old Sessionette 75 Mk 2 - no output

[Bushbaby]

A dear friend of mine, a guitarist from the late 60's from Cupid Inspiration (he rarely plays guitar now), recently picked up an old Sessionette 75 for next to nothing and it was indicated that the speaker wasn't working when he bought it. I did say to him that I had a spare suitable speaker and would clean it up, put in the speaker and as necessary have a look if there were any issues.  I noted that resistor R31 was indeed burnt out so i replaced with a an 470R 3 watt resistor.

https://www.award-session.com/pdfs/SG75_...ctions.pdf

I replaced the speaker with an Celestion G12B-150 8ohm speaker that I had spare and put the amp all back together again this afternoon. However, there is no output at all through the speaker or even headphones when connected?? Am I going to have to go from scratch to work through the amp to find out where the issue is? I was really hoping that this would be completely straightforward to get up and running again (thats how stupid I am!!).

Any help will be most appreciated.

Best Regards

Rob

[radioman]

Hi Rob,
I assume you checked that the original speaker was open circuit ?
If that indeed is the case, I wouldn't have risked connecting another 'speaker until I'd checked that there is no DC offset coming out of the power amp... in a DC-coupled circuit like this (i.e. no coupling capacitor) a fault with the power amp could fry the replacement 'speaker as well !

Andy

[Bushbaby]

Hi Andy
I checked the original speaker - no output whatsoever. I did connect the new speaker without checking first (numbskull on my part) but I'm pleased to say that its still working (thank goodness). It appears that this version of the amplifier has a ax120 power amp circuit board with Hitachi BUZ901P and BUZ905P so I hope they are not damaged as it appears that they are virtually irreplaceable. I've found this from Award Session who still deals with these amps

https://www.award-session.com/pdfs/sessi..._hum_1.pdf
so I need to check R9 but its a tiny little resistor to check. I assume I'll have to disconnect one end to check its current resistance.

Best Regards

Rob

[Bushbaby]

I have just measured R9 and its.....12.8kR NOT 12kR. As indicated its only a 1/4watt resistor. Will this really be an issue? Is a 2watt 12kR resistor a bit of an overkill in size?

Best Regards

Rob

[Bushbaby]

I'm wrong about the Mosfet trannies they are Green 2SJ82 & Black 2SK226.

Best Regards

[Murphyv310]

Hi Rob.

Yet again you are going around this the wrong way. If the original speaker was open circuit or burned out, why on earth did you try another speaker? Something has caused the speaker to blow, you have to find out why before the possibility of ruining a good speaker.
These amps run on a split HT rail and you need to take precautions so the amp doesn't blow itself up.
The resistor you talk about is a little high but not open circuit, plus most likely not the issue anyway, I wouldn't replace it with a 2 watt, I'd fit a 1 watt at the most but make sure its stood off the PCB by a few mm. Anyway most of these amps need what we call a blanket repair, one of the types of circuit that can self destruct if you don't. Also each HT line should be limited by a series resistor in series with the supplies, the speaker should be replaced with a suitable bulb as a load.
Each resistor needs to be checked and OUT of circuit, I'm sure I've explained this to you before.

You'll be out of your depth with this as you struggle with simple class A amps.

[Bushbaby]

Thanks Trevor
From what you are saying do you think I shouldn't touch this one at all? I feel a bit ashamed then that I've tried to help my fried with it.

Best Regards

Rob

[Murphyv310]

Hi Rob.
I honestly think with you not having the experience on DC coupled circuits it's best to get expert help with it. Also it's best to follow a certain procedure, limiting power supply current and loading the amp so it's not feeding a speaker but either a bulb or resistors the bulb though gives an instant visual display of a problem.
I'd send it to the manufacturer agent.

[Bushbaby]

Hi Trevor
I have taken your advice and have spoken to my friend (the owner) about this. I've now contacted Award- Session who indicate that they can repair these for a fixed cost (£69) plus parts. They do a new power amp replacement board at £59.99 which to be honest is quite reasonable considering that the old Hitachi Mosfet replacements could cost in excess of £100 assuming you can actually find the replacements in the first place.

Following on from this I am now going back to first principles (as we discussed today) and going to work on some very small projects / kits to understand what I am trying to do and if issues why those issues have occurred in the first place.

Speak soon and Best Regards

Rob

[Murphyv310]

Excellent Rob.
I wish you well.