Trace Elliot AH300 Bass Head Distored Output

[itek]

Hi guys, I'm new here I run a small electronics repair shop and recently started getting in to guitar amps and heads. I am really no expert and am kinda learning as I go. I have successfully repaired a couple Trace Elliot heads before, but this one is something new- it's an AH300, a real beast actually the fan blows hard enough to keep the sweat off your face in a hot club i guess hehe, anyway, It fires up OK, the preamp stage seems to be working correctly, but the output is constantly distorted and fades in and out. I tested each giant cap and ESR seems OK, capacitance shows ~5500, but voltage wise, after the amp is turned off one cap holds 70v, while the other is back at 0v. I poked around with a stick and nothing seems to affect the output, and there is no voltage at the outputs either. would that cap be bad or should I look at the output transistors; or am I totally lost? Thanks for any input!

[Jazz P Bass]

Here is the Service Manual.

[itek]

Thanks, will see what I can do with it..

[Enzo]

If the output fades in and out, connect the preamp out jack to the power amp in jack with a cord. ANy help? If there is an effects loop (FX loop) plug a cord from FX send to FX return. ANy help?

What is left in the caps after power down doesn't matter much. WHat does matter is what happens while power is on. Are both polarity power rails up to about the same voltage, and both are pretty much free of ripple? AT least both have the same amount of ripple.? And when a healthy amount of signal is feeding a load, are those power rails holding up OK?

And some advice: in general, don't focus laser-like on the problem always being a bad part. A good percentage of problems do not involve a bad part. Broken or loose connections are common, as are poor contact in the shunts of FX loop jacks and power amp jacks.

[itek]

wow thank you for the advice Enzo! I will check this stuff out and get back with results as soon as I can

[itek]

OK WOW so a long time has passed and I get to pull this thing out again! ) sorry to leave you hanging when you were so helpful. I have been testing now with a music input signal rather than a bass guitar, and it seems like the output is just plain distorted and wavy, especially bass notes, they get cut almost completely out. So both caps are at ~72v/-73v, very close to each other, although the 72v rail shows 92mv AC, while the -73v rail only shows 11mv AC. The both definitely seem to hold their voltages with signal driving a speaker. I have been re-checking solder points across both boards and so far still lost,, I really appreciate any input!
P.S. I have applied your advice to many repairs since!

[Enzo]

Really should apply a sine wave for distortion testing. I usually grab a line out from my stereo, tuned to FM music stations, but I can;t tell iof there is any distortion scoping that, unless it is gross distortion, like the entire half waveform misssing or severe clipping. Sine wave makes it easier to see.

Scope the 70v rails, see if the sine wave becomes impressed upon one.

But first, apply the test signal directly to the power amp, don;t come through the preamp. ANy difference?

You want to scope the output to see in what way the signal is distorted. CLipping? Symmetrical or not? Or one half of the waveform missing? Fading in and out makes me thing some resistor is open and a circuit node is charging up and then discharging.

[itek]

Really should apply a sine wave for distortion testing. I usually grab a line out from my stereo, tuned to FM music stations, but I can;t tell iof there is any distortion scoping that, unless it is gross distortion, like the entire half waveform misssing or severe clipping. Sine wave makes it easier to see.

Scope the 70v rails, see if the sine wave becomes impressed upon one.

But first, apply the test signal directly to the power amp, don;t come through the preamp. ANy difference?

You want to scope the output to see in what way the signal is distorted. CLipping? Symmetrical or not? Or one half of the waveform missing? Fading in and out makes me thing some resistor is open and a circuit node is charging up and then discharging.

I'll get a sine wave through it tomorrow and get my scope on it, its safe to hit my scope on those caps right? (sorry I'm very new at the scope, i just got a tektronix 465 and I'm having a hard time figuring out how to use it)
I tried the effects send/return loop and line in like you suggested with same results. I will report back tomorrow. Thanks!

[Enzo]

70v on a cap is no different from 70v on a stack of batteries or 70v coming from a 70v power supply. Set the input and probe of your scope for a 70v signal. And that would be on DC coupling. If we want to just look for ripple or signal on the powr rails, we set the scope input coupling to AC. Now it ignores the 70v of DC and only shows any AC that also exists there.

[robertmels]

Hi, I had a similar problem with my AH300. The output was very distorted while a speaker cabinet was attached. It also faded in and out, especiacially at lower frequencies while hitting the E-string.
I tested the headphones output and strangely the output was OK as long as there was no speaker attached to the amp's output. With a speaker attached, the headphones output was also distorted.
In the AH300 Circuit Diagram, it shows that the headphones output is directly drawn from the main speaker output.

I concluded that the output stage was apparently broken and I decided to replace all output MOSFET's. Since the original Hitachi 2SJ50's and 2SK135's are no longer produced and are quite rare now, I found good replacements with the same names, but from a different manufacturer; Magnatec (UK). I bought them for around 12 Euro/piece.
After replacement of both 2SJ50's and both 2SK135's the amp works like a charm. Maybe audio purists won't appreciate this Magnatec swap, but from a technical perspective, there is nothing wrong. For this type of amp (not HiFi but a working horse bass amp) the Magnatecs are really great and a relatively cheap solution.
I hope this solution helps other people in repairing their fabulous and vintage Trace Elliot amp and keep using it instead of new and modern stuff. Mail for details or working instructions.

[MarkusBass]

Maybe audio purists won't appreciate this Magnatec swap, but from a technical perspective, there is nothing wrong. For this type of amp (not HiFi but a working horse bass amp) the Magnatecs are really great and a relatively cheap solution.

I have nothing against Magnatec transistors but I would do the swap only when I'm 100% sure that the old transistors are really bad. Otherwise, you may spend 50 Euro and be still at the very begining of your task. There is no place for guessing. I would rather listen to Enzo's advise: use signal generator and an oscilloscope (and a dummy load). The power stage in this amp is very simple and checking what is wrong with it is really a simple task. It may be still a problem with the preamp.

Mark

[robertmels]

For anyone who is interested: I have e few unused (brand new) pairs of original Hitachi 2SJ50 and 2SK135 available. I can also help with the Magnatec substitutes. Mail for details.

[Steve Conner]

The Magnatec substitutes are the BUZ901 and BUZ906. Available from Newark and Farnell. They have the same die inside as all the other funny branded lateral MOSFETs you can get in the UK (Exicon, Alfet, Class-D etc)

For those who have the blind urge to mod, you can also get a BUZ901D and BUZ906D. They have two MOSFET dice inside and twice the current rating. I think I remember one forum member souping up his Trace Elliott with these.

[robertmels]

Hi, I had a similar problem with my AH300. The output was very distorted while a speaker cabinet was attached. It also faded in and out, especiacially at lower frequencies while hitting the E-string.
I tested the headphones output and strangely the output was OK as long as there was no speaker attached to the amp's output. With a speaker attached, the headphones output was also distorted.
In the AH300 Circuit Diagram, it shows that the headphones output is directly drawn from the main speaker output.

I concluded that the output stage was apparently broken and I decided to replace all output MOSFET's. Since the original Hitachi 2SJ50's and 2SK135's are no longer produced and are quite rare now, I found good replacements with the same names, but from a different manufacturer; Magnatec (UK). I bought them for around 12 Euro/piece.
After replacement of both 2SJ50's and both 2SK135's the amp works like a charm. Maybe audio purists won't appreciate this Magnatec swap, but from a technical perspective, there is nothing wrong. For this type of amp (not HiFi but a working horse bass amp) the Magnatecs are really great and a relatively cheap solution.
I hope this solution helps other people in repairing their fabulous and vintage Trace Elliot amp and keep using it instead of new and modern stuff. Mail for details or working instructions.

UPDATE: After more than 6 months, the Magnatec's are still running very well. I play several times a week at high volume with a 8x10" cabinet and works like a charm!

[robertmels]

UPDATE: After more than 6 months, the Magnatec's are still running very well. I play several times a week at high volume with a 8x10" cabinet and works like a charm!

Update March 2013: Still very, very, very OK! After a lot of rehearsals and high volume live shows the Magnatecs still do a great job!