TEAC A717 Phono Op Amp Problem

[Lectran]

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]

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]

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]

Ah I see!

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

MC=moving coil not ceramic

Probably just a cold solder joint.

Have fun, Hannes

[Lectran]

" 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]

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]

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