Eden VT.40 Distorting

[Hankbass]

I have an Eden VT.40 amplifier that is distorting slightly after a repair several years ago.
The amp thermo component came unglued and floated onto the output board and took out the 8099 and 8599 transistors and some caps.
I took it in for repair and didnt really notice that it was distorting until i hooked up my 4X10 cabinet. I have mostly been using a 15 Cab.
the distortion isnt as noticable with the 15 cab.
While checking for other issues on the output board i noticed that the 8099 and 8599 were replaced with MPS A06 and A56.
I have had no luck finding a schematic. but i did find a couple other suggestions. I have replaced the filter caps in the P.S. circuit
and found a 100uF cap blown on output board. all other components seem to be ok by ohm meter chk and visual check.

I am wondering if the substitution of the MPS A06 and A56 could be the cause.
Does anyone have experience with subbing the now out of production 8099 transistors.
Or have any suggetions of other things that could be the cause of the distortions.

[52 Bill]

Welcome to the place.

Without looking up the specific parts, I don't know if the substitution of the transistors is the cause of your problem or not, but generally speaking if the amp is working then the transistors are probably okay as subs. The amp may just need a little tweaking to get rid of the noise.

A better description of what you are hearing may help to figure it out. Does the distortion only happen with the one cabinet and not the other or is it just easier to hear with the 10's? Is the distortion constant with the audio signal or does it only happen with loud sounds? Does the distortion only happen when the signal starts to fade out?

[Hankbass]

The distortion is constant. I have tried lowering preamp gain as low as possible and still get the distortion. It is easier to hear with the 4x10 cab, but if i alter the tone for more highs it can be heard in the 15 cab as well.
It sounds like a minor clipping, as if the preamp gain is slightly overdriving the power out section.

What would a minor tweaking consist of?

[52 Bill]

What would a minor tweaking consist of?

Not saying that this is what your amp needs, but a misadjusted bias control could cause there to be crossover distortion in the power amp.

Have you tried listening to the signal coming from the line out or the FX loop send? Have you tried sending a clean signal into the FX loop return?

[Hankbass]

Thanks for the idea!
I just tried it and the distortion is not there when coming out of the FX loop.
And distortion is back when coming back through return of FX loop.
There is a pot in the middle of this board and i tried adjusting it little bit each way and got no change. Im assuming thats bias control but not sure.
I measured pot with ohm meter and it is .2 - 1 ohm.

[mozwell]

I don't have a VT.40 schematic, attached are some eden power amp schematics, one of which may be close enough for our purposes here.

WT400 (PM400) power amp uses MPS8099 & MPS8599, sch attached
WT1205 uses MPSA42 instead of MPS8099, and uses MPSA92 instead of MPS8599

MPSA06 is 80V NPN
MPSA42 is 200V NPN
MPSA56 is 80V PNP
MPSA92 is 300V PNP
MPS8099 is 80V NPN
MPS8599 is 80V PNP

So, MPSA06 & MPSA56 are good substitutes for MPS8099 & MPS8599

Near the power transistors, there should be some high wattage power resistors, most likely less than 0.50 ohm.

If you measured across the trimpot & got 1 ohm, i suggest the bias transistor is faulty, this will mean you have zero bias & are hearing crossover distortion (see post from 52 Bill)
Find the transistor connected to the trimpot & replace it. (you can see on the attached schematics how this is generally connected)
Then with power back on, adjust the bias to get maybe 10mA to 20mA through each power transistor. You can measure the voltage across the power transistor emitter resistors & do the math...
Let the amp warm up for 20 minutes & check & readjust the bias if necessary.
Do this bias adjustment with no load and with no signal applied.

If this was the cause of your fault, the distortion should now be gone

[Hankbass]

The power amp is in fact the PM 400. I replaced Q8 which was a mpsA06. and adjusted the bias pot to 7mv between TPA and TPB as stated on the schematic you provided. and also adjusted it to various settings between the range you suggested (10-20ma) from the .5ohm resistors R21 and R22. I noticed when set at 10ma from R21 or R22, - R23 and R24 were at 6ma. not sure if that means anything or not.

The Distortion is still present.

[52 Bill]

I noticed when set at 10ma from R21 or R22, - R23 and R24 were at 6ma. not sure if that means anything or not.

It means that the output transistors or the ballast resistors are not very well matched.

Have you meter tested all of the transistors and diodes in the power amp? Have you checked for open or off value resistors?

[mozwell]

I would check all emitter resistors are ok R21, R22, R23, R24, also the bootstrap circuit R25, C9, ZD1 & D2, R26, C10, ZD2, D3 are you getting +/- 5.6V on ZD1 & ZD2 wrt speaker out voltage
Also check C13, C14 are ok
As much as i dislike shotgunning as an approach, you could parallel an electrolytic (of the same type & value as the cap on the board) across each electrolytic cap on the power amp & see if that fixes your problem. It may be a dodgy electrolytic.
Temporarily desolder the drain from the input jfets Q1 & Q2 in case these are your issue
Try a 100uF bipolar electrolytic for C3, and a 22u bipolar for C4
As Bill said, check resistors to see if they are ok.
Do the voltages around the input diff pairs look ok, what is C-B-E voltage for each of Q3 to Q6

Can you inject a sine wave into the input & look at the output on a scope, that way we can see if its crossover distortion, or something else

[Hankbass]

I checked all the components. and they seem ok. R21, R22,R23 and R24 are at .51 Ohms as printed on resistors.


I attached pics of o scope wave forms. Seems like crossover distortion to me but not sure

[52 Bill]

I don't know what I'm looking at here. Is that a sine wave? Is that the power amp output? What is the signal level that we are seeing?

[mozwell]

Hank
I assume you have a reasonably "clean" sine wave input, and these are measurements on the speaker output of the power amp (no speaker connected)
I cant see from your pictures what the volts per division is.
It doesn't look like crossover distortion, which looks like a "flat spot" at zero where the AC goes from pos to neg & vice versa.

Lets go back a few steps & lets take some measurements, all with no signal applied & no speaker connected
1. positive power supply rail voltage wrt 0V, measure AC and DC volts
2. negative power supply rail voltage wrt 0V, measure AC and DC volts
3. power amp output voltage wrt 0V, measure AC and DC volts
4. ZD1 cathode voltage wrt power amp output, measure AC and DC volts
5. ZD2 anode voltage wrt power amp output, measure AC and DC volts
6. Q10 base voltage wrt power amp output, measure AC and DC volts
7. Q11 base voltage wrt power amp output, measure AC and DC volts
8. voltage across each of R21, R22, R23, R24 emitter resistors, DC volts
9. base, collector & emitter voltage of Q7 wrt 0V, DC volts
10. base, collector & emitter voltage of Q9 wrt 0V, DC volts

a failure of the bootstrap circuit could give the waveforms in your picture, with power off, check 6k8 2W R25 & R26 are ok, try replacing 1000u 10V C9 & C10

If you have spare transistors, maybe replace Q10 & Q11 (I dislike shotgunning, but it may be worth a go at this stage...)

[Hankbass]

Im sorry guys i was in a hurry when i replied with pics.

Those are from the output at the speaker jack with a speaker hooked up. I used a bass guitar as input (all I had).

I was in the process of measuring the voltages you said in previous post Mozwell and ended up shorting something out.
I burnt up Q3,4,5,6 and R6,9. (old eyes dont see so good even with glasses).

This is as far as i got
1. Positive Rail - 73.6vdc , 23mvac
2. Negative Rail - -73.6vdc 23mvac
3.Power amp out - 0vdc , 5.6mvac
4. ZD1 Anode - 5.72vdc
8. R21 - 5.4mvdc R22 - 5.4mvdc, R23 - 3.0mvdc, R24 - 3.0mvdc

I found matched sets of 8099 and 8599 on ebay. I also noticed that the shematic said mpsA42/A92 can be used. i will have to start all over checking components and reply back when i get that done.

Thanks for all of your very helpful advice. it is much appreciated.

[The Dude]

A bass guitar signal is full of harmonics and other assorted "nonsense" and isn't much help in diagnosing distortion problems such as this. It will never generate a pure sine wave. If you don't have a tone generator, you can use your tablet, phone, pc, etc. to play test tones freely available all over the web. I'd suggest you start there to give a true representation of what the amp is doing or not doing. Here is a page with a few, but they're everywhere if you search.

Download Audio Tone Files

[Hankbass]

OK I ended up having to replace Q1 thru Q6, Q7 & Q9, R14 & R18

I set the Bias between TP-A & TP-B at 7mvdc as stated on the schematic provided by Mozwell.

I get output on the amplifier same as before with the slight distortion.

I rechecked the voltages and below are the results.

1. positive power supply rail voltage wrt 0V, measure 20mvAC and 73.9vDC volts
2. negative power supply rail voltage wrt 0V, measure 20mvAC and 73.9DC volts
3. power amp output voltage wrt 0V, measure 5.6mvAC and 0vDC volts
4. ZD1 cathode voltage wrt power amp output, measure 6vAC and 5.72vDC volts
5. ZD2 anode voltage wrt power amp output, measure 6vAC and 5.72DC volts
6. Q10 base voltage wrt power amp output, measure 4.5mvAC and 1.1vDC volts
7. Q11 base voltage wrt power amp output, measure 4.6mvAC and 1.1vDC volts
8. voltage across each of R21 - 3.6mvdc, R22 - 3.6mvdc, R23 - 1.1mvdc, R24 - 1.1mvdc emitter resistors, DC volts
9. base-72.6vdc, collector- +1.4vdc & emitter-73.4 voltage of Q7 wrt 0V, DC volts
10. base-+72.7vdc, collector- -1.2vdc & emitter - +73.4vdc voltage of Q9 wrt 0V, DC volts

I also took pictures of waveforms taken on output with a speaker hooked up of 100hz and 250hz - the 250hz ones seem to develop the little hitch in there after a couple seconds of playing the tones suggested by The Dude.