TR-2055 project

Tandbergista buys a half-butchered TR-2055. Can it be restored?

A TR-2055 should look like this:

But mine looks like this:

Lets first take a look at what the Tandberg TR-2055 receiver is, or should be

Tandberg TR-2055 was produced in the mid-1970s and is one of the more powerful models in the TR-20xx range. It features FM radio, two tape inputs and two phono inputs with built in RIAA. Power output is 2 x 55W into 8 ohm.
Internal design is direct coupled. Separate rectifiers and filter capacitors for each power amplifier channel. The pre amplifier and radio section has a typical Tandberg style regulated supply. One big Norwegian-made transformer with separate windings for each power amp channel, the regulated pre amp supply and dial lamps.

I asked Venice AI about this model. The answer (slightly shortened) was nice:

TR-2055 is a vintage FM stereo receiver that was produced in the mid-1970s. This model is part of Tandberg’s line of high-quality audio equipment, which was known for its durability and excellent sound quality. The TR-2055, like other Tandberg receivers of its time, is often praised for its warm and natural sound, which is a characteristic sought after by audiophiles. These receivers are not only functional but also aesthetically pleasing, often featuring a blend of wood and silver that gives them a timeless appeal. For those interested in vintage audio equipment, the Tandberg TR-2055 is considered a valuable piece, and collectors often seek out these receivers for their sound quality and historical significance.

Venice AI also gave us its views of the company:

Tandberg was a company founded in 1933 by Vebjørn Tandberg in Oslo, Norway, and it became well-known for its innovative audio equipment, including radios, reel-to-reel tape recorders, and cassette decks. The brand was particularly admired for its attention to detail and the quality of its components.

If you want to read more about Tandberg the company, a very fascinating story, read here and here. Read more about the TR-20xx series later in this post.

This post is divided in several sections

The first part of this post will be about missing parts, specially knobs, and researching the different versions of this model – what are the differences? My new unit is a bit of a mystery model, maybe a transition model. I will write this post as the research and restoration progresses. The electronic restoration comes later, skip down if you want only that part.

The first challenge – missing parts

The seller was going to use it for spares. It is missing parts. Electronic components, like the relay, have been taken out. My plan is to try to restore it. The absent knobs will be a challenge.

Looking for knobs..

I am researching TR-20xx series knobs. Asking around in the Norwegian Tandberg community if anyone has a set of spares. I am also considering having them remade. Also searching for new knobs that could fit. The knob type is called stacked knobs and the potentiometers are concentric, that means two axles. But not all of them – the tone controls are concentric, while volume control,. speaker selector and tuning are normal, single axle type. Tandberg also used a four-pointed star hole in the inner knob that connects with the outer potentiometer axle. The stacked knobs you can buy today have a locking screw on the outer as well as the inner knob, while Tandberg knobs only lock on the outer.

If you know anything about these types of knobs, like if the “4-pointed star hole” for the inner knob is a standard or proprietary-Tandberg, you’re welcome to message me.

If I manage to have new knobs made, would you be interested in a set?
Should I make them like Mk I or Mk II?

Modern replacements, would it be possible? Not sure.

This TR-2055 is a mystery version

The Tandberg community is great. This TR-2055 research has very much been a team effort. Only TR-2075/2080 is supposed to have separate controls for left and right. I had a discussion with Ståle from Norway, one of the leading Tandberg experts in the community, he commented that maybe it is a early version, it is uncommon but he has found another unit like this. The photos below show the progression from the “normal” Mk I, to the “uncommon” Mk I to the Mk II.

I have a theory, that is possible that this is a late Mk I. So late that Tandberg considered TR-2075 type of tone controls for a new version and made a few such units, for which these concentric pots and TR-2075 style knobs were used (which we can not see, because they have been lost). But on later units they switched to single axle pots and the normal “single” Mk II type single knobs. Maybe you can call it a very early Mk II. But I am speculating.

Internal differences between versions?

Per M from a Norwegian Tandberg group on Facebook showed me the power amp board of another Mk I, below. I found some Mk II photos, also below.

I am not 100% clear about all the differences between versions. Restoration hasn’t started yet. First researching, which is very interesting thanks to the helpful guys in the Tandberg community, not only in Norway but in Poland, Australia and other places.

Another way to see the version is on the pre amp board.

Photos of my TR-2055

Pawel from Poland says:
“You may check the production time, at the blue level meters you should have the month and year , it’s usually 2-6 months before the unit production date. If it’s not marked you have the very early version with the German meters, not japanese 🙂 From time to time the boards are also dated, and the FM tuner was a little bit different in the early versions of TR2055”

We continue the discussion about this version. Pawel sees photo of my unit and says “Mark I , well preserved:) I my opinion the early stage of production. The main capacitors are Elna, later replaced by Siemens”

About Tandberg TR-2055 and the TR-20xx series

Tandberg TR-2055 specs:
Power is 55 Watt into 8 ohm. Tuner section is supposed to be outstanding. I think it was introduced 1976. The US brochure gives a price of $799. That converts to $4460 today! (I may be slightly off, read somewhere it is close to $3000, but could be depending on how you calculate). This was not a cheap receiver for sure.

The design is (like all TR-20xx models) direct coupled. That means a power amplifier supply that swings between positive and negative, with the output in between at zero volts. Tandberg had the TR-1040 before the TR-20xx series which also was direct coupled. For more info on the difference between direct coupling and capacitor coupling, see the TR-1010 restoration page.

A guide to the whole Tandberg TR-20xx series by the famous Tandberg aficioniado jdurbin1 in a hifi forum:

First-gen TR 20xx series: started in 1975 with the launch of the TR 2075 flagship, sold as “three components in one” message, i.e. you did not have to give up component-level performance when you bought a 2075. That first series also included ther TR 2025 and TR 2055. They can be distinguished by the knobs having deeper/square fluting and inset black pointers.

Gen 2 TR 20xx: 2075 is replaced by the 2075 MkII which incorporates numerous internal improvements made to the 2075 design over its production lifespan. New version of 2025 and 2055 are offered and a new model called the TR 2040 complete that series, which sees the older knob type replaced with a clear-anodized satin finish knob with shallower scalloped fluting. It was being exposed to the TR 2075 MkII in 1976 which turned me into a diehard Tandberg enhusiast (hence my avatar).

Gen 3 TR 20xx: blue displays are gone, replaced by amber. This was down to changing out the blue acrylic panel for a smoked gray/brown panel and similar change to the back panel colors for the two tuning meters. 2025 is replaced by 2030, 2040 is replaced by 2045, 2055 is replaced by 2060. The 2025 to 2030 and 2040 to 2045 changes are largely cosmetic and slight uptick in power rating. 2055 to 2060 is more drastic: they added AM to the 2060 (only model other than the three flagship models to have AM) and it went in the smaller chassis that the 2025/2030/2040/2045 all share. Also got pushbutton tuning. I’ve had an acquaintance that designed audiophile gear for a living lavish praise on the 2060, said it was the most accurate/neutral receiver from that era he had ever heard. Doesn’t seem like the 2060 gets that level of respect in terms of market value, though. Top of the line for this generation was the 2080 which launched in 1977/78 timeframe and continued in production through the introduction of the 3000-series.

(more info here)

Electronic restoration begins

I didn’t know anything about the state of the electronics in this TR-2055. Output transistors or rectifiers could be shorted or other problems. It didn’t seem safe to power it up on 240V. So I used my Philips lab power supply to first test the power amp. Each channel has its own rectifier and bank of filter capacitors, I wanted to test each channel separately. Connected the power supply on each side of the positive and negative filter capacitors and ground in between. Signal is fed via signal input to the power amp (from where it comes from the pre amp), through a small capacitor, from the Thandar signal generator (1 kHz sinus). The power amps actually work. A little surprising. I measured straight after the output transistors and the relay does not click in (it is not supposed to now, because the protection circuit isn’t getting power).

The pre amp has a Tandberg style regulated 25V supply. I tested it by feeding the 25V point on the pre amp board from the Philips supply and a signal (same signal on both channels). On the scope screen we could see the signal, but the pre amp has problems. The volume pot seemed seriously degraded, so I DeOxit’ed it, which also made it turn smoother. I then DeOxit’ed again and resoldered. Maybe someone has been there before, because of visible old flux, which I tried to clean up. The pre amp delivers relatively acceptable signal now, but I will get back to it later. Resoldered some joints, specially the input connectors and Hi Filter and Low Filter (after a dose of DeOxit), because these switches are in the signal path, as you see below. Dirty filter switches seem to be a Tandberg malady.

It had no relay. Where did it go? Let’s fit one in.

Testing the TR-2055 without load, straight after output transistors (before relay). It works.

Up to now ran it with my homemade current limiter (just a repurposed lamp). The amp seemed to work, but the protection circuit relay didn’t activate. Voltage on Q864 base was near zero (protection circuit is the lower part of the schematics below). Capacitor C863 was suspect. In circuit “leaky” on my capacitor tester. Soldered it out and it still indicated “leaky”. With a new 47uF 50V capacitor we had voltage there (but low). Also replaced the other capacitors in the circuit, C861 and C862. Now the protection circuit works. But I need to take it off the current limiter to test it properly.

On unrestricted power – the relay kicks in. The amp gets signal from Tape 1 and delivers signal out, though unloaded so far.

Measured all rails, +44V and -44V on both channels (43V according to schematics).

Repairing the FM scale string

The string had broken. Restringing a dial string can be tricky. Maybe most of you who want to read about Tandberg TR-2055 aren’t too interested in this part? If so, skip ahead. But it is quite interesting actually, and something often unavoidable when dealing with vintage radios and receivers. I’ve just done it a couple of times myself.

Should I just use a bit of new string and tie the two ends together? That was the plan.

How do you most efficiently, easily tie two strings together? I found this excellent instruction video: https://www.youtube.com/watch?v=zusMtS3rboU

The wheel on the FM pot meter originally had 4 turns, now it has 3.. I struggled to wind the string back after it unwinded itself, three turns will be enough. An advantage (for my convenience) is that the string became about 3 cm longer, so I used that to tie the broken ends together. Still was a bit loose so I made an extra turn on the axle of the knob, now three turns (I have seen other radios having several turns here, it seems Tandberg used only two turns). The string on the knob axle tended to slip a little at times before, so this was probably no disadvantage.
To attach the string after tying it together, I took off the wheel to the left, placed it in the groove and reattached the wheel. There is no spring on this dial string, so I didn’t want to force it over the wheel. Tightened the FM potmeter screws under the board. Lubricated moving axles. We’ll see later if I screwed up the FM tuning or not, but so far it turns and looks alright.

I have heard there is supposed to be a spring on the string. I’ll see if I can purchase one. If you know where to buy such, you’re welcome to message me.

Adjusting for 240V

The service manual contains instructions. Just move a “jumper cable” on the transformer. It wasn’t easy to get the solder to stick to the (so far) unused transformer terminals, even though I tried to clean them up before. But with a flux dispensing pen from Circuit Works it was much easier to solder.

Just to be safe, I powered it up slowly with a variac and measured one of the secondaries. The 240V conversion went well.

Tested it with a load today, and output is ok, although I didn’t challenge it with lots of power. FM reception was very good. I’ll get back to performance later, there is more to do first.

Closer check at capacitors and circuits

The input board, as I call it, surely someone had been there before. These jumpers (blue cables) and not perfectly fitted screws, that doesn’t look factory. Why?

The input board can seem tricky to get out. But actually it isn’t difficult, no violence required. Remove screws on the top of course, then you need to gently push the RCA connectors from the back, until they say ‘click’, and you can slide the whole board out to the right, the cables are long enough to follow, until you reach the cable connectors. Very clever construction.

Yes, someone has been here. The blue cables (and solder blobs below) are jumpers that bypass the level adjustment potentiometers on Tape 1 and Tape 2. All inputs have level adjustments except Phono 2 (inputs are Tape 1, Tape 2, Phono 1 and Phono 2). Why? I desoldered the jumpers and checked – the potentiometers work. I expected them to be faulty. One of them had a rather large spread between L and R, about 15%, maybe that is why? If you don’t want to adjust your incoming source signal, just set the pot to minimum, no need to bypass it. Or you lost the knobs (they are missing). I ordered dual trim pots from Alps and Bourns (from Mouser), 20k and 50k, the originals are 25k. We’ll see if I manage to replace these original pots on Tape 1 and Tape 2. Ordered a bunch, because I want a tolerance (btw L and R) of 5% or less, if possible.

I measured all caps in the input board (in circuit). They were mostly ok, but I will replace the first caps on all Tape inputs and the large 470uF caps in the Phono section.

New input level trim pots

20k or 50k? I chose in the end 50k Bourns. Original is 25k. I bought three different models, including by mistake a model with common ground, which won’t work here. But the chosen Bourns pots have separate grounds. I ordered several and use two with the best “spreads”.

There is a lot more to do with this TR-2055, but tonight I could test run it. Source is DAC and FM, speakers Hifi System 4 with Seas drivers, which you can read about here.
Amazingly, it works. The input level adjustments works perfectly too, the knob even moves in the right direction (max – min). It took some planning to get it right.

TR-2055 sound is clean, balanced, elegant. Similar to TR-2080, it is maybe not what you expect, specially if you have experience with earlier Tandberg models, like Huldra 10 and TR-1000, because they are fat and rich sounding, I compare them to chocolate dessert. The TR-2055 is more like strawberry sorbet.

More problems. A typical vintage receiver problem, channel imbalance. It is probably the volume potentiometer.
See the article Why do pots have channel imbalances at low volumes? at Tandbergista Talks.

To be continued..