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Tannoy R15 Subwoofer

E

eddie pugh

pfm Member
I'm after a bit of help from the gurus out there

I have the plate amp from a fairly old Tannoy R 15 subwoofer here and it's blowing fuses like no tomorrow.

Tannoy were very helpful and managed to dig up a circuit diagram.

https://dl.dropbox.com/u/71645468/Tannoy St200.pdf

This thing is allegedly good for about 300 watts It has 2 No TDA7294 Chip amps feeding a dual voice coil 15" driver

All I know so far is that if I put 2 new slow blow 6.3 amp fuses in the transformer output lines it releases a little whif of magic smaoke and blows its main input mains fuse T2 amp. It happens a little too fast for me to see which component emits the smoke

Before I got the circuit diagram I initially thought that each fused transformer output line powered a separate TDA7294 so I tried pulling one of the fuses to see if I coulsd home in on the fault. I was a bit surprised to find that if I only put one of the T 6.3 amp fuses in they blow with a nice blue flash leaving the mains input fuse intact

Does that give you something to go on.

I now see that each fused transformer secondary feeds a conventional bridge rectifier. The tranformer is a centre tapped +/- 32volt job if that's any use

I must say I was a bit surprised that such a massive sub had such modest amplification Made in China to boot


I sourced some replacement TDA7924's and swapped one that looked a bit dodgy. made no difference at all

Any help greatly appreciated.

eddie
 
That is the worst drawn schematic I have ever seen.

I would start with some resistance measurements while powered off, and see if you can find a short circuit.
 
Eddie, are you still up near St Stephens? If you want to pop down to Bennett St. we can check it with a 'scope.

Instantly-blown fuses I'd usually start looking at the main rectifier(s) for damage before anything else.


Edit: having had a look at teh schematic, PD is right - that's a fine exercise in obfuscation :eek:
 
Martin

Yes I'm still up on Lansdown just below the church. That's when I'm in the UK You must come up one evening and play with some of my toys. I thought you had moved out to the sticks.

Unfortunately this Tannoy plate amp weighs several kilos (mostly the transformer and backplate/heatsink frame) and lives in Riyad, Saudi Arabia where I spend most of my time these days. Also I can't power the damn thing up to measure anything as it keeps blowing the fuses.

I think you might be right about the 4 pin rectifier bridge which is what I was going to try next.

I agree with you and PD it's the worst circuit diagram I have ever seen but I have to say Tannoy were very helpful in making it available. You should see the diagram for the input board and filter circuit. If the fault was in there I would call it a day right now

The power amp board is really very basic with just 2 of those chip amps and support bits and I think part of it is just a regulated power supply to provide an 11 volt supply to the input board. There is also that relay that I think switches it to off/standby when there is no signal detected

I'll persevere for now and let you know how it develops

eddie
 
Update

I removed the diode bridge and made some tests. The bridge was fine but the board was an absolute short circuit across the bridge + /- points

I swapped the other TDA chip amp and I may have solved the problem. This chip amp has a distinct crack in the black body so must have seen some high temps

Fuses now all remain intact, little red/green Led lights up on the input board power switch, tranny warms up after a while so all looking good

Only a few millivolts of DC on the driver feeds

Fed it some line level input from my iPod and attached one pair of driver leads in turn to an old gash computer speaker with the volume right down low and it at least makes a sound. I'd rather not describe it as music. Speaker was only about 2" Don't quite know what to expect when feeding it bass below 100Hz but it popped and tizzed and responded to the volume control

Can't really test further without the big 15" so returned it to my friend, the owner to refit in the Tannoy box. Fingers crossed

eddie
 
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