It's my first time here so I don't know if I'm in the right place. Probably not, since it says to post a response, but here goes.
I am trying to refurbish my mc225. It's been around the block a few times, being hauled around Texas where I worked until retirement, and in the studio and on the road. Most of the voltages are within 10%. In fact, both plates of the 7025 which is used in the place of the circuit's 12AX7, are at 116 volts, which is only .8% high. The -21 volt grid biases rage from -20.8 to -20.9 volts, which puts it within an average 1% tolerance. Other plates are higher than called for. The V4 and V5 12BH7 plates are supposed to be 285 volts, but are 315 and 326 volts respectively, making their average 12% high. The plates on the 12AU7s max out at 10% high and the plates on the 7591 are within 7% as well as the 4-8 screen voltages.
The resistances on pins 2 of the 12AU7s averaged 32% low. Both plate resistances on the 7025 were 134K, which is 26% low. The maximum plate resistances on the 12AU7s after capacitor settling were within 1% as were the pin 6 voltages. The average resistance of all 4 12BH7 plates was within 5%. The average resistances for the 7591 plates were 5% high and pins 4 and 8 average was 3% high. The average resistance for pin 5 on the 7591s was .8% high.
For comparison, the resistance (not impedance) of the 70V leads of both output transformers were measured and found to be within 10% of each other. The 4, 8, and 16 ohm lines were also checked and found to be identical.
The problem shows itself with the outputs. A 1KHZ squarewave with a 10 ohm 25-watt resistor on each 8 ohm output produces only 5 volts peak to peak on the scope. It's a good, sharp square wave, but at that level, it should be. The resistors used to get warm in the good old days. Less than one watt rms might work just fine for some headphones but is pretty lousy for speakers. The other clue is that with no signal pins 5 of the 7591s range from .2 to .67 volts with an average of only .385 volts for all 4 when there should be .8 volts at pin 5 of each.
I have replaced all of the filter capacitors, along with all components of the bias circuits and changed the chokes to 68 ohm resistors, and replaced input and inter-stage coupling capacitors and the two cathode capacitors for the 7025. I have also tested the 7591 pin resistances for shorts or opens and found none.
In short, help! It's obvious I'm missing something. I feel like I did many years ago when I was a technician for a company in Odessa, Texas many years ago. I would be going nuts working on a mini computer and my boss Woody would come back, touch a thing or two, and the thing would start working. He would note that his fingers never left his hands. Is there another Woody in the house?
I want to sell this thing I used to like on eBay to raise money for a school fund for the kids of the kids I have had in ministry for the last 14 years. All I need is $2200.00 which a 79-year-old living on Social Security just can't do.
