was a problem with it i would have no output signal?
Op amps have various failure modes. One I worked on would whang to one power supply rail after a while, taking the amp output with it. Cause was a bad solder joint on the feedback input pin. Had been there 23 years. The power supply was disconnected from channel A and a label "Do not use Chan A" put on the back, so frustrated were previous repairmen.
Looking at various points with the AC meter and a radio can determine which stage is not performing. The transistors on the input are involved, too. Then comes the question - what is stupid about this stage? Transistor should be b to e junction 0.6 v, collector about midway between the rail and ground.
Hey, sorry its taken so long to get back, I had something else on my bench and couldn't get back to the sound tech, just had no room
Having a look at voltages at the moment...I have the main rail of 65V which is all good and then i have a second supply where i have -1.5V and +3.5V, so I think i need to look at the power supply...which i will do now...
I am still completely stuck on these wires though...I just cant work it out...there doesnt seem to be a place for them anywhere...!???
Having a look at voltages at the moment...I have the main rail of 65V which is all good and then i have a second supply where i have -1.5V and +3.5V, so I think i need to look at the power supply...which i will do now...
I am still completely stuck on these wires though...I just cant work it out...there doesnt seem to be a place for them anywhere...!???
I think I may indeed have a problem with the op amps...which are TL072N I am looking at some alternatives...but does anyone have any suggestions?
That looks perfect thanks mate. Well i worked out that those wires do in fact go to the thermal switch on the heat sink....on the schematic it appears that they are wired in series, but it is not the case...So hopefully replacement of the op amps will cure the gain problem...will let you know, thanks for the help
On op amps, Vout=A*(V1-V2) where v1 is the plus input and v2 is the inverting input. A is a big number. So usually, the inputs to the op amp have to be about the same number. If they are not, you have a solder or resistor problem. Gain of the op amp is Rf/Ri where Ri is the input resistor (usually to ground in the negative stage) and Rf is the feedback resistor. So if those resistors are off value the gain will be off.
I've found op amps dead or working, not usually low gain.
I've found op amps dead or working, not usually low gain.
I decided just to replace the op amps anyway, they were only 2 bucks each so i thought i may as well..
Its a bit of a task getting too channel two as the boards are stacked on top of each other..there is just no way you can get in there too probe anything.
Well definite improvement, i also plugged in a better pre amp which helped and have it running trough some 250 watt infinity tower speakers at the moment...these are power hungry speakers so it giving it a bit of a working but pretty confident it operating as it should now..apart from one thing..
I cannot get the second channel xlr input active...I had to repair the trace on the unbalanced input jack but i have traced continuity between the xlr in up-to the op amp,,,( i can sneak in the back corner of the unit without re cutting all wires but access is limited) ...and all is good...
puzzled again...had enough of lumping the heavy beast around on the bench for today though...Ideas welcome.
On another brand (Peavey PV1.3K) I found I could swap the A and B input boards which were stacked, to work on the B. I just had to unlace the harness to make it stretch the other way. Perhaps soundtech has a similar design. If not, maybe you can disconnect the top input board for a while and remove it for work on the bottom one.
I am afraid taking the top board out by unbolting all main outputs is the only way on this one..Its the only right way to service the amp to be honest as the bias pot for channel two is also not accessible...PINA!
But anyway, I pulled it out again and replaced the op amp again on channel two...as well as re touching a lot of solder joints and checking and cleaning everything i could..
Got the 2nd XLR back now, I do have a little feedback though when using either XLR input..this is possibly the female xlr too rca test lead i have made up..its only about 20cm long...but possibly the problem...or could it be the different op amps>?
One more thing, I have found one of the thermal switches on the heat sink to be dead short..I have hit it with a heat gun and it just wont open...What temp rating would i be looking at for a replacement? its marked T 95AR1U1
But anyway, I pulled it out again and replaced the op amp again on channel two...as well as re touching a lot of solder joints and checking and cleaning everything i could..
Got the 2nd XLR back now, I do have a little feedback though when using either XLR input..this is possibly the female xlr too rca test lead i have made up..its only about 20cm long...but possibly the problem...or could it be the different op amps>?
One more thing, I have found one of the thermal switches on the heat sink to be dead short..I have hit it with a heat gun and it just wont open...What temp rating would i be looking at for a replacement? its marked T 95AR1U1
If you clip lead a thermocouple from a thermometer to the working disk (in the air to make it quicker) and hit it with the heat gun, you can find out what temp it is supposed to be. My Sears autorange DVM has a yellow thermocouple input for that type of sensor.
I don't have any generic distributor sources of these thermal switchess. I would guess the manufacturer might be the cheapest one.
When I made an RCA plug to XLR plug to input to my Peavey CS800s, I made it six feet long. There was a slight bit of hum compared to a RCA to RCA cable going in an adapter in the 1/4 phone input, but no feedback. Perhaps you're putting in signal on the output jack?
I don't have any generic distributor sources of these thermal switchess. I would guess the manufacturer might be the cheapest one.
When I made an RCA plug to XLR plug to input to my Peavey CS800s, I made it six feet long. There was a slight bit of hum compared to a RCA to RCA cable going in an adapter in the 1/4 phone input, but no feedback. Perhaps you're putting in signal on the output jack?
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