Ashton Amp Repair Advice.

Hey electrical gurus!

So I've been given a broken amp. It just an Ashton GA50, nothing fancy. They guy who had it was complaining about scratchy pots and intermittent buzzing. I suggested some contact cleaner in the pots might be a good place to start and did say to bring it around and I'd do it for him.

He decided to have a crack himself. He gave them a spray and blew them out and left it all to dry. Plugged it in and had significant improvement.

Getting excited he started spraying everything again, including the power switch..... while it was still plugged in.

Buzz..pop!

So anyway, he's not bothered as it was just a spare one and he realises his mistake, but he's given it to me. If I fix it I can have it otherwise he said dump it.

I actually have another amp I could quite easily transplant into this box (which is better than the current one it's in) so if it's a write off, I can still make use of it.

I am really out of my depth with this kind of electronics. I've done some very basic stuff before with PCB's and I can solder fine, I get the basic concepts but I really don't know where to start here.

I think this is the correct schematic - it's very close at least from what I can tell. The fuses appear to be fine, they are like small old school automotive glass ones. I will double check with my multimeter.

http://schems.com/bmampscom/ashton/ashton_vp50.pdf

and what I mainly suspect is the two diodes in this picture - they are involved somehow in the power circuit and they do look a bit discoloured.

https://i.imgur.com/irgQZ8c.jpg

There is also what I think is a capacitor on the switch. I'm not sure why it would be there.. to make the light fade slowly when you turn it off?

https://i.imgur.com/E8joYxN.jpg

Anyway. Any advice or direction welcome. I am leaning toward just modding it with the other amp parts and making a franken amp. But maybe someone will have a good idea of what it needs......

Hey mate - I've built 5 valve amps from scratch following vintage schematics though.... no IC's or transistors like this one. If you are handy with a multimeter and understand the safety procedures, you can quite easily troubleshoot it. Safety is paramount here as you will be dealing with 240v AC on those switch terminals and somewhere around 600vdc on the secondaries of the power transformer. Only ever probe with one hand. Use an aligator clip from your meter negative to the chassis and keep the spare hand in your pocket.

Can you provide a bit more information on the current symptoms since he sprayed the switch? Was it the on/off or standby? Does it power up? The diodes look ok to me in that pic. The discolouration on the board will be from those 2W resistors. Anyway, if you can provide a bit more info, it should be fairly easy to work through.

Yeah I am very wary of mucking about at this voltage level. Basically it’s dead as a door nail. No sound no lights.

I don’t know that it has a standby circuit. It just has the one on/off switch on the fascia and a push switch to change channels.

Quote:

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Originally Posted by Sonic Mountain

Yeah I am very wary of mucking about at this voltage level. Basically it’s dead as a door nail. No sound no lights.

I don’t know that it has a standby circuit. It just has the one on/off switch on the fascia and a push switch to change channels.

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No sound no lights indicates no power do the heaters light up

The diodes are just downstream from the standby switch in the circuit you linked and if you say there's no standby switch then it's not the same circuit and the cap on the power switch will be there to reduce arcing when the switch is thrown

Marcel will almost certainly provide more info

I'm not an amp guy, and there was brief comment on safety from BigDaddy, but no one has specifically mentioned discharging each of the electrolytic caps in there before carrying out any trouble shooting & repair.

I hope you've done this Sonic.

Thanks guys...

Quote:

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Originally Posted by McCreed

I'm not an amp guy, and there was brief comment on safety from BigDaddy, but no one has specifically mentioned discharging each of the electrolytic caps in there before carrying out any trouble shooting & repair.

I hope you've done this Sonic.

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Haven't done anything more than turn it on and look for signs of life and have a look at the fuses. It came pre-dissasembled. I will have a look at doing this though - just had a bit of a read and seems important for Tube amps.. not sure on this, there don't appear to be any large capacitors, but better safe than sorry. Definitely a case of me knowing just enough to be dangerous :D

Quote:

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Originally Posted by dave.king1

No sound no lights indicates no power do the heaters light up

The diodes are just downstream from the standby switch in the circuit you linked and if you say there's no standby switch then it's not the same circuit and the cap on the power switch will be there to reduce arcing when the switch is thrown

Marcel will almost certainly provide more info

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Not sure how to identify 'heaters'? It's a solid state amp, not sure if that makes a difference.

Could well be the wrong diagram. I'll see what else I can find.

EDIT: Ah yes, I'm a dummy. That diagram is for a Valve amp. This one is definitely solid state.

Quote:

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Originally Posted by Sonic Mountain

EDIT: Ah yes, I'm a dummy. That diagram is for a Valve amp. This one is definitely solid state.

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Yep that was my thinking, the diagram was a hybrid beast so not valid

The user Valvehead in this post appears to have the Ashton schematics or at least some of them

https://www.guitargear.net.au/discus...?topic=40901.0

Ashton themselves may help

http://ashtonmusic.com/contact-2

Yeah I came across that thread as well. I'm guessing that my lack of any kind of qualification in this area will result in a big no. But I'll see, I've sent a request off.

Quote:

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...just had a bit of a read and seems important for Tube amps.. not sure on this, there don't appear to be any large capacitors...

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That schematic you linked shows 12AX7 & EL34 tubes. (???)

Yep - wrong schematic.

There will still be filter capacitors for the power supply which can poke you. Once identified, you need to check them for voltage and discharge if there is anything present as per McCreed's comment. Can you take a few wider pics of the guts? I assume there is a transformer in there....that may have fried on the short (are you sure all the fuses are OK?)

It may be as simple as replacing the switch. The contacts may have copped it when it shorted.

Here's some more pics. Just tried to get the whole board in sections. Let me know if you want to see something specific up closer.

Yeah I'll go over and make sure they are discharged before I do anything. This is exactly why I posted, I probabyl wouldn't have thought of that. I assume the two large black caps at the end of the board there would be prime suspects for discharging?

https://i.imgur.com/JyX0VBm.jpg

Yeah, built in transformer.

The fuses appear to be intact, but I will confirm that after work with the multimeter.

https://i.imgur.com/9yGhsV3.jpg

It'd be great if it just needed a switch replacement. That's the kind of achievable repair I was hoping for. I guess I just look for continuity when it's in the 'on' position?

https://i.imgur.com/EvwOygP.jpg
https://i.imgur.com/7zdmNlO.jpg
https://i.imgur.com/hs6VEfy.jpg
https://i.imgur.com/JoulsWk.jpg
https://i.imgur.com/dWknNt6.jpg
https://i.imgur.com/nzVLZdc.jpg
https://i.imgur.com/hubqcsC.jpg

Quote:

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Originally Posted by Sonic Mountain

Here's some more pics. Just tried to get the whole board in sections. Let me know if you want to see something specific up closer.

Yeah I'll go over and make sure they are discharged before I do anything. This is exactly why I posted, I probabyl wouldn't have thought of that. I assume the two large black caps at the end of the board there would be prime suspects for discharging?

https://i.imgur.com/JyX0VBm.jpg

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The filter caps are the ones that hold the big charges and at far higher voltages in a valve amp

Ten Commandments of Electrical Safety
(1) Beware of the lightning that lurks in an undischarged capacitor lest it cause thee to be bounced upon thy backside in a most ungainly manner.

(2) Cause thou the switch that supplies large quantities of juice to be opened and thusly tagged, so thy days may be long on this earthly vale of tears.

(3) Prove to thyself that all circuits that radiateth and upon which thou worketh are grounded lest they lift thee to high-frequency potential and cause thee to radiate also.

(4) Take care thou useth the proper method when thou taketh the measure of high-voltage circuits so that thou doth not incinerate both thee and the meter, for verily though thou hast no account number and can be easily replaced, the meter doth have one and as a consequence bringeth much woe upon the supply department.

(5)Tarry thee not amongst those who engage in intentional shocks for they are surely non-believers and are not long for this world.

(6) Take care thou tampereth not with interlocks and safety devices, for this incureth the wrath of thy seniors and bringeth the fury of the safety officer down upon thy head and shoulders.

(7) Work thee not on energized equipment, for if thou doeth, thy mates will surely be buying lunch without thee and thy space at the table will be filled by another.

(8) Verily, verily I say unto thee, never service high-voltage equipment alone, for electric cooking is a slothful process, and thou might sizzle in thy own fat for hours on end before thy Maker sees fit to end thy misery and drag thee into His fold.

(9) Trifle thee not with radioactive tubes and substances lest thou commence to glow in the dark like a lightning bug.

(10) Commit thee to memory the works of the prophets, which are written in the instruction books, which giveth the straight info and which consoleth thee, and thou cannot make mistakes.

Good pics. The heat shrinked covered bit in your 2nd pic may also be an in-line fuse. It appears to sit between the IEC power socket and the switch. Have a close look in there too. Those 2 big caps are the filter caps. The leads are underneath so u would have to lift the board to access them.

Regarding the switch, you can check that with your meter.

Hey BD.. do you mean this black block circled in red?

https://i.imgur.com/HF8ht17.jpg


That's a rubber cover that goes on the back of the switch.

Will discharge the caps and should get a chance to confirm the switch and fuses in a little while

Quote:

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Originally Posted by Sonic Mountain

https://i.imgur.com/JyX0VBm.jpg

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Be careful of the x2 transistors when you are lifting the board (see pic) - these are those black square looking things screwed to the aluminium heat sink. Q3 designates one of them in your pic. They will likely have white heat sink compound behind them to help couple them to the aluminium plate. Don't wipe that off.

No worries re that cover. I thought it may have been an inline fuse. Let's hope it is the switch.

Silver heatsink paste is available at Jaycar for a pittance if you need it.

I actually have a tube already from doing points to electronic ignition conversions on my various Vals.

OK so a bit of progress. The fuses and switch all appear fine according to the multimeter, the fuses have continuity and the switch only does when its in the 'on' position.

Will have to wait for a trip to Jaycar to get some bits and bobs. I'm going to make a little circuit with a lamp in it to discharge the caps with.

Very carefully removed the board so some more pics. Some definite discolouration in one spot.....

https://i.imgur.com/4dm4XxA.jpg

https://i.imgur.com/Cr66nLL.jpg

https://i.imgur.com/yF5HlcE.jpg

https://i.imgur.com/nKAaX7X.jpg

Quote:

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Originally Posted by Sonic Mountain

Some definite discolouration in one spot.....

https://i.imgur.com/yF5HlcE.jpg

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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

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.

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

Quote:

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Originally Posted by BigDaddy

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

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

Quote:

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Originally Posted by dave.king1

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

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

Quote:

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Originally Posted by BigDaddy

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.

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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?

https://i.imgur.com/r6Cg1V2.jpg

https://i.imgur.com/WWdmLtf.jpg

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.

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.

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.

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...

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.....

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.

Attachment 31945

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.

Quote:

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Originally Posted by 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.

Attachment 31945

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.

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

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