| Posted by Steve L. | November 09, 2020 - 07:13 am |
| Hi Jake, That sounds like an excellent background. Yes, not having the mica insulators would mean that the collectors were shorted to ground. Take heart though, the good news is that it should have blown only one or more fuses. It's likely that the output transistors would not be damaged. More good news is that TO-66 mica insulators still exist and are available at low cost from Mouser. (Just search "TO-66 mica insulator" on their website.) When installing, I usually coat the bottom of the transistor, apply the insulator to it (it will be held by the grease) and then coat the bottom of the insulator. When you replace the DC fuses, I recommend 2A instead of the original 1A. Also, it would be good to check the collector to emitter resistance of each power transistor. The main failure mode is a C-E short of typically less than 10 ohms. A value of 1Kohm or more would usually exonerate that failure mode. Let me know how it goes. We can get this! |
| Posted by Jacob B | October 14, 2020 - 08:14 am |
| Hi Steve. My degree is in EE. It has been awhile since I have really practiced though as I am more on the Manufacturing Engineering side of things in my job. I do have some basic knowledge still and I have a DMM. I can get access to a scope if need be. I have all of the switches in the correct location. I have the speaker wires correctly attached to Main and had that as the output. I may have screwed up bad though when removing the transistors and applying the heat sink paste.. I discarded the insulators after thinking that they were dried grease as I had forgotten about that detail in your document above. :( I pulled the transistors up a bit off of the metal plate and measured the top of them and got 1.5V. I had 0V when they were screwed down all the way. Perhaps I have shorted them out with my mistake..oi. Jake |
| Posted by Steve L. | October 13, 2020 - 03:03 pm |
| Hi Jake, Not to worry; we can get to bottom of this and I'm happy to help. So I can better assist, can you give me an idea of your experience in electronics? What test equipment do you have available---DMM? Oscilloscope? Do you have the 3070 manual? I take it you are applying audio to both channels (which inputs?) but cannot hear anything on either speaker. Is that correct? Here are the preliminary things I would check: Speakers switch set to Main or All and speakers connected to Main and Ground terminals; power light on; Input Selector set correctly; Off/Tape switch set to Off; Loudness switch off. With the DMM black lead on ground, you should see 40V on the case of two power transistors and 20V on the case of the other two. If one or both 40V tests is bad, check the fuses next to the output terminals. If fuse(s) are bad, there is a check we should make before replacing them... |
| Posted by Jacob B | October 13, 2020 - 09:00 am |
| Hi again Steve. I swapped those caps and also applied new thermal paste under those transistors. Sadly, the cap replacement did not resolve my issue. I am still getting no output through the speakers at all, not even noise. As mentioned before, I can still get some audio if I use the tape out and attach a blue tooth speaker. It appears that something that affects both channels must be broken. The caps were definitely leaking bad so replacing them was worth the effort either way. Jake |
| Posted by Steve L. | October 07, 2020 - 04:26 pm |
| Hi Jacob, Actually, when I said, "and it's mate, C5," I was referring to the 2000uF cap which is the output coupling cap in the right channel (not related to C305). C6 is the output coupling cap in the left channel. So I was saying it would be best to replace both C5 and C6 with a 4700uF value. Yes, C5 could be the issue you're trying to fix. I trust you've checked the fuses. They often blow when there's a problem in one of the power amps. In any case, I'll be happy to help if the cap replacement doesn't fix it. |
| Posted by Jacob B | October 07, 2020 - 02:46 pm |
| Hi Steve. Thanks for the prompt reply! When you say C5, you are referring to C305 correct? My C6 cap is blown and leaking which I am hoping is what caused the output to stop working. I still get photo output through the tape output which I have hooked up to a Bluetooth speaker temporarily. Hoping replacing the C6 cap does the trick! Jake |