MC2505 Autoformer Question - Load on DBT

thr33p4c

AK Subscriber
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I am working on an early MC2505 and working through a botched repair by previous tech. The multi section caps in the power supply are failing and this technician simply paralleled capacitors onto some of the multi section leads instead of replacing/re-stuffing the cans.

I also found that the Autoformers were completely bypassed. The tech simply ran the output straight to the 8 ohm tap, then continued to botch the repair by dumping a bunch of solder into the other taps to make the screws unusable.

I measured DCR on the Autoformer leads, no open connections and both L/R Channels measure exactly the same. After tracing the Autoformer leads I found the inputs disconnected, 8ohm lead detached on one channel and 16 ohm detached on the other. The tap tie downs that were still in place did not match channel to channel. After taking a closer look, I noticed that the Autoformer labels were different. One gold, one black, but same model number. The only thing that I can think of is that someone replaced one Autoformer at some point in the past but tied down the wrong connections to taps on the back of the amp on accident. This may have resulted in the last tech noticing a channel imbalance that could only be solved by bypassing the Autoformers.

I proceeded to reconnect the Autoformers and tested output on each tap. The signal was balanced and sounded fine. I tested 4/8/16 ohm taps with an equivalent load and found no issues.

The only thing that struck me as odd is the fact that my DBT shows more current being drawn by the amp when the Autoformers are in circuit. I am using a 150W clear bulb and as soon as I flip on the speaker switch (Autoformers in circuit). The filament of my dim bulb reacts and stays dimly lit, increasing as I increase volume on the input signal.

This is my first time working with a solid state amp with Autoformers and I do not know if this is normal.

When the Autoformers are in circuit does it put a load on the amp circuit? The dim bulb reacts as above even with no signal and no speakers connected.

If I bypass the dim bulb and run the amp straight into my Variac at 117VAC the power supply voltages are almost dead on with the schematic.

Trying to figure out if all of the above is normal or if the previous tech had a better reason to bypass the Autoformers other than channel imbalance due to connection error.
 
I don't run McIntosh amps on DBT's so I can't be sure, but I am sure because they are an Auto-Transformer which is a single winding with several taps on the way to ground, then its reasonable to assume they do present a small load on the amplifier.
 
Without an output loading, I wouldn't think so for a sound signal, but for dc offset, yes.
 
Check for dc offset with the autoformers disconnected, and if present, consult service manual.
 
You'd have to look to be sure, but IIRC the autoformers would show a 2.1 Ohm load to the amplifier on some models. It could be this impedance drawing current from the amplifier.

If there's no load on the outputs, then I'm not sure what it would be.
 
Modeling the arrangement in my head, it seems to me that the autoformers should be both transparent and "invisible" to the power stage output to be properly functional, they should draw no signal energy themselves.
Otoh, they will be a low resistance shunt to any DC offset appearing there and that would appear as unwanted DC power supply loading.

Someone with deep technical expertise would have to come in to explain how the autoformers actually function to couple the speaker load to the amp without being a signal load themselves.
 
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I've seen some similar repairs to Mc autoformer amps - some folks just shouldn't be mucking around inside these things.

OP - why not insert your ammeter in series with the AC inlet so you can obtain better data?
 
Thanks all for the comments.

Ill measure DC offset this weekend - check the trimmer settings based on SM and measure the change in current with and without autoformers in circuit.
 
I just took a look at the SM. There are no instructions for the 4 trimmers which I believe are Bias and DC Offset for each channel. On a quick search I found a post about instructions on measuring the current as you turn up a variac. This seems to indicate that the circuit does indeed draw current at rest but mine may be way out of whack.

http://audiokarma.org/forums/index.php?threads/mc2505-dc-balance-pot-adjustment.145993/

Any insight on trimmer settings on this unit would be greatly appreciated. SN is in the 10K01 -15K00 range
 
In the Akdatabase under service bulletins 1964 page 23 of 44 there is a mid 60s primer on setting the MC2505 bias.....remember back then all this semiconductor stuff was truely bordering on rocket science.
 
I an getting confused, the early 2505 had one adjustment per channel for bias plus the early pre-amp board has DC offset adjustment. The later and most common MC2505-2 had no adjustments and used the same pre-amp board as the 2100 and 2300.
 
I am trying to source a power transformer without luck. Audio Classic has it but just the core (not potted)
My 2505-2 has no adjustment.
 
Guys, thank you for getting me pointed in the right direction c_dk, that service bulletin note was just what I needed.

Checked bias on both channels. Its supposed to be 40mv. I had one channel at 150mv! I adjusted the bias trimmers to maximum and could only get them down to 111mv and 71mv. I could not find the instructions for DC offset but was able to figure it out. I probed between ground and the feed to each autoformer then adjusted the trimmers on the pre board. They were both way out showing between 100 - 300mv. As soon as I zeroed these in the dim bulb feedback went to nothing. Readjusted the bias to 40mv on each channel. Now with no input at idle not a peep on the Dim bulb. No change when bringing the autoformers in and out of circuit.

I think this one is solved. Does all of this make sense? High DC offset causing Autoformers to shunt the unwanted DC to ground loading up the circuit?

If so, I don't think I can ever assume that the tech before me knew what they were doing. From the looks of it, the perfectly good autoformers were bypassed, DC offset and bias were all out of whack, and on top of that, junk parts were used. I thinks its time to get on to the fun part and rebuild the power supply :)
 
Guys, thank you for getting me pointed in the right direction c_dk, that service bulletin note was just what I needed.

Checked bias on both channels. Its supposed to be 40mv. I had one channel at 150mv! I adjusted the bias trimmers to maximum and could only get them down to 111mv and 71mv. I could not find the instructions for DC offset but was able to figure it out. I probed between ground and the feed to each autoformer then adjusted the trimmers on the pre board. They were both way out showing between 100 - 300mv. As soon as I zeroed these in the dim bulb feedback went to nothing. Readjusted the bias to 40mv on each channel. Now with no input at idle not a peep on the Dim bulb. No change when bringing the autoformers in and out of circuit.

I think this one is solved. Does all of this make sense? High DC offset causing Autoformers to shunt the unwanted DC to ground loading up the circuit?

If so, I don't think I can ever assume that the tech before me knew what they were doing. From the looks of it, the perfectly good autoformers were bypassed, DC offset and bias were all out of whack, and on top of that, junk parts were used. I thinks its time to get on to the fun part and rebuild the power supply :)
Nice work, I'd say thats what was happening.....
I didn't realise you had the model with adjustable DC offset and Bias. Good work chasing that down.
 
Reading those old service bullitens reminds me how new all those "new" transitor designs were to the old old techs steeped in vacuum tube knowledge.
 
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