Tandberg Series 12 reel to reel power amp question - UK Vintage Radio Repair and Restoration Discussion Forum

john116 · 20th May 2014, 9:57 am

Hi - does anyone know which power amp circuit board is the LH channel? The service manual doesn't show which is which. I could work it out by tracing the speaker wires I guess, but thought someone on here might know.

Many thanks,

John

DOFFERY · 20th May 2014, 10:30 am

From memory only, the Left ch. is the one nearest to the side of the cabinet. Easy way to locate, remove the red wire (27v)from pin 10 on either amp. that amp will be dead .

Colin.

john116 · 20th May 2014, 1:02 pm

Colin - many thanks. Will try that.

John

john116 · 21st May 2014, 1:27 pm

Colin - you're right. Board nearest to the side of the case is the left channel. I've de-earthed the AC127 and AC128 transistors, and the left channel power amp is working again.

DOFFERY · 21st May 2014, 5:51 pm

Glad you found it John, the AC 128 is notorious at the best of times.

Cheers.

Colin.

ricard · 22nd May 2014, 7:54 am

Sounds like the classic tin whiskers problem then? You can probably 'zap' the transistor (there's a sticky thread in the Components forum section on how to do this) which will break the connection between the case and whatever electrode is short-circuiting. I've done that a couple of times with model 12 driver stages to good effect. Does require to remove the transistor to do the job though which is not fun given that those 12 power amp boards are a beast to dismantle in order to get at the solder tracks. Not one of Tandberg's great moments...

john116 · 22nd May 2014, 9:30 am

Ricard, I'll do that if the transistor(s) fail again. For the moment, de-earthing them worked.

I agree -those two power amp boards are a pain to remove, then once you've got through that hurdle, you have to remove the power transistors to remove the large heatsink metal plate, to finally get to the soldered connections. Did it once many years ago on another machine, prefer not to have to do it again.

J

20th May 2014, 9:57 am	#1
john116 Pentode Join Date: May 2008 Location: Sydney, Australia Posts: 156	Tandberg Series 12 reel to reel power amp question Hi - does anyone know which power amp circuit board is the LH channel? The service manual doesn't show which is which. I could work it out by tracing the speaker wires I guess, but thought someone on here might know. Many thanks, John

20th May 2014, 10:30 am	#2
DOFFERY Retired Dormant Member Join Date: Aug 2004 Location: Blackpool, Lancashire, UK. Posts: 1,488	Re: Tandberg Series 12 reel to reel power amp question From memory only, the Left ch. is the one nearest to the side of the cabinet. Easy way to locate, remove the red wire (27v)from pin 10 on either amp. that amp will be dead . Colin.

20th May 2014, 1:02 pm	#3
john116 Pentode Join Date: May 2008 Location: Sydney, Australia Posts: 156	Re: Tandberg Series 12 reel to reel power amp question Colin - many thanks. Will try that. John

21st May 2014, 1:27 pm	#4
john116 Pentode Join Date: May 2008 Location: Sydney, Australia Posts: 156	Re: Tandberg Series 12 reel to reel power amp question Colin - you're right. Board nearest to the side of the case is the left channel. I've de-earthed the AC127 and AC128 transistors, and the left channel power amp is working again.

21st May 2014, 5:51 pm	#5
DOFFERY Retired Dormant Member Join Date: Aug 2004 Location: Blackpool, Lancashire, UK. Posts: 1,488	Re: Tandberg Series 12 reel to reel power amp question Glad you found it John, the AC 128 is notorious at the best of times. Cheers. Colin.

22nd May 2014, 7:54 am	#6
ricard Octode Join Date: Mar 2006 Location: Lund, Sweden Posts: 1,632	Re: Tandberg Series 12 reel to reel power amp question Sounds like the classic tin whiskers problem then? You can probably 'zap' the transistor (there's a sticky thread in the Components forum section on how to do this) which will break the connection between the case and whatever electrode is short-circuiting. I've done that a couple of times with model 12 driver stages to good effect. Does require to remove the transistor to do the job though which is not fun given that those 12 power amp boards are a beast to dismantle in order to get at the solder tracks. Not one of Tandberg's great moments...

22nd May 2014, 9:30 am	#7
john116 Pentode Join Date: May 2008 Location: Sydney, Australia Posts: 156	Re: Tandberg Series 12 reel to reel power amp question Ricard, I'll do that if the transistor(s) fail again. For the moment, de-earthing them worked. I agree -those two power amp boards are a pain to remove, then once you've got through that hurdle, you have to remove the power transistors to remove the large heatsink metal plate, to finally get to the soldered connections. Did it once many years ago on another machine, prefer not to have to do it again. J