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-   -   TEAC A717 Phono Op Amp Problem (http://www.diyaudio.com/forums/solid-state/133885-teac-a717-phono-op-amp-problem.html)

Lectran 27th November 2008 12:08 PM

TEAC A717 Phono Op Amp Problem
 
I've encountered a TEAC A717 that has shot noise in one Phono channel when either Ceramic or Magnetic settings are fed by a known good record player.

The biasing on the 845808 Op Amp is wrong. The working channel shows the inputs to be biased at 7.8 volts and the noisey channel to be biased at 0 volts, the opposite of what I expected to find.

Any help with a circuit diagram, an identifiable number for the Op Amp and any previous experience with this amp would be appreciated.

It's performance and power (100 watts per channel) on the non-Phono channels has justified the effort in recovering the dead left channel (shorted Power Transistors) so far.

Lectran

h_a 27th November 2008 01:31 PM

That amp has really an input for ceramic cartridges?? If so, then the amp is at least from the 60ies...? That contradicts a bit with your mentioning of an opamp, so please explain!

Have fun, Hannes

jaycee 28th November 2008 10:16 AM

I stripped a Teac A-919DC down, and this had both MM and MC phono input, using a 6458DS op-amp, and some 2SK170 jfets. It's from the mid-80's at a guess.

h_a 28th November 2008 10:58 AM

Ah I see!

The amp is from the 80ies but the gentleman is a bit older :D

MC=moving coil not ceramic ;)

Probably just a cold solder joint.

Have fun, Hannes

Lectran 28th November 2008 02:02 PM

Teac A-919DC
 
" Post #3
I stripped a Teac A-919DC down, and this had both MM and MC phono input, using a 6458DS op-amp, and some 2SK170 jfets. It's from the mid-80's at a guess."

Very helpful, many thanks.

All soldered joints are fine. I think the phono channel has been overloaded by having a higher output device plugged into it and possibly blown the JFets, at least.

Lectran

jaycee 28th November 2008 05:16 PM

This is quite probably the case if your TEAC was like mine - the MM/MC setting was a knob on the front panel ! I thought this was absolutely absurd as it's a setting you are not going to want to change once connected up, and doing so can cause damage as you indicated.

I must admit that overall I didn't think much of the amp's design. It had a single pair of 2SC2565/2SA1095 with 71V rails, and inverted right hand channel for some reason. It did work but the temptation to strip it for parts and build something new was too great!

Lectran 29th November 2008 07:07 PM

Info
 
Thanks for the info.

The rails as I recall them were +- 50 volts.

I replaced the power transistors on the blown channel with those used in the KD-261 which are higher power, voltage, current, Hfe etc, etc.

Lectran


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