I have an AA-1800 that I purchased new (as a kit), and assembled it at that time. I just looked at the receipt and it cost me $419.97 in August of 1982. Copyright on manuals is 1980, so I guess that's about the date of the original design.
I used it for ~15 years, and then it sat unused until recently. I hooked it back up several months ago and it worked fine, and then after a few months, it seemed that one channel was only giving a very low output - like missing most of the input signal. I finally struggled to lift it down from the top of the equipment cabinet, and reconnected it on my workbench and now it seems to work fine again. I hate dealing with intermittent problems... Maybe I'll look at the input coax cables.
It is a bit amazing that it works so well since it is approaching 40 years in age and nothing has been replaced. Not sure if I want to bother with replacing capacitors yet. I do plan to keep it in use in my basement as I clean things out and rearrange things - since I have nothing else to do while stuck mostly at home.
If anyone needs copies of the documentation, I finally located the 120 page assembly manual, assembly diagrams, and schematic, and I can try to scan portions of that if anyone needs it.
There are instructions for setting the output amplifier bias and zero voltage as a part of the final checkout. I'm wondering if redoing that would be recommended, or if it falls into the category of not fixing it if it isn't broke?
I used it for ~15 years, and then it sat unused until recently. I hooked it back up several months ago and it worked fine, and then after a few months, it seemed that one channel was only giving a very low output - like missing most of the input signal. I finally struggled to lift it down from the top of the equipment cabinet, and reconnected it on my workbench and now it seems to work fine again. I hate dealing with intermittent problems... Maybe I'll look at the input coax cables.
It is a bit amazing that it works so well since it is approaching 40 years in age and nothing has been replaced. Not sure if I want to bother with replacing capacitors yet. I do plan to keep it in use in my basement as I clean things out and rearrange things - since I have nothing else to do while stuck mostly at home.
If anyone needs copies of the documentation, I finally located the 120 page assembly manual, assembly diagrams, and schematic, and I can try to scan portions of that if anyone needs it.
There are instructions for setting the output amplifier bias and zero voltage as a part of the final checkout. I'm wondering if redoing that would be recommended, or if it falls into the category of not fixing it if it isn't broke?
Potentiometers can lose contact wiper to track sitting on the shelf due to oxidation. Oxygen never sleeps. Worth a check on idle current and zero voltage.
AA-1800 Bias Adjustment
I went through the Bias Adjustment process and it was relatively straightforward - but the potentiometers are a bit touchy, and I'm almost tempted to replace them, but finding ones that would fit the same as the old ones would likely be a problem. I wrote up the process after extracting it from the assembly manual, and thought it might be useful to others.
Heathkit AA-1800 Adjustment procedures
Note: This information was extracted from the assembly manual, and are steps recommended after assembly. There are numerous voltage checks that are likely not necessary, if the amplifier has been working properly and you just need to adjust bias and offset.
Voltage Check: Check for + and – 100v to ground at each main power supply filter capacitor. There are numerous warnings throughout the adjustment to be careful, and recommendations to discharge these capacitors with a 500 ohm 10-watt resistor after various steps.
Remove the 6 screws holding each amplifier assembly (circuit board and attached heat sink) from the main chassis so that they are more accessible. Originally there were extension cables to allow the amplifier assembly to be further from the chassis, but it’s possible to perform the adjustments without the extension cables. It will help to disconnect the RCA input to the amplifier circuit board also perhaps also the connection to the thermal sensor attached to the heat sink. I found it easier to disconnect one side, do the adjustments on that side, and then reattach that side before doing the other side.
After the amplifier is separated from the main chassis, the potentiometers are more readily accessible. The OFFSET Adjustment Control R123 is accessible through a hold in each main amplifier circuit board, using a small screwdriver.
The BIAS Adjustment Control R127 is mounted on the component side of that circuit board near the edge and can be reached with your finger (being very careful of the +100v supply voltages present nearby). You can turn it by touching the plastic knurled outside edge of that variable resistor. You can also turn it with a small tuning wand or screwdriver, but because of the way is it mounted, you can easily push it over if using a screwdriver.
There are two test points (TP-A and TP-B) on the foil side of each main amplifier circuit board. These should have been created by leaving some excess length to the resistor lead when it was soldered to the board at this location, and then bending that into a loop that serves as the test point. Those points are also marked on the foil side of the circuit board.
Initial adjustment was to set BIAS Adjustment R127 fully counterclockwise, and OFFSET Adjustment R123 to the center of its rotation. If resetting, probably best to start from where they are.
Voltage Check: can likely can be skipped. It called for checking voltage between either TP-A or TP-B to ground when turning the amplifier on, it might momentarily swing up or down, it should go to zero after about 5 seconds.
Probably best to make sure amplifier has had time to warm up before proceeding with adjustments.
Next, it recommends that the BIAS be (initially) set at 50 mV, with the negative voltmeter lead to TP-B, and other lead to TP-A. Adjust BIAS control R127 to get a 50 mV reading.
Then connect voltmeter to rear panel speaker output binding posts on the channel you are adjusting. Adjust OFFSET control R123 to produce a DC Voltage reading of Zero at the output terminals. This adjustment is touchy, but I was able to get it to be less than 1 mV.
Voltage Check: Again, I think could be skipped if amplifier is working properly. Connect negative lead of voltmeter to ground. Then touch other volt meter lead to the collector (case) of large heatsink mounted output transistor Q117, looking for a reading of about 45 volts. Check for similar reading on case of Q121.
Then check voltage at collectors of Q118 and Q122, where you should have a reading of negative 45 volts.
Then again, connect volt meter to TP-A & TP-B. Turn Amplifier on, and all to warm up again (if necessary) for 20 minutes to stabilize. Then readjust BIAS Adjustment R127 for a bias of 35 mV between the two test points.
Repeat procedure above for the other channel.
I went through the Bias Adjustment process and it was relatively straightforward - but the potentiometers are a bit touchy, and I'm almost tempted to replace them, but finding ones that would fit the same as the old ones would likely be a problem. I wrote up the process after extracting it from the assembly manual, and thought it might be useful to others.
Heathkit AA-1800 Adjustment procedures
Note: This information was extracted from the assembly manual, and are steps recommended after assembly. There are numerous voltage checks that are likely not necessary, if the amplifier has been working properly and you just need to adjust bias and offset.
Voltage Check: Check for + and – 100v to ground at each main power supply filter capacitor. There are numerous warnings throughout the adjustment to be careful, and recommendations to discharge these capacitors with a 500 ohm 10-watt resistor after various steps.
Remove the 6 screws holding each amplifier assembly (circuit board and attached heat sink) from the main chassis so that they are more accessible. Originally there were extension cables to allow the amplifier assembly to be further from the chassis, but it’s possible to perform the adjustments without the extension cables. It will help to disconnect the RCA input to the amplifier circuit board also perhaps also the connection to the thermal sensor attached to the heat sink. I found it easier to disconnect one side, do the adjustments on that side, and then reattach that side before doing the other side.
After the amplifier is separated from the main chassis, the potentiometers are more readily accessible. The OFFSET Adjustment Control R123 is accessible through a hold in each main amplifier circuit board, using a small screwdriver.
The BIAS Adjustment Control R127 is mounted on the component side of that circuit board near the edge and can be reached with your finger (being very careful of the +100v supply voltages present nearby). You can turn it by touching the plastic knurled outside edge of that variable resistor. You can also turn it with a small tuning wand or screwdriver, but because of the way is it mounted, you can easily push it over if using a screwdriver.
There are two test points (TP-A and TP-B) on the foil side of each main amplifier circuit board. These should have been created by leaving some excess length to the resistor lead when it was soldered to the board at this location, and then bending that into a loop that serves as the test point. Those points are also marked on the foil side of the circuit board.
Initial adjustment was to set BIAS Adjustment R127 fully counterclockwise, and OFFSET Adjustment R123 to the center of its rotation. If resetting, probably best to start from where they are.
Voltage Check: can likely can be skipped. It called for checking voltage between either TP-A or TP-B to ground when turning the amplifier on, it might momentarily swing up or down, it should go to zero after about 5 seconds.
Probably best to make sure amplifier has had time to warm up before proceeding with adjustments.
Next, it recommends that the BIAS be (initially) set at 50 mV, with the negative voltmeter lead to TP-B, and other lead to TP-A. Adjust BIAS control R127 to get a 50 mV reading.
Then connect voltmeter to rear panel speaker output binding posts on the channel you are adjusting. Adjust OFFSET control R123 to produce a DC Voltage reading of Zero at the output terminals. This adjustment is touchy, but I was able to get it to be less than 1 mV.
Voltage Check: Again, I think could be skipped if amplifier is working properly. Connect negative lead of voltmeter to ground. Then touch other volt meter lead to the collector (case) of large heatsink mounted output transistor Q117, looking for a reading of about 45 volts. Check for similar reading on case of Q121.
Then check voltage at collectors of Q118 and Q122, where you should have a reading of negative 45 volts.
Then again, connect volt meter to TP-A & TP-B. Turn Amplifier on, and all to warm up again (if necessary) for 20 minutes to stabilize. Then readjust BIAS Adjustment R127 for a bias of 35 mV between the two test points.
Repeat procedure above for the other channel.
Hi Ben, I just picked up an aa1800 and replaced all output and drivers I checked all other components against the good channel and there must be a transistor that tests good under tester voltages that leaks with real operating voltage. This unit was designed by a sadist. The components are between the heatsink and the pcb. If I could take you up on your offer of the schematic I would be eternally grateful. My e mail is clindhome@gmail.com. This is such a nice unit I would like to save it. Thank you
I missed you message from above.
I still have the Heathkit Amp, and the manuals. I reset the bias just to get is as close to zero as possible. Also seemed to solve the input issue - apparently the variable resistor used to limit the input signal was just dirty.
If you need information from the manuals, just let me know.
I still have the Heathkit Amp, and the manuals. I reset the bias just to get is as close to zero as possible. Also seemed to solve the input issue - apparently the variable resistor used to limit the input signal was just dirty.
If you need information from the manuals, just let me know.
I was able to obtain a build manual. There was two different fuses installed by someone. One fuse was not right and burned up some components and the board. Now I'm looking for an output board or a parts amp.
Why do you need a PCB assembly?
A circuit board can be repaired reliably even if there is a hole burned through. It's work, but can be done. You can also create a new PCB. Depopulate the old one and use it for a drilling guide on a copper clad blank. Draw in the pads, connect the dots keeping to the same routing and etch it. I have saved a few amplifiers, receivers and other equipment by doing this. Lately I draw the PCB up in a PCB routing software, like Eagle or DipTrace. I thien send it out to a board house and get something back superior to the original.
-Chris
A circuit board can be repaired reliably even if there is a hole burned through. It's work, but can be done. You can also create a new PCB. Depopulate the old one and use it for a drilling guide on a copper clad blank. Draw in the pads, connect the dots keeping to the same routing and etch it. I have saved a few amplifiers, receivers and other equipment by doing this. Lately I draw the PCB up in a PCB routing software, like Eagle or DipTrace. I thien send it out to a board house and get something back superior to the original.
-Chris
