Ashton Amp Repair Advice.

[Sonic Mountain]

Another way you can discharge the caps is with the multimeter. Select high voltage DC (>600vdc), hook one leg of a 1K resistor (1w) to the positive lead on the meter. Touch the neg lead to ground and the free leg of the resistor to the cap positive. Doing ut this way both discharges the cap and shows you viabthe meter if there is any voltage present. I modded a cheap meter for this process with a couple of banana sockets. Works a treat.

I’ve got a pic of mine on my phone but can’t seem to attach it! I don’t use image hosting so I could try PM or something

All good. I have some other things on this evening so I'll have a look at that tomorrow.

You'd almost describe that as a guilty blush.

Looking at the circuit you previously linked that had a solid state power circuit I suspect this will probably be very similar, why re-engineer what you don't have to

Yeah that part of the diagram looked the same to me.

[Sonic Mountain]

Now you have that board out, you can check continuity of the power transformer windings with the multimeter. Look at both the primary and secondary sides. If the switch was working and the fuses are fine, the transformer is the next thing in line. Primary first (should directly connect to the switch), then secondaries which will connect to the board near those big caps and that bridge rectifying diode (BD1). Looks like they connect via molex plugs. You can do this b4 you discharge the caps as they are not connected to the tranny.

Ok.. I think I have continuity in the windings but I'm not sure I'm checking it correctly. Photo here and a diagram to make it a bit clearer what I'm looking at. 2 blue wires coming out of the transformer to the fuses prior to the PCB appear to have continuity.. that would be the secondary winding's? On the other side both blue wires are soldered to the switch. Should I un solder and test between those 2?

[BigDaddy]

Perfect. Check each secondary to the center tap (black wire) - IE One probe on a blue wire and the other on the black. Check for continuity and also measure the resistance. It would be best to unsolder the switch to check the primaries - Those you can just put the probes onto both wires as there is no center tap. If they are all good, we can move onto reassembly and taking voltage measurements.

It would be great to try to get a schematic to help troubleshoot - I had a quick search on the net but couldn't find one for that model. The other thing is to check the indicator light bulb - that may be buggered and there is actually HV present off the transformer with the problem lying downstream.

[Sonic Mountain]

Yeah I wondered about the indicator light. It's actually contained in the switch itself, so not sure how to go about testing. Perhaps un-solder the switch and put some temporary power from a battery to check it?

I've had no luck finding a schematic. I've sent a request off to Ashton but I have my doubts they'll be very helpful.

Will report back on windings once I have a chance to look at it better.

[BigDaddy]

Goodo - also can you read the values on that blue 'cap' across the switch? I can't make it out in your pics. It may actually be a thermistor as it looks like one to me. It's in the right spot for one too.

[Marcel]

Just like to point out that the Blue disc on the back of the power switch is NOT a cap.... It is a MOV surge suppressor. A MOV (Metal Oxide Varistor) is a voltage surge suppression device. It limits voltage spikes to manageable pre-determined voltages. A 275V MOV will limit incoming voltage spikes to 275V which is pretty reasonable on a 240VAC supply. Main reason they are installed is when little sister switches on/off her hair dryer in the bathroom little Jonny's amp in the bedroom next door doesn't cope a voltage spike and blow up.... Pretty much mandatory for solid state amps and amps with fussy solid state bits inside, never fitted to real tube only amps.

Those diodes that have discoloration on the board around them and big resistors beside them are most likely Zenner diodes being used to regulate the +ve and -ve DC voltage rails. When powered up a multimeter can be used to see if they are okay by measuring the voltage across them. They should be similar voltages of opposite polarity.... i.e. Both would typically be 18VDC or 24VDC or 15VDC or whatever with the -ve on the side with the dark band around it. The actual voltage is a design thing by Ashton, but both should be the same. There will be large caps before and after. The power chain typically goes - transformer, rectifier, basic filter DC regulation caps, Zener stage, fine DC regulation filter caps, pre-amp stages. The power amp will usually take its power off the basic filter DC regulation caps.

That's my 2c so far...

[Sonic Mountain]

Cheers Marcel.

Good to know re surge suppressor.

I haven't had any time to investigate further...also not sure if I should poke around in it while powered up. I can take all the precautions mentioned in this thread and still do something dumb.....

[BigDaddy]

Now I've worked out how to resize and post pics, here's a pic of my capacitor bleeder that I use for my amp building and repairs. You can get those bits at Jaycar. Having one like this is great as you can check and discharge in one step.



100K resistor across x2 banana plugs into the meter. Both leads are alligator type on the ends. I put a small strip of timber in between both plugs to get the spacing right and superglued the lot together. The cable tie was to pull it all together.

[Marcel]

Now I've worked out how to resize and post pics, here's a pic of my capacitor bleeder that I use for my amp building and repairs. You can get those bits at Jaycar. Having one like this is great as you can check and discharge in one step.



100K resistor across x2 banana plugs into the meter. Both leads are alligator type on the ends. I put a small strip of timber in between both plugs to get the spacing right and superglued the lot together. The cable tie was to pull it all together.

That's a pretty neat idea.... I like it a lot...

[DrNomis_44]

You might be able to find a service manual on the internet for this amp.