Gingerbread Cookie Day

There are no actual gingerbread cookies in the blog, but I was at a loss for a title. I looked up national commemorative holidays, and settled on National Gingerbread Cookie Day on November 21st. (It was also Goldie Hawn’s birthday.) Making and decorating many, many dozens of gingerbread cookies for the holiday season was a family tradition when I was growing up. (This also shows how long I’ve been writing this - because it’s been well over a week since I started writing. Alas for slow writing.)

Puppies and Mushrooms

The recent rains have made dog walking challenging. Here are the guys in Indianola during a slightly dryer moment.

The rain has also produced quite a few fungal fruiting bodies, a.k.a. mushrooms. I don’t have the mushroom identification skills to say which ones these are.

The MCI 16-track Recorder

A few weeks ago I ran tests on a power supply from an MCI 2” tape machine. Tape is now very much old-school, but there are still studios using it for its warmth and unique sound.

The tests I ran showed that the power supply is working as designed, so the issue is somewhere else in the machine.

Here’s the motherboard with the daughter boards removed for access to the test points in the upper right corner. The expected voltages are conveniently printed on them. Interestingly none of these expected voltages was present, and the fan on the transport power supply wasn’t running while the fan on the audio power supply was running. Since I had already shown that the power supply is working when it’s tested separately, I started looking for something systematic that was preventing supply voltage from getting to it.

Here’s where the power cable plugs in, and also where the two power supply modules plug in. The multipin connectors both provide 120V AC to the modules and bring the DC voltage supplies back from them. The two pins in the upper left of each connector are the AC line. The audio connector has the expected 120V, but the deck connector doesn’t. I traced the problem to the power switch.

Here’s the underside of the power switch. The purple wires are for the audio supply, and the orange are for the transport. The contacts on the orange side apparently weren’t connecting with each other. Once we wiggled the switch a bit, the transport got its AC supply back on, the fan came back on, and the DC supplies came back on. I reinstalled all the daughter boards, and the transport lights came on but there was no torque or back tension on the tape. We realized that the auto-locator needed to be plugged in, and we did that, but it still didn’t work. Later the customer removed and reseated the torque control daughter board, and the machine is working again. Whew! I’m glad he solved it. I didn’t write a haiku for it because the ultimate completion of the repair wasn’t on my watch.

The Crown Preamp

For the most part, the process of replacing electrolytic capacitors is not a difficult one, though there can be some annoying quirks.

This preamp has two printed circuit boards. The one hanging at upper right was fairly simple to access so that its caps can be replaced. The bigger one at the left, with all the pots and switches, is a bigger deal. I had numerous opportunities to curse at Crown’s design engineers or to speculate about what illicit substances they had been smoking or ingesting. This preamp was designed to look good from the outside, and they pretty much ignored how difficult it would be to take it apart to repair.

You may recall seeing the front panel previously. The area around the volume control is covered in an oily residue. I had thought that it had resulted from someone spraying the pot from the outside with WD-40 or something similar. It turned out to be something even more regrettable.

The volume control turns out to not actually be a pot. Instead it’s a stepped attenuator. It’s a 32-position switch fitted with two sets of graduated precision resistors. Since it’s a multi-position switch, it needs lubrication.

Someone at Crown really thought that “more is better” and then doubled down with “if more is better, than even more has to be better still” when it came to lubricating this switch. They used at least twenty times more grease than they needed, and they forgot about the effects of time and gravity. Here’s just some of it on the back of the faceplate as I removed it to access the underside of the big circuit board. There was more lubricant… a lot more. The thinner component of it had oozed all along the joint between the bottom plate and the faceplate, and had migrated through the switch mounting hole and out onto the front of the faceplate as we saw above.

I got the faceplate off, and found that there was also a huge glob of grease on the board itself. At that point I determined that the attenuator needed to be removed so that all the old grease could be taken off. It’s never simple, is it?

All of this was so that I could reach the underside of the board, desolder the original capacitors, and solder in new ones. It’s like walking a mile to change a lightbulb and then having to walk back: the journey takes a lot more effort than the task itself. So I got the caps changed, the attenuator cleaned up and relubricated (with much less lubricant) and reinstalled, and I got it put back together for testing.

The moment of turning a unit back on after working on it is always a little fraught with tension. In this case it powered up just fine, but it didn’t have any sound. I was somewhat disheartened.

There had been several points along the way that I had some cause to make grumpy comments about Crown, and now I made some more. I had the circuit diagram and the board layout graphic, but the circuit as built did not fully match the diagram. In particular the component that was overheating was not shown on the diagram, and a different component was shown on the board layout. After a while I identified that the problem was a short on the little phono preamp board at the back.

Here’s the phono board. I pulled it out and powered it up with a bench supply in place of the +18V / -18V supply from the preamp. Bit by bit I crossed the suspects off the list, until at long last I found the issue with one of the capacitors I had replaced. In the circuit it measured OK. Out of the circuit it measured OK, but the short was resolved when I took it out. I replaced it with another one and everything was fine. I didn’t see any solder bridges or anything else to explain this.

Once I got that sorted, it was all happy. Finally it got a haiku:

vintage Crown preamp
cleaned corrosion and recapped
now it all tests clean

After that saga we need a puppy break, right?

Car Rides

Tyr has discovered the joy of sticking his head out the window as we drive.

So has Odin.

These discoveries have made car rides more enjoyable for us, because the pups typically end up wrestling and playing and barking at each other in the back seat. Having both windows open far enough to let them amuse themselves with the wind may be a little chilly in the fall, but it’s still a better option than the barking.

There were quite a few other repairs completed in the last few weeks, a surprising number actually considering how unfocussed and tense I’ve been feeling. The queue is shortening. If you are still waiting, consider the rugby scrum I showed last month, and imagine your project wearing a rugby jersey and pushing its way to the bench at the center.

As always, this blog is 100% Alden and Tyr and Odin. No little electronic neural networks were consulted or listened to.

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Stay warm, remember that the solstice is coming and the days will soon start lengthening, and before we know it the leaves will be on the trees again.

Before I go, I’d like to make a special shoutout to Garry and Michael. Garry asked almost plaintively if I was going to drop him of the blog now that his repair is done, and I assured him that he could stay as long as he wanted. Thanks for the visit and the hugs, guys. You did my heart good.

Bright blessings,

Alden