nyjb
Junior Member
Posts: 54 
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Post by nyjb on Dec 20, 2018 15:47:34 GMT
Hello,
I am working on a 478 and stumped. I'm much more familiar with 50s Seeburgs, so the circuit boards are a new challenge. Perhaps there's a common problem corresponding to my symptoms.
The amplifier was producing sound in one channel. The sound system tests for the other channel produced no readings.
I found two blown 3 amp fuses, and began to locate the problem by trying the driver boards in each channel, swapping them and started to get speaker overload. I couldn't find a pattern, nor could I reliably reproduce results, but I usually lost a fuse or got the overload. It is like chasing a ghost.
Subsequently, I found that the speakers had been connected to the wrong part of the audio terminal strip, and one of the small speakers was open. Perhaps that's a clue.
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Post by Ron Rich on Dec 20, 2018 16:08:21 GMT
Yes--Improperly connected speakers will cause problems--I am not familiar with RO amps, but if I were trbl shooting it, I would look at the driver, and output transistors--- ?? Ron Rich
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Post by jukenorman on Dec 20, 2018 16:37:43 GMT
Yes as Ron says, check the output transistors - 3055s, nice and easy to change, they are in sockets. The driver boards are mounted in edge connectors and the soldering of these connectors to the main board is prone to problems, I have found, so if you are looking for a generalisation, you could check that. It's not actually a particularly difficult amp to work on.
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nyjb
Junior Member
Posts: 54
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Post by nyjb on Dec 20, 2018 19:48:11 GMT
That's very interesting jukenorman. I hadn't thought of the edge connectors themselves. Although the symptoms seems to appear arbitrarily, I know that there is a logical explanation. Since the problem seems to move everytime I touch something, that'd would make sense.
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nyjb
Junior Member
Posts: 54
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Post by nyjb on Dec 20, 2018 20:15:14 GMT
By "main board," do you mean the thermal plate? I've replaced the transistors already-- which I know doesn't guarantee anything.
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Post by jukenorman on Dec 20, 2018 22:22:49 GMT
Actually, I think I meant the pre-amp board (now that I've had a look at the manual!). I've had an amp that would bring up the overload lamp from time to time but when I powered it up on the bench it would be fine - took me ages to find it was poor solder joints on edge connectors (they plug in, don't they?). It didn't blow fuses though. Fuses would be pointing to the output transistors. The last one of these amps I looked at, the same solder connections looked a bit iffy too.
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nyjb
Junior Member
Posts: 54
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Post by nyjb on Dec 26, 2018 20:23:43 GMT
The edge connectors don't plug in, if by "edge connectors" you mean the clips that hold the board in place.
Meanwhile, I checked all the transistors, replaced a bad one (a new one, at that) and got it up and going. Then I closed the amp cover. When I turned it on again, the big noise had returned. I'm looking for anything loose, or with damaged covering. Are there any "hot spots" where a problem is likely to show up here?
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Post by Ron Rich on Dec 26, 2018 20:40:09 GMT
Once again--sounds like bad connection(s?)-- The "edge connectors" are, can be, anywhere a "board" is plugged in-- check the solder joints, on these connectors -- they often become "poor", "cold soldered", or "dry soldered".
Ron Rich
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Post by jukenorman on Dec 27, 2018 9:59:58 GMT
The solder joints on the connectors on the pre-amp board was what I meant. My memory of the exact arrangement was a bit confused! In any case, you will be able to power up the amplifier in situ with the front panel open which should allow you to determine which connectors are a problem by "jiggling" them.
Norman.
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nyjb
Junior Member
Posts: 54
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Post by nyjb on Dec 28, 2018 17:50:44 GMT
Wow! I took a look with a magnifying glass and it's amazing what you can miss without it. The circuit boards are a whole new world. Not sure it's a world I want to live in, but I appreciate the guidance while I'm here.
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