Help with studio 350 offset voltage adjustment

[Brendan m]

With supply voltage at 70V I am unable to adjust the offset voltage, VR1 has no effect, the output offset voltage swings wildly positive and negative, I can hear oscillations.When the supply voltage is reduced to 60V adjustment works perfectly ie: with safety resistors in place, set VR2 anticlockwise to 200 ohms , adjust VR1 for a output dc voltage of 0V.

I am thinking a 2sa 1084 or Bf469,bf470,mje340 breaking down past a certain voltage?I have checked the pin outs of the 2sa 1084 transistors.

Any help would be appreciated.

 

[Ian Finch]

You are going to need a schematic, either linked or posted for others to follow you. There is at least one other Silicon Chip studio 350 thread here that you could post in or reference some details.
http://www.diyaudio.com/forums/solid-state/61413-studio-350-bias-problem.html
https://www.google.com.au/search?q=...ywti9M%2C_&usg=__ynZ_EX5-c2SKkoqU7cDw9bBI810=

Transistors that break down with excess voltage applied, will fail completely - there is no temporary state that can be recovered from unless the current flowing is restricted in some way. If you test your transistors in a multimeter test socket or use a cheap kit transitor tester, you'll soon see if the semis, at least the smaller ones, are functional.

As you say you have oscillation, is your output Zobel network in place and soldered correctly? MJL 21193/4 genuine parts? BF469/70 bought on Ebay perhaps? Was it a retail kit or did you source the parts and etch the board yourself?

 

[restorer-john]

Transistors often breakdown with excessive voltage and appear perfectly fine afterwards on normal cheap low voltage testers.

I have several examples of output transistors (particularly Ring emitters and diffused emitter types) that the failure mode is only discovered using my curve tracer at high voltages under their rated limits. They test perfectly at low voltages.

 

[Ian Finch]

Transistors often breakdown with excessive voltage and appear perfectly fine afterwards on normal cheap low voltage testers....

Exactly, with many power transistors, which is why I drew attention to the smaller ones. I use about 0.5A Ic to check power transistors on a heatsink and that seems a fair compromise that still shows up problems. You can get hot spots developing from second breakdown in conventional power transistors too. I don't think this is what the OP is talking about though, when there is oscillation occurring with presumably no load on a newly built amplifier where the supply voltages are within spec. and SOA.

If there really is a transistor breakdown problem in a proven design with a good 12 year track record, then apart from errors published by Silicon Chip or in the kit instructions, PCB faults or assembly errors, the next area of suspicion is fake components, noting that Renesas 2SA1084 transistors have been supplied in the past with reversed pinout! That would be obvious with DC measurements around the input stage but who knows what thru-hole components are being supplied now - Altronics kits at least, are now packaged in Asia.

For larger power amplifiers, the cost to get serious with decent test equipment is worth it. I bought a 100MHz Tektronix 'scope for $150 5 years ago. 'best test instrument investment I've made to date - great for finding where oscillation problems begin.

 

[Brendan m]

You are going to need a schematic, either linked or posted for others to follow you. There is at least one other Silicon Chip studio 350 thread here that you could post in or reference some details.

Transistors that break down with excess voltage applied, will fail completely - there is no temporary state that can be recovered from unless the current flowing is restricted in some way. If you test your transistors in a multimeter test socket or use a cheap kit transitor tester, you'll soon see if the semis, at least the smaller ones, are functional.

As you say you have oscillation, is your output Zobel network in place and soldered correctly? MJL 21193/4 genuine parts? BF469/70 bought on Ebay perhaps? Was it a retail kit or did you source the parts and etch the board yourself?

Problem has been solved (the output offset voltage adjustment, haven't gone further) for some reason I presumed BF459 transistors were equivalent to
BF469 transistors. 🙄

I need to redo the pcb but I try and use: on semiconductor, Motorola, nxp semiconductor.

 

[Ian Finch]

Aha!.. Just beware of buying Motorola parts now. They became On Semi in 2000, so if you like buying fakes in the guise of NOS parts, there are plenty out there to go around.
Replacements for BF469/70 are Fairchild KSC3503/A1381 as genuine BF series video driver transistors have completely dried up, other than from vintage spares companies charging hefty premiums. The last quantity of these I bought several years ago looked very convincing and aged, but broke down at about 115V and had very low Ft and Hfe.

Take care and good luck with completion 🙂