Tandberg TR-2075, one channel; now you hear it, now you don't..

steveUK

steveUK

Active Member
Although it was ok at first, the right hand channel on a Tandberg TR-2075 sometimes does not work (in my bedroom), so I moved the receiver to my workshop and did a thorough overall/service of the protection relay and the receiver then worked fine. I took it back to the bedroom and all was fine - for a while. Now, the right hand speaker doesn't work again in the bedroom. So I took the receiver to my workshop and guess what? yes, the right hand speaker works fine in there. I've swapped the right and left speakers in the bedroom and so proven that they are ok. When it works, it sounds fine. Tapping the relay does not invoke an on/off situation, in fact I'm pretty sure that the protection relay is not the problem, if it ever was. Switching the receiver on and off does not appear to affect the fault. The speakers in the bedroom are specified as 6 ohms and the speakers in the workshop are specified as 4 ohms.

Any ideas?
 
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I presume when swapping speakers you swapped speaker cables at the receiver side.

define preamp or main amp by crosswiring pre out - main in.

If you have headphones , use headphones to exclude the speaker selector switch from the problem.

(Yes I know it is all obvious but sometimes it just gets overlooked....).

Finding such problems are a PITA and the relay is mounted in such way you do not want to change it without being sure (guess you also have this relay mounted "above the circuit card")

Apart from measuring output signal in the receiver itself, a way to exclude the relay is having wired up the contacts externally and short them out while they are supposed to be shorted by the relay itself, anyway.
 
Bad solder joints maybe? Maybe the shock and vibrations of you moving it to your workshop is causing a joint to make connection and then when you move it back to your room the joint stops making connection. Try banging on the receiver while it's on, not to hard though, don't want to put your fist through it. ;)
 
Thanks guys for your input and suggestions. I dare say that when I do find it, it will be one of these obvious things; poor switch contacts etc - even though I have addressed these issues, but you can never be sure that attacking a switch with Servisol (UK's equivalent to deoxit) will do the job as it's not easy to get the stuff to go throughout the whole workings of the switch (ie all contacts) without applying so much as to swamp the whole area, I don't like doing that. Anyway, I will keep you good guys informed in due course.
 
Ok, after an overnight stay in the workshop, the receiver works fine in there. Took it into the bedroom, it works fine in there too. So it's back to monitoring it and see what happens.
 
Update. It hadn't been on for a while and on switching it on today, the problematic channel was 'working' albeit at a very reduced volume, whereas before it was all or nothing. I see this as being good in a way as it may be a pointer to the problem. Just to recap: When it works, there is nothing wrong with the sound, you wouldn’t know there was an issue. In its 'new', reduced volume mode, there's a high noise level when you turn it up to hear what’s going on. That’s a higher noise level than normal I’m talking about, and as the volume is increased distortion is apparent, particularly in the bass frequencies. That’s on AM or FM and with the receiver set to mono (it’s the same on stereo).

I have swapped over the 'preamp out' and 'power amp in' break in connections and proven that the fault is in the power amp section as both preamps are producing a good signal.

I have cleaned and exercised all switches and pots thoroughly.

I have removed the protection relay and cleaned and tweaked its contacts for proper operation ie properly open or ‘nicely made’ with a bit of pressure.

It is not my speakers that are the problem, all checked.

My plan is to get it on the bench and scope it to compare channels and see where the signal is being lost.

But for now, any more ideas anyone? I’m thinking it sounds like a faulty electrolytic or something. Dry joint maybe, but I’ve tapped around with the lid off and not discovered anything yet, but obviously it could still be the case.
 
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You will have to measure, indeed. Myself I had an intermitting problem too, for which I still do not know the real cause. Anyway, it IS possible a defective or wrongly steered transistor drives down the voltage all the way to minus 40 or so (preventing the relay to come up) without destroying the drivers/outputs.

I did put some info in another TR2075 thread not long ago, over here on AK.

In my case, I was not really sure since the amp reacted on cold spray for the differential pairs as well the voltage amp.
I changed out all small transistors if I remember well (with the exception of drivers and output transistors.
Biggest risk items for intermitting behavior I thought was the differential pairs Q901/903, and Q907/911 (going by one channel out of the schematic) or the voltage amp Q909.

In my case, Q901/903 had a hfe difference of 1:2 so it was clearly compromised but replacing did not solve the problem.

Be aware IF the voltage was driven down or up fully without proper operation of the amp, (some) transistors may have suffered reverse base-emitter junction current and so they would be quality compromised theoretically, behaving not normal but still not be the initial cause of the problem.
So, finding a clearly defective transistor as "being the culprit" is nice (in case you find it) but replacing adjacent ones might be a safe bet also.

You also may suspect c901 for leakage, or the 15 volts zener, measure if the voltage is minus 15 volts +/- 10% over there.
 
For sure get a good look at the solder work on the bottom side of the power amp board, with magnification if possible. And, make sure the power supply rail fuses are good & that the fuseholders are still tight. Shouldn't be too hard to track down once you have it on the bench.

John
 
Thanks guys.

BTW John, this isn't the one that I need the parts for, I have two. I guess you realised that though.
 
I didn't but answer would have been the same :-)

BTW, if you haven't already done it, work over that preamp record switch at the far right of the lower filter switch area diligently with deox - that thing is notorious for strangling signal before it passes on to the power amp. Dropped or weak channels, etc.

John
 
Thanks John. I think I read previously somewhere - by yourself I guess - that that was a known weak spot. It's currently behaving itself. I hate intermittent faults...

This set, the one with the intermittent fault, is all complete. The other one has the heatsink and parts missing.
 
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With two of them on hand, I think I'd be tempted to do some judicious board swaps one at a time until I isolated the one with the fault, if I couldn't pin it down otherwise.

I still need to do a head count on 2075's on hand vs. my parts stash & get back to you on what you need for that unit. The heat sinks should be ok, don't think I've ever had to replace one of those so I have to have extras from parted units. Not as sure on the rest.


John
 
John,

To be honest I haven't got 'down and dirty' with it yet. When I do I'll be stressing and pressing on PCBs and components and using a strong eyeglass to look for dry joints etc all over. Only after a thorough going over of that nature will I start swapping boards around as that is a big task in itself. It's definitely not the preamp record switch though. I'm lucky because at the moment it's in 'failure mode', but you can bet that as soon as I have transported it to my workshop it will have reverted to 'working mode' again. That's intermittent faults for you!

Oh, just a thought, I noticed this morning that when I touch the preamp to power amp link for the left channel (the working one) there's a hum/buzz through the speaker. When the fault on the RH channel is there (ie now) touching the link on the RH channel does not result in a similar noise in the RH speaker. I guess that confirms that the fault is in the power amp?
 
Unless the preamp is pulling that signal line to ground (unlikely), yes - it's in the power amp.

Connections between the amp and speaker terminals are pretty reliable, can't remember I had any trouble in that path but it's worth making sure the rotary speaker select switch is deoxidized (it's open frame type so easy to get to the contacts). That and broken solder joints on the board are about it, very little chance the soldered wire harness connections at the terminals or switch have failed.

John
 
down and dirty is lick a finger and touch input to power amp .. try both channels .. speakers should hum/buzz ..f so go back to vol pot and try same thing .
or get technical with a signal injector ..
 
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