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Monday, May 11, 2015

Beomaster 4400 (2419): Tuner Repaired

The Beomaster 4400 that I am rebuilding right now seems to be running on all cylinders again. But a little more work was needed: 

After restoring the 15V rail the tuner came alive, but with a very weak output, while a standard 1Vpp signal piped into the tape input yielded a normal output response. So the problem was definitely not in the amp section. So I poked around on the IF board (#2). First I suspected a broken filter crystal, but the three turned out to be o.k. But while I was messing around out of a sudden the tuner started blasting full volume, indicating another broken trace/bad contact issue. I cleaned the contacts of the two board-to-board connectors and plugged it back in...no change. So I re-soldered all the contacts of the plugs and I also replaced the two electrolytic capacitor on this board with quality Japanese 105C types while I had it out. I also put in new headers and jacks for the signal meter connectors. They looked strangely corroded with some white coating. Here is a picture of the rebuilt board:

After I plugged everything back together the tuner came to life at full volume, but only on the presets. The main FM dial still did not work. This indicated some more contact issues, since the FM dial is nothing but another tuning voltage delivering trimmer in the context of the circuit diagram. Just looks more important...;-). I finally figured out that the culprit was a crack in the switches PCB (#6):

This was a pretty big one. It separated the 4.7V end of the main dial 100k pot from the voltage source, and so the pot was not able to change the tuning voltage from the 26V on the other end, i.e. the tuner was permanently set to 108 MHz independently of the pot setting.

I installed a jumper (grey) to bridge the crack (the grey/white wire goes to the pot):

This brought the FM dial back to life. Then I epoxied the PCB back together using a couple PCB snippets that I had laying around from designing Beogram 4000 scale illumination LED boards:

Here is a nice shot of the tuning indicators after the repair. Both tuning lights work again, and the stereo indicator is on. Beautiful! Beolove!

One the two capacitors that I replaced on the IF board was completely dead. It showed the characteristic cracks in the can, and measured 25nF instead of 4.7uF. This makes this Beomaster 4400 a definite candidate for a recap:

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