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Rotel Amp Troubleshooting

P

pearce_jj

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May 14, 2010
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Picked up a Vintage(ish) amp - RA-935BX. Condition is very good inside and out, however it doesn't power up. When connected to the mains there is a loud buzzing (8R resistors are fitted across the speaker outputs in lieu of actual speakers) from transformer/rectifier-ish locations and an immediate rise in transformer temperature. I've only left it connected for a few seconds because of this.

Just need some ideas on where to start really. There is DC resistance across primary and secondary windings of the quite substantial transformer, which is supposed to be delivering 2x 29.2V AC (at 180* presumably, since these feed each side of the bridge rectifier). The internal fuses are not blown.
 
Thanks, they look OK at least.

Transformer spits and crackles even with both output fuses (centre-tapped) removed. DC resistance looks OK on both sides. I do have a 500V tester, but it only reads in M-Ohms so I can't get much of any use other than to confirm that at least the primary-to-secondary isolation is in tact.

So, I guess the insulation on the primary side is degraded somehow. There's no sign of any previous repairs and the PCB condition is excellent, so hopefully a new transformer and some caps are all that will be needed :)
 
I see it's a toroidal type. Certainly sounds like it needs replacing. Obviously you checked for the correct mains voltage setting :)
 
Yes, good spot it's multi tapped. This unit has always been in the UK and was wired up as 240v. The secondaries are 28.3V but it's quite compact for a 300VA unit, 110x51mm. Model is TT-279GF, unavailable from Rotel sadly.

Thinking of going with a 230V/30V unit from Maplin If it will fit:
http://www.maplin.co.uk/p/2x30v-300va-toroidal-transformer-a58qj

Otherwise I might have to fit af 250VA unit perhaps.
 
I would get one of the same physical size and live with any minor loss of maximum power. No point in buying one that doesn't fit in the box.
 
Paul, thanks. I don't understand where the rating plate info comes from as the factory PSU is definitely no more than 225VA, so I've gone with that.
 
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