Randall V2 power amp problem

[QJerry]

i have got a problem with V2 head - every 2 - 3 weeks one of 2SK1058 blowns up - after replacing with a new one everyfing is ok - amp is working again till the next time...
is there anything i can do? some upgrade maybe?
best regards/sorry for my english

[J M Fahey]

Don't know what this means, please post the schematic.

[QJerry]

V2 poweramp

that's all i've got

[J M Fahey]

Thanks.
Is it always the same one?
Specifically: the one you just replaced?
Maybe it's a fake *or* it's not properly mounted and heat transfer to the heat sink is not good enough.
I suspect more the first possibility; just google "counterfeit transistors" and you'll drop from your chair.
*Maybe* you have to order a replacement from Randall (although at a higher price than from a regular shop) to be sure it's "good" (or at least as good as the others).
Good luck.

[Steve Conner]

I got more interesting results by googling Transistores Falsificados.

Another possibility is: Maybe you need to replace all of the 2SK1058 with a matched set. If they're not matched, one could be overloaded while the other one does hardly anything.

[bsco]

Just joined the form a few days ago. I have a Randall V2 here as well with a bad hum which is adjustable with the master volume control. All other controls can be turned down and only the master volume turned up, the amp hums and it gets louder if you crank up the master volume. Sounds like a power supply hum only that this one is adjustable. Did you ever get your Randall head repaired after??I had an Eden Bass that used to do what your did and it turned out to be a flaky electrolytic capacitor in the power supply..The 162 was always ok...the 1058 used to blow out on me. I am hoping someone can help me out with my problem here.

[J M Fahey]

Start by googling its schematic *and* ask Randall.
Post it here.
Thanks.

[bsco]

I tried searching for a schematic...there are lots that come up but this one doesn't...I will see if I can get something from Randall. If I do, I will be sure to post it. That amp has four tubes in it. I swapped them out so the tubes were not the problem. So I have a problem on a board somewhere.
Thanks....

[J M Fahey]

You already limited its source to a relatively small part of the preamp (or the Power Supply which feeds it) between most tone controls and Master Volume, but to advance further we need a "map".
Good luck.

[bsco]

Looks like I might have found the problem. One of the 8200uf/80V capacitors in the power supply is bad...I am going to order two and replace both of them..Once they go in I'll have to check to see if anything else is causing problems...The only thing with this amp is that everything had to come out of the chassis in order to get at these capacitors....a very time comsuming ordeal and in order to test it, everything has to do back together and all the harnessing has to be re-connected. It will be a few weeks before I get these parts in. Once I install them I'll post here how everything went..Thanks for the replies to my post....

[bsco]

Thanks for the compliment...I finally received those caps back a week or two ago and replaced them. They were the problem.. The amp works perfectly. There is no hum there at all. One of the caps had zero capaciatance while the other had it's rated value...but I went ahead and replaced both of them anyway..... if one is after going and they are both original, then the other one might go soon as well... I hope this solution also helps somebody else in need...

[Jazz P Bass]

Glad you have it repaired.

[mikeydee77]

Thanks for the compliment...I finally received those caps back a week or two ago and replaced them. They were the problem.. The amp works perfectly. There is no hum there at all. One of the caps had zero capaciatance while the other had it's rated value...but I went ahead and replaced both of them anyway..... if one is after going and they are both original, then the other one might go soon as well... I hope this solution also helps somebody else in need...

Thanks for posting this bsco. I have the same issue. Did you get a schematic from Randall? I have already requested a schematic one from UK distributor soundtech.co.uk but I wondered if you ever received one and could share?

Mike

[bsco]

Hey Mike....I do have some schematics....I'll go through what I have here and see if I have the proper ones....If I do, I can post them for you....I tried to post something before and it wouldn't go up....I'll try again and if it doesn't work then I can email them to you.....
Cheers,
Bernie

[mikeydee77]

Thanks BSCO. I managed to find this one which covers T2 / V2 which might be good enough for my needs.

T2_Schematic.pdf