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Post by prime on Jul 16, 2023 16:28:01 GMT
Hi guys i have only 20% volume on the left side, looked at cart connections, checked all speaker connections. Im thinking amp issue but before removing it , i tried reversing the three prong cart plug and either way its the left side that has an issue, does this confirm it is amp and not cart or cart wiring? thanks
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Post by jukeboxmarty on Jul 16, 2023 20:38:17 GMT
I would agree, it would seem to indicate the amp.
*If you're sure about the speakers*
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Post by jukenorman on Jul 16, 2023 21:08:19 GMT
There's a fair bit of info on that amplifier with that kind of issue already in the forum history!
Norman.
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Post by prime on Jul 16, 2023 22:53:01 GMT
I searched 477 Max and only got about 8 posts, none describing volume loss on one channel, the manual does not discuss this issue, it has tests for all boards except drivers etc. I reversed the driver boards but still had the issue on the left side, I tested the speaker connector down to the speakers, i get good continuity on both sides. I removed the right woofer leads and put them on the left woofer, when using right channel very loud. It seems i am only getting a little amplification to the left side, beside the driver boards what other boards amplify the channels? I think i may have found the issue, i have marked the pin that has no continuity with the 8 pin connector on the right, its the only pin on both left side connectors that has no continuity, I will reflow entire board to see if i can get a connection.  |
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Post by jukenorman on Jul 17, 2023 16:06:55 GMT
That particular amplifier was used in many more models than the 477.
Norman.
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Post by prime on Jul 17, 2023 23:38:56 GMT
After extensive testing i have confirmed it is a short in one of the driver boards, i found a fuse was removed and when i replaced it , the unit showed short with dim watt. I thought the easiest way to find the issue was to start disconnecting boards, I stared with one driver, still short, i switched driver , no short, i moved the driver to the other slot, no short and when single driver in each position i get full sound left then full sound right, I believe this confirms defective board.. I cannot find any defective part comparing values from good board to bad, any thoughts....pete I think my red transistor is shorted, does anyone know what the replacement is?
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Post by jukenorman on Jul 18, 2023 19:51:28 GMT
TIP41C and TIP42C - you would probably need to replace both driver transistors even though only one may be short, if you use these.
Norman.
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Post by prime on Jul 19, 2023 21:40:29 GMT
Hi Norman, I dont like to give up but the amp is unforgiving, I replaced the power transistors and started up with a dim watt, first i used it in the channel that was operating properly, dim watt showed no short , used full power got music. Moved it to the problem side, dim watt seemed ok for about 5 seconds then got brighter at the same time a very load noise coming from speaker . I turned it off but find that board is now shorted again, obviously the issue is further down the line , Im not sure if it makes sense to remove each board, remove components and test each one or just replace the amp. Not sure which board would have caused a loud sound coming through speaker with no record on the turntable. Do you or anyone here have an amp for sale/ pete
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Post by jukenorman on Jul 20, 2023 9:55:55 GMT
Hi Pete,
When you say you replaced the power transistors, I presume you mean the driver transistors since it was these you asked about? Did you set the biasing as instructed in the manual after you replaced the transistors? Always a good idea to (the very first thing Ron tried to teach you) RTFM - page 63! I suspect that will resolve your issue. You could always check the two potentiometers first just to make sure they're OK.
Norman.
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Post by prime on Jul 20, 2023 18:01:47 GMT
I do mean the driver transistors, I read the manual but was afraid to do the setting with power on, i guess with the speaker socket detached and volume at low, the short issue will not be an issue? pete
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Post by jukenorman on Jul 20, 2023 19:18:54 GMT
Come on Pete, if you read the manual you should have seen that it specifically states that the procedure is to be carried out when the driver board has been repaired! You must follow the instructions to the absolute letter and you will see that certain actions are to be carried out before you power up. Doing that (particularly the potentiometer settings) should have ensured that the transistors are set well back from optimum biasing which you set after power up.
Norman.
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Post by prime on Jul 20, 2023 19:39:33 GMT
When i reinstalled the board after replacing the transistors i turned the R33 to minimum and the R25 to center, I put the board in on dim watt to see if there was a short, Im following the instructions to the letter, i have no way of confirming the board has no short except for checking all components and replacing defective ones. When i put it in it seemed to cause an issue again. All i can do is check the board for faults, if i dont find any assume the board is fine and install as earlier with no speaker or volume and set bias and hope its ok.
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Post by jukenorman on Jul 20, 2023 20:54:22 GMT
You have me totally confused Pete! You didn't mention anything earlier about setting the pots - and how could you possibly have followed the instructions to the letter when you stated that "a very load noise coming from speaker" which should have been disconnected!
Norman.
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Post by prime on Jul 20, 2023 22:22:55 GMT
Sorry for being so vague, I thought i needed to confirm i had the short in the board solved before doing any of the recommended settings. I installed the board in question after replacing transistors and setting resistors as recommended in manual using dim watt and found the light dimmed but within 5 seconds it started to brighten and the noise appeared. I will reconfirm board with just testing all components and then will install in amp on good side with pickup removed , speaker connections removed and volume at zero, i will start with dim watt just in case. After making settings i will reconnect speakers etc and again with dim watt confirm no short. If all is good i will then install board on the side that had all of the issues in the first place.
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Post by prime on Jul 22, 2023 14:08:19 GMT
Hi guys repaired (tested both boards) all good but i dont know how to test PEC-8 , set R33 to minimum, set R25 to center, powered up no speaker or input also amp volume at zero. clipped negative meter lead to chassis, put positive lead into pin 1, powered on , moved R25 slowly to left and then right, was able to get o vlt, tried R33 cannot get constant voltage, voltage appears then goes or OL ,tried to move lead no luck just keeps popping in then out, dont know how to test, test socket , on the other board installed in same slot can not get 0 voltage at R25. tested all four trimmers all show very close to specs.
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