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Repairing an audio amplifier

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roboticmisinfo

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Hi guys,

I've just bought a broken Denon PMA250 II Audio Amplifier off eBay, and I've tested it and found that the transformer is kaputzed. It's only putting out 1.5 V AC so I'm guessing there's something a bit wrong there. I'd like to get a new transformer for it, but I don't know what the output is. The amplifier says 130W power consumption on the back, and I've heard somebody with a Denon PMA 250SE say that it's transformer is 2 X 30 VA output, but that was for a different model so I don't want to rely on that.

Also, I've tried taking the transformer into Maplin, but the thick students in there (IMO not qualified to be working in an electronics hobbyist shop) "didn't know what the output was", "didn't stock big transformers", and "didn't know where else I could get one..." derrrr..... helpful!

I wondered if you guys could offer any help here? Even if somebody could point me towards some schematics, or where I could get hold of them. Thanks, Mike
 
Well for a start the consumption, or even the power rating, isn't of much help to you - you first need to find the voltage outputs required. Also a faulty transformer won't put out 1.5V, it will either put out the correct voltage, nothing at all, or SMOKE!!. Check the resistance of the windings, you may find the primary is O/C - possibly because it has an internal thermal fuse?.
 
Hmm... yeah I see where you're coming from, but it is putting out 1.5V so I'm a bit confussled why it's doing that.... anyway I'll check the primary windings resistance later and let you know. Don't fancy trying to pull the transformer to bits though, as it's casing looks a bit difficult to get into, and there's glue everywhere..... I understand you can't just calculate the required voltage from the power consumption, but I thought someone might've had experience with this amplifier, or maybe have some schematics I could refer to...

Thanks,

Mike
 
roboticmisinfo said:
Hmm... yeah I see where you're coming from, but it is putting out 1.5V so I'm a bit confussled why it's doing that.... anyway I'll check the primary windings resistance later and let you know. Don't fancy trying to pull the transformer to bits though, as it's casing looks a bit difficult to get into, and there's glue everywhere..... I understand you can't just calculate the required voltage from the power consumption, but I thought someone might've had experience with this amplifier, or maybe have some schematics I could refer to...

What's the output spec of the amp (watts) and what speaker impedance (ohms) - and does it have multiple windings (which is common).

EDIT: Just checked the specs, it's a very conservative 30W and 8 ohms, the 'very conservative' part makes it too hard to guess.

The Denon site lists a Product support number - 01753 688495
 
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Thanks Nigel, I shall ring them tomorrow to confirm. I had a go at getting the transformer to bits, but after taking the metal casing off, the rest of it seems to be encapsulated and getting into it would probably mean destruction hehe. RS do two 2 x 30V transformers (100VA and 200VA). I'm pretty sure that's what I will need as I've done quite a bit of googling... if I get one I'll go for the bigger one to be on the safe side. Would probably never run it even to half it's output anyway, but just in case.
 
Wow!! I finally got through, and they gave me the number of their spare parts dealer. I rang it, and asked for the specifications of the transformer. He said "I' can't tell you the output voltage, but I can sell you one. They are £152....". I think he knew perfectly well what the output voltage was but he wouldn't tell me as he knew I would go somewhere else. I promptly told him that I could buy 2 or 3 new amplifiers for that price and hung up.......
 
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Hello again. I've tried fitting a 230V - 24V transformer, and it works. Thought it didn't work at first, as I tried it with 1 speaker & it was doing some very strange things, distorting, flicking on and off, and increasing/decreasing in volume, but when I connected the other speaker up it worked fine. Is this normal? I also find that it takes 10 seconds after switching the power on before the power relay clicks on and I get a sound in the right speaker, then another 10 second delay before the left speaker comes on.
 
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What do your HT rails read?, 30W to 8 ohms requires 44V p-p, and as it's very conservative, I would imagine the rails need to be something like +/-30V, perhaps more?.
 
roboticmisinfo said:
Ermm.... sorry I'll be honest you've lost me a bit here. What do you mean by HT?

HT is 'High Tension', originally used for the power rail in valve amplifiers, but carried on to transistor amps even though it's not normally 'high' - so the voltage on the power rails for the amp.
 
OK, cool. It all works fine now - just the mystery with the 10 sec delay in the right channel & 20 secs in the left after powering up. Thanks for your help.
 
Hello. I have carried out more testing and yes, there is a soft start circuit (only a few seconds though). I've commissioned my new Alesis Monitors now, and the left & right channel delay seems to have stopped... strange. I wonder if that crappy bookshelf speaker had something inside it causing the delay? :S Anyway, I was wondering, would mains cable be good enough for the speaker cables? and I have some cheap audio connectors running from my soundcard to my amp. Should I upgrade either of these, and what sort of money should I be spending on cables to get a good enough sound for mixing?

Thanks,

Mike
 
I use "indoor/outdoor" extension cords for my speaker wires; usually 14 AWG (stranded). My speaker lines are no more than 25 ft.

I cut BOTH ends off to eliminate the risk that it might ever get plugged in.

Your primary concern in connecting to your sound card is the quality of the card. Seldom are they good enough that cable matters. You need to have a good enough quality cable that you don't have intermittent connections. I would match the metallurgy on the sound card; if it's gold, use gold plated connectors. If it's tin, use tin plated connectors.
 
I like the power cord idea, I'll have to remember that one. Though if you need higher quality or longer runs then that you're going to want to look for individually shielded cables.
 
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