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Feedback blew my phono stage?

Ok....I don't know what's going on here but here's my set-up
HALCRO
On the right hand side is my 'nude' Victor TT-101 DD turntable supported on spikes surrounded by 3 three tonearms on separate solid bronze armpods.
The 3 arms are connected separately to the Halcro DM10 preamp's phonostage via 2.5 metre long balanced Cardas Golden Ref phono cables.
With MM cartridges (I have around 30).....there are no problems.
With LOMC cartridges however......if I rest the stylus on the record without the motor turned on......I can induce a low frequency feedback through the speakers if I turn up the volume sufficiently?
Nothing I did to support the turntable differently.....could eliminate this problem so I have simply been playing MM cartridges through this turntable.
Recently.....I mounted an Acutex 420STR MM cartridge on the SAEC tonearm and discovered that it produced the same feedback symptoms as the LOMCs?
In experimenting with differing support methods for the Victor.....I forgot to turn the volume down before hitting the 'mute' button to 'OFF'....and an almighty noise knocked out my phono stage and tripped the protection on the left channel Halcro monoblock.
The 'Balanced' XLR inputs for the phonostage now produce no output whilst the RCA inputs produce a feeble output in the right channel only.

Any Gurus here with some ideas for the cause of this problem would be much appreciated?
Incidentally......no such problems with the 3 tonearms on the Raven AC connected to the same preamp inputs....even with LOMCs?
128x128halcro
mosin
410 posts
 
I had a somewhat similar problem when running a prototype drive for a strain gauge cartridge. I unexpectedly blew out a tube in my linestage. The root of the problem was poor grounding in one of the prototype's RCA connectors that caused hum.

I don't know if my experience helps you, but could it be a purely mechanical problem that is exacerbated by the requirements of a moving coil cartridge that are not common to a moving magnet one?
halcro's avatar
halcro OP
2,664 posts
 
The Halcro DM10 went to my local technician who....when he opened it up FOUND THIS
The Halcro designer in Adelaide said that the thick paint overlay was done to protect against patent infringement 'copying'....and also to prevent oxidisation?
My local technician said it's probably more likely there to prevent anyone other than Halcro, from working on the circuit boards?
In any case.....without the circuit diagram.....my local technician could not repair the unit so off to Adelaide the preamp went.

Today the Halcro designer writes:-
There were 3 faulty parts. Each input has a common mode choke to reduce noise. Both the phono stage chokes were damaged. There was also an electrolytic capacitor which had failed. Some of the other caps were also showing signs of aging (these parts are known to degrade over time). I took the liberty of replacing all of these caps as a preventative maintenance measure.

I have seen the choke fail on an unbalanced input before. This was caused by a fault in the equipment which was connected to it. If there is a large voltage between the ground and signal connection these chokes can fail. Perhaps you could check your turntable? I must say it is hard to imagine how a turn table could cause this problem.
Perhaps I need to tell him of the 'feedback problem' :-(
And how do I ensure it doesn't happen again?
Or could perhaps some faulty parts have caused the 'feedback' in the first place?
nandric's avatar
nandric
4,086 posts
 
Dear Henry, Everyone knows what 'wishful thinking' means but no one want to give up this dream about the 'Nirvana for cheap'. That is, I think, why so many of us own +/- 20 of those old MM carts. The fact that there are no problems of any kind with your Raven may entail the answer to your question.
Kind regards,
dover
2,250 posts
 
Wouldn't you be better off to sell it whilst it's still going. What happens if the designer gets run over.
halcro's avatar
halcro OP
2,664 posts
 
What happens if the designer gets run over?
That's exactly what I've been wondering Dover.........
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