We had two McIntosh MC275-IV amps, a version from early 2000s, in the shop recently for service. One amp had a bit more damage than the other. We serviced both quickly and they are sounding great.
One of the amps came in with a power tube that had a distinct white coloration inside. The white substance is called getter. It is normally a shiny silver/gray. Getter is a powered metal that reacts strongly to oxygen. The getter is fired during manufacturing when the vacuum tube is sealed. As a result it flashes onto the inside of the glass. In the process it helps evacuate the tube. When it turns white it indicates that the tube has lost it’s internal vacuum. The photo below, credit to Tube Depot, shows a good tube on the right. The tube on the left has lost its vacuum. There is an excellent discussion of tube ailments like this and others on the Tube Depot web site.
We tested all of the amp’s tubes on our Hickock tube tester and not surprisingly that fogged tube was bad. Bad tubes often indicate issues inside, and this was no exception. The amp needed replacement of screen grid resistors and several other bias resistors. The first photo shows some of the rework.
The 2nd amplifier had a bit of oil on the top surface and the customer wanted it checked out. Part of the reason that McIntosh amplifiers ares so beefy is the high quality of the transformer wiring. The transformers are then potted in tar. This helps avoid any vibrations or noise. In this case, the oil was simply a bit of residue from the potting tar that had leaked out onto the surface. We cleaned and polished the chrome and the amp looks as good as new.
When a unit comes into the shop, we always check for applicable service bulletins. In this case this amplifier was due to have mod made per a posted bulletin. The bulletin calls for replacement of a key pair of capacitor. We quickly performed that work, shown in the last photo above.
We love the MC275 and enjoyed listening to this pair while we had them in the shop. If you have a McIntosh MC275-IV in need of service, contact Bob the Tech Audio at our service link.
