Does anyone know cap values for a HEQ5 motherboard? :(

[Joseki]

Hi, it's been a while since I started a thread but I was hoping someone might be able to help me try and fix my HEQ5 motherboard.

I guess it's likely I'll have to buy another motherboard but I would like to try and fix it.  - this wasn't caused by reversed polarity btw as far as I know and even after I heard a loud 'pop' the mount was able to slew, so I have hope (but then smoke started coming out of the mount and I realised it couldn't have been my noisy neighbour after all ).

I took the mount to pieces and one of the capacitors had exploded ('A' in the photo), another one (labeled 'B' in the photo below) looked like I should replace it too.

Replacing capacitor 'B' should be easy enough but there's nothing left of 'A' to know what capacity to use.  - the plastic cover of the capacitor had melted.  I do know it's larger than the small caps (which are 47 uF) and smaller than the two medium sized caps - 470 uF.

from the opticstar website http://www.opticstar.com/Run/Astronomy/Astro-Accessories-Mounts-Skywatcher.asp?p=0_10_5_2_0_60

I can see it's the only capacitor on the board that there is only one of, other than the massive one (1000 uF).

So, a bit of a long shot but does anyone have any idea what value cap I could try?  At this stage I'm quite ok risking the board on 'best guesses' and 'hunches' - I take full responsibility for this one  I may end up guessing based on the know dimensions and capacities of other caps on the board, looking at published component specs on the web and scaling (since different brands of caps apparently have different dimensions - to - capacity relationships).  The rubber base of the blown capacitor is still attached and may give some indication of dimensions, allowing for a bit of expansion 'post explosion'.

btw, this is an upgrade motherboard for a 2nd hand HEQ5 I bought.  I got the upgrade kit from someone selling on a kit that they had ordered in by mistake so no-one that has them in stock has any responsibility to help me.  

Why it blew, I'm not sure but my best guess is that I did it with my diy power tank.  - a deep cycle leisure battery in a tool box.  I was getting dodgy behaviour and lost connections with the handset so I added a dc - dc step up converter circuit to step up to 13.8v  http://www.ebay.co.uk/itm/2Pcs-XL6009-DC-DC-Adjustable-Step-up-boost-Power-Converter-Module-Replace-LM2577-/271532707165?pt=LH_DefaultDomain_3&hash=item3f389c4d5d

- which worked brilliantly until this happened.  It might have gotten damaged last week though when I dropped the powertank on my toe   - it seemed to be okay but when the mount board blew this dc-dc circuit had also blown up. - all with fuses and decent wiring btw.  - Next time I'll keep it more simple.  

At least it's given me the motivation to work on my dob mount.

[Joseki]

hmm. just noticed some caps on the board are rated 35v, some 25v and some 16v which probably complicates the whole make a guess approach.

p.s. just reading up on capacitors it seems it should be okay as long as the voltage rating is larger than the voltage I'm running the circuit on, e.g just go for 25v and I should be fine.

[leelee970]

the voltage rating is just the max at which the capacitor can operate at - it also means the physical size can increase.

so if say 22uF 16V needed replacing then you could replace it with a 22uF 35V - as long as it could physically fit on the board and inside the casing.

Lee

[leelee970]

one thing to remember when replacing an electrolytic capacitor is to make a note of its polarity on the board. you don't want to reverse it.

Lee 

[faulksy]

dont worry about capity size. as you say if its 35v then thats the minimum you need, if you exceed that value it will be better, as you no capicitors are rated in farhads so the same again, if you exceed that capacitance or physical size it will be better. the reason if they are charged at say 35v it can only recieve 35v so no problem.

[Dr_Ju_ju]

Hi John,

If they will fit, use 35v caps, also if possible get s reputable manufacturer e.g. Panasonic, Rubycon etc.

Also you state that you step up the source voltage to 13.8, why ?  as this will be used as the source for the motors, that probably means that they would attempt to take more current, which would have knock-on effects on the drivers & their power sources .....

[leelee970]

You could also give Bernard a call - Modern Astronomy. He is so helpful and may be able to help.

Lee

[Joseki]

Thanks for the advice - I'm a bit of a dunce with electronics, but dumb enough to meddle..  I've been trying to narrow it down using the Farnell website, filtering it on diameter, operating temperature and pin spacing, assuming the dimensions are about the same as Panasonic ones.  Best guess from that is 68uF or 100uF.  I'll see if Bernard can help, thanks Lee.

The reason I started mucking around with step up circuits was when I just used the leisure battery the mount would sometimes just stall/stop responding and slew off to random locations during the 3 star alignment stage. Some suggestions on this forum of people with similar problems was that it was the power supply not being a clean 13.8v so I thought this might fix it.

[Joseki]

dont worry about capity size. as you say if its 35v then thats the minimum you need, if you exceed that value it will be better, as you no capicitors are rated in farhads so the same again, if you exceed that capacitance or physical size it will be better. the reason if they are charged at say 35v it can only recieve 35v so no problem.

Hi faulksy, I just read your post again, so you're saying I could just use a 470uF capacitor? (I think it's safe to say 470 uF will exceed the capacity of the blown component).  There should be plenty of space around that component and I'll probably have to buy a pack of 10 anyway. 

edit: (looks online), yes it seems you can do it generally, thanks for that advice.  Maybe I should replace the other 16v cap whilst I'm at it. 

[Lonestar70]

Hi Joseki,

Just had a look in my HEQ5 and it is fitted with a 220uF 40v dc capacitor.

This one would do you as a replacement: -

http://au.rs-online.com/web/p/aluminium-capacitors/8621719/

It is 50v working and is a high ripple component... ideal for use in switching motor drivers.

The 470uF ones should also be replaced as at least one of them is showing signs of blowing.

Use any voltage above 16v... a 470uF from the same range as the above would be ideal.

Running the mount at 13.8v would not cause this... I suspect the step up converter... which can output a lot higher voltage when not under load.

A fully charged 12volt battery will be at 13.8v and the mount needs between 11v and 15v according to the manual.

In practice most S.W mounts need a minimum of 12.5v or things start to get unreliable... I run mine at 13.8v from a Maplin stabilised mains power supply. (see FLO)

Hope this helps.

Best regards.

Sandy.

[Joseki]

Thanks Sandy, I wasn't expecting anyone to go to the trouble of opening up their mount, that's really nice of you.  

I think you're right about the step up circuit.  The leisure batteries start out around 13v when fully charged so perhaps I was just unlucky before.  I do have one of those 13.8v bench supplies but no obs & I don't like running an extension cable out the back door.  I might have to get an external power socket put in for my back garden - it'd be cheaper than buying replacement motherboards..

cheers,

John

[Joseki]

Hi, I just wanted to say that thanks to everyones help I've just finished replacing those two caps.  A bit ugly, but I put it all back together and it all works very nicely happy bunny here.

[Lonestar70]

Hi, I just wanted to say that thanks to everyones help I've just finished replacing those two caps.  A bit ugly, but I put it all back together and it all works very nicely happy bunny here.

Hi John,

That's good to hear and I am happy to have been of some small assistance... it is, after all, what this place is all about.

Clear Skies.

Sandy.

[leelee970]

Fantastic news John.

Lee

[faulksy]

Hi, I just wanted to say that thanks to everyones help I've just finished replacing those two caps.  A bit ugly, but I put it all back together and it all works very nicely happy bunny here.

thats brilliant news, was just about to add make sure you get the polarity right but you obviously new that. great result