Counterpoint SA-100 with Exicon MOSFETs

[woodman]

Hi,

I don't often plead for help, but I've got a Counterpoint SA-100 with a blown channel that's been sitting around for a few years. I finally decided to get it up and running again and purchased a full compliment of Exicon 20N16 and 20P16 to replace the CPT10N12 & 10P12.

I don't know why, but I seem to have damaged the Exicons between the gate and sources - both the N & P types.

I did this only on one channel because the other channel had a working set of the original MOSFETs. Just for a sanity check, I insalled a working set of devices from a Counterpoint SA-220 (RFM 10N15 and IRFP 9241) in the blown channel and it works fine - so I know for sure both channels work and that the issue is with the MOSFETS.

I thought that by slowly bringing the amp up on a variac wold keep me from damaging the Exicons in case there were some bias voltage problems. I brought the rails only up to +/- 10V or so. The gate voltage went up to around 1.5V on the N side, and suddenly went way down. I noticed this morning that the 1A safety fuse I installed in the rail was blown, so I replaced it. The other rail fuse had a 1ohm resistor so I could monitor current. I just cracked the variac open (for 300mv at rails) after replacing the fuse and now the devices are drawing as much current as they can - this means a short somewhere. I measured the N and P MOSFETs with the diode tester and I'm getting strange numbers.

What did I do wrong? Am I mis-handling the Mosfets somehow? I'm totally not MOSFET savvy so it may be something really rudimentary that I neglected to do. I have one more channels worth of Exicons, but I don't want to destroy these too.

I know Chris might say that I'm abusing good MOSFETS by installing them in a Counterpoint, but I really just want this to work without having to re-engineer it, and the Exicons were (I thought) the easiest way to get it to work.

Thanks for your help,

[ilimzn]

The Exicon MOSFETs have a much lower threshold voltage than the originals or the usual replacements. The bias arrangements in the SA100 may not be appropriate...

[Leolabs]

LATFET vs HEXFET????

[woodman]

Originally posted by ilimzn
The Exicon MOSFETs have a much lower threshold voltage than the originals or the usual replacements. The bias arrangements in the SA100 may not be appropriate...

Yeah, but with 10V source to drain and 1.5V at the gate I would imagine the MOSFET would be perfectly happy with this.

Originally posted by Leolabs
LATFET vs HEXFET????

Is that the difference - I really don't know.

According to the Altavista Audio site, the exicon MOSFETs can be used. I understand that they need to be biased differently than the original MOSFETs, but I thought I was being careful by limiting the voltage.

Someday I'll get this working...

[Miles Prower]

Originally posted by woodman
Hi,
I don't know why, but I seem to have damaged the Exicons between the gate and sources - both the N & P types.

How did you handle your MOSFETs? You have to remember that the insulation between the gate and substrate is very thin, has a whoppingly high impedance, and, therefore, is highly susceptable to damage from static electricity. Especially during wintertime when dry air is dried even more by central heating. It doesn't take very much, after all, to see and feel a spark means that you have several thousand volts. It takes way less to ruin a MOSFET. Furthermore, these don't very often poof at once, but rather have partial gate damage that only gets worse with time.

Tp prevent that, it's essential to not remove the shorting collars from the leads until ready to install, make sure you use a grounding strap when you handle them, make certain that the equipment is grounded and that there is no residual charge on PS filter capacitors.

Did you do that?

[djk]

The pin out of the lateral FETs is different.

[anatech]

Hi James,

I know Chris might say that I'm abusing good MOSFETS by installing them in a Counterpoint,

Well, yes. I imagine these would sound better than the IR devices once they are connected properly and biased properly.

Other things to be aware of:
1) The relay shorts the gates to ground until the circuit times out. You can pull the relays for testing.
2) You may need to change the gate snubber resistor values.
3) Check the darn zeners. Do you need a different value?
4) Most important. Do not tighten the devices until the sockets almost break as recommended in the service manual.

You may be able to get away with a lower bias current than teh original parts. This way you can still use the bias circuit (with adjusted values for lower gate voltage). You may need to mess with the bias emitter resistor to adjust the correction.

Now that you've done this much, do the other channel. Fix the voltage amp stage design issues. You will have to match these outputs very closely as you do with the IR parts.

-Chris (am I really that infamous?? )

[woodman]

Originally posted by Miles Prower


How did you handle your MOSFETs? You have to remember that the insulation between the gate and substrate is very thin, has a whoppingly high impedance, and, therefore, is highly susceptable to damage from static electricity. Especially during wintertime when dry air is dried even more by central heating. It doesn't take very much, after all, to see and feel a spark means that you have several thousand volts. It takes way less to ruin a MOSFET. Furthermore, these don't very often poof at once, but rather have partial gate damage that only gets worse with time.

Tp prevent that, it's essential to not remove the shorting collars from the leads until ready to install, make sure you use a grounding strap when you handle them, make certain that the equipment is grounded and that there is no residual charge on PS filter capacitors.

Did you do that?

Umm...no. I stupidly thought that the protection diode exempted me from practicing any of the ESD training that I've had for the past 10years. I'll have to bust out the wrist strap now. Also, I bought the Exicon's on Ebay and they were unceremoniusly placed in ESD bags without the black conductive foam - they could have been bad before I got to them....


[QUOTE]Originally posted by anatech
Hi James,

Well, yes. I imagine these would sound better than the IR devices once they are connected properly and biased properly.

Other things to be aware of:

1) The relay shorts the gates to ground until the circuit times out. You can pull the relays for testing.

Yup, did that.

2) You may need to change the gate snubber resistor values.

Did that too - though I used the Moscode 300 output stage as a guide.

3) Check the darn zeners. Do you need a different value?

Zeners were good. They're 10V zeners and I probably needed a smaller value.

4) Most important. Do not tighten the devices until the sockets almost break as recommended in the service manual.

I only tighten them as much as they need to be.

You may be able to get away with a lower bias current than teh original parts. This way you can still use the bias circuit (with adjusted values for lower gate voltage). You may need to mess with the bias emitter resistor to adjust the correction.

I probably should have done this but I figured limiting the power via the variac would protect the MOSFETs.

Now that you've done this much, do the other channel. Fix the voltage amp stage design issues. You will have to match these outputs very closely as you do with the IR parts.

Match them? I thought lateral MOSFETs didn't need to be matched. Besides I was gonna hang some ballast resistors on the sources - would that have worked?

-Chris (am I really that infamous?? )

Famous, not infamous
BTW - let me know when you're willing to share your BJT SA-100 mod.

[anatech]

Hi James,

I only tighten them as much as they need to be.

I'm sure that you are aware how tight transistors ought to be. Most of what I posted was for the benefit of others. Wasn't sure you knew about the relays.

Did that too - though I used the Moscode 300 output stage as a guide.

Should be close enough for Rock & Roll!

I probably should have done this but I figured limiting the power via the variac would protect the MOSFETs.

Actually, no. If you increase the AC voltage slowly you may reach a point where they oscillate or conduct heavily. They then discharge all the energy in the filter caps. Insta poof! An open gate resistor will do this.

Match them? I thought lateral MOSFETs didn't need to be matched

I'm not sure on that. I normally err on the side of caution. I am use to matching transistors on everything. We had to match our own mosfets for Adcom service. I'm betting many didn't do that because I asked for the procedure and it took a while to get it.

Besides I was gonna hang some ballast resistors on the sources - would that have worked?

Not with this design. Ballast resistors will really adversely affect the sound quality. There is no feedback from the output.

BTW - let me know when you're willing to share your BJT SA-100 mod.

I've made huge gains in the sound quality but I'm still not happy. Work has been slow due to a head injury about 1 1/2 years ago. I'm still trying to learn what doesn't make sense anymore. I do have prototypes made and I'm playing with them. Changes to every circuit, but you can still use the PCB. I use bipolars for outputs instead. They still have the Counterpoint sound, but much cleaner, cooler and more stable. Forgot quieter and better sound. SY heard one.

-Chris

[ilimzn]

Originally posted by djk
The pin out of the lateral FETs is different.

I'll re-quote this here. S and D are exchanged on lateral with respect to vertical MOS, did you check this?. Laterals also have the D-S protection diode which would present a near short if S anad D are reversed. Also, and unfortunately, I know this from experience, forgetting to reverse S and D in a vertical to lateral MOS conversion, will usually destroy the lateral FETs, the reverse protection diode can fry.