Hi all. I recently bought a Dynacord MV75 supposedly in full working order. I turned it on, and it did this:
https://www.youtube.com/watch?v=AH-4Xp6Pza4
Does anyone have any idea what's happening there? Also, what exactly is that contraption on the corner of the transformer? It has 2 screws at an adjustable distance; could the fix be as easy as changing the gap?
Here's a screen capture in case you can't see video
https://www.youtube.com/watch?v=AH-4Xp6Pza4
Does anyone have any idea what's happening there? Also, what exactly is that contraption on the corner of the transformer? It has 2 screws at an adjustable distance; could the fix be as easy as changing the gap?
Here's a screen capture in case you can't see video
The device on the edge of the (output?) transformer is a spark gap. This is a very simple overvoltage protection device.
I failed to locate any schematics for the amplifier online, but it seems that it is located on the output transformer. I would expect it is meant to protect the OPT against overvoltage incidents with disconnected loads or with the output stage driven into serious clipping.
Was there any load connected when the video was recorded? From the display tube it seems that the amp was driven to its limits before the sparking occurred. You should never drive a tube amplifier without a proper load present, especially not into clipping.
Regards,
Rundmaus
I failed to locate any schematics for the amplifier online, but it seems that it is located on the output transformer. I would expect it is meant to protect the OPT against overvoltage incidents with disconnected loads or with the output stage driven into serious clipping.
Was there any load connected when the video was recorded? From the display tube it seems that the amp was driven to its limits before the sparking occurred. You should never drive a tube amplifier without a proper load present, especially not into clipping.
Regards,
Rundmaus
Hi Rundmaus, thank you for that helpful info! No, I did not have a load connected so that could be the problem. I did make sure all levels were at minimum, but nonetheless you're right.
The MV75 uses an old speaker out connection that I didn't have; it looks like a 3-pin banana plug. I mainly wanted to check if it was DOA when it arrived, so I did that quick on/off test. I haven't touched it since.
I think this MV75 is very similar to the MV120 (in fact, on the front of my amp it says "MV75/120"). MV120 schematics can be found here:
http://www.peel.dk/Dynacord/pdf/MV120 (36-20.000) (1960).pdf
I'll try to hunt down that odd connector before trying it again. On a related note, can anyone tell me where to find these connectors (speaker as well as these 4-pin input plugs)? I don't even know what they're called, so it's hard tracking them down.
The MV75 uses an old speaker out connection that I didn't have; it looks like a 3-pin banana plug. I mainly wanted to check if it was DOA when it arrived, so I did that quick on/off test. I haven't touched it since.
I think this MV75 is very similar to the MV120 (in fact, on the front of my amp it says "MV75/120"). MV120 schematics can be found here:
http://www.peel.dk/Dynacord/pdf/MV120 (36-20.000) (1960).pdf
I'll try to hunt down that odd connector before trying it again. On a related note, can anyone tell me where to find these connectors (speaker as well as these 4-pin input plugs)? I don't even know what they're called, so it's hard tracking them down.
DID-YOU-CONNECT-A-SPEAKER?
I guess not.
What were you thinking?
That spark gap overvoltage protector just saved your output transformer, thus the amplifier, from instantly turning into landfill garbage.
1) connect the proper speaker
2) set all controls to 0
3) turn amp on, carefully watching for smoke/sparks/buzzing/overheating. that amp is 50/60 years old
4) start rising controls, nothing at the input yet, listening for hum/hiss/buzz/pops/crackle at the speaker, anything which shows it's alive.
5) set all controls to 0 again, inject some music and slowly start rising controls until you get sound.
I guess not.
What were you thinking?
That spark gap overvoltage protector just saved your output transformer, thus the amplifier, from instantly turning into landfill garbage.
1) connect the proper speaker
2) set all controls to 0
3) turn amp on, carefully watching for smoke/sparks/buzzing/overheating. that amp is 50/60 years old
4) start rising controls, nothing at the input yet, listening for hum/hiss/buzz/pops/crackle at the speaker, anything which shows it's alive.
5) set all controls to 0 again, inject some music and slowly start rising controls until you get sound.
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