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 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.