Eden WT800 low output problem

[Mick Bailey]

I'm working on an Eden WT800 bass amp and I'm currently out of ideas as to why the output is so low;

The amp layout is a preamp which splits to two separate 400w/4 Ohm power amps. It can be run stereo, or bridged mono into 8 ohms.

DC voltages to the op amps, tube heater and power amp read fine (though I don't know if the tube -HV and +HV are correct at -59v and +59v), and all controls function correctly. Output is undistorted. A request on the Eden forum for test voltages didn't get any response.

My test conditions are using a 1v/1Khz signal source and scope, no load and all EQ controls set to flat.

I've traced the signal path through with the following observations; the output from the 12AX7 is slightly less than unity. Output from U1 pin 7 is 0.5v compressor on, 7.5v compressor off (Gain control full). Signal traces trough to U7/U8 OK and level reduces off the Balance pot before hitting U9. This particular amp does not have the later gain recovery modifications around U9 (it has the values shown on the first schematic). All connectors and jack sockets have been cleaned and checked. Power resistors also checked for solder joint problems. Under any conditions the maximum output from the preamp is 0.95v, but the output limit LEDs hanging off U10-Q15/Q17 light when the output reaches 0.8v.

Neither power amp will swing more than plus/minus 13v with a 1v input signal to the preamp and master on full, despite gain clipping LED and output limit LEDs being illuminated. Voltage rails on the power amps read plus/minus 78.5v.

I'm unfamiliar with this particular amp and I've stared at it so long it hurts. Does anyone have any ideas, or test voltages?

I'm concluding that either the preamp is way down, or both power amps have identical faults.

[olddawg]

Can you open the MV and inject a signal there to drive the power amps to clipping.

[booj]

I have a WT800 that I've beeen working on since March with a similar problem. I'm supposed to get with Eden ( They actually called me back) tomorrow morning. If I get some info I'll let ya know!

[Amp Kat]

Eden Customer service is not the best at least not in my experience and maybe they have gotten better but I doubt they tell you anything worth beans except to send it to them for repair. Amazed you even got schemos because they didn't used to give them out but maybe they have to. Every Eden I've ever fixed had power supply issues and the components checked good and voltages were present but nada on the outs. I replaced the filter caps and a resistor and it came back up.

I would check D6,D7,D8 & D11 and make sure you are getting +/- 15 vdc for the op-amp rails and 12 volts for the heaters.

Also try isolating as another poster suggested by taking the send out to another amps return and see if the preamp is working ok then try injecting a signal to the return and see if it works. If it does you're loop jacks are bad so do try patching a cord from send to return 1st to verify the jacks are ok.

[g1]

You could try removing the 4 muting FET's (two J112's at each power amp input). Sometimes with a common mute signal, one bad FET will make them all turn on.

[booj]

Amazed you even got schemos because they didn't used to give them out but maybe they have to. Every Eden I've ever fixed had power supply issues and the components checked good and voltages were present but nada on the outs. I replaced the filter caps and a resistor and it came back up.

I'm not hijacking the thread, but I appreciate the info. I'll check into that or be prepared to if
I get a chance to talk to someone in the morning. I agree about thier lack of giving enthusiastic support. My repair belongs to George Casas, Bassist for Gloria Estefon. I told them that. If it gets a response so be it. I'll post the results!

[MarkusBass]

Amazed you even got schemos because they didn't used to give them out but maybe they have to.

I'm amazed that you say so because the schematic was published many times on this forum and also Eden gives it to anyone who needs it.

Also try isolating as another poster suggested by taking the send out to another amps return and see if the preamp is working ok then try injecting a signal to the return and see if it works. If it does you're loop jacks are bad so do try patching a cord from send to return 1st to verify the jacks are ok.

Yes, send the signal from the preamp to another power amp. Also send the signal from the generator directly to the power amp (you may need more than 1 Vpp). This is usual way to isolate the problem.

My test conditions are using a 1v/1Khz signal source and scope, no load and all EQ controls set to flat.

Mick, you haven't provide information whether this is peak-peak or RMS voltage. If this is peak-peak then there is nothing wrong with the amp. Also, if the amp seems to be OK, why don't you try it with a dummy load and with the signal feed directly into one of the power amps?

I'm concluding that either the preamp is way down, or both power amps have identical faults.

As I said, this may be easily verified by checking separately the preamp and the power amp.

Mark

[Jazz P Bass]

" Originally Posted by Mick Bailey
My test conditions are using a 1v/1Khz signal source and scope, no load and all EQ controls set to flat."
Isn't a 1v input signal a bit excessive.
A typical guitar pickup outputs what, 100mv?

[Mick Bailey]

With the preamp signals disconnected and injecting a variable amplitude sine wave at each power amp input I get 148v peak-to-peak output (no load) just before before the amp clips with a 4v peak-to-peak signal input. So this looks pretty healthy to me.

If I inject a signal on either C37 or C40 (just to the left of U9 on the schematic) with the balance control disconnected, at the point when the clipping LEDs operate I'm reading 1v input. This is giving a power amp output of 124v (right hand channel), but only 65v on the left hand channel.
In bridged mode the amp is showing serious crossover distortion.

Strangely, the signal level into both power amps is identical when measured on each individual power amp PCB and I've repeated the experiment a few times just to be certain.

So my next step is to try to resolve why this is the case before continuing with any other troubleshooting. My signal generator has a low impedance output so I would expect if there was any loading issue with the preamp then this would also show up with the signal generator connected.

Think I'll try swapping the inputs over next to see the fault switches channels.

Re-checked all of the DC voltages and these are fine.

[g1]

Sounds like you at least have a problem with the left channel. Right channel output seems in line with 400W into 4 ohms. 40V rms (approx. 113V p.-p.) into 4 ohms would be 400 watts. The drawing for the power amp says gain is 40 so I would think you should get full power with 1V rms into the power amp.
So the 65V P-P on the left channel is low. Get it up to par with the right channel before worrying about the limit lights, bridging, or input signal levels.
Also wondering how you are floating your scope ground to check bridge operation.

Edit: realized you are getting 2 completely different results but both with equal signal levels at the inputs. Makes no sense. Could be one way feedback loop is in circuit and one way isn't?

[12Bass]

When bridged mode is engaged, an inverted signal taken from the left power amp is sent into the right power amp via a 27 K resistor (which is located on the PS board near the left front, IIRC). This is shown on page 13 of the schematic pdf as connected to "FEEDBACK". I recall having some difficulties understanding what was going on with this part of the circuit. The value of this resistor determines the relative balance in output between the two power sections. Hope this helps!

[booj]

Well, I said I'd post if Eden got back to me on my problem (similar, but not the same as this posters). They suggested a new power module. Said they'd call back in an hour. That was 8-22 and 1PM CST. Still waiting.....

[Mick Bailey]

I've removed the left amp for closer inspection and it's previously had a burnout on part of the -VCC track at some time, so I'm thinking the previous trauma has not fully been sorted out. It's interesting that the LH amp will perform well when fed directly from a signal generator (and not from the preamp), but perhaps the low impedance of the SG is acting in its favour. I'm intending to remove the muting transistors as per g-one's post to see if there is something subtle going on there. I'm in agreement that the low output on this side should be the priority at this stage before looking at bridging etc.

I've got a scope converter for PC which does low frequency floating differential measurement, otherwise my 465B for everything else.

The feedback resistor info is interesting; there is a 27k resistor epoxied on to the RH board with one end tacked to a flying lead which disappears onto the PSU board. I'd overlooked this on the schematic and thought it was a botched mod that needed investigating at some point. The marked space where the resistor originally came from on the PSU has a zero-ohm jumper 'resistor' instead. Don't know why, but the change just swaps the resistor from one end of the wire to the other (?)

There are quite a lot of tacked on wires here and there all over this amp, but they mostly appear to tie up with the schematic. Probably lots of upgrade/changes to save the cost of a PCB redesign. Pretty amateur soldering, though.

[Mick Bailey]

Power amp sorted!

Removed the mute FETs and no improvement, but further investigation led to C4. Measures OK on the capacitance meter but ESR is off the scale. Nice output on that channel now and the weirdness in bridge mode has disappeared. Still doesn't seem quite enough output from the preamp, but my simple signal generator only outputs 1khz so perhaps with a full spectrum bass signal it will be OK. Next project is to build a better SG.

Really do appreciate the assistance with this.

[Mick Bailey]

The owner dropped by to try the amp with his bass and cab and it's all working fine. The amp had previously shown preamp problems as well as the overall low output - the input LED was showing clipping around the 10 o'clock position and the output LEDs coming on at the same time with the master at about the same position. Now all the controls work correctly.

I investigated the offending cap and it measures 50 Ohms ESR. Presumably, this is enough to modulate the 0v rail to some degree which could affect the preamp headroom.

The RH power amp C4 has also been replaced in the past, so this could be a typical failure point. Maybe this component is something to look at if anyone is troubled by the Gain LED lighting up at low levels.