Ashdown MAG300, can't get the DC off the output

[nsubulysses]

I have a MAG300 I really cannot figure out. There is DC on the speaker jack. When I got this amp initially it had a shorted bridge rectifier and shorted output transistors. Some of the other transistors in the output section were shorted too, but it's hard to remember which ones since I started this project a while ago and have gone on vacation since.

Regardless, I have replaced: Bridge rectifier, all output transistors (TR16, TR17, TR11, TR3), as well as TR13 and TR8, TR6 and TR7, TR14 and TR12, and also TR15.

Emitter resistors (R4, 13, 20 and 23) all measure .3 ohms. But at all junctions of those resistors I get positive DC voltage, even on the V- emitter side of R4 and R13. not sure if this is correct. Is V+ and V- supposed to zero out here and leave only AC?? I am not confident on what voltages should be expected at certain places in the circuit like I would be with a tube output section. I get my V+/- voltages respectively at the collectors of the output transistors.

Any help in the right direction much appreciated. I hate to give up, but I have no other thoughts on how to realize the problem, so this is my last resort.

The bulb limiter does not light with no load attached, but when one is hooked up it will light brightly.

I can check voltages at any point in the circuit and relay them

041 - MAG300 Change.pdf

[Jazz P Bass]

Were all of those transistors blown?

You must check that all resistors are still good.

This is a Class A/B output so the output transistors should be barely On.
The bases of TR16/ 17 should read about +0.55 Vdc.
There driver, TR13, should have a base of +1.1 Vdc.

The negative rail transistors, the same, but negative.

With a negative voltage on the output (do not connect a load) I am sure you will not find these voltages.
Base, Emitter, Collector voltages on every transistor may lead to the problem.

[J M Fahey]

But at all junctions of those resistors I get positive DC voltage, even on the V- emitter side of R4 and R13. not sure if this is correct. Is V+ and V- supposed to zero out here and leave only AC??

You *should* ... if the amp were repaired, which it is not

Thanks God that amp is relatively simple and of a classic design and all transistors and their legs are clearly identified in the schematic, which makes Forum communication possible.

What JPB says about voltages is right, of course, but you are not sure about what to expect.

1) even if the output transistors are now fine, the whole SS amp is (different from tubes) DC coupled, we don't have those nice useful audio coupling DC separating caps which divide a Tube amp into neat independent blocks which can be repaired one by one, but all parts are interconnected and interdependent.

Think of it as if "a guy in Oklahoma catches a cold and then Obama in Washington sneezes" .

Treating Obama in that case is not enough if you don't also find who caught that cold somewhere else.

In this case, those output transistors are showing +V voltages, because "somebody" is making them do that ... which may be bad itself or having somebody else forcing it do so ... and so on.

Maddening? No, but you will have to measure beyond the apparent culprits.

2) plug the amp into the lamp limiter (which will stay there until we finish), no signal, all pots on 0, no speaker.
We work on the unmodified original schematic , there's time later for mods.

When I want the voltage on a transistor leg, I will ask for transistor number + leg number (good schematic idea) such as : "voltage to ground on Tr2-3" or "voltage from Tr2-2 to Tr2-3" .
In the latter case, also indicate polarity, such as "Tr2-2 is 600 mV more negative than Tr2-3" , etc.

3) Universal "expected voltage rules"
a) a base must be some 600/700mV more positive than the emitter for NPN, negative in PNP.
b) a collector must be at least 2V more positive than the emitter for NPN, negative in PNP
that's it

To that add that in audio circuits, we *usually* expect collector to emitter voltages to be about half the full end to end voltage available, so as to allow for the largest sinewave possible.

Ok, armed with that, we can start measuring voltages in that amp and find weird ones.

This is much faster and to the point that endlessly pulling parts, measuring outside, remounting, etc.

Last suggestion: download the transistors datasheets basically to check pinouts, draw them all in a sheet you tape visibly by your workbench.



4) Post whatever's asked here:
voltages normally to ground unless specifically asked leg to leg , in any case remember polarity.
a) remeasure output rail (speaker out hot) voltage .
b) measure voltage at Tr2-3 / Tr4-3 .
c) at Tr1-2
d) at Tr5-2
e) Tr12-2
f) Tr7-2

There's more to follow, but post these first.

[g1]

When I want the voltage on a transistor leg, I will ask for transistor number + leg number (good schematic idea)

I assume those leg numbers are actually labelled on the circuit board? Otherwise we will have a problem. Why don't they just use the standard E,B,& C designations on both the schematic and circuit board?

[nsubulysses]

Here are my findings. More than what was asked, but maybe you were gonna ask for those voltages next.

TR13
B 66.4
C 67.1
E 66.6

TR8
B 66.6
C -68
E 67.4

Tip of speaker jack is 66.3V

TR2
E 1.1
B 9.0
C -67.2

TR4
E 1.1
B 0.545
C -65.2

TR1
E .545
B .012
C -65.3

TR5
E 9.02
B 16.94
C -67.5

TR12
E 67.1
B 66.5
C 67.1

TR7
E -48.2
B -67.3
C 66.7

[nsubulysses]

I assume those leg numbers are actually labelled on the circuit board? Otherwise we will have a problem. Why don't they just use the standard E,B,& C designations on both the schematic and circuit board?

yeah that was confusing me a little so I just went with E,B,C. The 1,2,3 kept changing with it's E,B,C correspondence.

The emitter always has the arrow right? Regardless of if it is physically on the top or bottom of the transistor on the schematic?

[g1]

Yes the arrow is always emitter, base always shown in middle. Arrow pointing away from transistor means NPN, pointing inward means PNP.
So those numbers are not printed on the board? The reason I thought it would get weird is that a standard 2N3904 is EBC when looking at the face, from left to right. Yet they call out 321, backwards. So if those numbers are not printed on the board it would just make things more difficult.

[nsubulysses]

the numbers are not printed on the board, only the schematic

[g1]

So you still need to look up the pin-out for each type of transistor to do measurements. They might just as well called out ebc on schematic. Or use 123 as seen from front, not from back.

[nsubulysses]

I checked the datasheets for all the transistors I metered. So I believe they are correct as far as E,B and C. Do some of the measurements look strange to you, other than that the amp is not repaired properly??

[g1]

TR8 is the first thing jumping out at me. Base to emitter junction says it is turned on, yet none of the voltage present at collector is getting to emitter. Check it is good, correct type, and installed with proper orientation.

[nsubulysses]

I had TR7 and TR8 in backwards whoops. No wonder I've been stuck on this amp for so long. Still DC on the speaker jack however.

TR8 voltages are the same. It checks ok with the diode function of my DMM.

TR7 is now
B -66
C 66
E -66.8

Seems to be disobeying Mr Fahey's 3a and 3b rules

[olddawg]

I had TR7 and TR8 in backwards whoops. No wonder I've been stuck on this amp for so long. Still DC on the speaker jack however.

TR8 voltages are the same. It checks ok with the diode function of my DMM.

TR7 is now
B -66
C 66
E -66.8

Seems to be disobeying Mr Fahey's 3a and 3b rules

Now check the basing and orientation of every part you replaced. Look for open resistors and dry caps.

[J M Fahey]

Yes.
Given what's happened please triple check (double is not enough ) all transistors are correctly oriented, hand draw in a sheet of paper each transistor type (many are repeated) the 1-2-3 and e-b-c labelling so when in doubt you just rise your head and recheck.

For example, TR1/2/4/5/14 are all the same.
Also all Power transistors probably have the same pinout, although it must be the "japanese" ECB one and not the more common Euro/American BCE one. But check the datasheets.

And on the PCB itself you might add a small dot showing the collector leg (I use white correcting pen because it's very visible)

And to save time, besides the measurements asked for, check all transistors, slowly and one by one to check the "universal rules" and post which don't follow them, because those are "red flags" which mean something is wrong.

As Napoleon once said to his very anxious Valet before a battle: "dress me slowly, because I'm in a hurry".

Or like old Carpenters say: "measure twice and cut once".

EDIT: remeasure Tr7 b to e for better precision (use the 2V scale) and EC (200V scale) , it seems to be open.
That alone would slam the output towards the +V rail (hint ... hint ...)

Also measure voltage on the 2V scale across R10 (47r).
If value is <200mV, switch to the 200mV scale for more precision.

[nsubulysses]

TR7 B to E - 1.05V
C to E - 131.2V

Voltage drop over r10 is 0, even on mV scale

I will have to order a few parts and get back to this. Thank you for your help so far.