Tweet
Hi everyone, got a Drawmer 1960 compressor in for repair recently.
Owner had just bought this secondhand cheaply because it was only partially functional. It had been "repaired" before. This is an early unit (date codes point to 1988), with the potted black box modules. Appears these had failed which was apparently common, and somebody tore the whole thing to pieces "troubleshooting", taking out many, MANY pads, traces and connectors in the process, before they finally discovered the modules were at fault and installed replacements. This is my best guess because the damaged sections were seemingly at random, but the currently installed modules have a couple of long pins that have been folded over to bridge lifted pads, that wouldn't have been possible with the original factory modules which would have had pins cut short after installation.
Anyway, after tracking down all the faults, most of which had been caused by the earlier "repair" work, the B+ supply is oscillating. All parts (with the exception of the TL783 adjustable high voltage regulator - though in situ it performs fine, with phantom voltage well regulated and accurate) test within tolerance, and electrolytic caps have been replaced.
In the process of troubleshooting, I discovered that removing the 10n cap (2n2 on the schematic, silkscreen shows 10n) between the MPSA42 diff pair error amp collectors cured the oscillation, and the regulated output is solid in use. Relevant section of the schem is on page 7 of the PDF.
Now clearly, at some point in time this circuit functioned correctly as designed. I've been unable to identify why it no longer functions correctly, so I'm second-guessing myself.
Anyone have any thoughts? Thanks.
EDIT: should add, the oscillation with the cap in place is roughly 430Hz, 5vp-p sawtooth, riding on the 200V B+. Removing the cap subdues the oscillation entirely, it doesn't simply move it outside the audio band or anything like that.
Owner had just bought this secondhand cheaply because it was only partially functional. It had been "repaired" before. This is an early unit (date codes point to 1988), with the potted black box modules. Appears these had failed which was apparently common, and somebody tore the whole thing to pieces "troubleshooting", taking out many, MANY pads, traces and connectors in the process, before they finally discovered the modules were at fault and installed replacements. This is my best guess because the damaged sections were seemingly at random, but the currently installed modules have a couple of long pins that have been folded over to bridge lifted pads, that wouldn't have been possible with the original factory modules which would have had pins cut short after installation.
Anyway, after tracking down all the faults, most of which had been caused by the earlier "repair" work, the B+ supply is oscillating. All parts (with the exception of the TL783 adjustable high voltage regulator - though in situ it performs fine, with phantom voltage well regulated and accurate) test within tolerance, and electrolytic caps have been replaced.
In the process of troubleshooting, I discovered that removing the 10n cap (2n2 on the schematic, silkscreen shows 10n) between the MPSA42 diff pair error amp collectors cured the oscillation, and the regulated output is solid in use. Relevant section of the schem is on page 7 of the PDF.
Now clearly, at some point in time this circuit functioned correctly as designed. I've been unable to identify why it no longer functions correctly, so I'm second-guessing myself.
Anyone have any thoughts? Thanks.
EDIT: should add, the oscillation with the cap in place is roughly 430Hz, 5vp-p sawtooth, riding on the 200V B+. Removing the cap subdues the oscillation entirely, it doesn't simply move it outside the audio band or anything like that.
Comment