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Post by prime on Aug 3, 2024 22:24:32 GMT
I have a MRA-3 set up on a bench ,i connected a speaker to give load, i have an adapter that feeds the 120 and 6 vac. My question can i just connect left or right rca from my turntable to test sound or is there an issue with the mute plug not installed.
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Post by jukeboxmarty on Aug 4, 2024 0:22:17 GMT
If you didn't use a field coil speaker you will need to compensate for the field coil using a resistor.
Also, if not using a stock speaker setup you will need to jumper J3 pin 6 (Speaker Socket) to ground.
A turntable into the RCA input should work just fine, weather it's a magnetic or ceramic cartridge.
If the AVC circuits in the amp are intact, and the corresponding 6SL7 and 6SK7 tubes are present, you will need to jumper pin 3 of J4 (AVC/Mute Socket) to ground to release the squelch.
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Post by prime on Aug 4, 2024 0:54:48 GMT
I have a speaker i picked up a while ago for a C, i didnt like how hot the resistor got so when i found an original Seeburg with a field coil i replaced it. It a Radio Shack 40-1318 with a 6000ohm 25 watt resistor added to it. Im using it for testing amps, hopefully it will work ok.
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Post by prime on Aug 4, 2024 2:04:14 GMT
I jumped pin 3 on J4 to chassis, no luck. Im trying to get this amp going, it was in a C and in storage for 25 years. Not sure if it worked then, its been completely recapped , all resistors checked for obvious issues, all tubes tested and all connections checked for loose wires or touching where they shouldn't, i tried the amp ,no sound at all, i tried another amp in its place and it worked perfectly so i know everything else is ok. After testing plate voltage on 6L6 plate found 547vdc, checked pin 8 on 5U4 and found same, after which I found R1 1200 ohm 10 watt resistor open. I replaced it but find the new resistor heats very quickly so I feel whatever damaged it is still an issue. I was just trying to see if sound had returned at least. Not sure if the speaker im using as a dummy is ok.
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Post by jukeboxmarty on Aug 4, 2024 3:14:53 GMT
I have a speaker i picked up a while ago for a C, i didnt like how hot the resistor got so when i found an original Seeburg with a field coil i replaced it. It a Radio Shack 40-1318 with a 6000ohm 25 watt resistor added to it. Im using it for testing amps, hopefully it will work ok.
That speaker should work. And yes, that 6K ohm resistor will get VERY hot!
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Post by jukeboxmarty on Aug 4, 2024 3:17:32 GMT
...the new resistor heats very quickly so I feel whatever damaged it is still an issue.
Have the electrolytics been replaced? One or more may be shorted.
Careful you don't fry that transformer!
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Post by prime on Aug 4, 2024 11:13:09 GMT
All of the caps electrolytics and paper, all polarity has been confirmed, all resistors have been checked for any dead short but i have not removed one lead from each resistor, only some that had readings off the scope. ive looked very close to all leads but find nothing touching. When i replaced the 1200 ohm 10 watt resistor i only had two 5 watt 2200 ohm available for now, they have been twisted together giving me a 1100 ohm reading. I have ordered a 1200 25watt but will not get it for a while, i realize this should lower the voltage a bit but felt it should work for now. When i power up the unit, about 20 seconds in i get a low volume high pitch coming from the speaker, the new R1 resister gets hot , when i remove power i hear a sound from the speaker like something is slowing down, kind of like when you are tuning a ham radio
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Post by prime on Aug 4, 2024 14:42:55 GMT
I find the sound issue coming from the speaker is not present when one of the 6sl7s is removed, i tried the other 6sl7 in the same socket sound back so not the tube, am i correct in thinking the issue is with something connected to that socket or the 6l6 it feeds? The link is the sound i hear youtube.com/shorts/Ji9jER0EPY4?si=z4V6spzvuehm7uX3 ( I see that is not where the issue is) |
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Post by jukeboxmarty on Aug 4, 2024 15:58:00 GMT
The sound you are hearing is some sort of feedback oscillation; it can be caused from a variety of reasons.
My guess is that some component is not wired in correctly or is open or shorted, but that's just a wild guess.
It would be hard to diagnose this issue without an oscilloscope and the prerequisite knowledge of what to look for.
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