leigh
Well-known member
The situation:
Modding the group modules in a Trident Series 65. Switched out a bunch of ICs from TL072 to LME49710. Power the board back up, and suddenly the VU meters for the groups are all floating at -7dB, without any signal present.
I get out the oscilloscope and see what's going on: there's an oscillation on the meter feed line, at about 7MHz. It swings to about a 0.6V peak, + and -.
If I disconnect the meter line, the IC feeding it (IC2, for those reading along on the schematic) stops oscillating.
I could just swap IC2 back to a TL071, and the oscillation stops. Unfortunately, that IC is not just functioning as a meter driver, it is also in the main signal path, so I'd prefer to keep it as an LME49710.
Two strange twists:
1. The output oscillation is at 7MHz, but IC2 has been bandwidth limited to 300KHz (negative feedback path consists of a 16k resistor in parallel with a 33 pico cap).
2. On the original Trident schematic, a buildout resistor R93 is shown in series with the meter feed, with a value of 3k6. However, in the actual console, although that resistor marking appears on the PCB, it has been replaced by a wire jumper.
The group VU meters are Componex 2LX. I can't find any specs on those - is it possible they opted for a model of VU meter that contains that standard 3k6 resistor internally?
Also, I should note, in testing if I add a 100ohm buildout resistor in series with the meter feed, the oscillation stops. The difference in meter readings with and without that 100ohm resistor is only about 0.1VU, so that may be an acceptable solution.
Modding the group modules in a Trident Series 65. Switched out a bunch of ICs from TL072 to LME49710. Power the board back up, and suddenly the VU meters for the groups are all floating at -7dB, without any signal present.
I get out the oscilloscope and see what's going on: there's an oscillation on the meter feed line, at about 7MHz. It swings to about a 0.6V peak, + and -.
If I disconnect the meter line, the IC feeding it (IC2, for those reading along on the schematic) stops oscillating.
I could just swap IC2 back to a TL071, and the oscillation stops. Unfortunately, that IC is not just functioning as a meter driver, it is also in the main signal path, so I'd prefer to keep it as an LME49710.
Two strange twists:
1. The output oscillation is at 7MHz, but IC2 has been bandwidth limited to 300KHz (negative feedback path consists of a 16k resistor in parallel with a 33 pico cap).
2. On the original Trident schematic, a buildout resistor R93 is shown in series with the meter feed, with a value of 3k6. However, in the actual console, although that resistor marking appears on the PCB, it has been replaced by a wire jumper.
The group VU meters are Componex 2LX. I can't find any specs on those - is it possible they opted for a model of VU meter that contains that standard 3k6 resistor internally?
Also, I should note, in testing if I add a 100ohm buildout resistor in series with the meter feed, the oscillation stops. The difference in meter readings with and without that 100ohm resistor is only about 0.1VU, so that may be an acceptable solution.