Hello AK
I'm trying to bring this British piece back to life but I'm a little stumped with the cause of the problem - it's a bit of an uncommon receiver. There was one thread on AK that suggested there might be a few of you that know the piece. I'd appreciate a little guidance if any of you have experience with it, or the inclination to help.
The schematic is available at http://www.audiomisc.co.uk/Armstrong/600/600faq.html. I've also included some photos.
There's no audio output. I've tested/observed the following things.
The function of the tuner on power-down and the transient quiescent current are interesting but I don't know what to make of it.
There is a cap on the mains side of the transformer (across the power switch). The component tester recognised it as a functioning cap (measured in situ) but the values were out. It doesn't look like the source of a short.
Any thoughts on where to go from here would be much appreciated.
Cheers, Jon
I'm trying to bring this British piece back to life but I'm a little stumped with the cause of the problem - it's a bit of an uncommon receiver. There was one thread on AK that suggested there might be a few of you that know the piece. I'd appreciate a little guidance if any of you have experience with it, or the inclination to help.
The schematic is available at http://www.audiomisc.co.uk/Armstrong/600/600faq.html. I've also included some photos.
There's no audio output. I've tested/observed the following things.
- Power-up on DBT - looks fairly ordinary: 28W bulb glows while caps fill and then dims to almost imperceptible glow. (Measurements were made using 70W globe, which only gives a dim pulse during power-up.)
- Voltage supplies for power stage appear to be just under 75% or expected voltages (57V at +ve side of main, 3300 microF filter cap instead of 82; 28V at +ve side of 4000 microF speaker caps, instead of 40V).
- Power transistors appear to be OK - OC measured E-C.
- There is some DC offset on outputs. The outputs are cap-coupled. I measured them with a 57Ohm load. They start off quite high as the caps charge but drop off quite quickly. Once they settled down there was a fluctuating voltage, generally up to 30mV (sometimes up to 50) across the load.
- Quiescent current was measured on resistor 226 (L and R). These are 0.68, not 0.47Ohm as shown on schematic. During power-up the voltage rises to about 13mV (just under the 13.6 expected for 20mA stated in schematic) but soon drops to zero as caps charge. I understand that the corresponding resistors (227 L and R) for the other outputs should read the same (albeit on the C rather than E side of the output) but they show no voltage at all on power-up or after. It is hard to see how any real DC voltage is coming from the output transistors.
- Resistor 23 (just under the rectifier in the schematic; 3rd photo) runs hot. It gets too hot to touch quickly, gives off a noticeable smell, and has darkened the insulation on the wires leading to it. I suspect that this is linked to the problem.
- The two caps that are parallel to a pair of the rectifier diodes are darkened (4th photo). However, DC voltage across the bridge takes expected steps of 0, 24 and 58V (although it should go up to 82V instead of 58V).
- The tuner section appears generally not to be working. However, on power-off (while it is running on charge from caps) it appears to start working - at least the LED indicator for locking into a station appears to come to life and respond to tuning. This is brief and only last while there is charge in the caps. This may also point to the cause of the problem.
- During the course of making measurements I tripped the earth return on the house fuse box a couple of times. The overload on the power board I used didn't respond.
The function of the tuner on power-down and the transient quiescent current are interesting but I don't know what to make of it.
There is a cap on the mains side of the transformer (across the power switch). The component tester recognised it as a functioning cap (measured in situ) but the values were out. It doesn't look like the source of a short.
Any thoughts on where to go from here would be much appreciated.
Cheers, Jon