Randall RH150 low output

[g1]

They are usually called 'ballast' resistors and serve a couple functions. For now we are concerned with their 'fuse' function.
Check them like you checked the fuses, they will either be a very low resistance, near zero ohms, or a much higher resistance, 'open'.

Signal path is very straightforward, from SIG A, through C7, then on to R3,4,5 to gates of TR1,2,&3 and out their source terminals. In tube jargon I guess you would say they are acting like cathode followers. R6,7,&8 complete the circuit from the Fets to the output line.
The signal routes through C9 and on to R15 etc. mirror image of top for the bottom half.

[LarBal]

Ok Thanks!

Looking at the Fets, from left to right, the pins are 1,2,3 > gate, drain, source , correct?
Looking at unused pads on my board(for the 300W version), I can clearly see a trace from one end of the .1R go to the middle pin of the Fet, or pin 2, which must be the Source, according to the schemetic.
So I'm instantly confused there. Pin 2 must be the source, not the drain?
Here are my measurements, with one probe at the junction of the diodes, D3 and D5, and the other on the respective pins of the Fets left to right, which I'm calling 1,2,3

TR2
1 - 1.5K
2 - .2
3 - .4

TR1
1 - 134
2 - .2
3 - .4

TR6
1 - 6K
2 - .2
3 - 789

TR5
1 - 6K
2 - .2
3 - 789

The pin 2 , .2 measurements, which visually tell me this is the correct spot to test the .1R resistors, tell me that these resistors read OK, but I'm unsure as to the descrepancies with the other pins.

[Jazz P Bass]

Open your eyes my friend.

TR1 & TR2 are shorted Source to Drain.

[g1]

What he said ^^^^ .
Pin2 is the source, and looks like source to drain is shorted on TR1 & 2. That would account for the blown fuse. Removing them and rechecking out of circuit may show only one is shorted, but replace them both anyway.
And just out of curiosity, where did you see pin2 called out as drain?

[LarBal]

Ok Great! So I had my pinouts wrong, and Sorce is shorted to drain, which denotes failure of the Mosfet(s).

It seems every google search for "Mosfet Pinout" yielded Gate1 Drain2 Source3
https://www.espruino.com/mosfets?print

I'd love to order from Mouser, as I also need to recap my personal Peavey Studio 40 , and need some radials

Looks like this is the part? link

Will any of these work? I need some help narrowing down my search : link

[g1]

Like it said on that page "always consult the datasheet". There is no generic pinout, just 'typical', which is kind of meaningless unless you only need it to work 'usually'.
For any semiconductor, just google the number on it and you will most likely find the datasheet. Some times there is an implied '2S' or '2N' prefix not shown. In this case the labelled K1058 is a 2SK1058.
Your first link from mouser is the part. It is obsolete.
I'm not sure about using any of those others in your second mouser link as a direct replacement. Maybe someone else will comment.
If it were me, I'd buy the used ones Dalbani Corp. is selling on Ebay. They are a reputable seller of real parts. The new ones listed coming from China are likely not real exact parts.

[LarBal]

Ok great, so I'm looking at these : linkhttps://www.ebay.com/itm/2SK1058-Ori...4a78f#viTabs_0

[g1]

Yes, those are the ones I was looking at.

[LarBal]

Got the fuses today, looks like I got the right ones

They Mosfets are on their way too, got 2 of them for $12.

What do I need to know about mounting the Mosfets to the heatsink? Do I need to use thermal paste? An insulator? I'm assuming there's one on there, but here's a couple of pics.

[The Dude]

You can clearly see the insulator and heat sink compound in your pic. Mount the new devices just as the old ones are. Evenly coat the back of the new devices, but don't overdo it.

[LarBal]

https://www.mouser.com/ProductDetail...52bS5HRHbi0%3dhttps://www.mouser.com/ProductDetail...52bS5HRHbi0%3d

How about this stuff?

[g1]

That looks fine. However, if the old stuff isn't dried out there looks to be enough there that you could reuse it. It only needs a very thin layer. Just be careful with the mica insulator as they crack easily. Try to do it so the output device comes away from the mica spacer leaving the spacer attached to the heatsink.

[LarBal]

Allright, I got the 2 shorted Mosfets off and the new ones installed, along with the fuses for the +60V rail.
To recap, TR1 and TR2 were bad, we determined, and the fuses for the +60V rail had blown also.
Now, I've got both + and - 60V rails up and running, and I have +60V on pins 3, the drain, of TR1 and TR2, and -60V on pins3 of the other side, TR5 and TR6 (there are empty spots for TR3 on one side, and TR4 on the other for the 300W version of this amp. Mines 150W)

So that's good, but still no sound.

Upon first power up, the fuses blew, but I also had neglected to reconnect some of the quick connects...the speaker connects and the connect for the +60V rail. Maybe it was shortingf to something when it was dangling.

What measurements should I be looking for now?
edit: also of note, when I first powered on(before I knew the fuses had blown), I was getting a little bit of sound from the amp, as I had been all along(whisper levers, at least).
When I got the fuses up and got both rails going, I got zero sound from amp. Also, the side of the heatsink with the Mosfets I didn't change got really hot, and the side with the replaced ones did not.
G3Poweramp_large.pdf

[LarBal]

Any other thoughts here?

[g1]

So the heatsink is still getting hot, or that was just when the wiring wasn't all hooked up?
Since that side got hot, have you redone the diode or resistance checks like in post #17 ?