Setting bias on Tandberg 12 - Page 4 - UK Vintage Radio Repair and Restoration Discussion Forum

Eichberger · 8th Jul 2017, 10:59 am

Another thing I've noticed is the sharp 'knee' as the plot goes from attenuation to the bump ( 5kHz @ 1.785ips for example). The eq curve in the service manual is a gentle increase so there must be something else happening.

TIMTAPE · 8th Jul 2017, 11:47 am

Agreed. For 1.875ips that sharp dip around 5kHz followed by the rise is odd.

What signal do the plots represent? Left channel? Right channel? Both summed?

Eichberger · 8th Jul 2017, 2:28 pm

It's the left channel. I'm fairly sure the right channel was the same but I'll do a proper plot when I have the chance.

The plot is done by recording white noise (eq'ed to be flat up to about 18kHz) and then playing back into my Mac via a quality audio interface.

TIMTAPE · 8th Jul 2017, 3:22 pm

Ok, just confirming it isn't right and left summed where deep dips such as in your plots would be a possibility especially at the slow tape speeds.

Eichberger · 23rd Sep 2017, 5:50 pm

Right, had a bit of time off this and am getting back on the horse. Figured I'd start by changing any suspect capacitors. As well as the high frequency roll off there is a lot of low frequency hum so I'll have a look at the two big electrolytics in the power supply as a starting point.

Getting the main audio circuit board out looks like a challenge but I'm actually quite excited about digging a bit deeper into this refurbishing thing!

I was thinking of completely disconnecting the speakers and power amp as I do not have any use for them. Will this cause any problem elsewhere?

8th Jul 2017, 10:59 am	#61
Eichberger Triode Join Date: Apr 2017 Location: Winchester, Hampshire, UK. Posts: 42	Re: Setting bias on Tandberg 12 Another thing I've noticed is the sharp 'knee' as the plot goes from attenuation to the bump ( 5kHz @ 1.785ips for example). The eq curve in the service manual is a gentle increase so there must be something else happening.

8th Jul 2017, 11:47 am	#62
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,969	Re: Setting bias on Tandberg 12 Agreed. For 1.875ips that sharp dip around 5kHz followed by the rise is odd. What signal do the plots represent? Left channel? Right channel? Both summed?

8th Jul 2017, 2:28 pm	#63
Eichberger Triode Join Date: Apr 2017 Location: Winchester, Hampshire, UK. Posts: 42	Re: Setting bias on Tandberg 12 It's the left channel. I'm fairly sure the right channel was the same but I'll do a proper plot when I have the chance. The plot is done by recording white noise (eq'ed to be flat up to about 18kHz) and then playing back into my Mac via a quality audio interface.

8th Jul 2017, 3:22 pm	#64
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,969	Re: Setting bias on Tandberg 12 Ok, just confirming it isn't right and left summed where deep dips such as in your plots would be a possibility especially at the slow tape speeds.

23rd Sep 2017, 5:50 pm	#65
Eichberger Triode Join Date: Apr 2017 Location: Winchester, Hampshire, UK. Posts: 42	Re: Setting bias on Tandberg 12 Right, had a bit of time off this and am getting back on the horse. Figured I'd start by changing any suspect capacitors. As well as the high frequency roll off there is a lot of low frequency hum so I'll have a look at the two big electrolytics in the power supply as a starting point. Getting the main audio circuit board out looks like a challenge but I'm actually quite excited about digging a bit deeper into this refurbishing thing! I was thinking of completely disconnecting the speakers and power amp as I do not have any use for them. Will this cause any problem elsewhere?