Function generator repair

I have TTi TG330 function generator that I use every now and then. It is fairly nice device, capable to frequencies from 0.003 Hz to 3 MHz in sinusoidal, trapezoid or square waveforms, including sweeps and 120MHz counter. I do not need it every day or even every week but when I do, it is worth every single cent of its price.

Unfortunately it broke a while back. Turning it on did absolutely nothing. I checked the main fuse but no, it was okay. So time to take it apart and see what is going on then.

Its a bit difficult to tell from the picture but the top cover has turned slightly yellowish near rear. Generator is fairly warm during use so that isn't exactly surprising.

Covers are held by simple plastic locking tabs instead of screws. Take these off from corners and cover comes handily off.

The insides. Note that generator is shown here upside down; the board seen on bottom is actually towards the top of the cover in normal orientation. I'm not exactly happy about that large and heavy transformer hanging in there during normal use.

All the capacitors seem to be okay, so it was time to start probing the board. It did not take long to find out that fuses, circled in picture below (marked FS1 and FS2) both seem to be blown. Weird, considering that main fuse (earlier in power supply chain) was intact.

These fuses are on secondary side of transformer so there should not be any (very) high voltages there, but one still must be very very careful when probing around these devices. Even when powered off there might be dangerous voltages around.

I could not find schematic for this model, but I did find schematic for model 550 which, while different, suggested that these fuses might be 500mA (if my memory serves, this was few weeks back now). Not something I had handy, but a quick trip to Sp-Elektroniikka (local parts store) I was able to acquire suitable SMD replacements. The space is a bit cramped but I was still able to replace these fuses with little trouble.

Since the original fuses were both blown I was a bit suspicious if this would work; if there is some issue further down the chain, new fuses might blow again immediately.

They didn't, and generator seems to be working fine again.

So I put it back together and it is serving me well again. No idea why those fuses have blown, but as long as it doesn't happen again I am happy. Although I do not keep the generator on 24/7 anymore.