Have one of these in the shop. Customer had brought to another shop prior to bringing here.
The problem I noted off the get-go was that the front panel LED doesn't light at all and the amplifier won't pass any signal and appears to be in mute.
Taking a closer look at this front end PCB, it looks like Q1, Q2, Q4 and Q5 were removed for some reason.
The PIC doesn't have any observable clock on pin 1. I don't know if the PIC is bad, or if the board doesn't work because of the missing transistors.
If anyone is familiar with this particular VTL preamp, can you tell me if these transistors are present on other serial number units?
The problem I noted off the get-go was that the front panel LED doesn't light at all and the amplifier won't pass any signal and appears to be in mute.
Taking a closer look at this front end PCB, it looks like Q1, Q2, Q4 and Q5 were removed for some reason.
The PIC doesn't have any observable clock on pin 1. I don't know if the PIC is bad, or if the board doesn't work because of the missing transistors.
If anyone is familiar with this particular VTL preamp, can you tell me if these transistors are present on other serial number units?
I did some searching and found this photo. https://www.usaudiomart.com/details/649641597-vtl-tl-25/images/2811096/ Hard to tell but it does look like those transistors are populated. I was going to say maybe they were part of the muting circuit but the picture makes it clear. The motorized pot goes to that white connector. It's an H bridge to drive the motor.
Thanks. Motor driver makes sense. No idea why anyone would have removed them.
The PIC shows what I thought was a clock on pin 1, but I see there are two OSC pins at 26 and 27 now that I take a closer look. I did not see any clock signal on any pin when I probed with a scope. Either the crystal is dead or the PIC is dead. Crystal would probably be my first guess.
I doubt the transistors are reverse mounted, as the holes have no leads protruding.
The PIC shows what I thought was a clock on pin 1, but I see there are two OSC pins at 26 and 27 now that I take a closer look. I did not see any clock signal on any pin when I probed with a scope. Either the crystal is dead or the PIC is dead. Crystal would probably be my first guess.
I doubt the transistors are reverse mounted, as the holes have no leads protruding.
I just discovered that if I connect pins 14 and 15 together, the unit comes out of mute and audio passes through both channels. I still can't see a clock signal on either of the clock pins. There's a 4MHz crystal in there.
Yeah, interesting, there are 18 pins on the physical chip. But the part number references to this data sheet. Oh wait, the diagram on the left is also DIP:
Yeah, interesting, there are 18 pins on the physical chip. But the part number references to this data sheet. Oh wait, the diagram on the left is also DIP: