Pre/Power Amp Confusions/Problems with Yamaha Bass Cabinet - diyAudio
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Old 25th October 2009, 09:03 PM   #1
ix66 is offline ix66  United Kingdom
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Default Pre/Power Amp Confusions/Problems with Yamaha Bass Cabinet

Please could anyone make suggestions as to my problem here, or if I am doing anything wrong.

I have recently acquired a Yamaha YBA-100 bass amp. This consists of a seperate head pre-amp/control unit (BE-200) and Speaker/Power-amp cabinet (BS100) with 2.7 ohm load (3 x 8 ohm drivers).

The power-amp/speaker cab works very well when fed with line-out from my other guitar amp. However, if I plug the BE-200 pre-amp/control unit into this, all I get is a very loud mains hum, which actually becomes less (or less bassy perhaps) if I turn the volumes up.

I do not understand much about electronics, but can see that work has been done on the pre-amp. In the photos below, one can see the thicker white wires(mains cable), these seem to have been added/tampered with.

Can anybody see, just from magnifying my photos, what may be the trouble? I have searched endlessly for schematics, but with no success. Is it possible the pre/amp has been converted into a pre and power amp..? I am a bit unsure as to why there are TWO transformers in the pre-amp, and one of them seems to be a very large 30v-0v-30v unit.

PICTURES:

Complete Unit
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Pre-Amp/Control Unit
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Rear of Pre-Amp/Control Unit
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Power Amp
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Old 27th October 2009, 06:00 PM   #2
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It sounds as if your pre-amp is missing a ground connection.
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Old 27th October 2009, 06:10 PM   #3
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It would appear as if there is a power amp added into the pre-amp, which is the fibreglass board in the middle. It is not original in my opinion.

I cannot see properly from the photo but it looks as if the previous owner wired the power amp output to No.1 output.

You may find that he was using the pre-amp box as a power amp also. It may just be that he left the wiring from the pre-amp to the remaining outputs No2, 3, and four. Try each of these connections to the bass amp and see if they work.

What this guy may have had in mind is that the pre-amp (plus bulit in power amp served as lead or vocal and the pre-amp connected directly to the bass amp/speaker served as bass. Try it.
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Old 28th October 2009, 12:46 PM   #4
ix66 is offline ix66  United Kingdom
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Nico, I have a feeling you may be right. The transformer is made by Repanco (English) and that main amp board is made by is a TUAC TD150, also English. I suspect that any components in the original would have been from Japan.

I plugged in a speaker and a guitar and it played ok, though not with any great volume.

I have tried the other outputs to the speaker/power-amp, but one gets the same mains hum noise. I have a feeling the actual pre-amp circuitry may have been removed, though the adaptations have been done so well, that it is hard to notice.
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Old 28th October 2009, 06:58 PM   #5
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send me some close up shots, let me see what I can spot
e-mail: nico_ras@relkomsa.net
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Old 31st October 2009, 03:52 PM   #6
ix66 is offline ix66  United Kingdom
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This is no longer an issue, as I have decided to return the amp from whence it came... Thanks for the advice anyway.
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