Recently did a recap and refurb of a craigslist SX-650, which was all electrolytics except for the tuner board and a variety of suggested transistors in power amp board and equalizers boards. When I was done, I was getting intermittent protection circuit triggering that I couldn't resolved.
Q14 & Q15 were replaced, I could measure the voltage on Q15's collector start at 32V, discharge down to 8V and the relay would engage on the way, then it would intermittently kick back up to 32V. I had around 21V on the cathode of D7, D8, & D9 steady during all of the protection circuit occurrances and I couldn't figure out why Q15 was seemingly unbiasing.
I did two things that after the fact that seem to have helped:
1) Tapped the solder joins on the high power/high voltage (HOT!) transistors on the power amp GWR-101 board with the soldering iron to reflow, in case they had some heat induced stress. Still had occasional, though less often, protection circuit triggers.
2) Removed and replaced D12, the relay coil inductive suppression diode on GWR-101 with a 1N4148 I had in a spare parts box.
The diode seemed to do the trick. I checked the old diode with my DMM and it tested well to the 'diode test' mode, though if it was an intermittent P-N junction problem, I wouldn't expect it to do anything.
My hypothesis is that years of inductive kickback on the coil caused some latent damage to that relay that was only prevalent in use.
Hope this helps someone else in the future.
Q14 & Q15 were replaced, I could measure the voltage on Q15's collector start at 32V, discharge down to 8V and the relay would engage on the way, then it would intermittently kick back up to 32V. I had around 21V on the cathode of D7, D8, & D9 steady during all of the protection circuit occurrances and I couldn't figure out why Q15 was seemingly unbiasing.
I did two things that after the fact that seem to have helped:
1) Tapped the solder joins on the high power/high voltage (HOT!) transistors on the power amp GWR-101 board with the soldering iron to reflow, in case they had some heat induced stress. Still had occasional, though less often, protection circuit triggers.
2) Removed and replaced D12, the relay coil inductive suppression diode on GWR-101 with a 1N4148 I had in a spare parts box.
The diode seemed to do the trick. I checked the old diode with my DMM and it tested well to the 'diode test' mode, though if it was an intermittent P-N junction problem, I wouldn't expect it to do anything.
My hypothesis is that years of inductive kickback on the coil caused some latent damage to that relay that was only prevalent in use.
Hope this helps someone else in the future.
