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I received one of our Ashdown ABM900 Bass amps in with no functional Drive Circuit. Verified it on the bench, where feeding input signal to where I'm getting mid-meter reading from my burst pnk noise signal, switching in the Drive control and rotating it CW to engage the tube overdrive, the output drops to nearly nothing.
So, removed the input signal and measured the tube supply circuit, where there should be 250VAC hitting the Eurostyle terminal blocks. Only reading 125VAC with no tube installed. First tried swapping out ECC83/12AX7 tubes, with no change. Plugging the tube back in, the DC supply dropped to less than 50VDC. Powered down, and measured the DCR on the 250VAC winding. Measures open circuit. Stuck my sharpened probes thru the insulation of the thin gauge wires just leaving the fiberglass sleeving of the Toridal Xfmr. I got one of the two leads to read between the terminal strip and the probed wire, but trying the same on the other lead, I get no continuity.
Well, that sucks! I see only two obvious solutions. One, being very expensive...order a replacement huge power xfmr from Ashdown in the UK, or....find a low current 250VAC (or close) EI power transformer, find a 'convenient' mounting location in the chassis and wire it in to replace this open winding.
Any other obvious suggestions?
I don't actually have the schematics for the ABM 900, only that of the ABM 500, which I've attached. below
ABM500-Sch.pdf
So, removed the input signal and measured the tube supply circuit, where there should be 250VAC hitting the Eurostyle terminal blocks. Only reading 125VAC with no tube installed. First tried swapping out ECC83/12AX7 tubes, with no change. Plugging the tube back in, the DC supply dropped to less than 50VDC. Powered down, and measured the DCR on the 250VAC winding. Measures open circuit. Stuck my sharpened probes thru the insulation of the thin gauge wires just leaving the fiberglass sleeving of the Toridal Xfmr. I got one of the two leads to read between the terminal strip and the probed wire, but trying the same on the other lead, I get no continuity.
Well, that sucks! I see only two obvious solutions. One, being very expensive...order a replacement huge power xfmr from Ashdown in the UK, or....find a low current 250VAC (or close) EI power transformer, find a 'convenient' mounting location in the chassis and wire it in to replace this open winding.
Any other obvious suggestions?
I don't actually have the schematics for the ABM 900, only that of the ABM 500, which I've attached. below
ABM500-Sch.pdf
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