plaurids Posted October 17, 2022 Author Report Share Posted October 17, 2022 (edited) Have just replaced the R31-34 resistors in the following fashion to keep these parts minimally coherent after one of the R34's leads broke: R31-32: 8.2kOhm, 5W, 5% (Dale). R33-34: 8.2kOhm, 3W, 5% (Philips). These are the same ratings as in the stock parts, recalling that I couldn't figure out the latter's actual power ratings. I've used the only SRM-T1 schematic I have for that rating's reference, but there R31-34 are all rated as 15kOhm. Unfortunately, as expected, the bias voltage problem remains unchanged. Update (October 31st 2022): I messed up the pins last time I made the measurements, sorry. Actually, the problem manifests itself like this: the balance voltage measurements (i.e. measured between +/- and ground, and adjusted by the TVR2 trimpots) as one sweeps through TVR2 are (with the current tube pair): -235V to -330V on the left channel; -243V to -330V on the right channel. All balance voltage measurements were made with the ground (black) voltmeter probe in the ground pin. I'm currently somewhat in the middle TVR2 setting on both channels, measuring -283V (left) and -285V (right). The offset voltage measurements (i.e. measured between + and -, and adjusted by the TVR1 trimpots) on both channels can be brought down to +/- 2V around zero at best after drifting, which is still too unstable (it should be +/- 1V around zero at worst). As before: Tube swapping/rolling does nothing; No electrolytic caps are shorted or bloated; Couldn't identify any short points in the main PCB or in the CCS module's PCB; Currently looking for broken copper traces in the main PCB, particularly around C9-10 and R31-34 (where the +/-48V rails are derived from the HV rails), but haven't found anything yet. Will update again here if that happens. One aspect I hadn't paid attention to before, which was raised privately to me by Dominik Stritt (aka Firschi in the Head-Fi forum), is the relative brightness of the D3-D4 LEDs. All four are lit, but D4 (next to the offset TVR1 trimpots) is much weaker than D3 on both channels. Notice that in my unit these LEDs are marked the opposite as in the SRM-T1 schematic (i.e. the LEDs next to TVR1 are marked D3 in the latter). Tube swapping/rolling doesn't change the behavior of D3-4 either. For the time being, I'm performing all tests with the stock Gold Aero tubes, since the above problem makes impossible to be certain of these really being worn out. In retrospect, I cannot ascertain since when this has been happening. It certainly happened after installing the CCS module and the ensuing successful rebiasing more than a year ago, but I haven't made any measurements between then and my bias measurements about two weeks ago. I could seriously use some help right now, for I have no idea what's going on... Any suggestions are more than welcome! Edited November 2, 2022 by plaurids Quote Link to comment Share on other sites More sharing options...
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