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Marshal JMP 2099

Started by gbono, August 25, 2014, 02:40:26 AM

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gbono

Quote from: Roly on August 29, 2014, 09:44:29 AM
Yeah, well, um, that all looks pretty smooth.

So, (Q) by disconnecting the output autotranny, and with all the output transistors etc otherwise fully connected, we now have an amp that is DC stable and not drawing excessive current???

(or am I missing something?)

Note in your marked up schematic the BC184 should not have a jumper across CE.............

Enzo

By verifying the rest of the amp works without it.  That would demonstrate that the transformer and only the transformer causes the amp to malfunction.


To test the part on its own, well, it is a transformer.  Put some AC across two points on a winding and verify that appropriate AC results on the other taps.  It appears to be an autotransformer, so there is only one large winding.

You can also go to RG Keen's Geofex web site and look up the transformer tester there and make one.  It is a simple gadget.

Roly




Measure the resistance, between terminations (lowest ohms), and to the frame (very high ohms).


Shorted turns test;


Visit: http://www.ozvalveamps.org/repairs/trannytest.htm

-

The jumper was to show how to disable the bias network, voltages are without jumper or autotranny (but it has been overtaken by the autotranny discovery).

If you say theory and practice don't agree you haven't applied enough theory.

gbono

#18
Here is a picture of the transformer in situ. The gray wire was connected to the 4 ohm tap. If you look at the schematic one side of the transformer is connected to ground. So how does this work? Every "tap" is going to and does have continuity to ground. How should the output transistors connect to this autotransformer or the output impedance switch?





Roly

see attach

This is used as a step-up tranny to match 8 and 16 ohm loads to the 4 ohm nominal output impedance, which the amp drives all the time.

This could be a more conventional double-wound type, but for economy, and because primary-secondary isolation isn't required, they can combine the primary with the secondary in an "auto-transformer".

As you have noticed, this presents an impedance (AC resistance) of 4 ohms to the amp output, but it also presents the winding resistance, which will only be a few tenths of an ohm.

In the event of any DC offset in the amp the output current will rise dramatically.  I don't consider this a robust design without an output coupling cap to keep any DC off the autotranny in-built DC short (or at least put the fuse on the amp side).

Given how critical this offset might be, I only see trims for bias and AC gain.   ???  (which is currently where your problem seems to be)




I may have observed of the 741 op-amps in the front that you can do a hell of a lot better for a few dollars these days.  These are instrumentation quality, they were never intended or designed for amplifying audio.  Today a buck will buy you a drop-in op-amp that is almost Hi-Fi perfect.  The line starts at TL071 and gets better from there.
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

OK, with that wire off, the transformer is disconnected.  Now does the amp power up OK?   And if so, connect a speaker between the gray wire and ground, sound come out?

gbono

Quote from: Enzo on September 01, 2014, 07:23:12 PM
OK, with that wire off, the transformer is disconnected.  Now does the amp power up OK?   And if so, connect a speaker between the gray wire and ground, sound come out?

Yes the amp works fine with the autoxfmr disconnected. So with 10mV offset the issue has to be a shorted turn to chassis ground - no?

Enzo

Doesn't have to be to ground, even just winding to winding.  That will make it stop working like a transformer and become a hunk of wire.

10mv offset is nothing by itself, but short the amp output, and the amp circuit will try to correct it into the load.

Roly

A "shorted turn" is an AC condition and has no effect on DC operation; it would not cause excessive standing current draw, only under AC signal drive.

A "short to frame (ground)" is both a DC and AC condition, but it is also very rare and unlikely in this situation.  Neon-flash test the autotranny by all means, but I don't think that is where the basic problem lies.  At the moment it looks like it's voltage offset related, but there is a deeper issue.

{maybe it's just a phase I'm going through, but} I think this is a design flaw, that the guys at Marshal didn't think through the implications of a much greater sensitivity to any output offset voltage.  I can only guess what the DC resistance of the bottom winding is, but I expect it wil be only about an ohm or two, maybe less, so we have an output current to offset of 1mA/mV,  1 amp per volt if it's one ohm, more if it's less.  Don't like.}


It's a bit drastic but I'd be considering modding the amp by inserting a suitable large electro between the amp output and the auto tranny, and moving the fuse to the amp output side too (that isn't thought through either).


Where is this (minimal) 10mV offset coming from, anyway.  (I'm sure my own Twin-50's have about three times this much offset).
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

In defense of the design, we do not see a steady stream of all the 2099s coming in for service.  There are other SS amps with autoformer OTs.   And this is not much diferent from a ordinary SS amp driving a CV transformer.


J M Fahey

#25
Quote from: gbono on August 31, 2014, 09:42:03 PM
Here is a picture of the transformer in situ. The gray wire was connected to the 4 ohm tap. If you look at the schematic one side of the transformer is connected to ground. So how does this work? Every "tap" is going to and does have continuity to ground. How should the output transistors connect to this autotransformer or the output impedance switch?
1) please post transformer physical size.
Just the "roof"  rectangle is enough.
Millimeters preferred (as in 32mm x 94 mm or wharever)  but worst case inches are acceptable ;)

2) since it´s an autoformer, it´s just one long tapped winding.
One end goes to ground/0 and the others provide different voltages but all have DC continuity.
I said DC because they carry different AC voltages.

The amp direct output is 100W into 4 ohms, so it goes to the 4 ohms inpit/output (transformers are perfectly reversible) and proper voltages appear at the other taps.

As in: the amp produces clean 20V RMS.
If you do the math, 20^2/4=100W RMS
The 8 ohms tap rises that to 28.3V RMS and the 16 ohms one to 40V RMS .

Once you "see"  it that way, it´s logic and simple.

EDIT:
a) I don´t think the transformer is bad.
Very low voltages around it, the transistors have built in reverse protection diodes, etc.
b) I suspect you still use the lamp bulb limoter.
P suggest:
turn it on with no load applied
measure you have 10mV or some other real low offset
connect the grey wire to 4 ohms tap and nothing else.
I guess nothing bad will happen, at most offset will lower a bit. or nothing
if everything, connect the speaker to the proper rap
if everything fine, play some music at low volume (say 1 W)
if everything fine, plug the amp straight in the wall and repeat tests, I guess  everything will be fine.
blast away :)

gbono

The core is 100mm x 80mm and 30mm "thick". The windings are about 66mm in diameter.............

gbono

Okay this is a bit embarrassing ...... when I hooked up the light bulb limiter I also monitored the output DC offset. The DC offset is quite large until the amp biases up and then quickly drops to 10mV. Amp works fine  :dbtu:

g1

  This is with or without the transformer?

J M Fahey

Quote from: gbono on September 08, 2014, 11:23:04 PM
The core is 100mm x 80mm and 30mm "thick". The windings are about 66mm in diameter.............

Thanks :)