bias a trace elliot ta100r

[lowell]

Which exactly transistors do you mean? Did you replaced 2SK135 with IRFP? This is not allowed - they are completely different type and one cannot replace the other. The first one is lateral, and the other is vertical. You may read about differences between these two here: http://www12.fairchildsemi.com/an/AN/AN-558.pdf - check that the gate needs a turn on voltage, otherwise MOSFET is not conducting. So you need eg. +/-1.5V on gates (on P1 trimmer).

Mark in this article it says that lateral MOSFETS are enhancement mode but it doesn't clearly state that vertical MOSFETS are depletion mode. Is that what you're saying? That vertical MOSFETS are depletion mode? Finally, I don't see the "mode" listed in the datasheets for the IRF. I DO however see that the G/S threshold voltage is +-2v. So that tells us that they're enhancement mode right? The 2SK135 is also enhancement mode so not sure, maybe I'm missing something.

I will add the 1.5k resistors and dummy load and see what results I get. Am I looking for TR4 and TR6 collectors to a positive and negative voltage? That makes sense to me, this way the gate/source voltages will be ON.

[MarkusBass]

Mark in this article it says that lateral MOSFETS are enhancement mode but it doesn't clearly state that vertical MOSFETS are depletion mode. Is that what you're saying? That vertical MOSFETS are depletion mode? Finally, I don't see the "mode" listed in the datasheets for the IRF. I DO however see that the G/S threshold voltage is +-2v. So that tells us that they're enhancement mode right? The 2SK135 is also enhancement mode so not sure, maybe I'm missing something.

All I'm saying is that 2SK.. is a lateral MOSFET and IRFP is vertical. You cannot replace one with other. If you use IRFP in this amp, you will get a thermal runaway and the amp will not work to long. If you are looking for general information about MOSFETs, you can look here: MOSFET - Wikipedia, the free encyclopedia , or just type "lateral versus vertical MOSFETs" in google. You cannot expect all information to be in the datasheet.

Am I looking for TR4 and TR6 collectors to a positive and negative voltage? That makes sense to me, this way the gate/source voltages will be ON.

On top P1 trimmer pin (from V+ rail side) you need positive voltage and on the other (from V- rail side) you need negative voltage.
Have you check the gate resistors? You could check also all the diodes - desolder them and check with external power supply and a resistor.

Mark

[g1]

Maybe these "new" mosfets have gone bad or were damaged during or after installation? ... What do you all think of replacing with these?

IRFP-9240
IRFP-9240 MOSFET | Mouser

IRFP-240
IRFP-240 MOSFET | Mouser

Lowell: It would seem you have now replaced the output devices twice? Have you used the stock type J50 & K135 either time?
It would be helpful if you would say which type devices you have tried for replacements.
It is possible incorrect subs have been the whole problem since the original output devices were replaced.

[lowell]

Hi g-one. I have not been able to find the J50/K135 transistors. On various sites I've found that these should work: 2sj201/2sk1530.

Mark thanks for the info on that, I guess I'd need to redesign a bit, add a heatsinked Vbe multiplier so as to limit runaway with vertical mosfets. And I'm not going to do that.

Again, back to just using 1.5k resistors for now. I'll report back!

[MarkusBass]

Hi g-one. I have not been able to find the J50/K135 transistors. On various sites I've found that these should work: 2sj201/2sk1530.

They will work but they are in a completely different package, which could cause some problems when trying to use them. They also require slightly higher voltage on gates to work properly (so same small changes to the amp may be required). If you are looking for better equivalents, you may take a look at BUZ901/906. I also see (quite often) 2SK135 on ebay. In Poland I can even buy them in several shops - hard to believe that you cannot buy them in the States.

Mark thanks for the info on that, I guess I'd need to redesign a bit, add a heatsinked Vbe multiplier so as to limit runaway with vertical mosfets. And I'm not going to do that.

Yes, you would need to rediesgn the amp. In my opinion this is not needed since you can buy either original transistors, or eqivalents. What about the transistors that were originally in the amp, are they shorted?

Mark

[lowell]

They will work but they are in a completely different package, which could cause some problems when trying to use them. They also require slightly higher voltage on gates to work properly (so same small changes to the amp may be required). If you are looking for better equivalents, you may take a look at BUZ901/906. I also see (quite often) 2SK135 on ebay. In Poland I can even buy them in several shops - hard to believe that you cannot buy them in the States.

I checked for them and they seemed quit expensive. In any case I've had these replacements installed already, on the heatsink, with thermal pads and wires running to the board. I'll look into the BUZ901/906. I think I looked for those already and couldnt' find any either but will check again.

In any case at this point I have the 1.5k 10w resistors in and there is STILL 70mv on the output.

[MarkusBass]

Take a look here:

BUZ901 - SEMELAB - N CHANNEL MOSFET, 200V, 8A, TO | Newark Products

BUZ906 - SEMELAB - P CHANNEL MOSFET, -200V, 8A, TO | Newark

Mark

[52 Bill]

Thanks for the post Mark. If you look at the data sheet, it's all in Russian maybe?

[MarkusBass]

Thanks for the post Mark. If you look at the data sheet, it's all in Russian maybe?

This is possible (I cannot check it today - I have a computer without Acrobat Reader). I think you can find on the net some information about the factory in Russia. This may be the same story as with valves - every good valve was manufactured in Russia. The Russian language is not that difficult, especially in a datasheet there is a lot of symbols, or drawings that you can easily recognise.
If you want the datasheet in English, you may go to Profusionplc: Lateral Mosfet
I think that some of the transistors listed there are equivalents to the ones that Lowell is searching for.
You may also ask Km6xz to purchase aa batch of BUZ9xx - they must be cheap there (and he stays in Petersburg - most probably close to the factory).

Mark

[J M Fahey]

I think they linked to the Russian Distributor page by mistake.
Semelab/Magnatec is British.
Their proper page is:
TT electronics Semelab Limited - Magnatec Datasheets

[lowell]

Well at this point I have the 1.5k 10w resistors in there and no power mosfets. I am very confused at results I'm getting.

I have ZD3, ZD4, ZD5, ZD6 all lifted
There is -40mv on the output (no speakers connected)
On D7 anode and D10 cathode I have -51v (I have P1 set at zero ohms)

I began checking voltage drops across R23, R21, R22
I got ~500mv across R23 and thought this was half of what it SHOULD be. I checked voltage drops across R26, D6, R22 string and it all looked good, therefore TR5 is most likely fine - so I replaced TR6. I now have 1v across R23. However, still -51v at P1.

So I'm now questioning if TR4 is working. I am getting 590mv across R21 and Vb/e on TR4 is -560mv. So I'm thinking that it is fine. I also lifted C17 and it didn't help.

So everything is checking out fine, BUT, -51v on the junction of TR4 and TR6. I'm very confused.

[Steve Conner]

That's right, the Semelab parts (or at least the silicon chips inside them) are made right here in Scotland. Exicon and all of the other little brands use the same chips, so they are equivalent.

The only other people still making lateral MOSFETs are Hitachi, the 2SK1058/2SJ162 etc. These aren't drop-in replacements due to a different threshold voltage.

[lowell]

*bump*

[lowell]

So I have the resistors in place of the mosfets. I also have 22k 1/2w resistors in place of the drivers. I still had a bit of offset at P1 when it was set to zero resistance. I then decided that I'd continue this "lets see what results we get as we eliminate variables." So I disconnected the 56k feedback resistor and this modification allowed for 0v offset at P1. SOooo... I then checked that TR5 was on and drawing current.. and it was. The B/E junction was ~ 600mv as well. Finish this post in a bit.

[MarkusBass]

I remember my advise to put resistors in place of MOSFETs. But I don't know who advised you to put 22k resistors in place of drivers (do you mean TR4, TR6?). They, togehter with TR5, form a diffenrential amp with a current source. So you removed the current source and made it "non-differential" and now you complain that it does not work. Of course you may say that a high value resistor is kind of a current source but it is not the way to fix the amp. You are now triyng to solve problems that you created and not the real one.
This amp is a very simple amp and if you have such problems with it, you could replace all semiconductors (I mean 5 transistors and 8-9 diodes in the output stage).

PS:the problem with 40mV on the output may be caused by rails (uneven voltages), or uneven 1.5k resistors. Have you considered this?

Mark