Hi, when I am using my 1970’s Armstrong amplifier with a laptop line out connected to tape in, I get distortion in the speakers after about half a hour. If I turn the amplifier off and on again, it works again for a few seconds.
However, when using the “tape out “as an audio input, it work perfectly fine. I don’t have a turntable handy to test any of the turntable inputs. I do have a multimeter and soldering iron, and I can do small repairs.
However, when using the “tape out “as an audio input, it work perfectly fine. I don’t have a turntable handy to test any of the turntable inputs. I do have a multimeter and soldering iron, and I can do small repairs.
It seems there are quirks to these designs in their selector switching arrangements which may have something to do with the problem, since evidently it's not in the amplifier section.
The Armstrong 500 Series - Amplifier Circuit Diagrams.
Check this site for interest and come back if you find out some more regarding the behaviour of switches, sockets or even preamp, if possible. I suspect the usual old cap. story in the power supply but let's see what's going on better.
The Armstrong 500 Series - Amplifier Circuit Diagrams.
Check this site for interest and come back if you find out some more regarding the behaviour of switches, sockets or even preamp, if possible. I suspect the usual old cap. story in the power supply but let's see what's going on better.
Last edited:
Like Ian I would suspect a cap too. One question though. Are both channels affected or just one ? If you look at the circuit for the preamp, C201 (as far as I can see) doesn't have a bleed resistor on the input side. Depending on the source component (your laptop) it is possible this cap could end up reverse biased and if so that would then leak current, alter the bias of the preamp stages and so cause distortion.
A simple test is to measure the voltage on each side of C201 and confirm that the cap is never reverse biased. If this is the problem area then even the act of connecting the meter to the cap could "cure" the problem. And again, if this is the problem then a permanent fix is dead easy
Have a measure.
A simple test is to measure the voltage on each side of C201 and confirm that the cap is never reverse biased. If this is the problem area then even the act of connecting the meter to the cap could "cure" the problem. And again, if this is the problem then a permanent fix is dead easy
Have a measure.
There is no distortion when using the tape monitor button, so i'm suspecting it could be something to do with the tone controls. I've cleaned all the opts and switches with contact cleaner, and visually checked all the caps.
How would I go about finding C201? I'm useless at reading schematics!
How would I go about finding C201? I'm useless at reading schematics!
It's no more difficult than finding a road on a map. I must say, though, that this is the first schematic I've seen that refers to some parts (such as resistors and capacitors) with numerals only (201 instead of C201, or 203 instead of R203). Jargon is "reference designator", usually abbreviated to "refdes".
Click on the schematic's image "C.16 Control Unit" in Ian's link, slightly less than far left there's a component designated "201", this is C201. The image to the right of the schematic is of the board layout, C201 is the lower left circle. C201 should be in the same location on the actual pcb.
Click on the schematic's image "C.16 Control Unit" in Ian's link, slightly less than far left there's a component designated "201", this is C201. The image to the right of the schematic is of the board layout, C201 is the lower left circle. C201 should be in the same location on the actual pcb.
Last edited:
And what could be a big big clue as I mentioned earlier
Are both channels affected or just one ?
Both. I have found C201, and will wait for the distortion to appear before testing.
That narrows things down a lot hopefully.
When you measure around C201 remember there will be another identical channel so check both. If it is a bias issue of the type I mentioned earlier then both channels would be affected.
If that test reveals nothing (but I'm hoping it will) then check this.
Do you see on the same circuit a resistor marked 207. Its near the top left. That resistor carries the supply voltage for the preamp. When working measure and record the voltage at each end. When the amp goes faulty compare those voltages and see if they have dropped. The power supply that feeds this resistor will be common to both channels so see what that shows.
Thats a very interesting result re 201. The 47 and 45 volts sounds OK
Is there voltage on both sides of the cap. As you look at the circuit there should really be zero volts DC on the left side of 201 and some value (determined by the component values) on the right side.
If there is voltage on the left side then you need to add a "bleed" resistor from the left side of 201 to ground (the lower end of resistor 202 would be the ideal place). Something like a 100K resistor would be good.
I'm surprised your laptop allows this to occur though... and there are other possibilities. Have you measured the DC voltage on the laptop output without it being plugged into the amp. It should be zero really.
This is worth pursuing. To test the theory virtually any resistors can be used from say 1K to 300K. You need to add the resistors (one for each channel) from the left hand end of 201 to ground as mentioned above. Doing that should definitely result in you measuring zero volts DC on the left (our new resistor) side of the cap.
If it still distorts after a while then we do some more tests
Another thought... laptops and ipods etc use what are called Class D (or switching) amplifier output stages. If these are headphone outputs then they can need and rely on the low impedance of the headphones as part of the necessary output filter that the Class D amp needs. If that's missing then there could be a lot of high frequency noise present that could be upsetting the bias. Its to high to hear and won't read accurately on a meter.
So try the resistor first and see if there is any improvement.
If it still distorts after a while then we do some more tests
Another thought... laptops and ipods etc use what are called Class D (or switching) amplifier output stages. If these are headphone outputs then they can need and rely on the low impedance of the headphones as part of the necessary output filter that the Class D amp needs. If that's missing then there could be a lot of high frequency noise present that could be upsetting the bias. Its to high to hear and won't read accurately on a meter.
So try the resistor first and see if there is any improvement.
Oh no, thats terrible. So we were on to a winner and then disaster struck.
The good news is the amp is pretty simple and so if you want to have a go at fixing it then there is a good chance of success.
What you must do now before just switching on and hoping is to fit a "bulb tester". This is just a 100 watt mains filament bulb wired in series with the mains and it will protect the amp from further damage if there is a problem. The bulb should be lit dimly if at all when the amp is on.
So if you want to cary on
The good news is the amp is pretty simple and so if you want to have a go at fixing it then there is a good chance of success.
What you must do now before just switching on and hoping is to fit a "bulb tester". This is just a 100 watt mains filament bulb wired in series with the mains and it will protect the amp from further damage if there is a problem. The bulb should be lit dimly if at all when the amp is on.
So if you want to cary on
I'l head out tomorrow morning to get a 100W bulb. Is it normal for the transformer to show 2 ohms on the secondary and 23 ohms on the primary? Would the transformer drawing to much current be the only thing that blew the 400mA on the back of the unit?
I can't find anything that looks like it has smoked, although the massive resistor(298) on Z17 power supply looks a little black on the top although it still measuring close to it's original 2.7 ohms (3.5).
Pictures of the inside and the PCI style cards: http://i.imgur.com/AMBpt.jpg http://i.imgur.com/TuU45.jpg
I can't find anything that looks like it has smoked, although the massive resistor(298) on Z17 power supply looks a little black on the top although it still measuring close to it's original 2.7 ohms (3.5).
Pictures of the inside and the PCI style cards: http://i.imgur.com/AMBpt.jpg http://i.imgur.com/TuU45.jpg
Last edited:
Try a 60 watt if you haven't a 100. The tranny will be fine and as whizgeek says, 2.5 and 23 ohms are fine. Yes, the 400ma fuse blew because too much current was drawn from the power supply (the secondary side), possibly by a failed output transistor.
Some more basic tests to do before switching on again. First though, make sure that there is no DC voltage (as in caps charged up) across C300 in the PSU.
1. TR29 and TR30 on the power supply. Check these for being short circuit. Use your meter on the "diode" test range. If any lead of these transistors reads short circuit to any other lead on the same transistor (meter would show 0.00or very very low) then we need to investigate. Any readings of .450 or higher we will take to be OK.
This part of the circuit is a voltage regulator and it provides a stable -37 volts from the -70 volts.
2. Check the transistors making up the output stage in both channels in the same way as above looking for shorts. These are TR25 and TR27 which are the main outputs and TR34 and TR36 which are the drivers.
3. Do you see the resistors 1284 and 1285 connected to the output transistors. These are likely failure items when the output transistors fail so check them and compare to the other channel. (no values on the circuit)
If there is a fault it will be in these areas.
Before switching on with the bulb tester... remove the fuses F1 and F2 that feed the power amp stages. Now switch on and quickly measure and confirm that the -37 volts is correct by measuring on the appropriate side of the fuseholders. If it is high then we have a problem in the regulator that must be fixed first.
Some more basic tests to do before switching on again. First though, make sure that there is no DC voltage (as in caps charged up) across C300 in the PSU.
1. TR29 and TR30 on the power supply. Check these for being short circuit. Use your meter on the "diode" test range. If any lead of these transistors reads short circuit to any other lead on the same transistor (meter would show 0.00or very very low) then we need to investigate. Any readings of .450 or higher we will take to be OK.
This part of the circuit is a voltage regulator and it provides a stable -37 volts from the -70 volts.
2. Check the transistors making up the output stage in both channels in the same way as above looking for shorts. These are TR25 and TR27 which are the main outputs and TR34 and TR36 which are the drivers.
3. Do you see the resistors 1284 and 1285 connected to the output transistors. These are likely failure items when the output transistors fail so check them and compare to the other channel. (no values on the circuit
)If there is a fault it will be in these areas.
Before switching on with the bulb tester... remove the fuses F1 and F2 that feed the power amp stages. Now switch on and quickly measure and confirm that the -37 volts is correct by measuring on the appropriate side of the fuseholders. If it is high then we have a problem in the regulator that must be fixed first.
TR29 and TR30 are showing 3.70 and 3.36 on the 20K resistance range on my multimeter. However, TR25A is showing 0.70 on the 20K range of my multimeter. (removed from the circuit)
It was a nice sounding amplifier, although the AL102 transistor looks very hard to find, and I do not know if it's worth fixing...
It was a nice sounding amplifier, although the AL102 transistor looks very hard to find, and I do not know if it's worth fixing...
Only you know if its worth fixing
It is all discrete circuitry and an easy fix no matter what is wrong, its just a case of being methodical. I don't know what your skills are which is why I have kept it to basic simple tests. If any parts have failed then they will be in these areas. Transistors usually fail short so all up to now seems "OK" as far as that goes.
Maybe try the bulb tester and see what really happens. The -37 volt line is first to check. Then replace fuses to output stages one at a time and see.
It is all discrete circuitry and an easy fix no matter what is wrong, its just a case of being methodical. I don't know what your skills are which is why I have kept it to basic simple tests. If any parts have failed then they will be in these areas. Transistors usually fail short so all up to now seems "OK" as far as that goes.
Maybe try the bulb tester and see what really happens. The -37 volt line is first to check. Then replace fuses to output stages one at a time and see.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.