I'm having a real weird issue with my Altec 1593B. I have two 1593B amps, one sounds great but the other has no bias on the power transistors at all. At very low volumes the sound is distorted, as expected, but as the volume is turned up the transistors are biased and starts to sound better and at loud volume sounds fine, since the bias gets close to the operating point.
I have attached the schematic. Here is what I've done so far:
- both circuit boards (the part on the schematic surrounded by the dotted lines) work on the good amp. But neither board work on the bad amp. So it's definitely not the board.
- with no signal, on the bad amp, the base to ground voltage of Q1 and Q3 are around .5V as expected from the circuit. But the emitter to ground voltage which should be around 30mV is zero.
- I tested all transistors and they are all fine. Still, I replaced all transistors with a new set and still the same. I also replaced the zener CR5 for the 15V line and the diodes CR2 and CR3.
- all collector voltages are fine at 31V, the OPT looks OK and it's probably OK since it plays fine at loud volumes. The stabistor measure 1.3V as expected. The power resistors measure fine and are not open.
- I re-soldered all connections from the circuit board connector and the connections on the power transistors. All grounds are fine.
Don't know what I'm missing here. Any help is appreciated.
not a tech, but going out on a limb here. if you had a signal tracer. you could work your way from the source to the output transisters at low volume till you find the point the distortion comes in.
It's hard to read the print , but it looks like CR1 and it's dropping resistor supplies 1.2V bias through the center tap of the driver transformer. Inspect the resistor / diodes verify you have 1.2V.........GC
It's hard to read the print , but it looks like CR1 and it's dropping resistor supplies 1.2V bias through the center tap of the driver transformer. Inspect the resistor / diodes verify you have 1.2V.........GC
Yes, that was the problem. I had checked the CR1 a few times and the voltage was 1.3V. But when I took it out of the circuit it just had a 0.7V drop. I tied 2 diodes together and got around 1.1V and it worked fine. I need to get the right stabistor. I tried 2 diodes and 1.1V is too low. Tried 3, got 1.5V...too high. Anyone tried a battery based bias?
Common silicon diodes have .6 V drop but, not exactly....try different types to get the required 1.2V
drop .....I would replace the dropping resistor too......
That circuit is interesting in that it's SS with driver and output transformers. How does it sound compared
to complimentary symmetry or quasi complimentary SS amps ????.............GC
Common silicon diodes have .6 V drop but, not exactly....try different types to get the required 1.2V
drop .....I would replace the dropping resistor too......
That circuit is interesting in that it's SS with driver and output transformers. How does it sound compared
to complimentary symmetry or quasi complimentary SS amps ????.............GC
I already did it. Connected 2 diodes back to back and got 1V. Tried 3 and got 1.5V. I need to play around to get something closer.
I haven't set them up in my main system so I can really tell you how they sound. I have an Altec 1003B running the mids from 250 - 7Khz with a phenolic driver and a super tweeter on top. I bi-amp with a couple of huge Bozak Symphonies doing the bass. The huge 1003B horns are very revealing and personally I found out over many variations of amps that it's really not tube vs SS that matters, but the topology. I tried many full and quasi complementary designs in SS and found that quasi sounds nicer but not as revealing as my BGW 250D fully complementary. Those are incredibly clear and revealing. I'm curious about the 1593B because of the combination of transformer inter-stage and OPT transformer. I should have the 1593b amps setup tonight and I'll let you know how it goes.
Originally Posted by 604man
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