MTX Road Thunder RT-404 help

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Hey folks,

Okay..firstly I admit I think the issue I have is totally my own doing :rolleyes:, but could really do with some assistance.
Im reasonable at electronics, and understand the principles, but its been a long while since I've dabbled at fault finding and I don't have a scope either which makes it a bit harder. willing to give it a go and learn as I go :D

I've had a solid-as-a-rock MTX RT-404, 4 ch amp installed for a few years and its been as good as gold.
Recently I noticed that only 2 or 3 of the speakers were working.
Subsequently I found that by turning the gain adjustment, the speakers all worked again.
So, this weekend, I figured a quick clean/blast with some circuit board cleaner and re-soldering any dodgy/dry joints.

So..cleaned, re-soldered and went to put it back together...looked at the FETs and rectifiers and the heat transfer strip looked past in a flurry of activity I cleaned and removed the strip and re-applied some heat transfer PASTE (not the strip).
Now, I know this was a bad thing to do (TO-220 package and the tag is connected to the base(?)), but unfortunately I only remembered this after it blew the 2 x 25A fuses :eek:

So, where I am now with it after grabbing my multi-meter.

It looks in good condition. No burnt tracks or fused legs Ive seen on some of the posts. I think my saving grace was the short to earth was so clean (via the TO-220 case) that it popped the fuses and rectifier pretty quickly before some of the other components were damaged/heated up sufficiently to get damaged!

My initial diagnosis is that it appears the rectifier is shorted (FEP16DTA).
Putting the meter on diode setting, all combinations show 0v (tested on another diode and I can get the 0.4V forward, and 0V reverse).
So replace this I think?

Secondly, the second rectifier (U1620 I think), seems to be a 'ultra-fast' rectifier. It has A K A on the package.
I get 0V between pins 1 and 3 (A -A)
and ~0.4 with COM (black) on pin 2 and V (red) on 1 and the same for 3 (I think this looks OK)?

Finally all the FET's read the same, so they are ALL either OK or ALL blown.
As an example FET201 and FET202 (My knowledge is limited, but I assume theres an output FET and another one?)

Pins 1 & 2 = 0.5K and rising (longer I leave it on there the higher it reads)
Pins 2 & 3 = As above
Pins 1 & 3 = 0.22K (steady)

FET202: (assume this is the O/P?)
Pins 1 & 2 = 0.424K (steady)
Pins 2 & 3 = 0.2K (steady)
Pins 1 & 3 = 0.22K (steady)

While I remember, there are 2 large caps on the board (4700uF @ 25V), and they look OK, but the top is domed. My experience to date has been that this indicates that theyve gotten hot at some point. Worth replacing if its cheap and easy to do?

I can post a few pics if its useful, but hopefully Ive done some basic fault finding (have read Perry's posts/website for some good tips) to give someone an idea of if Im on the right tracks.

BTW, not that it makes much difference, but Im in Australia, so dont have some of the same resources that the US and UK does. We do have RS components, but already tried looking for the 2 rectifiers and no results....eBay may be my only option.

Anyway, appreciate any help people can give (and sorry for the long post!):)


Many thanks for the reply Perry.

Hopefully theres enough info in my first post to make diagnosis easy(ier)?

One quick question, the FEP16DTA appears to be 'glass passivated' whereas the more readily available FEP16DT isnt.

What would be the effect of using the non-glass passivated version?
I read that it means less leakage, but real world..what does it manifest itself as? (background noise/hum/amp not working 100% efficiently?)

Once again, many many thanks (soldering iron at the ready!)

The glass passification is part of the production process and isn't important. The DT isn't the same as the DTA. If you can't find the DTA, use the MUR1620CTR.


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