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-   -   Depressed Amplifier Chassis - K5881 (http://www.world-designs.co.uk/forum/showthread.php?t=8029)

gbjbatlh 7th June 2021 05:19 PM

Depressed Amplifier Chassis - K5881
 
Hello All - I have recently bought a K5881 with the central, power supply transformer sitting at a "jaunty angle". I suspect at sometime in the life of the amplifier a friendly courier has put down (dropped) the amplifier with no care of the consequences. Result? - the considerable weight of the power supply transformer has deformed to the top plate of the chassis causing it to sit at an angle. All seems to operate as normal, this is not a new injury.
My question(s) - the best way to straighten the top plate and any recommendations on how to strengthen the top plate? OR - should I leave it be?
Grateful thanks for any suggestions, recommendations and help.

Dave999 8th June 2021 09:26 AM

Re: Depressed Amplifier Chassis - K5881
 
eminently fixable but in theory you will need to remove most of the internals

you can leave sockets and wireing if they are out of the way of where you need to work just get the txs and the boards out

next find a workshop with hydraulic press

amp face down on rubber mat, use the press with a flat round foot on the plunger. a square one might mark at the corners

press the chassis back to where it should be

any workshop that does automotive repair will have a floor mount or bench mounted press. good for a couple of tonnes

this works on chrome bumpers off 60s/ 70s cars. so should work on your chassis

other tempting option is to try to lever it with a pri-bar or screw driver... this invariable ends in tears as the section by the lever deforms and stretches. The bit you lever against and the bit you are trying to correct both bend
you end up with the middle of the low section too high and the sides of the low section 50% higher than they were but still too low and not right.

the chassis is a bit thick for panel beating and that tends to damage paint.

other option uses a different type of press in reverse and you pull the low section by its mounting holes. again this tends to get the mounting holes back to where they should be but it leaves the rest behind, usually used for getting car door hinge mounts back to where they should be after a crash and before panel work is re done....pressing is best.

home method

flat bench bolted to floor or with something exceedingly heavy on it or held down by timber fence post between it and a window lintel or roof joist Be careful do not jack the roof off you garage.

support amp chasssis from the inside, on specially cut section of kitchen work bench on the cup of a trolley jack
jack the kinked bit into the base of the bench
the flat kitchen bench is usually robust enough to apply pressure over a wide enough area

used his method to straighten the rear of a lambretta that had been stacked vertically on its rear/number plate with many others in a van for its trip from spain or italy into the UK.
had to build a wooden fixture to jack against,bolted to engine mount, moved the rear back again and in line by 2-3 inches, side panels now fit.

proper press is probably best

Dave

Pingushome 8th June 2021 10:28 AM

Re: Depressed Amplifier Chassis - K5881
 
Hi,
As Dave says you will have to remove TXs and internal bits and clamp firmly to a large block of wood. The chassis is thick metal so a small club hammer and a metal rod or steel bar (to make it easier to work inside the chassis) will be needed but you may be able to gently smooth out the bent bit from the inside.
It won’t be pretty and will need a repaint probably but if you can’t use a press this is the next best option.
Personally I’d strip it all out and go with the hydraulic press from the local corner garage (nearly all garages have one to replace bearings and bushes) as Dave suggests. With luck and a bit of a skilled user it’ll be almost as good as new with little or no paint loss either.
If you need build notes you can get then thro the forum, just ask.
Martin

bob orbell 8th June 2021 01:35 PM

Re: Depressed Amplifier Chassis - K5881
 
Martin, stick to what you now best, because metal work is obviously not your forte, sorry to be blunt but, a press will do nowt, the only way is to replace. Bob

Pingushome 8th June 2021 04:22 PM

Re: Depressed Amplifier Chassis - K5881
 
Bob youíve not seen some of the metal benders I have, trust me 3 or 4 tons on a press will get it more or less straight. He only wants to level up the TX. Iíve had them straighten out thick wrought iron gates (3/16 bar). Far beyond my skill tho
All those misspent years watching the cut and shut merchants straightening up bent chassis back in my teenage years, all highly illegal now tho.
Always worth a try before buying a new bit, after all nothing to loose
But I totally agree my metal bending skills are very minimal limited hammer and dolly
Martin

John Caswell 8th June 2021 04:42 PM

Re: Depressed Amplifier Chassis - K5881
 
It might even be cheaper to get a new aluminium chassis made and painted a la WD88VA. But I second the visit to car bodyshop some of them are surprisingly helpful and relatively low cost.

John

bob orbell 8th June 2021 06:19 PM

Re: Depressed Amplifier Chassis - K5881
 
Yes guy's, if it was Aluminium it could be pressed straight for the simple reason Aluminium has very little spring retention, steel will simply return to its present position after squashing even with 1000 tones pressure, to get this back straight one must use the equal and opposite procedure and this is impossible as the bent sides will not allow this, as John said, it will be cheaper to have one made from Aluminium with all the benefits that this material brings. Bob

Greg. 8th June 2021 09:53 PM

Re: Depressed Amplifier Chassis - K5881
 
Bob is right. Pressing will not work. With 1.5mm mild steel, it is unlikely any repair effort will be completely satisfactory.

A newly made aluminium copy would be good but I expect it would be cost prohibitive for a one off order.

What about retaining the existing chassis but cut out the damaged top plate, leaving a lip around the edges to allow fixing of a new cut and painted top plate? Depending on capability, this could be a DIY project or, it could be relatively affordable to contract a fabricator to do it for you. Well worth considering IMHO. Personally I could not hack living with a functioning amp that has a wonky visual appearance. Iíd need to correct that.

gbjbatlh 9th June 2021 10:14 AM

Re: Depressed Amplifier Chassis - K5881
 
2 Attachment(s)
Many thanks to all for your suggested chassis straightening remedies, they are all worthy of consideration. I have included two pictures of the offending transformer - hard to capture and does not show the depression in the top plate that can be felt with finger tips.

It's occurred to me an alternative solution would be to "hide" the transformer with a full "safety cage" with a foot-print the size of the top plate. This would leave the working amplifier as-is and add a safety feature. Has anyone experience of building a safety cage for any amplifier that would assist the design/build process?

Richard 9th June 2021 10:53 AM

Re: Depressed Amplifier Chassis - K5881
 
Surely the answer is "it depends" on how damaged the chassis is. If it's a gentle smooth deformation with the TX just looking a bit wonky there's every chance it can be straightened. If the edges of the TX have creased the top plate it would be much harder and need refinishing and repainting after straightening. Can you post a pic?


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