Testing an OPT

I have a Mcintosh MC2105 that had lost power in one channel. I worked on it for quite a while but was was not able to find the problem. I left it with a local audio repair tech for a look. He spent a little time and concluded that one of the OPTs was faulty. Said he found no resistance between the common speaker post and the three other outputs. Likely the output wiring had fused. He had replaced a couple of suspect power transistors but only charged me a couple of dollars for the parts he purchased.

Replacement OPTs are very expensive, if you can even find one. The amp is in good shape cosmetically, so I thought I would part-it-out and buy something else. Before doing so, I double checked the speaker post readings and found little difference between the channels.

I have no reason to think the tech has an ulterior motive: he's generally respected, he didn't make any money for his work, and did not suggest doing further repairs. But, I would like to verity this diagnosis before I start disassembling the amp and would appreciate any advice on how to go about testing it.

Thanks
 
Sorry but your Tech is mistaken.
To begin with, "no resistance" s meaningless, it may mean "open" or "shorted".

IF he found zero or very low resistance (say 0.1 or 0.2 ohm DCR) between terminals,it is to be expected, that amplifier has an output transformer and such very low values are expected.
You will NOT measure anything even close to 4-8-16 ohm there.

Service manual is available at Hi Fi Engine but it requires registration, so you get it and post it here.
If too large for this Forum, post it somewhere else, say Google Drive or something and leave download link here.
 
Here's the MC2105 service manual from HiFi Engine. My goal here is to find a diagnostic method to verify that the target OPT is defective. Best if I could do that with the transformer in-circuit. However, I could de-solder the secondary outputs without too much hassle. Thanks
 

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Diag is easy: the opt is just an impedance matching auto-transformer outside of any critical circuit
  1. Use a voltmeter to measure the output terminal. Any DC? No? Good, carry on. Yes? Bad news, amp blown.
  2. Connect some headphone. Any sound? Yes? Good, carry on
  3. Connect some sacrificial speakers between the ground and the "center" of the speaker switch S101. This is the true output of the amp, the thick line in the schematic. Any sound? Yes? Bad news: 1 or both OPT dead, unless it's a contact problem; try DeOxit.
 
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If speaker switch is "OFF" and headphone working well (both channel), the amplifier part is OK.

Switch this to "ON".
Is both channel power level meter working?
If yes, and shows -more or less- same level, the amplifier channels are OK.

If "bad" channel level meter shows -much- less level (or nothing), the OPT may degraded.
If shows more level, as Zung said: contact problem in S101 speaker switch.
 
Both channels provide ac signal through the input section at the headphone jack. H'ever, no ac signal present at the right speaker output posts. Speaker switch is OK, Physical layout of the amp prevents powered voltage checks (ac and dc) beyond the input section (headphone jack). All output power components on the right side check out OK with power off. (PCBs and heat-sink mounted components both). Removeable power PCBs; were swapped and right channel continued to fail while left channel (using the right PCB) worked fine. As noted in the original post, a couple of possibly suspect right-side output transistors were replaced. Therefore, everything points to the right OPT. Have not yet checked right channel posts for dc voltage but will do so as soon as I get a chance.

Many thanks
 
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This amplifier power sections designed for about 2 Ohm load (usually 2.5 Ohm)
The 043-694 autoformer transform 16/8/4 Ohm load (loudspeaker) to 2.5 Ohm.

It's a potted double C-core autotransformer.
https://hometheaterhifi.com/volume_14_1/images/mcintosh-tour-photo-10.jpg

If it's destroyed and unobtainable, can be omitted. In this case the amplifier would working, but the loading would be higher than optimal, so output power will be decreasing.
 
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With power to the amp off, both sides are the same. Com to: 4 - .06R; 8R- 0.7R; 16 - 1.0 R;.

With amp and speakers on; 0 gain, no signal in: left channel shows all three left outputs at 26 R; right channel all three at 4R. Also with power and speakers on, left outputs all show 0.027 vdc;; right channel, all three -0.014 vdc.
 
Fine on most amps, but not too useful here: there can be NO DC across a transformer primary, even less such a low DCR one.
If any, it straight blows fuses.
On the contrary; it's only useful here: it's an auto-transformer directly coupled to the output stage. Any DC means a blown amp. Look at the schematic.
euro21 is right: this amp was designed for 2.5 ohms way back when the power transistors couldn't stand high voltage.
 
With power to the amp off, both sides are the same. Com to: 4 - .06R; 8R- 0.7R; 16 - 1.0 R;.

With amp and speakers on; 0 gain, no signal in: left channel shows all three left outputs at 26 R; right channel all three at 4R. Also with power and speakers on, left outputs all show 0.027 vdc;; right channel, all three -0.014 vdc.
Good news, maybe the OPT are OK.
Now try:
  1. Continuity tests between the true output/thick line/junction of R147 & R159 and the speaker switch, then on to the output terminals.
  2. Continuity tests between the amp grounds and the output terminal ground

NB.1: resistance measurement with power on is affected by the output offset. Don't do this.
NB.2: output offset is a bit high, but OK
 
On the contrary; it's only useful here: it's an auto-transformer directly coupled to the output stage. Any DC means a blown amp. Look at the schematic.
euro21 is right: this amp was designed for 2.5 ohms way back when the power transistors couldn't stand high voltage.
No, you can NOT have DC across a short, what this transformer primary essentially is.

Basic Ohm´s Law.
 
Good news, maybe the OPT are OK.
Now try:
  1. Continuity tests between the true output/thick line/junction of R147 & R159 and the speaker switch, then on to the output terminals. - YES
  2. Continuity tests between the amp grounds and the output terminal ground - YES

NB.1: resistance measurement with power on is affected by the output offset. Don't do this.
NB.2: output offset is a bit high, but OK
 
It's getting a bit difficult for remote diag.
Let's define some experimental conditions:
  • Input: AC either from a signal generator, about 0.1V, or some music from any source, telephone, CD, etc., at some moderate level
  • The test should be carried out either with an AC voltmeter (report the number), or a speaker
Feed some signal to the input, and see if anything comes out of points 1, 3, 5 and 2, 4, 6 in the annotated schematic below.

1687528324626.png
 
Zung, thanks for the assistance. I've run into a snag which will have to be fixed. I put signal into the device and tested both channels at pin 11; both showed appropriate ac voltage. Moving to the speaker switch, I poked around looking for ac voltage and managed blow the fuse. I changed the fuse and tried to power-on with the speaker switch on; blew a couple more fuses. Then tried to power-on with the speaker switch off. It stayed on for a minute then burned out a couple of resistors; but didn't blow the fuse. The resistors that burned out were 162, 134 & 130. Once it's fixed, to get an ac voltage reading at your #2 spot, I'll have to remove the plug-in board (043-899), attach a dmm lead to the out leg-of R148, replace the plug-in board, then test again..

Background: A few months ago (just to be sure) I replaced the speaker switch with a recommended after market substitute. That worked OK. When I got the amp back from the tech a couple of weeks ago, the switch movement operation was labored. Probably the tech damaged the switch mechanically, then I shorted it poking around. I might have the original switch around; but I probably don't have those resistor values in .5 watt; so I'll have to get some parts. Probably take about a week.

Thanks again.