Marshall MG250DFX Current Output Check/Power Check Jack Darr

[Hawk]

I've been reading Jack Darr's book, Electric Guitar Amplifier Handbook. I love  it!

1. He refers to hooking up an ammeter to the collector of the output transistor to measure current while slowly bringing up your variac and keeping an eye on the current (I'm pretending the amp is blowing fuses and trying to discover the problem). I chose to use the MG250DFX as my practice mule, so I can't physically unhook an output transistor collector and hook in my metere. Instead, I'm dealing with the TDA7294 DMOS audio amp. Do I break the wire connection to Pin 14 and hook in the ammeter there??? (This amp works so I'm just getting used to being hands on and am not trying to fix it, at least not this part--some quick dying of notes on distortion channel which I'll deal with later).

2. On the clean channel, I injected a 1 KHz, 50 mv signal into the amp. I hooked up a Weber VST Dummy Load on 8 ohm impedance. I hooked up an rms voltmeter across it. I turned up the master volume to full and slowly brought up the gain until the top of the wave started to flatten then pulled it back a little. At that point I found approx. 3.8 volts across the dummy load of 8 ohms. Using the formula, Esquared/8 I come up with 3.8x3.8/8=1.08 Watts. What does this reveal? It's obviously not the full output power of the amp.

Thanks!

[Roly]

1.
No offense to Jack Darr, but I've been a professional tech all of my life (now retired) and this is not how I would do it.

Firstly, current measurements are always a bit more difficult than voltage measurements because you have to break the circuit somewhere and insert your ammeter.  In this case I would measure the voltage across the emitter resistor as a proxy for the current; but then I wouldn't be doing that either because I'd be using a limiting lamp in the mains supply.

Some people like using a Variac to bring the voltage up on an amplifier under test, but I don't; never owned one.  The primary reason is that a Variac is basically a transformer, and as such it still represents a pretty "stiff" supply to the amp, even on low settings.  The limiting lamp in contrast presents a pretty soggy supply that is very easy for a faulty amplifier to pull down without excessive current and resulting damage.  Use a Variac by all means, but include a limiting lamp between it and the amp under test.

Using a limiting lamp and not cutting any of the internal wiring should meet your (perfectly reasonable) concerns.  I avoid doing that, cutting tracks etc, if at all possible, and almost all the time it is possible.


2.
Your reasoning and math are okay, but you can't be sure that it's the output stage that is clipping, and if it looks "soft" or rounded at all then it probably isn't.  Output stage clipping is normally quite abrupt flat-topping (and flat-bottoming), usually with some mains hum superimposed.  To confirm output power I would go in as close to the power amp input as possible, say the Fx Return socket if there is one (or in Ye Olde Days lift the lead to then top of the (Master) volume control and inject my signal generator there.  This will typically require a level of 100mV to 1V.

If the amp is known to be in good health, makes a bloody loud noise in normal operation, then this power measurement clearly can't be right, and to my mind the most likely reason is that the clipping you are observing is occurring somewhere in the preamp and the power amp(s) are not being driven to anything like full capacity.


A point to watch is that modern DMM's can't always be trusted to give faithful AC readings at other than power mains frequencies (not that I think this is your problem here).

[Hawk]

Thanks Roly for taking the time. Instead of a limiting lamp I have inserted a household wattmeter which shows current draw (I like to know what current the amp is drawing. Or I've also inserted an ammeter between the fuse leads to make sure this wattmeter is accurate and indeed it is). Maybe it's time to trust the limiter, and I have used one in the past, and it's just with all that glowing I find it hard to tell if it's on the cusp of drawing too much current and the fuse will blow! Guess that comes with experience. I can't tell you how much I appreciate this site and people like yourself as this is such an isolating business and it's hard to find someone locally who you can "apprentice with" as there aren't that many of us around. thanks!

Later today I will inject the signal at the master volume, great idea!--I also picked up a an RCA VoltOhmyst but not sure if that will give me a more accurate reading than the modern DMM. From what I've read it won't due to loading. What would you suggest? I value you your years of experience!

[Hawk]

On the schematic the master volume feeds to seperate/somewhat identical circuits feeding two TDA7293's.

1. If this is considered a Stereo set-up what makes it sound like Stereo? Wouldn't connecting a single TDA to two speakers give us the same sound? Why two circuits?

2. The schematic is confusing! It shows Pain L, Pain R (what do those stand for? Pan Left, Pan Right?), and then two variable resistors as if there are two master volume controls which there are not. What am I missing here?

Thanks!

[Roly]

In this post I have calculated a table of various values with various limiting lamp wattages, including the current that they will pass on full mains (240VAC) voltage.  You can estimate the maximum current on 115VAC by doubling these values, or actually calculate for whatever lamp you have to hand.

E.g.

60 watt 115VAC

P = E * I

I = P/E

60/115 = 0.52173913 amp

So an amplifier with a 3/4A or 1A power fuse simply cannot blow it because the lamp doesn't pass enough current even on full 115V mains voltage.  A half-amp fuse should eventually blow, but with such a marginal overload it could take tens of minutes.


{I have a number of calculators around me, not to mention the ones in Windoze and my text editor, but I've always had a slide rule or two around the bench, and strangely they have never had a flat battery or needed rebooting.  :lmao: }




Ah yes, this is a Vacuume Tube VoltMeter or VTVM, and quite a respectable instrument despite its vintage.

VTVM's can all be assumed to have the same input specs.  Ordinary VOM's are rated in ohms per volt (of setting).  So an automotive VOM might have a robust 1mA meter movement and be rated at only 1k ohm per volt, or 20k input resistance on its 20V range.  An average moving coil instrument would be 10k ohms per volt (100uA movement), while a quality instrument might be 20k or 40k per volt (of range setting).

In contrast VTVM's and oscilloscopes have attenuator inputs and are almost always present a flat 11 megohms on all ranges.  For almost all uses this can be considered to be a negligible loading.  While cheap DMM's are quite similar they typically have an input resistance of only 1 megohm, so the VTVM is quite superior.

The AC side is a bit different and if you look at the circuit above you will see that the AC path is about 1 megohm (near enough; 900+324+150=1374k).  Note also that DC is blocked by an 0.1uF cap, and that the 6AL5 double diode rectifier should be flat well into the supersonic or low RF range.

About the only thing that VTVM's lack is low AC voltage sensitivity, but a small external x10/x100 op-amp would turn it into a very acceptable audio millivoltmeter.



1.
The Fx module.  It will use the DSP to produce two signals from the single input to sound like stereo, e.g. chorus.

2.
Pain L/R = Power amp in Left/Right

The Master volume pot will be a stereo type, two pots on the same shaft.  Here they are shown as "VR13a" and "VR13b" but in Ye Olde Days stereo (or even four-gang) pots were shown as mechanically coupled with a skewed dotted line between the wipers, ditto switch pots, etc.

[g1]

   Seeing as this is your "test mule", you should be warned about the 7293 output IC's.
  At any time they are disconnected, before re-connection their power supplies must be discharged.  If not, they can be damaged.  So, if you disconnect their board connectors, you must discharge their supply caps before reconnection.

[Hawk]

Roly and G1 thanks great info and I soaked it up (no pun intended). I'll resurrect my limiter and put my analog voltmeter to good use. Thanks also for the cap discharge info! :dbtu:
What heat would you use when soldering/desoldering on pcb's?

[Enzo]

Tools are tools, and like any tool, it needs to be used properly and in a good context.  I often recommend bulb limiters, they are especially easy to understand, and for the novice, they are simple and cheap to make.

But in the last 40 years of running a electronics bench professionally, I have never had nor used one.  I use a variac.  The variac is not complete without a current meter.   When I bring up an amp on it, I watch the current.  If it tries to ramp up the current when the voltage is still only 15-20 volts or something, I can back right off, and the amp has received no more than it would through a bulb.   I won't belabor it.

But a variac is much more than a substitute for a bulb limiter.   I can take a working amp and run it at reduced voltage to check for instabilities, without the bulb affecting the operation.   I can check operation of SMPS under various conditions.   I have had amps that were OK up to maybe 90v. but draw heavily at full voltage.   The variac allows me to work on the amp while at 90 volts.   The list goes on.