Tandberg 3012A-sound only from headphones

ET16

Well-Known Member
I've been going through some amps that I have, and I moved along to my Tandberg 30112A. I was adjusting the idle when the probe slipped. I didn't think much about it, at first, but I then found that I had no reading at the speaker terminals. Hooking it up, I have sound from headphones, but nothing from speakers. I don't see anything blown.

I don't know what I did.
 
That is a typical symptom of a failed (open) output bleeding enough signal through to power headphones, but nowhere near enough to move a speaker cone. It could also be a loss of signal through a marginal connection, such as a protection relay contact, speaker selector switch, or solder joint.

Also, those are FET outputs, so testing will get a little more interesting.
 
Interesting that you mention a relay. I head a click of a relay when the probe slipped. The relay doesn't activate now.
 
That probe slip may have done some damage, and could involve the output stage(s), protection circuit, or power supplies. R615/616 are interestingly close to a ground plane on the PS board, and if the probe shorted the ±57V supplies to ground, the power supply might be the first place to start looking. Are the supply voltages present, and close the indications on the diagram? The ±57V supplies can be measured directly at the main filter caps (15,000µF).
 
I'm not sure how to proceed. I find that the R410 pot crackles when turned.
I may need to take it in.
 
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The relay won't engage if there's excess DC offset in the output stage which is likely from the sound of things. As Watthour said, these are FET outputs (very well-respected Hitachi brand lateral MOSFET's, to be specific) and replacements - esp. matched ones as the factory used - are hard to come by though substitutes do exist. Hopefully you only damaged something ahead of the finals, not the FET's themselves. It would take some voltage checking in the driver and final stages to isolate the issue, assuming the power supply voltages are all present at the correct values.

This is not an amp you want to destroy, and it should only be worked on with caution using a dim bulb fixture to current-limit things and you'd want to de-couple the output stage while working on the driver stage if that's where the issue lies. I've forgotten the detail for that on the 3012/3012A, on the 3006A/3009A etc. there are 3-pin connectors that route signal between the driver stage and outputs that can be unplugged, and the FET's themselves can be removed if necessary (but keep track which ones go where).

John
 
John,

I think I'm going to need to send it out. I think that the outputs may be okay as the heat sinks seem normal.

I was thinking of sending to Approved Audio Service in CT or Rick Rupert in PA.
 
There's no load across the outputs with the relay staying open, so heat wouldn't necessarily be generated. That said, sending it out may be your best option regardless. Hope the news is good!

John
 
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