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Orange AD200B Mk III with excessive AC Buzz

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  • Orange AD200B Mk III with excessive AC Buzz

    One of the many Orange AD200 amps in our rental inventory arrived at my shop, this one an AD200B Mk III. I plugged it in with speaker connected to see what I had to deal with. INDEED it had strong AC Buzz that was present as soon as you turned up the Master Volume pot (as well as turning up the missing Gain Knob for more buzz).

    So, pulled the chassis out of the cabinet, which revealed the front panel had been damaged, as was the power transformer. Looked like this was dropped in its' road case somewhere along the way with client/roadies. I first tackled the stretched power xfmr mounting feet with a variety of long reach tools being used to transfer hammer blows in attempt to straighten the mounting flanges. Ended up using a cold chisel on the joint of the bracket to finally get it back to original shape, then put the core bolt back in (which I had to remove for access) and restored the mounting screws to torque it down.

    Then using several small vises and machinist clamps, did my best to straighten the bent front panel back into shape. I decided NOT to remove the panel to give best access to what still wouldn't have come out flat, but at least it now looks undamaged unless you look close.

    I next checked tubes to see if any were involved in this horrendous Buzz. I had connected my Amber 3501a Audio Analyzer & scope to see what I was dealing with. I didn't stop to take photos of the rich and full RF spikes filling the space between 60Hz spikes. I had the amp standing on the Output Xfmr side with a block to stabilize it, There was less of the noise spikes with the input preamp tube removed, but all still very strong, and unusable as a bass amp. All the tubes were fine.

    I took power supply readings, and all were nominal.

    Click image for larger version  Name:	AD200B MKIII FP-1.jpg Views:	0 Size:	443.3 KB ID:	997894 Click image for larger version  Name:	Insides-1.jpg Views:	0 Size:	1.53 MB ID:	997896 Click image for larger version  Name:	AD200 Chassis Wiring-15.jpg Views:	0 Size:	395.6 KB ID:	997898

    I had both this nasty AC Buzz present, as well as oscillation that I could get to come and go with the input Gain and Treble control positions, as well as flexing the stacked First Power Supply Filter stage as well as the smallest filter cap in the power supply array (smallest one, blue sleeve. I tried shoving in some packing foam which showed there was issues with the cap leads soldered into the PCB. While that foam stopped the RF buzz waveform and Oscillation, I could get it back with the greatest of ease just tapping on the filter caps and the chassis.

    So, I needed to remove this main PCB from the chassis. Sigh........ So, I took an array of photos so I had a roadmap to follow when it came time to put it all back together.

    After I had finally removed the PCB from the chassis so I could turn it over, expecting to see fractured solder joints on the snap-in power supply caps, They all looked well soldered into place. Now the smallest one in that group, it had 0.080" dia solder pads!! THAT never ceases to disgust me seeing such disregard of designing/creating a PCB layout and NOT putting sufficiently large solder pads so one can desolder a part for replacement some time in its life!! I de-soldered and re-soldered the five caps in that group, then put it back together. Didn't change a thing! Still had the aggressive AC Buzz. I also found I could apply force to the two input stage filter caps and get the buzz to stop. I then fetched some Fish Paper to be used a shim and forced them under once side of the cap to tweak their position enough to see NO MORE RF waveform. I could still hit the caps to make it come back, so this wasn't a solution, just a demonstration to tell me I had to remove the PCB again, and this time fully desolder and remove the caps.

    Click image for larger version  Name:	AD200 Chassis Wiring-2.jpg Views:	0 Size:	451.0 KB ID:	997900 Click image for larger version  Name:	AD200 Chassis Wiring-5.jpg Views:	0 Size:	395.1 KB ID:	997902 Click image for larger version  Name:	AD200 Chassis Wiring-6.jpg Views:	0 Size:	437.9 KB ID:	997904

    To add to the fun, one of the two fuse clips for F2 TD12A fuse shows a serious burn thru the PCB and one side of its soldered mounting damaged. I've seen this damage before on AD200's. Not shown in any of the schematics I have on this amp, but those two BLU locking Fast-On Female terminals carry the AC Heater of the amp. The white wire that is twisted in that 3-wire bundle is the other side of the 12.6VAC on the Secondary side of the Pwr Xfmr. So I did my best to clean up that mess.

    After fully desoldering the two stacked 1st stage power supply filter caps, and the next filter cap following, I cleaned up the PCB and didn't see any damage, but, rest assured, Lead-Free Solder is used in the manufacturing of the board. I've run into this problem before having nicely soldered filter caps in place yet misbehaving like crazy to make you desolder and resolder caps like these to try and restore order.

    I did finally succeed in getting the AC Buzz out, other than a single spike riding on residual 60Hz hum with Master and Gain pots fully CW, but no more of the forest of spikes between those 16.67mS noise spikes. AND NO Oscillation. I could still tap on the power supply caps and get transient noise to emit. Best I could do.

    With this approach to building tube amps, there's a lot to be said for Discrete Wiring and NOT using single or multiple PCB's to mount all the parts. I had one Victory Kraken VX amp that I could never solve its AC Buzz problem. And a nightmare to service with so many PCB's sandwiched into its small chassis. Sent it back to the UK, though never did get an explanation from them as to where the problem was.
    Attached Files
    Last edited by nevetslab; 04-19-2024, 06:46 PM.
    Logic is an organized way of going wrong with confidence
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