With no load connected, is there any DC at the output?  What is the DC voltage drop across each of R737, R738 ?

How much each output device conducts is related to the bias circuit.
As you have made modifications, the stock schematic does not show the whole picture.
Post a schematic that includes all your circuit changes.

Now the edits have rendered my question redundant. 
In any case, your answers in posts #2 and #4 are all correct.

Yes that is it, touching 2 component leads with the same probe.
But also, the meter is reconfigured as a short when you are in the current (Amps or mA) ranges.  That is why we must 'break' the circuit and insert the meter in series to measure current.
If you do a current measurement, then forget and try to measure voltage with the meter still set for current, you are essentially putting a piece of wire across the points your 2 probes connect to.  Hopefully the meter fuse will save the piece of gear you are working on.

Good work.  Mainly I wanted to impress the idea of always asking 'from where?'
Whether you are talking about driving distance, or voltages, the point of reference should always be the first thing established.
With voltages, it is usually assumed to be 'with respect to ground', but it doesn't hurt to ask.  And it's very helpful to be able to juggle the reference point around in your head when you are visualizing a circuit.

Quote from: saturated on March 24, 2025, 02:10:57 PMI'm thinking it has something to do with my meter's (internal resistance?) 

Yes.  You could look it up, or you could solve for X: 
8.76/12.80 = x/(x+6)

edit:  my mistake, should have read 13.8, not 12.8, see below.

A nice visual reminder that voltages are always in respect (or reference) to something else.  In this case it was ground (common) but it does not necessarily have to be.

For bonus points:
1) what are voltages at A and C with respect to B.
2) what are voltages at B and C with respect to A.
 

Quote from: saturated on March 19, 2025, 08:41:56 PMI got to thinking I guess these guys flash sixty times a second. 

The LED elements run on DC.  Under the plastic cover of the bulb is a circuit board with the electronics.

Here's one version of schematic.  4 transistors, driver transformer, output transformer. 

Quote from: DPM309 on March 15, 2025, 11:57:50 AMThe schematic shows them as 2N5988 (PNP) and 2N5991 (NPN). The markings on the transistors are TIP42B (PNP) and TIP41B (NPN). The ones I purchased were TIP42B and TIP41B which are good replacements.

I responded to your post over at MEF forum, but will include it here for others who may be working on these amps.
The TIP41 and 42 have BCE pinout.  The original 2N5988 and 5991 have ECB pinout, so the TIP's will not work as replacements.
There are fake datasheets out there showing ON Semi 2N5988 with TO3P case and incorrect pinout, but those transistors do not exist.
The real 2N5988 and 5991 were TO-225 case with ECB pinout.

Edit:  received response that this amp had been working and came with TIP41 and TIP42 outputs.
So can only conclude there are more than one version and be sure to check where the traces of the output devices are going, as there seems to be only 1 version of schematic available, and it does not match the later version.

Quote from: J M Fahey on February 20, 2025, 05:42:01 PMThat´s why each of the channels is already BTL, you can´t undo that.

Now if you wire both BTL amps in parallel, you can get twice the current but peak voltage does not change.

I'm still wondering if Ch.1 and Ch.2 can be put together in regular bridged configuration, rather than the parallel PBTL.
If so, it would give full chip output into 8 ohms (rather than the 2 ohms of PBTL).
The reason I raise the question is that if it were possible, that may be what the Pandora is doing and how they get that high power figure into 8 ohms.  If so, I guess the amp would be marked 'minimum load 8 ohms'.  (edit:  minimum load 8 ohms confirmed here:  https://lichtlaermaudio.com/shop/pandora )

Quote from: Tassieviking on February 20, 2025, 11:08:49 AMPBTL is just 2 amplifiers working opposite to each other, one puts out a positive signal while the other one a negative and then it reverses during each sinewave.

You are describing BTL.
PBTL is something new to me, putting 2 BTL amps in parallel.
It gets confusing in that TI still refers to the 2 'stereo' sides as BTL.  I guess that is how they are made in the chip, but if you can't access each non BTL amp seperately it is kind of pointless to call them out as such.

So each 'side' of the 3118 is called 'BTL' but you can run the 2 sides in parallel configuration which is called 'PBTL'.
In PBTL configuration you can drive a load half the impedance of what each side is capable of.
Regular BTL requires double the load impedance that each amp seperately would be capable of.
So the 3118 can drive 4 ohms per side, or 2 ohms in PBTL configuration.

Quote from: joecool85 on February 19, 2025, 11:53:28 AMThis chip does 2x BTL outputs.  At 24v, 8ohm, it puts out a solid 33w per BTL output with 1% THD.  At 4ohm, it is 60w.  This chip does offer PBTL, but he graphs only show 2ohm and 3ohm.  At 2ohm you can do an impressive 120w.  This makes sense because each side of the chip would see 4ohms (making 60w), adding up to 120w.  PBTL allows you to run lower ohm loads and higher wattage.  But my understanding is that running PBTL with 8ohm would still only yield about 30w, not 60w.

PBTL is parallel running of the 2 sides, something new to me.  But is there anything saying you can't run the 2 sides in series (standard BTL)?  That would give full power into double the impedance.

One more question, in the very first sentence of the datasheet, what does TI mean by "4 ohm BTL load" ? 
A load is a load is a load, isn't it? 

Maybe this one (corrected version)

Quote from: saturated on February 02, 2025, 05:43:03 AMAt night in the small town it got quiet with the exception of an audible hum 
Sometimes I would have the windows open I was in an upstairs room at the end of the house.  This (real or perceived) hum/pitch/monotone frequency tormented me to some end.  In fact I made a few nocturnal excursions around town looking for it.  I would drive around to different areas and shut the engine (slant six 66 Dart) off and sit and try to determine where it was coming from.
Spoiler alert: I never did find it or find out anything.

Tinnitus is not always a ringing and can be a hum.  I have it at 104 Hz.
Most of the time I don't notice, but sometimes it gets bad.

Sometimes the hum can be external though.  For a long time there were people driving around in Windsor, searching for the hum, like you.   

https://www.detroitnews.com/story/news/local/wayne-county/2020/07/28/windsor-residents-rejoice-after-mysterious-hum-goes-silent-after-decade/5529975002/