I got my hands on an Ultrapath battery powered valve preamp, it's already got new batteries (the whole device is battery powered, both the heater and the plates of the ECC86) but one channel has a much lower rail voltage and audible distortion.
Things I've tried so far:
What I find strange is that if I remove the LED in that channel, the B+ goes back to normal, roughly +25VDC, with the LED in circuit it drops to ~17V (measured without tubes).
When turning both channels on, on the working channel the LED pops right on while the defective one comes on much slower and stays dimmer.
Things I've tried so far:
- Remove and measure C1-C5 OK
- Check all solder joints OK
- Measured all R's OK
- Measure PSU voltage without the preamp unit attached OK
- Swapped valves between the channels
- Measured coil resistance on the primary of the output transformers - identical between channels
What I find strange is that if I remove the LED in that channel, the B+ goes back to normal, roughly +25VDC, with the LED in circuit it drops to ~17V (measured without tubes).
When turning both channels on, on the working channel the LED pops right on while the defective one comes on much slower and stays dimmer.
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Just remove the LED if you can't find the problem. Check around R3 for shorts etc.
Do both channels use the same batteries, or does each channel have its own batteries?
The entire schematic is not accurately posted, since there is no power switch shown.
Maybe something omitted is related to the problem.
Do both channels use the same batteries, or does each channel have its own batteries?
The entire schematic is not accurately posted, since there is no power switch shown.
Maybe something omitted is related to the problem.
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Looks like the problem is in the PSU - the LED on the charging circuit on the right channel is also dim when the charging circuit is energized (shows similar behaviour to the power LED on the preamp itself).
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How weird, one of the battery fuses (not on the schematic, itself, however mentioned further in the build manual) looks fine but is measuring 17k ohms...
F2, F3, F4, F5 fuse 1/2A 2AG pigtail fuse according to the manual.
F2, F3, F4, F5 fuse 1/2A 2AG pigtail fuse according to the manual.