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Measuring bias current on Tandberg TR 1000

Crystalboy

Crystalboy

Member
I'm finishing off a few projects laying around, recap of two Tandberg TR 1000 receivers. Before I'm going to seal up the cabinets I'd thought I would check the bias current. Reading the manual for this procedure on this capacitor coupled amplifier makes me contemplative. I don't understand what this "strap" could be on the rectifier board? I tried googling the Norwegian word for it (which I didn't understand at all, even though I'm Swedish and know it okay-ish) and no results came up. I'm thinking I need to detach something from the board, but it’s really vague. I have done this measurement on newer Tandberg receivers, and that was very straightforward and just measuring across a emitter resistor on the power amplifier boards. I attached a couple of pictures of the schematic. Let me know if I should posts some pictures inside the receiver as well. Any help greatly appreciated.
 

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Strap is a solder bridge between two points exactly where you circled them. Desolder the bridge and put your DMM in series there for current adjustment. Very simple. It's good to have new trimmer potentiometers in place before the adjustment. There are also several tantalum caps, which are good to get replaced as well, maybe you noticed them.
 
Mr. Nick said:
Strap is a solder bridge between two points exactly where you circled them. Desolder the bridge and put your DMM in series there for current adjustment. Very simple. It's good to have new trimmer potentiometers in place before the adjustment. There are also several tantalum caps, which are good to get replaced as well, maybe you noticed them.
Click to expand...

Thanks for clearing that up! I’ve contemplated replacing the trim pots but thought I’d try them out and see how they work first. The tantalums I haven’t thought about though, didn’t think they usually go bad do they?
 
There are numerous examples here of blown amplifier stages due to failed bias trim pot. I suggest you replace them with modern equivalent, such as Bourns trimmer. Bear in mind these are 50 years old components. Tantalums do drift and often these fail shorted. If you search AK, you will draw a conclusion it's a good idea to replace them. Near power switch there's a current limiting resistor to FM Stereo lamp, suggest you upgrade it with something like 5 watter, mine looked like bit cooked, so better replace that as well.
 
Mr. Nick said:
There are numerous examples here of blown amplifier stages due to failed bias trim pot. I suggest you replace them with modern equivalent, such as Bourns trimmer. Bear in mind these are 50 years old components. Tantalums do drift and often these fail shorted. If you search AK, you will draw a conclusion it's a good idea to replace them. Near power switch there's a current limiting resistor to FM Stereo lamp, suggest you upgrade it with something like 5 watter, mine looked like bit cooked, so better replace that as well.
Click to expand...

Yeah, what the heck I'm gonna make a big order on Mouser this weekend I'll just throw in some more capacitors then. What did you replace your tantalums with? My conclusion from other threads seems that Nichicon UKL would be appropriate. Also on the RIAA, I replaced the lone electrolytic capacitor (220uF 10V) on C301 with a Nichicon UKZ, would a low noise capacitor be more appropriate there?
 
Good choice! I used an electrolytic. UKL would be just fine. Consider how much an electrolytic cap evolved in 50 years. Low noise or not, I doubt you hear the difference between them.
When you replace the trimmers, before you start I would write down the resistances of the original trimmers in circuit and adjust the new trimmers to match the original resistance once you have soldered the new trimmers in.
Maybe you are already aware, trimmers have polarity and if you "blindly" adjust anti-clockwise them to start the process as stated in many service manuals the effect might be the opposite, max out the current and the amp stage will be torched.
 
Imo and 2¢... if you haven't touched the output transistors I wouldn't play with bias if the readings are good. Instead though, if no adjustment for DCv offset.. I'd measure for that at the speaker terminals. just another hint at the health of the unit.
 
^This is a capacitor coupled receiver. The fresh output caps will take care of the DC.
There's still elevated risk to torch the amp, if the trimmers are not replaced.
 
If anyone's looking info on how to actually set the bias for TR-1000/TR-1010 (like I did as a new owner), it can be measured from the emitter resistors too. The target voltage is 15mv, as printed in the schematics. Tested and confirmed.
 
For the record, after restoration I needed to increase bias voltage to 22mv to get rid off crossover distortion (could be lower, but running my first listening test). Didn't measure the current. Same thing happened with my HK730, no doubt because of new transistors in driver boards driving original outputs. Or that is my current hypothesis.
 
An addition to this zombie thread. After a full restoration, no need to set the bias above 15mv. Soundstaging is phenomenal! :rockon:
 
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