MC2505 Power Transformer Testing

I know I read the posts. There was no mention of evidence of the transformer over heating and just to have go open seem to justified actually testing it.
 
Yes, it is overheating. I did not mention it because the info is not relevant to most. I removed the transformer out of the chassis due to overheating.
 
Here is the Power Transformer wiring.

Does it matter to take (move) the pair of wires with the green marker connect to circuit breaker then the pair with the red marker to the S5?

Thanks

upload_2019-3-15_21-8-6.png
 
Last edited:
I would think that if the output winding has an open circuit it could go into oscillation and get very warm.
 
Another interesting wiring: Schematic Vs Actual

As you can see the cap C79, the schematic has the NEGATIVE to GROUND but on the actual amp the POSITIVE is connected to ground. (see red circle on the photo)

Can someone verify this for me?


upload_2019-3-20_15-9-31.png

upload_2019-3-20_15-10-34.png
 
They were always flippant with how they would draw their symbols.......I guess they just expected a seasoned tech to realize that to filter a negative voltage the + had to go to chassis ground.
 
It is kind of a hoot though reading the first service bulliten for their first transistor amp the first MC2505s.......they did do some hand holding for all those grizzled tube techs and the new fangled transistor unit.
 
Hi c_dk,

Do you have any insight for this?

Here is the Power Transformer wiring.

Does it matter to take (move) the pair of wires with the green marker connect to circuit breaker then the pair with the red marker to the S5?

Thanks

index.php
 
Plan to leave the large can filter cap in place and install this one underneath.
Do your think it will do the job?

upload_2019-4-2_20-4-24.png
 
Have you found a replacement transformer?

I had a MC2250 sitting around for decades with a partially melted power transformer from a lightning hit.

Found a toriod with the correct values and installed it inside the metal case after I depotted the original......have used it now for years as a shop amp. I can not remember the supplier but it was very very reasonable.
 
Yes sir, I got the power transformer.

Power is restored and now there are many other issues: Buzzing and whining sound when engage the speaker switch and the heat sink (the red mark) get hot very quickly. I narrow down to the right output board that generate the buzz and whining noise.



upload_2019-4-2_21-41-57.png
 
The potential effect would be less headroom / peak output power, less of a buffer for the rectifier, probably will not have any audible hum however.

Why not replace them with the correct capacitors?
 
I see your direction but given a choice I would just leave it until you're armed with the correct caps. If you ever choose to sell (we all do eventually), the 8k caps will lead people to believe the circuit-butcher has been in there.
 
Buzzing and whining sound when engage the speaker switch, it is coming from the autoformer. Disconnect the autoformer from the speaker switch and the buzzing noise disappear. It is time for sourcing the autoformer.
 
Back
Top Bottom