Ashdown EVO III 900 Power Xfmr potted core

[nevetslab]

While selecting another amp from the accumulated pile of gear to repair, now set up in the warehouse with a temporary shop, I brought over an Ashdown EVO III 900 Bass Amp, with the complaint of it distorting. After I pried off the domed hole plugs hiding the top and bottom chassis mtg screws thru the wooden housing, I tried to slide the chassis out of the cabinet. Didn't want to come out easily. When I finally got it out, I found the toroidal power xfmr was sitting up off the floor of the chassis. Odd. I disconnected the ribbon cable that wrapped around it from the rear panel PCB, and removed the xfmr core bolt and removed it, now lifting the xfmr up and over onto it's side.

The potted center had come unattached,and now was half-way thru the bottom of the xfmr, trying to exit. I didn't think to stop and take a photo of that before reaching for my 2lb copper sledge hammer to knock the core back into place. It was after that when I stopped to take photos. Though I first disconnected the high voltage wires from the two power amp modules, leaving the LV wires attached, and plugged the amp into the variac/power analyzer to check if there is any damage to the windings, now having hammered that potted center back into position. Fortunately, all seems ok. I hope.

For the moment, I've put the core bolt/nut/washers back into place, but, now that the potted center has broken free, I now need to track down a proper metal dish & rubber washer, with a longer core bolt/nut/washer to it's properly mounted. My initial search yesterday didn't reveal any suitable sources, while what I DID find was made from 18ga steel, instead of 14ga like we normally find for a 1KVA xfmr like this (or similar VA size). I then thought about the Toroids in the GK 1001RB and 2001RB's. I guess I'll give GK a holler to see if I can buy a hold-down dish, rubber washer and core bolt stack as used in the 2001RB.

[Jon Snell]

Araldite or an epoxy glue will fill and seal the errant plug.
Hardware is easy; https://uk.rs-online.com/web/c/power...mounting-kits/ if you need to purchase a set as well as fixing the plug.

[Enzo]

Betting the amp was dropped flat on its head at some point.

[The Dude]

Not sure if this would work without measurements, but check this out.

https://www.digikey.com/product-deta...SABEgIfjPD_BwE

[nevetslab]

Not sure if this would work without measurements, but check this out.

https://www.digikey.com/product-deta...SABEgIfjPD_BwE

That's the same part I was looking at. Mouser has it for $4.63 ea, and was just in the process of ordering a couple of those, while adding to the parts order so I don't get clobbered by shpg fees every time I need something else. I was going to call Gallien Kruger for one of their xfmr mtg kits, but I still don't have my telephone here from my shop down the street. And, the GK bolt is a carriage bolt in the chassis, so I would have had to change that. Thanks.

[jondoe]

I had a 600 watt model where the cracked core had rubbed through the insulation and shorted the turnings. Their replacements are not potted.

[nevetslab]

After finding very low output on this ABM EVO III 900 Bass Amp, with signal passing thru the preamp ok, it seemed, and finding it on the Line Output jack on the front panel, the signal on both power amps was very low, and would change to triangular waveform above 4V Out. I didn't find any cause for it on the power amp modules, though after cranking the input level up another 10dB, now around 300-400mV, and pegging the input level meter, I suddenly got full output from the power amps. So, I pulled the preamp PCB's out, finding a few bad solder joints, and also cleaned the input pot, the output pot and the Shape pushbutton, which was very intermittent. Put that back together, and now have solid unchanging levels. But, I wasn't getting signal into the amp from the rear panel Line Input jack nor the Effects Return jack. They appeared to pass thru an active circuit below the four jacks, above the Neutrik NL4 Combo Output connectors.

Checked the ribbon cable and had continuity on it, so extracted that rear panel PCB, which requires unmounting the power xfmr, enough to slip that PCB assy out, then removing two of the fans that were preventing the output cable from the left-most power amp module, and pulled the assembly out. Repaired a couple solder joints, then replaced the TL072 Op Amp that was used to buffer the signal passing thru the Line Input jack and Effects Return jack. Put it back together, restored the power xfmr mounting and tried again.

Still no signal from the rear panel jacks. The signal from them IS making it to the front panel PCB, but that's as far as it goes. I don't have service documents on all of this model. I have enough to have navigated thru most of the amp, as well as that of the power amp modules (Bipolar xstrs as opposed to the BUZ901/905 MosFET's).

I've at least cured the amp of the low level distortion problem that led it to the repair pile. And, received the 1kVA hold-down toroidal xfmr mounting hardware, so the amp IS functional. As long as you don't need the Effects Send/Return jacks or Line input jack.

Does anyone have the schematic of that rear panel PCB assy, as well as the Preamp circuitry that uses the 10-conductor ribbon cable that plugs into PL-5 near the Output level pot?