Help Fixing my Sessionette 75 AMP Power Module

[monto]

hi all, at the moment I'm troubleshooting my award session sessionette 75 (Model SG75 from around 1985) guitar amplifier, and ended up at the power module board, where I have removed the two power mosfets (BUZ901P N-Channel, and BUZ905P P-Channel)... as pictured below:



http://www.farnell.com/datasheets/637905.pdf
http://www.farnell.com/datasheets/69258.pdf

I did some tests on the power mosfets (via articles found online) using my digital multimeter set in the 'diode' position. I came to the conclusion that something does seem to be up with at least one of the power mosfets, so I am now looking to replace them.

Here is a picture of my AX120P power module board from my sessionette 75 amplifier (with the power mosfets removed):



I did manage to find a schematic of the AX120P power module board, but the quality is not so good, but here it is:



so my question is (note: I don't claim to be an expert at this stuff, but ok with using a soldering iron/multimeter, performing tests/readings on components etc):

I had a read around, and noticed in a previous thread on this forum that rather than purchasing new BUZ901P/BUZ906P power mosfets, I might be able to use the cheaper 2SK1058 (K1058) and 2SJ162 (J162) power mosfets instead:

http://documentation.renesas.com/doc..._2sk1056ds.pdf
http://documentation.renesas.com/doc...7_2sj160ds.pdf

I checked the physical dimensions and it looks like they will fit to the pcb/heatsink ok.
I was wondering if anyone could offer any advice regarding if this all sounds ok? I notice that there is a trim pot (RV1) on the AX120P power module board. Is that something I should check/adjust when installing new power mosfets? also, because the K1058/J162 are cheaper in price than the BUZ901P/BUZ906P, should I expect lesser performance in terms of say... noise, reliability, stability, overall sound, etc?

Failing all that, i would be happy to use some other alternative compatible power mosfets if anyone has any suggestions? all said and done though, if it is recommended i just stick with the more expensive BUZ901P/BUZ906P combination, i would do that, but just wanted to check here first for advice, because it's nice to have alternatives. many thanks!

[monto]

i looked into this further and found that it looks like the sessionette 75 amplifier originally used (now obsolete) Hitachi 2SK226/2SJ82 mosfets (see pic below):



...and looking at the datasheet for those:

2SK225-2SK226-2SK227.pdf

it seems that the 2SK1058 (K1058) and 2SJ162 (J162) mosfets are a seemingly very well suited replacement, closely matching the specs of the original Hitachi mosfets ... however, for anyone like me using an eighties sessionette 75 amplifier with the AX120P power module, you would need to drill a new hole in each of your sessionette power module heatsinks to allow mounting of the new replacement mosfets, as they use a single screw hole to attach to the heatsink, as opposed to the original Hitachi mosfets which use dual screw holes (as you can see in the above picture).

[J M Fahey]

Also re re re check pinout

[monto]

hey again all, I wanted to add that during my looking around the internet for information about the AX120P power module board (as found in a number of mid-eighties sessionette 75 (SG 75) amplifiers), I stumbled upon this information below, from the amp designer/s:

"...If you have ANY problems with [the AX120P Power module], then check R9 (12k) is actually 12K. They can go high or open circuit! Replace it with a 2W resistor, as a matter of course, to prevent future failure! ..."

Using my multimeter, I tested the R9 (12k) resistor on my AX120P power module board, and instead of being 12k, I noticed it had drifted to above 13k, so im going to replace it with a 2W or 3W 12k resistor instead. Note for anyone wanting to test their R9 resistor, you can do that while it is still attached to the pcb (ie. you can measure it in-circuit). do consider that, even if your R9 resistor measures ok, it seems a good idea to replace it with a higher wattage resistor anyway, just for peace of mind.

also note, a 2W or 3W resistor will most likely be larger in physical size than the more common/'regular' sized 0.25W resistors, but don't worry, what you can do is: fit it on your AX120P power module board in a vertical position, instead of the horizontal position. see below for an example picture of what I mean:

[purveyor]

R9 - Solved a humming power section. Thanks!

"...If you have ANY problems with [the AX120P Power module], then check R9 (12k) is actually 12K. They can go high or open circuit! Replace it with a 2W resistor, as a matter of course, to prevent future failure! ..."

R9 - Solved a humming power section. Thanks! - Mario

[rvin28]

A friend of mine has just picked up a 1984 Sessionette 75 and it does appear that there is an issue with the MOSFETS in particular the 2SK226 (its an AX120P power module). Does anyone know if the 2SK1058 (K1058) and 2SJ162 (J162) actually work as a good replacement and are there any other (I'm very much aware of R9 issues) modifications that need to made with the addition of these MOSFETS?

Best Regards

Rob

[Jon Snell]

If you have a short circuit between the Source and the Drain after placing your probes between the Source and the Gate to discharge the voltage, it is faulty otherwise they are fine.
Use either makes of power fet but only of the same types.
The bias preset MUST be adjusted to set the quiescent current to the correct amount; 75mA is in my experience too high. I find 35mA is sufficient. Set with No Load and No Signal. Replacing the fets will change the quiescent current due to differences in manufacturing techniques and cut off points.
If F1 has been soldered in, remove the fuse and holder, replace them with new. Soldering affects the fuse!
The earlier models has a MOS248 module fitted, these are now totally obsolete and a replacement module must be made for them.

A little history for you;
Totally un-repairable but the Sessionette 100 use the discrete versionsas per your module.

[rvin28]

Many thanks for your reply. I have tested the original MOSFETS (see attached) and also tested for the diodes between the source and drain and found this:

2SK226 (Black) N - Channel MOSFET.
- source to drain = 0.1358 V
- drain to source = 0.0L V
2SJ82 (Green) P - Channel MOSFET
- source to drain = 0.0L V
- drain to source = 0.2858 V

I'm no expert here (or electronics wise, hence my request) but my thought were the 2SK226 value was very low??
One thing I did note was that the fuse in the power module had blown before any testing which does indicate a problem. I too, checked the R9 and found it to 12.8kR, a wee bit too high.

Any help is greatly appreciated.

Best Regards

Rob

[Jon Snell]

From your readings, I would say they were fine.
12.8k is within 5% so a good reading.
Check you have both + and - supplies.
Put the transistors back in and power it up without a load.
What is the voltage on the fuse? Should be near to zero.

If you get stuck and are in the UK, I will happily repair the module for you at the price of parts plus postage.
PM me if this will help you.

[rvin28]

I soldered back up and connected my DMM to measure the voltage across the fuse connectors (no fuse in place) and it measured at 0.0007V DC, I have my DMM before I powered up the amp. If I add a fuse, shall I measure the output at the speaker connectors WITHOUT a speaker connected to see what the output is. I don't want to damage the new speaker I've added just in case.

If this doesn't work I will be happy to PM you with regards to repairing the module for me.

Best Regards

Rob

[Mick Bailey]

....If I add a fuse, shall I measure the output at the speaker connectors WITHOUT a speaker connected to see what the output is.

That's what Jon refers to when he says power it up without a load. It protects the amp to a large degree if something is wrong - especially if the amp swings towards one voltage rail. Disconnect the speaker, insert the fuse, then power up the amp and measure the DC across the output terminals.

[rvin28]

Many thanks and will do.

Best Regards

Rob

[rvin28]

I've just added a 3.15A fuse and tested the output - zero volts DC and no fuse blown. My next thought is to put a signal through the amp and measure the output again as I'm very worried about damaging the new speaker that I've added if some thing is faulty with the system (maybe I'm completely wrong here). Your thoughts will be most welcome.

Best Regards

Rob

[Jon Snell]

I have been known to temporailly fit a 5Watt car bulb in line with the speaker, by removing the 3.15A fuse and fitting a lamp on leads. This will allow the amplifier to work but not destroy the speaker if it fails under test.
This will save you the cost of a fuse and will attenuate the audio feeding the speaker at high volumes whilst protecting the speaker. All that will happen if the amplifier fails is DC across the lamp which will fail, not your speaker.

[g1]

Also should be using load resistor rather than speaker until you are sure amp is working correctly.