In a prior life L'il Mar must have been loved, because she was most certainly used.
Sometimes I forget that the components that I work on are electro-mechanical devices, and often their most challenging problems are caused by old fashioned physical wear and tear. Caps dry out, contacts tarnish and transistors die in lightning strikes, but switches and pots succumb to the human touch. And sometimes just moving a component can be the difference between working and not.
In our last episode we had made some progress: the rear channel of the Marantz 4220 Quadradial receiver had come to life after replacing 8 transistors in the tone control ciruitry. But the front channels remained distorted and the balance control sliders seemed... wrong.
The next trip to the bench started with pulling the tone control card to expose the slender strip of a card that holds the 3 sliders: front, fader and rear balance control. Here is the front slider:
In my brief experience refurbishing vintage gear I have grown to dislike slide controls. While satisfying ergonomically they are harder to clean and much more fragile that the typical pot. A quick check with the meter indicated the front was completely gone. I removed it from the board and took it apart and discovered the tiny contact was missing. Evidently it broke off and eventually fell out the slit. The fader in the middle was functioning, but the rear was marginal and the sonic mid point was skewed left. I put the front slider back and left the question of 'to slide or not to slide' until another day.
Look, Ma! No tines!
While useful to discover, unfortunately the sliders were not the source of distortion in the front left channel. Something else was afoot. At this point the A Team (eg. Mike) decided I had suffered enough and pulled out the 'scope. He walked through the very dense tone control circuitry and isolated the distortion to a section of the card between the bass and treble controls. He then flexed the card and the distortion healed. Eureka! Sort of.
Above, at the lower right is the tone control circuit board, dangling. Just above it you can see the front fader attached to the face plate.
The tone control/preamp circuit board is very dense and two sided, and there was most likely a faulty solder joint between the sides causing our distortion. Less than perfect soldering is probably the second most common cause of failure in vintage gear (behind faulty caps). But it is by far the most common cause of failure in modern gear. If your home theater receiver quits working, try picking it up and shaking it. It's liable to work for a few more weeks after 'treatment'. (Try at your own risk. We are all adults here, right?) The constant thermal ebb and flow along with occasionally moving a component will eventually cause weak solder work to fail. And sometimes just another move will fix the problem, albeit temporarily. A few years ago I fixed the kids' modern Sherwood receiver by merely shipping it in for warranty work. Sherwood said it worked fine, and it did for about a month on return.
Mike spent a solid 10 minutes hitting every solder in the tone control of area and especially the rivets and the very complex pots, and finally the front left channel lost its distortion.
At this point I celebrated a bit, but as we all know that is a very dangerous thing to do. Yes, the saga of L'il Mar is not over by a long shot...