help with SS amp repair (Fender Stage 1600)

[HTH]

ok, I got this Fender Stage 1600 combo to repair - it's my brother's flat mate's amp. I usually never touch solid state gear, but I'm attempting this as a favour but have really no idea where to start.

I've tried powering it up and it's totally dead (speakers are fine btw). There are no external fuses, but I'm assuming there will be some internally (will check those first).

However, assuming that there will be blown fuses there must be a cause for this. I'm thinking that the output mosfets have blown or have caused the amp to stop working by overheating and blowing a fuse (this appears to be a common fault on SS guitar amps, right?)

Anyone?

[52 Bill]

Start by downloading the schematic from the Fender support site.

The common problem with those amps is shorted power transistors. They use plastic cased Darlingtons, probably TIP142 and TIP147.

Open it up and look for a blown fuse. If it is blown check the outputs for shorts and proceed from there. The outputs are wired in parallel so if one is shorted they all may read bad, until you remove the bad one(s).

[HTH]

Start by downloading the schematic from the Fender support site.

The common problem with those amps is shorted power transistors. They use plastic cased Darlingtons, probably TIP142 and TIP147.

Open it up and look for a blown fuse. If it is blown check the outputs for shorts and proceed from there. The outputs are wired in parallel so if one is shorted they all may read bad, until you remove the bad one(s).

Is it usually a case of the power transistors running too close to max spec, or poor heatsinking? Any suggestions for a beefed-up replacement for the TIP142 and TIP147?

[R.G.]

1. Make yourself a light bulb limiter. This places an ordinary incandescent lamp in series with the AC power line to the amp. It can be used to prevent the amp from instantly blowing fuses and thereby preventing you from measuring anything.
2. Test the output devices for shorts. If even one is shorted, assume that every single semiconductor in the power amp section may be damaged.
3. Get the power supply working correctly first. Disconnect the power amp from it and make sure the power amp is operating right. NOTHING works if the power supply isn't right.
4. Then get the power amp working correctly. The light bulb limiter is invaluable for this. The brighter the light, the more current the amp's drawing. A good amp will pulse the bulb brightly at start up then settle down to a dim glow.
5. Once the power amp works, fiddle with the preamp stages if there's still something wrong.

[Enzo]

Is it usually a case of the power transistors running too close to max spec, or poor heatsinking? Any suggestions for a beefed-up replacement for the TIP142 and TIP147?

Why assume it is some sort of systematic failure needing modification? The TIP142/147 pair have been successfully powering amplifiers for various amp makers for years and years. Parts just sometimes fail.

What Bill said: he didn't say these amps commonly blow up their power transistors. What he was saying was that WHEN one of these blows up, then power transistors is the most likely place.

[HTH]

Thanks for the suggestions guys, always appreciated.

I opened up the amp, measured 240vac going to the mains socket, then on to the power on/off switch and finally to the board hooking up to the mains tx primary.

Unhooked all the secondaries and there is nothing - a few mA across each secondary. The schematic shows that I should be getting 68vac across one secondary and 18vac across the other.

Seems like the mains transformer is screwed, right?

[HTH]

Fender UK spares dept want £105 ($166 at today's exchange rate) for a replacement mains tx. Thats just stupid money and accounts for a third of the whole amp's retail cost. I've spoken to the guy that owns the amp and we've agreed that it's not an economical repair at the above price.

So, I'd appreciate it if anyone could suggest an alternative source for a mains tx, he's willing to spend upto £50.00 on a mains tx.

From the schematic it looks like I just need secondaries of 18-0-18 and 68-0-68, but I'm not sure on the current requirements - anyone?

Even two seperate mains tx's would work as 18-0-18 would be an easy tx to source, the 68-0-68 would be tougher. Like I said before though, I'd not sure on the current requirements in order to select the correct mains tx's.

Any help appreciated, I'd really like to get this amp working as a challenge to myself more than anything - I'm doing this for beer money afterall since its a friend of my brother's.

[52 Bill]

The schematic shows an external fuse and in-rush limiter in the primary wiring as well as an thermal fuse inside the transformer. Have you checked all of these?

Have you actually checked the transformer primary windings for continuity?

[Enzo]

My thought as well, did you check the transformer itself for continuity.

As to an odd transformer in place of a new Fender one, try looking at dead stareo receivers with similar power levels. The transformers in them are usually still OK.

You could also find similar power levels in some other brand guitar amp an get one of their transformers. Maybe a PV costs less than Fender in Eu. Or a Behringer something.

SOlid state power amps - at least the common push pull bipolar ones - will all tend to have the same voltage and current ratings for a given power output power.

[Steve Conner]

It won't be 68-0-68: the TIP142/147 can't even remotely stand the DC rail voltages that would imply. It'll be either just a single 68V winding, or if it has a CT it'll be 34-0-34.

Most any electronics place will sell some kind of transformer with two 35V windings that you can connect as 35-0-35. And a separate 18-0-18 too, no doubt. Maplin used to sell a toroidal transformer with 35-0-35 and 2x 12V for audio amps.

As for power rating, just take it equal to the output power Fender quote for the amp. Or choose one that weighs about the same Or yes, get one out of a stereo picked from a dumpster or whatever.

You could always dig inside the existing transformer to bypass the thermal fuse if it's blown. But I'd want to know why it blew.

And yes, it is relatively common for the output transistors to blow out, and the resulting short circuit across the DC rails takes other stuff out with them. There are ways of stopping it, but they all eat into the watts-per-dollar ratio.

[HTH]

thanks for all the help and suggestions, however the mains surge (thermister?) had one side broken off the pcb and was stopping the circuit completing on the primary side. all fixed now. oh, and the fuse was actually on the circuit board not inside the mains tx (glad I didn't open the PT looking for a fuse, doh!).

Had a play of the amp and the clean channel is ok if you dial in some of the reverb and trem from the DSP section. The OD channel could be ok if all you play is hardcore metal, but even on full bass there isn't quite enough 'chunk', and this is through my 4x12 with bass-cone G12H-30s.

Anyone got ideas on what might beef up the bass end on the OD2 channel for that 'Dimebag' bottom end? - I know this guy loves his metal (Pantera, Slayer, Machinehead etc...) and was underwhelmed by the tone out of this amp, but can't afford to upgrade it. It would be cool if I could give it back to him both working AND sounding better.

Cheers!

[Steve Conner]

A Furman parametric EQ in the effects loop, I guess, since that's what Dimebag used.

Having said that, they probably limited the bottom end on purpose to save the speakers from destruction and make the amp subjectively louder. Fender know fine well that kids love what the forum leaders here call "Wanker woofie drone", but they also know that you get more "Heard-over-a-drummerness" per watt by going light on bass and saving the amp's power for midrange.

[Enzo]

Glad you fixed it HTH, but the thermal fuse Steve refered to is not the same as the system fuse. many transformers have a fuse tucked in between their windings - it opens at a temperature rather than at a current. That is a separate issue from the fuse on the circuit board that responds to current.

[HTH]

Glad you fixed it HTH, but the thermal fuse Steve refered to is not the same as the system fuse. many transformers have a fuse tucked in between their windings - it opens at a temperature rather than at a current. That is a separate issue from the fuse on the circuit board that responds to current.

Enzo, I'm always looking to learn the most I can from any repair situation and with that in mind can you tell me where on the schematic it shows the thermal fuse in the mains tx? - I can't see it on the schematic.

As for the tone, I agree that the amp is likely voiced so that there's no excess bass to get the most 'cut' from the amp. However, the clean channel has lots of bass, more than required (typical Fender clean tone) but the OD channel has nowhere near enough bass even on full bass settings.

I'm sure that putting a Furman parametric EQ in the loop would sort out the lack of bass, but I'm wanting to increase the bass by modding the amp - I'm not gonna buy a GEQ for the guy, and it seems he's too broke to buy one himself.

Anyone got suggestions?

[Enzo]

The thermal fuse is part of the transformer, it will not be on the schematic. YOu can dig them out and even replace them, but as manufatured, thermal fuses inside the transformers were never intended as a replaceable component. Once in a while some OEM will show a little symbol inside the transformer to indicate its present, but it won;t be called out as a part - just part of the transformer symbol. If I can recall where one is, I;ll post it.