Restoring a Threshold FET 10e - a few questions

[GaryB]

Hi,
I seem to have turned into a collector of old Threshold preamps. Over the last couple of years I bought both an SL10 and then an FET 10/hl preamp and with some minor restoration found them both to be excellent.

I recently came across an FET 10e and FEt 10pe (line and phono) and picked them up and I'm in the process of restoring them. I am going through the usual routine of replacing all the power supply electrolytics, changing the ps diodes to soft recovery types and adding snubbers.

One question I had is about the proper bias for the gain stage output transistors. Both line and phone stages use a universal gain stage which was shared by Zen Mod in an earlier thread New (to me) Threshold FET-10 HL (What to refurbish?)

In the line stage, the bias for the different modules is between 12-15ma. In the phone stage, the bias seems to be a bit lower, being between 8-10ma.
It's probably not too critical but I figure I should at least balance the left and right channels so they're running at the same operating point. Anyone have a suggestion for what that should be?

Besides replacing the electrolytics, I found that the 4.7uf tantalum caps in the servo loop needed some attention. A few of them seem to have gotten a bit leaky causing some but not all of the servos to not quite zero things out. Replacing them with some new 10uf caps worked nicely in the line stage. I also did this in the phono stage. But due to the high DC gain of your typical RIAA stage, you don't get stable DC even with the servo. There is a slow DC drift which I've seen in other phono stages and which has also been described by Syn08 in his HPS 5.1 thread. Can anyone confirm that this is normal for this design?

Thanks,
---Gary

[GaryB]

Since I haven't seen this posted before, here's the schematic of the 10pe phone stage. The gain block is shown in the previous post.
---Gary

[Blues]

I have a suggestion, Gary...how about replacing that old pot with your Warpspeed?

[Zen Mod]

bias : you can't do much except changing semis with matched ones ; only that will give same bias in diff channels

servo : have you C8 implemented ?

[GaryB]

Originally Posted by Zen Mod

bias : you can't do much except changing semis with matched ones ; only that will give same bias in diff channels

servo : have you C8 implemented ?

ZM,
The gain module for the phono stage uses C3, C4, and C8. The line stage does not use them. I did replace C8 on the phono modules. The photo below is taken from a.wayne's pictures of his FET 9e but it uses the same gain modules as the 10pe. C8 is the electrolytic in the bottom left corner, while C3 and C4 are the mica caps along the bottom edge of the board below the transistors with heat sinks.

Regarding the question of whether or not some drift of the DC level of the phono modules is "normal", I'm thinking that it must be since Threshold includes a 10uf output cap even though the modules are servo'd.

One other thing to note is that the schematic differs from the board in one respect. The schematic in the 1st post doesn't show any adjustments. But there is a square blue ten turn pot on the board just above the servo op amp. At first I thought that this pot might allow control of the bias current. Tracing out the circuit, it turns out this is in series with R10 - the output resistor from the servo op amp. My guess is that this resistor is set so that the op amp is operating roughly in the middle of its voltage range.

As I stare at the circuit, I realize that I don't fully understand how the servo works. I'm used to seeing servos fed into the "-" input. But this is being fed into the current sources for the input stage. Nominally, the bases of transistors Q3 and Q4 are fixed by the turn on voltage of D3 / D4. But changing the op amp output will cause more or less current to flow through the diodes and slightly change the voltage of D3 / D4. Which in turn will increase the input current and the current of the next stage. I don't see how the loop closes. Can someone explain how this helps zero the output?

Thanks,
---Gary

[Zen Mod]

it will alter base currents of both Q3 and Q4 , thus altering current through them

rest is logical

[GaryB]

Originally Posted by Zen Mod

it will alter base currents of both Q3 and Q4 , thus altering current through them

rest is logical

ZM,
Maybe I need a 2nd cup of coffee. I got the first part with the base currents of Q3 and Q4 being altered. It's the "rest is logical" that I'm missing . I'll stare at it some more and see if it makes sense later.

---Gary

[Blues]

Originally Posted by GaryB

...One other thing to note is that the schematic differs from the board in one respect. The schematic in the 1st post doesn't show any adjustments. But there is a square blue ten turn pot on the board just above the servo op amp. At first I thought that this pot might allow control of the bias current. Tracing out the circuit, it turns out this is in series with R10 - the output resistor from the servo op amp. My guess is that this resistor is set so that the op amp is operating roughly in the middle of its voltage range...---Gary

Gary,

If you trace the blue trim pot its outer pins 1 & 3 might be connected to pins 1 and 5 of the servo opamp (do a search for LF351N datasheet) and is used for its input offset null adjustment. Pin 2 wiper of the pot would be to -Vcc.

[GaryB]

Originally Posted by Blues

Gary,

If you trace the blue trim pot its outer pins 1 & 3 might be connected to pins 1 and 5 of the servo opamp (do a search for LF351N datasheet) and is used for its input offset null adjustment. Pin 2 wiper of the pot would be to -Vcc.

No - that's the 1st thing I checked and it is definitely not wired up that way. As I said in my previous post, it's wired up to the output of the opamp (pin 6) and is in series with resistor R10.

---Gary

[GaryB]

I'm happy to report that after keeping the phono stage powered up for a couple of days, the servos in the phono stage have settled down. I guess that the new electrolytic caps in the servo needed some time to settle down. Now the servo is perfectly quiet and doesn't jump around the way it did when I first replaced the caps. I guess there is something to this burn in phenomena . . .

---Gary