Fellow Drake Enthusiasts,
Just to recap the past several months, I had a problem with the fan in one of my L7 linear amps. It would work but after several minutes it would start making noise and then just stop. After fault isolating and finding nothing defective, I incorrectly assumed it was the fan. After replacing the fan with a new, original, or improved version, replacing the thermostat twice, replacing the dropping resistor twice, and replacing the fan circuit leads, I was on the verge of running out of alternatives. Then a friend (thanks Woody!) indicated he had a similar experience and used a different fan, with success. I followed in his footsteps and, without having any logic for doing so, replaced the fan with a new model that had a similar current rating as the original. As part of the replacement process, I changed the series dropping resistor from 470 ohms to 300 ohms. The particular fan that I used was an IMC Boxer, model 4715RS-12W-B10. The result? It worked and has continued to work with over 6 hours of testing so hopefully this is a permanent solution. Moral of the story? Replacing the fan with an original was not a solution as one would have expected. Why? Well, although I bought the fans from different sources (one was directly from the manufacturer), and they had identical part numbers, something in the manufacturing process changed. I talked with a rep on the phone who informed me that the 450S fan was a sleeve model category, while the 450 was a ball bearing category. Since the company has change hands 2-3 times in the last 8-10 years, hard to know what happened when. So even though the 450 and 450S had identical part numbers, they both must have had some production line variances perhaps in the lubricant used in the fans. Obviously, the heat from the L7 was too much for these fans. Hence, if one needs to replace the fan, considering alternate models of the fan should be a priority. There might be some experimentation, however, to get the right balance between slow speed cooling that is quiet yet effective, versus when the thermostat closes to activate the high speed configuration. Selecting the right fan and dropping resistor is a trial and error process but it shouldn't take too long. I hope the journey that I have been on will shorten your path should you have to replace the fan. 73, Evan, K9SQG
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