Ashdown ABM900 250VAC winding for tube bad

[nevetslab]

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

[mozz]

How much current do you need? You can get cheap DC/AC converters from Ali express.

[Jon Snell]

Have you got 30v AC between FS1 & FS2?

How can you have half 250v AC on P5 if the winding is O/C.

[nevetslab]

Have you got 30v AC between FS1 & FS2?

How can you have half 250v AC on P5 if the winding is O/C.

Yes, I have all of the remaining secondary voltages on the windings, just don't have the required 250VAC on the HV winding for running the plate supply for the tube. With the fuse in place on the preamp PCB, and the tube installed, there's barely 40VDC on the plates, so my first task today is to disconnect those two wires from the secondary and run 350VDC from my small Kikisui Power Supply to verify the Drive circuit is working as it should be. Then, the task becomes the most sensible solution to having a reliable 250VAC (or close) voltage source to restore the amp.

HOW can I have half? No load, and whatever has failed inside the lead terminations of the xfmr. I have one of the two leads reading 0 ohms DC, while the other, (piercing the insulation of the lead exiting the fiberglass sleeves out of the xfmr windings at the top to the other end of the wire), it measures OPEN CIRCUIT. So, there's a partial circuit being completed out of sight but can't run the circuit. Simple as that.

[Helmholtz]

I have one of the two leads reading 0 ohms DC,...

Not clear what you mean here.

You might be measuring a "ghost voltage" caused by capacitance between the turns of the open winding.
To verify, load the winding with a 100k resistor and measure voltage again.
Should be much lower now.

[g1]

How can you have half 250v AC on P5 if the winding is O/C.

It's only there with no load, and is probably due to capacitive coupling in the winding. Basically a 'ghost' reading.

[nevetslab]

Not clear what you mean here.

You might be measuring a "ghost voltage" caused by capacitance between the turns of the open winding.
To verify, load the winding with a 100k resistor and measure voltage again.
Should be much lower now.

Under the nominal load for that 250VAC winding, tube connected, the plate voltage is less than 50VDC. That IS the circuit the supply is feeding, and the tube doesn't work as a result. With an external DC power supply connected (past the Fuse and xfmr windings disconnected, I have a working Drive Circuit. I think that spells it out.

That prior measurement with power off, measuring continuity between the Eurostrip terminal having the two 250VAC winding leads connected, I found only one of the two leads where they exit the sleeves at the power transformer showing continuity, while the other shows open circuit. Measuring across the two connections at the terminal strip with the leads attached, it measures open circuit, even at the highest resistive setting.

[Jon Snell]

The reason I asked about the twin 15volt output was to suggest a 240v primary and two 15volt secondaries at, say 5va, in reverse will provide 240volts from your present 30volt supply.
Transformers don't care which is primary as long as the voltage is correct.
Select one like these, they have a single screw fixing and easy to use; https://uk.rs-online.com/web/c/power...983,4294837670
Mouser will sell them as well.

[nevetslab]

The reason I asked about the twin 15volt output was to suggest a 240v primary and two 15volt secondaries at, say 5va, in reverse will provide 240volts from your present 30volt supply.
Transformers don't care which is primary as long as the voltage is correct.
Select one like these, they have a single screw fixing and easy to use; https://uk.rs-online.com/web/c/power...983,4294837670
Mouser will sell them as well.

Clever idea. I haven't looked up this transformer yet, as I just placed an order for a Triad VPS230-110 transformer, and have a mounting space worked out to the right of the preamp PCB behnd the far right side of the chassis up front. Moving the inrush AC PCB (large Thermistor on it) to mount off the rear chassis standoff, and add additional one at the back end, clears space for this vertically mounted Triad xfmr. Might even be able to unwind and re-route the pair of BRN transformer wires from the 250VAC secondary of the Toroid. As the current load is low on this winding, I'm sure the rated 240V will be higher for the light load the tube circuit uses to run the plate voltage (in either case).

Thanks for your input.

[Jon Snell]

I have made many valve foot pedals using 240v mains input, isolated with a 12volt secondary and then a smaller but same voltage rating transformer produces 300v DC for the valves.

[nevetslab]

Yesterday the Triad VPS230-110 25VA Power Transformer arrived, it being selected for the task of providing around 250VAC (or less) from the 120VAC applied to the dual primary wired in parallel for 120VAC in, and the output wired in series to feed the existing pair of BRN wires to the 350VDC power supply circuit to run the single 12AX7 tube used in the Drive Circuit. I found after staring at the chassis of this Ashdown ABM900 Amp, the least-intrusive location for this small transformer was in the right front corner of the chassis, previously occupied by the small in-rush circuit board sporting the large Thermistor. After removing that PCB, and moving it into the space between the large Toridal Power Transformer and the edge of the chassis, I tried to remove the pressed-in M3 Spacer without any success. When the transformer arrived, I was pleased to find it actually WASN'T in the way, where it actually sat below the primary/secondary wiring terminals out of harms way.

So, armed with calipers, adjustable 4" machinist square, scribe and center punch, I marked up the chassis, punched the hole marks for the transformer and the other support end of the inrush circuit board and drilled out the mounting holes, deburred them and mounted the transformer and circuit board.



I then mounted the transformer with #8-32 hardware, and a 1/4" tall #4-40 threaded standoff and secured the back end of the inrush PCB. Next, I unplugged the pair of switched Primary transformer wires from the rocker switch, slid back the clear vinyl covers of the 14-16AWG Female Quick-Disconnect terminals, found I could lay a bead of solder onto the crimp area where I could then tack-solder the AC wires to feed the primary of this little transformer, and installed those wires, struggling a little bit with the clear vinyl cover to get it back over the added wire for each terminal, and plugged those back into place.

I didn't have any in-line fuse holders, but after looking thru what I had in panel-mount fuse holders, came up with a suitable fuse holder to wire in and insulate to serve the task of primary protection of this added power transformer. After wiring up the primary and secondary windings, primary being parallel for 120VAC and Secondary being series connected for twice the voltage (hopefully), I extended the secondary wires to join the cut-off wires from the original 250VAC winding to the power supply circuit of the tube. When I looked at the result, I wasn't so happy with the choice of this Triad VPS230-110 transformer. With 120VAC in, I only found the series-connected Secondary achieved 205.1VAC and 283.6VDC on the first filter stage to run the triode plate circuit. Even removing the tube, it only came up a couple volts, This transformer only cost around $16, while another one I was looking at a Hammond 261C6 250VAC-45mA Sec/120VAC Primary xfmr for $60.



When I had checked out the Drive Circuit on the amp, discovering the 250VAC winding of the bit Toroid had failed, I first used my Kikisui supply that only goes as high as 300VDC, which at least told me that was where the problem was with the power supply circuit, and the amp worked find with that somewhat lower voltage. I then substituted that for my HP 712C which goes as high as 500VDC, dialed in 350VDC which is the nominal voltage used at that first stage of the power supply circuit. So, I could have spent more $$ to get it where it should have been, though chances are, I wouldn't have had 250VAC from it either, though I'll never know. I didn't hear any noticeable difference with the drive circuit with 300VDC vs 350VDC. I also found this was the toroidal transformer that had broken free of its potted center in 2022, and I had to replace the original xfmr mounting with the large mounting dish and longer mounting bolt from the bottom of the chassis. I wonder if that mechanical failure back then had anything to do with this 250VAC winding failure of late? Never will know. But the solution here works. The amount of labor to install it was way more than the cost of the little transformer! And the amp is back up and running.

Triad VPS230-110 Pwr Xfmr.pdf