Yaesu FT-902DM "Restoration" Part Two

Hi,

After a little testing everything tested out pretty well, RX was sensitive and working well, all the controls worked nicely with no "scratchiness" at all or needing to "rock" switches to get them to work.

Anyone who restores or repairs older transceivers will recognise these problems ! - usually quite easy to fix with the appropriate specialised cleaners - Corrosion X - I first starting to use this when I noticed that the great Kenwood restoration guru Jim, KE7OAY used it - it is really very good!) put it in a dropper dispenser and it lasts for ever and DeOxit - I recommend not getting an aerosol but getting a dropper bottle or a 'pump action' bottle - a lot easier to user and doesn't 'spray' everywhere - wasting what is a very expensive liquid !

I'll use a combination of these on the switches and pots when I clean them up - I do when I'm restoring others equipment, as a preventative and restorative measure.

So after checking I decided to replace the bias and HT electrolytic's everything checked out OK after slowly bringing up the voltage (see previous post) but these components are over  30 years old so replacement with newer, better tolerance 105deg components with better voltage ratings where available will help to preserve the radio for another 30 years hopefully !!

So after taking precautions to make sure that the radio (and its electrolytics!!) were fully discharged and the transceiver not plugged into the mains - we begin......

So board PB1717 (Rectifier C Board) first -

Before Capacitor change

Having a really close look at this board - overall its in good condition. There was no real worries about the capacitors, although the middle capacitor on the lower row (you can just see it in this picture) did look a little discolored compared to the others, no sign of stress such as bulging sides or tops.

Other components looked in good condition, although as always with this age of Yaesu equipment (or at least the ones I've worked on) the components seem to have been inserted in the board by someone who wanted to get home early on a Friday afternoon, they are not vertical and overall pretty poorly placed.

You might think a little OCD (?) but actually vertical components help heat flow and aid reliability so I usually (very carefully!) align them.
  




So on to removing the capacitors. Yaesu put these capacitors in with a small amount of 'glue' around each one as the radio could be used mobile (!) and it was an attempt to stop vibration based failure.

My approach is to use my desolder station (a god send - if you regularly repair or restore get a decent one!) to remove the capacitors. I do one at a time and start with the big ones.

removed electrolytics

Not quite sure that the photograph (left) shows it but the capacitor on the right of the photo has a very slight 'bulge' on its base, the left hand one is flat, so perhaps just as well I took the trouble to power the transceiver up carefully and I'm replacing the capacitors !!

The white / yellow stuff is the glue that Yaesu used. The trick to getting the capacitors out of this stuff is to desolder the legs carefully from the PCB and check that the legs are free, and then carefully - very carefully gentle rock the capacitor back and forth - don't force it - you may crack or damage the PCB :( 




Just for 'fun' I thought i would check the ESR and capacitance using my Peak ESR+ tester (another great bit of kit!) - 

As you can see the ESR is .49 ohms and its capacity is just over 28uF - not too bad for a 30 + year old capacitor !!!

But I'm replacing it anyway.....

So here is a comparison to the replacement capacitor as you can see its value is pretty much spot on and the ESR is slightly better at .47ohms.

The more observant amongst you will notice also that it is a higher voltage rating - 400V rather than 350V.

If possible (without going mad) I like to up the voltage rating a little, and of course improve the temperature rating to +105deg - not sure what the original ones were but most probably +85deg. This will again help aid reliability and longevity


I wont bore you with lots of pictures of removed capacitors etc.

Remember to clean the 'glue' residue off the board before mounting the new capacitors, I use a combination of a PCB 'tooth' pick tool and a little PCB cleaner - works well.

The board has the polarity of the capacitors marked with + and - , so be sure to follow those correctly, if your thinking of doing this - you'll know that the white stripe (with the - marking!) points to the minus lead - this is also the shorter of the two leads (but don't always rely on that - sometime they are trimmed incorrectly!!)

So what was the result for the Rectifier C board ?

Finished board

Again the more observant will note that I haven't change the axial capacitor at the right of the board - simple reason is that I didn't have one in stock ! - I checked it using the peak ESR+ meter and its ESR and capacitance was fine so I decided to leave it. I've order some new ones and when they arrive I'll replace it.

On a final note - always use the best capacitors you can find, a brand name like Rubycon, Nichicon (which I try to use) etc. - others may disagree

A word of caution though - use a reputable supplier - they're are fakes out there and eBay is awash with 'good value' components. I only purchase through RS Components - they might be a little more expensive (in some cases they are a lot cheaper!) but the peace of mind is priceless - especially if your doing the work for someone else !!

Now onto the next board ........