Maintenance on Tandberg 3009A monos

[Mike Russell]

I recently picked up a pair of Tandberg 3009A monos from their original owner. They work/sound great but the seller mentioned that they haven't been serviced in approx. 20 years. I have three questions: 1) Given how well they appear to be working now, but considering how long they've gone without care, how important is it to do some preventative maintenance (replace caps, etc.)? 2) If important, what maintenance would your recommend? 3) Given the Tandberg reputation, how important is it for this kind of maintenance to be performed by a Tandberg expert? Is it something a reasonably competent tech but not someone super familiar with Tandberg units could do? (I'm certain not qualified, myself.) Many thanks for all your helpful guidance.

 

[jdurbin1]

So, the part you want to be uber careful about on these is preserving the very difficult to replace Hitachi output MOSFET's. Also the design uses what Tandberg called feed-forward rather than global negative feedback, and how the bias and DC offset are adjusted is a bit different than some more traditional amplifiers.

It's worth replacing the smaller electrolytic caps and at least testing the big pair for leakage/ESR/capacitance, which will take a good tech with the right tools. It's also worth renewing the thermal grease between the output transistors and heat sink which is probably pretty baked at this age, which can start to interfere with good heat transfer to the heat sink.

IIRC the circuit diagram for this model is available, and it should cover the important parts of the process. You may need to make a small fixture for the DC offset adjustment, the manual recommends soldering a pair of LED's anode to cathode (and vice-versa) and there's usually a place on the board where that can be connected with a small 2-pin connector so you can rough adjust the offset using the LED's as visual feedback, then do fine-adjustment with a meter connected as well.

John

 

[Mike Russell]

Much obliged, John. In your experience (or knowledge), what's something to listen for as the first signs that things may be failing? Thanks again.

 

[jdurbin1]

Taking longer than normal to come out of protection after power up is probably the first sign. Means the DC offset is increasing or isn't as stable as it normally should be. I've never listened to one that was starting to fail though, so...

John