Tandberg TR-2075 turn-off pop and restoration

[CosmoK]

I've recently revived a TR-2075 and it sounds great. However, I have a loud pop that visibly moves the woofers when I turn the receiver off without disengaging the speakers via the speaker selector switch.

This is a non-issue for me, since I'm used to turning down the volume and turning the speakers off, but it poses a risk if a friend or kid is operating the receiver. I'm trying to narrow down the fix for the shutdown pop.

There's less than 2 mV DC on the speakers, I just recapped everything except the tuner. It occurs on both channels and for both the A and B outputs. It occurs whether I've been playing it for 1 minute or 2 hours.

Identified some possibilities from previous posts, but how do I determine the most likely cause?
(1) A bad snub capacitor
(2) Bad connectors, the TR-2075 has a service bulletin on soldering the signal cables between boards instead of relying on the old snap-in connectors.
(3) A failing relay
(4) A bad cap somewhere (but, I've replaced all the electrolytics).
(5) Something else?

So where would you start in tracking this down? Thanks!

 

[avionic]

@jdurbin1

 

[CosmoK]

Quick update: The pop only occurs when I turn the unit off in the FM mode (with or without the tape monitor depressed). So, I think it's a leaky cap from the FM tuner. I didn't change those caps because I really didn't want to mess with the alignment, since the FM sounds pretty good.

The other tricks I read about for trouble shooting are: (1) remove the pre-out main-in jumpers to isolate the problem (this helped me figure out it was the the tuner going through the pre not the main amp) and (2) try turning the unit off using a surge protector, which might diagnose a bad snubber cap, I guess.

 

[jdurbin1]

I would NOT solder those preamp connections personally, but I do always thoroughly spritz them with Deox and run them in and out of the jacks repeatedly so the Deox can help scrub either side of the connection (since you can't really get inside the jack to clean those surfaces directly).

I've never found a leaky cap in one of these tuners, but I guess that's possible. I'd do a lot of other things before chasing that though, including making sure all the board grounds are secure, whether wired or those places where the screw holding the board in place is also clamping a ground trace against the chassis under the board. I'd have automatically replaced the 1000uF/63 filter cap on the regulator board, and inspected every single header where harnesses connect boards to look for cracked solder. I'd also do that same deox plug/unplug/replug treatment on those header pins/harnesses. And, if the preamp board has Frako caps on it (also the power amp board), those would have been replaced at this point. All of those things are known issues for the 2075.

If it's still popping and all those preventative treatments have been done, can check for DC offset on the tuner signal coming off the FM board but like I said I've never seen that particular issue.

John

 

[CosmoK]

Thanks, I found an "extra" screw on my work bench... I'll take a look inside and see if the tuner has unsecured ground. That would be an easy fix.

My next step will be cleaning the connectors and checking for cracked solder. I definitely wont solder the connectors... it would be a pain to mess with for future servicing, and I dont think the delicate pads would stand up to many repeated heatings.

 

[jdurbin1]

Yeah, it is a known weak point on these earlier 2075's (they changed to a more robust jack/plug solution on the MkII and 2080) but losing that serviceability by hard-wiring is a real bitch. I've been able to avoid that more drastic solution so far.

John

 

[CosmoK]

So it turned out to be the new stereo light (24V/50mA). I unplugged it and the pop was gone (along with arcing in the relay). It looks like the right rating for both the 2075 and 2075 mk II, but mine was a little long and was pushing up against the housing, maybe that was causing a problem?

 

[jdurbin1]

Was it the T (tubular) glass shape instead of G (globe)? I can't see that causing electrical problems, but def. wouldn't have fit well. That lighting board doesn't physically impact the tuner board either.

How sure are you the lamp is 24V/50mA? Those are a lot harder to source than they used to be, and I've run into lamp places that literally don't care and will ship something they think is close (like when I ordered some 12V/50mA fuse lamps and received a 5W lamp with completely different electrical ratings and a different p/n. If that lamp was pulling excess current from the FM board, I could see some side effects though usually it just fries a transistor and that's that.

I assume it was a lamp, not an LED replacement?

John

 

[CosmoK]

Yes, it looks to be a tube (T) lamp. It has 24V/50mA stamped into the steel base and I bought it from a reputable AK'er. I'll try a new lamp when I get a chance and check some voltages in the vicinity.

Its gonna be hard taking it back to the bench though because I'm enjoying listening to it so much and the pop is gone for now.

Maybe a bad lamp, maybe something else. Either way, you've saved me a ton of time pointing me in the right direction.

Come to think of it, the lights were unplugged when I got it. So maybe there was some problem in the past. One output was also fried and the amp PCB was broken in a corner and there was no power cable... so there could be more to the story than aging components...

 

[jdurbin1]

Yeah that's the wrong lamp, let me know if you need the correct globe style - same electrical ratings. Tandberg did use those T 3-1/4 type for some of the open reel deck meters, IIRC and may have used them for FM stereo lamps on some older receivers but they're too tall for the 2055/2075/2080 FM stereo application.

John

 

[CosmoK]

Restoration notes:
When I finish a project, I like to post a few notes about oddities, etc.

(1) Take some pictures when disassembling these Tandberg receivers, they come apart really easily once you figure it out, but it can be tough to remember where all the little 5 mm hex screws go. I love the unique internal layout and design of this receiver. It's been jokingly called lutefisk engineering. Well, my grandpa, a T/Sgt in the signal corps during WWII, loved electronics AND lutefisk.

(2) There are a combination of radial and axial electrolytic caps. Open the unit up and look before ordering, otherwise things might not fit. A lot of the axial caps are standing up and can be replaced by radials, but some are laying down, so need an axial replacement.

(3) Check (and double check) the capacitor heights. Several boards, especially the RIAA board on the back panel, have VERY little clearance.

(4) Desoldering was easier than usual because most pins were straight, not bent flush with the PCB. The pads are delicate so heat gently.

(5) As @jdurbin1 says: deoxit those wire connectors while you're in there. I didn't realize this was a problem-area until I was done. The good news: those wire connectors are awesome for removing boards easily.

(6) There's a lot of great info on restoring the TR receivers (mostly provided by @jdurbin1). This thread on restoring a TR-2080 by @dlucy was very useful, as the TR-2075 shares several boards and design features with the 2080.

(7) These Mini Gold Plated Insulated 5-Way Binding Post Banana Jacks can replace the speaker terminal screws without any disassembly. I left the original pads. One caveat is that you need to put the nut on the jack before screwing it in to ensure good contact with the pad (I had one post that wouldn't make contact until I figured this out). Normally, you'd want the nut on the inside, but that wont work in this unit without modifications.

Popping noise status:
My family gets mad when our stereo cabinet is empty, so I have to finish a CR-1020 restoration before I can take the TR-2075 back to the bench and figure out the FM stereo light popping issue. Stay tuned.