Huldra 10

Tandberg top model receiver 1972-77

Considered to be the warmest sounding receiver from Tandberg

Huldra 10 where first seen in the catalog for 1972-1973 together with TR-1000 and TA-300. How Huldra 10s amplification compares to TA-300, TR-200, TR-1000/1010 and later TR-1040/1055 can be seen here.

  • 2x25W/8Ω single supply push-pull power amp
  • 61VDC single power rail
  • Seperate transformer windings and rectifier to supply 25VDC to radio/preamp
  • Up until serial number 471500 Huldra 10 had TBA231 op-amp in the riaa/phono stage — then changed it in favour of a descrete 8 pcs BC149B transistor circuit.
  • Output transistors: MJE3055/MJE2955 in a Darlington Emitter Follower configuration with MPS-U06/MPS-U56
  • Output capasitors: 4700µF changed to 3300µF on later units
  • Later models had many 22µF caps values upped to 47µF throughout all printed circuit boards with no changes made to the schematics.
  • From s/n 483170 accuracy for the FM stereo decoder are better using IC on the main board. But if sound quality for the FM receiver are of importance we are supposed to look for serial numbers from 477000 to 484000 still using transformers. Else later models (as in newer) are noted to sound better for amplification duties.
  • W53.5 H12 D30.5 cm, 8.2 kg

General service

This Huldra 10 where received as partial functioning. General cleaning of all switches with Arrow Lotoxane and oiling of the axel for AM tuner where done.

Axel where really stuck and afer some jiggeling what made it come loose where to remove the E-ring on the axel and then press the axel down into the houseing of the variable capacitor while turning back and forth. Take care not to damage the tiny fragile brass and aluminium gears. Had to shorten the AM dial cord 2-3mm to get better grip.

Modern 240V mains need 240V transformer setting

Mains wire for the transformer where moved from the 220V tab to the 240V position (outer tabs) and 25V PSU adjusted accordingly (VR R1002). Actually after the primary winding change voltage dropped to 25.1V.

Quiescent current where now low around 5-6mV after 15 minutes warmup. Adjusted to 11mV (60 mA) according to service manual. There are some that recommend turning it a notch higher to 18mV (100mA) for a cleaner sound. Better familiarize myself with stock setting before setting out on that journey.

"One of the best World-Band Hi-Fi Receivers ever!"

Tom's Radio World

Power Supply Upgrade

Kemet slit foil Original 4700µF capacitor where replaced with a 10.000µF based on multiple recommendations. This give measurable lower ripple noise and bass reproduction benefit from it. The fuse and rectifier are capable to handle the added inrush current at power-on.

A new hole had to be drilled for the large cap. Actually, the 10.000µF cap are smaller than the original, but have a different foot print. To ensure correct spacing I used the new cap as a compass inserting one pin to my designated hole and scraped an arc on the area where the new hole where to be drilled.

1.5µF/100V film cap where soldered on underside of PCB over the big cap terminals using free holes left from the original cap. Small and large caps combined supposedly give a better broadband circuit having different self-resonace frequencies.

LED back-light for radio scale

Only a single bulb out of eight 6.3V/320mA E10 bulbs for the radio scale panel where lighting. Rather than replacing all of them with new bulbs the idea to use LED open up the possibility to have this powerful 5.8V secondary transformer winding powering a Raspberry Pi (RPi) mini computer to give Internett radio in place of the now closed Norwegian FM radio network.

H10 Line level mod White LED where first tested with decent result. Front glass on these old Tandberg receivers are tinted blue. While they normally fade when exposed to UV light this specimen still have a clear deep blue color which really pop when scale where backlit with blue LEDs. For AM scale there are now total of 16 LED and 6 for the FM scale. Huldra, the old dame emblem will have her own LED up her back — planned to be lid up when there are an Internett connection.

While LED functioned fine on AC current flickering where notisble. An 3 ampere rectifier where installed clamped to the top edge of the chassis, right over the main rectifier. With a 1000µF cap the new circuit now output 7.8V DC with no load.

Note: Brown wire to center tap for 4.2VAC indicator bulbs are removed from transformer side, taking these bulbs out of circuit. Later they be feed with positive DC current with the Brown wire connected to negative terminal on this new PSU.

H10 LED mod 75Ω resistor where used for each pair of LED. Tests after installation now show we can add at least another 75Ω in series with the finished LED strip and still have the blue come trought even in bright daylight. While bright blue looks really cool it soon gets tiresome — dimmed down is proper way to go one think.
220mA (1.8W) current draw from all these LEDs are much less what a single bulb used to consume where total load where around 2.5A (15.5W).

5V DC to power a mini computer

A LM350 voltage regulator where installed after the new rectifier (for the LED) before I understood more tests should have been performed beforehand. As it turn out voltage drop are considerable from idling at 7.8V DC. Test now unveil that without voltage regulation I can only get some 700mA/5VDC. This would need a LDO (low dropout regulator) rather than the LM350 regulator to be useful. H10 5V mod
Loading the rectifier harder with a 2.2Ω resistor voltage now drops down to 4.09VDC ≈ 1.9A.

To keep these figures indicator bulbs also have to be converted to LED, or removed from circuit with it's total load of 575mA when all bulbs are on.

Not sure this will perform as planned with RPi B3+ needing close to 980mA at full throttle, — idling at some 350mA. Then add a dedicated DAC+ DSP hat from HiFiBerry system will work close to the edge with regards to keeping voltage within specification.

emitter resistor value * quiescent current = voltage over emitter resistor

For Huldra 10 the default setting are: 0.18Ω * 60mA = 10.8mV ≈ 11mV

measured voltage over emitter resistor / 0.18 = quiescent current

"Closest I came to tube sound without the hassle of tubes"

"Tandberg did well with the transition from tubes to transistors"

anonymous forum members

H10 output cap H10 pin spacing

AUX & Tape Output Level

Not planning to use this Huldra 10 with any wintage tape machines bringing Line Output levels up closer to modern specification seems a good idea. Current configuration has 40kΩ output impedance and 5 times lower output gain than most modern gear. Higher output will prove very useful if Huldra 10s RIAA/phono amplifier are to be used as a stand alone device.

 AUX Out — Remove R561, R562 (47kΩ, leave emty) and replace the two 330kΩ resistors R567, R568 with 10kΩ.
 TAPE Out — Remove R565 and R566 (47kΩ, leave emty) and replace the two 330kΩ resistors R571, R572 with 10kΩ

PCB revision 41160-6 found in this reciver the resistors R567, R568, R571, R572 where 280kΩ

H10 Line level mod

For hi-fi gear there are some that recommend vintage carbon resistors in this positions. For period correct resistors 10kΩ values can be fount in the AM tuner at R319, R329, R335, R336 and a few others. They could be replaced with modern resistors, or just removed if AM tuner aren't in use. I chose to replace them. Not finding R319 instead snagged R313, part of a divider network I got a little jittery ending up with a burned transistor if simply removed.

One aberration for the line out mod could be to mount RCA sockets with it's own 10kΩ resistor and leave the 330kΩ in place. Then mount a switch to the 47kΩ resistor going to ground (those we removed). This setup will then let us hook up and configure the DIN socket for vintage gear switching in the 47kΩ, and use RCA for modern without the 47kΩ to ground. With only one device physically connected at any time thought. Schematics for TR-1000 actually show similar setup, but the 330kΩ resistor are 150kΩ with 47kkΩ always in circuit to ground with seperate 10kΩ resistors inline for the RCA sockets.

Utilizing KISS one simply keep one output unmodified.

Huldra 10 as Preamp — 3 possible scenarios

  • As Phono preamp and Radio — Use Tape out with line level modifications mentioned elsewhere on this page. Aux out can also be used but will not have Tape as possible source. Tape cannot be selected as source with this setup.
  • Traditional Pre-amp — With input selector and volume/tone controls. Lift or double the wire coming from preamp going to power amp and connect to a new set of RCA connectors with a 22µF cap i series with signal and 220kΩ resistor from signal to ground after the cap. The cap are important as there are a large DC offset on the preamp output the coupling cap are intended to block.
  • Less traditional — Recommended for sound quality (coloration) and ease of implementation. Use the headphone out with two optional 50Ω/1W resistors over the L/GND and R/GND terminals to create a line level output still utelizing the power amp. please see drawing below

    H10 preamp mod This concept that have been a known way for studios to record electric guitar and bass amps since early years of recordings where first advocated used with Tandberg receivers by Kjell B. Whom also recommended lowering the quiescent current to 6mA (1,1 mV over R 721-722) when used as headphone/pre-amp with no speakers connected. This should have the output stage work in Class A up to 200 mV from where it become Class AB. Although there are reports that crossover distortion might become a problem when lowering the quiescent current this low.

  • This last setup has actually been reviewed on print back in 5/2004 Fidelity #10. The article where an exercise comparing vintage 70's Huldra 10 to at the time modern Krell KAV-400xi to learn if the world of hi-fi had moved forward. H10 as an amp with original power supply fell apart due to low wattage for the test setup, but as an preamp it held up remarcable well.

    TIP: This technique can possible be used as poor man's Neve/SSL style color box in your recording studio. If you favorite amp are missing headphone out use a load box for guitar amp. Or make one as shown in the drawing above. Higher wattage amp would of course need resistors capable to handle the power.

Switch OFF radio tuners when used for amplification

The initial idea where to find a way to shut down any oscillators when H10 where used for amplification only. Several hours were wasted to trace main PCB. Sadly revision 41160-6 PCB layout does not show up in any of the service manuals I found online.

Interesting I found the screening bar going over the PCB front to back do not all go to ground as expected. The one separating PSU and preamp actually feed 25V to one part of the AM tuner. But the actual AM oscillator takes 25V from the push button switching system in front. So does the FM tuner.

H10 PSU bulb As it turns out Tandberg had the same idea that turning off the radios completely is preferred. This can be verified by watching the small bulb inside the 25V power supply. The more current the system draw the more this bulb will light up. From barely glowing when any of the line level Phono/Tape/Aux are selected. To very visible when any of the AM band are active. And full glow when FM with presets are selected.
Actually this can be a useful trick when troubleshooting contact problem in the switch gear. Watch the bulb for kontact verification.

While Huldra 9 has the serial number stamped on metal backplate Huldra 10 made use of a simple paper sticker on the back. If the sticker are missing as it often is you usually find an identical sticker on the main board inside the receiver. Although none of the revision -6 board with Decoder 5 we encountered had this sticker.
Wonder if not TR series got branded like so to insert Tandbergs Radiofabrikk into the model name? Fabrication branches where also called by their abbreviation TR.
TR1 and TR2 at Kjelsås, TR-3 at Kjeller, TR-4 at Skullerud, TR-6 at Notodden, TR-7 Haddington in Skottland
Keeping the well renowned model name Huldra 10 had at least four upgrades through its lifespan from 1972 to 1977. Where new discrete transistor phono amp and several changes to better the FM tuner where introduced.
While Tandberg already having many years of experience with transistor radio tuners from the Sølvsuper 11, HiFi-FM and portable devices Huldra 10 where the first from the Huldra serie receivers to leave tubes behind.

Table show capacitive reactance for the two common output coupling capacitor values:

20 40 80 160 320 640 1280 2560 5120 10240 20480 Hz
3300µF 2.41 1.21 0.60 0.30 0.15 0.08 0.04 0.02 0.01 0.05 0
4700µF 1.69 0.85 0.42 0.21 0.11 0.05 0.03 0.01 0.01 0 0
0.8 0.4 0.2 0.1 0.05 0.02 0.01 0.01 0.00 0 0
Fill inn the green box to experiment with various capacitor values. For instance are 2200µF used in some Sølvsuper receivers.
C719/C720 coupling capasitors
Although not being particular 
accurate applying calculations
fc=1/2πRC for a RC high pass
filter using a typical speaker 
resistance as the resistor 
value tell us there are negligibly 
difference in cutoff frequency 
between the two capacitor values 
typically used on the power amp 

    3300µF  4700µF  |  1000uF
4Ω   12 Hz  8.5 Hz  |  39.8 Hz
6Ω    8 Hz  5.6 Hz  |  26.5 Hz
8Ω    6 Hz  4.2 Hz  |  19.9 Hz

Understanding this less than adequate
and simplified calculation we antici-
pate that the real speaker impedance 
will influence about equal for both 
cap values.

RC High Pass 1st Order High Pass Filter  fc = 1/(2πRC)    Ω

Table list -3dB cutoff frequencies for E12 series capacitor values into given impedance
nF 1 1.2 1.5 1.8 2.2 2.7 3.3 3.9 4.7 5.6 6.8 8.2
Hz - - - - - - - - - - - -
nF 10 12 15 18 22 27 33 39 47 56 68 82
Hz - - - - - - - - - - - -
nF 100 120 150 180 220 270 330 390 470 560 680 820
Hz - - - - - - - - - - - -
µF 1.0 1.2 1.5 1.8 2.2 2.7 3.3 3.9 4.7 5.6 6.8 8.2
Hz - - - - - - - - - - - -
µF 10 12 15 18 22 27 33 39 47 56 68 82
Hz - - - - - - - - - - - -
µF 100 120 150 180 220 270 330 390 470 560 680 820
Hz - - - - - - - - - - - -
µF 1000 1200 1500 1800 2200 2700 3300 3900 4700 5600 6800 8200
Hz - - - - - - - - - - - -

No sound, one channel missing or distorted sound - Switches

While most modern receiver make use of relays or rotary selector switches these seventies Tandberg receivers use push button switches with an arm to mechanically release any switch already locked in. This mechanic need cleaning and lubrication as dust and hair accumulate over time and hinder all the tiny parts to move freely.

But what differ the most are how these switches are implemented electronically. Where each switch either select or pass on the signal. Meaning if one input is not working properly the contact problem could possibly be located on a switch further up the signal path not passing on the signal as intended. For instance the Phono switch select between Radio or Phone. Which are then passed on the Aux switch and from there to the Tape switch before going to the volume control and preamp.

These switches are actually designed to be taken apart and serviced. Although air pressured cleaning fluid are the popular way to do these repair. My absolute favorite are Lotoxane with its long open time let me soak and dissolve old grease and oxidation without drying out and create a new layer of dirt on parts and contact elements. I use the liquid version and apply air pressure afterwards from separate container.
Capacitors order list for Tandberg Huldra 10
All capacitor are electrolytic type unless noted.

I like to keep as many original components as possible. But
for safety reason and possibly being those caps that had most 
stress through nearly 50 years I replace all electrolytic caps
on power supply and power amp except the input coupling caps 
which are some special selected types compared to power rail 
caps with identical value.

I'm unsure all 2.2µF used to block DC on inputs on each module 
truly are tantal type. Dielectric are unknown and it might be 
better to call them drop capacitor. They do have similar mushroom 
shape and dipping process into liquid insolation layer as typical 
tantal does. Anyway, they are part of the sound and we better 
keep the originals if possible. If replaced try film capacitors.

NOTE: Several 22µF values have 47µF mounted on PCB on newer 
      revision without schematic being changed.

Power Supply  #41150
	C1006 - 4700µF  70V (*) 
	C1005 -  220µF  50V 
	C1004 -   10uF 100V (35V)
	C1003 -   22uF  40V (47µF original on PCB)
	C1001 -  100µF  50V (feed stereo indicator bulb)(**) 
	*  rectifier and fuse seem to hold if increased to 
	   10.000µF which give better ripple measurement 
	   and bass performance
	** Tantal on PCB but not on schematic parts list 

Power Amp  #41115  takes 61V from PSU
	C725  -  470µF  63V possible has to do with power mute circuit
	C701  -   22µF  63V input signal
	C705  -   22µF  63V
	C707  -   47µF 100V
	C709  -  250µF  40V
	C719  - 4700µF  70V* 
	* manual list 40V as minimum voltage for this cap
	  later H10 have 3300µF same as TR-1000
	C702  -   22µF  63V input signal
	C706  -   22µF  63V
	C708  -   47µF 100V 
	C710  -  250µF  40V 
	C720  - 4700µF  70V
	* manual list 40V as minimum voltage for this cap
	  later H10 have 3300µF same as TR-1000

RIIA AMP  #41201  35V
	C503  -   10µF  25V
	C511  -  100µF  25V
	C513  -  100µF  25V
	C504  -   10µF  25V
	C512  -  100µF  25V
	C514  -  100µF  25V

RF Board #41160  35V
	C551  -   2.2µF  35V input signal - Tantal
	C555  -    50µF  40V
	C557  -    10µF  25V

	C552  -   2.2µF  35V input signal - Tantal
	C556  -    50µF  40V
	C558  -    10µF  25V

PreAmp  #41150  35V
	C605  -   2.2µF 100V signal
	C609  -    10µF 100V signal
	C621  -    10µF 100V signal
	C623  -   2.2µF 100V signal - Tantal
	C625  -    50µF  40V
	C635  -   2.2µF  35V signal - Tantal
	C637  -   100µF  25V signal
	C606  -   2.2µF 100V signal
	C610  -    10µF 100V signal
	C622  -    10µF 100V signal
	C624  -   2.2µF 100V signal - Tantal
	C626  -    50µF  40V
	C636  -   2.2µF  35V signal - Tantal
	C638  -   100µF  25V signal 
MJE3055 / MJE2955 in TO-220-3 package are still available from On Semiconductor and STMicroelectronics. Note that pin configuration might be different on modern revisions.

Transistor cross-reference for BC149B NPN Si X09 cap

Tandberg list following equivalents: BC109B, BC109C, BC149C, BC173B, BC173C, else table below list possible replacements.
Type Pc W Vcb Vce Veb Ic Tj Ft Cc Hfe
A747B  0.25  50  45  6  0.2  125  150  5  200
A747C  0.25  50  45  6  0.2  125  150  5  250
A748B  0.25  30  20  5  0.2  125  150  5  200
A748C  0.25  30  20  5  0.2  125  150  5  420
A749B  0.25  30  20  5  0.2  125  150  5  200
A749C  0.25  30  20  5  0.2  125  150  5  420
BC147B  0.25  50  45  6  0.2  125  150  5  200
BC148B  0.25  30  20  5  0.2  125  150  5  200
BC148C  0.25  30  20  5  0.2  125  150  5  420
BC149  0.25  30  20  5  0.2  125  150  5  200
BC149B  0.25  30  20  5  0.2  125  150  5  200
BC149C  0.25  30  20  5  0.2  125  150  5  420

How to get more omf from the power amp:

Add more capacitance up from the original 4700µF in the power supply (PSU).

Leaving the voltage selctor in the 230V setting in regions width 240 main voltage could add approx one watt, — if combined with larger cap in the PSU.

Using the H10 as a mono amp will add a lot of power as it free up current from the main transformer now only running a single channel. I.e. implementing another H10 for dual mono or even bi-amping. With a estimate around 22% more gain we could have some 2x30W/8Ω with two H10.

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Some comprehensive Tandberg archives here

Norsk Radiohistorisk Forening

Sverre Holm list the Huldra series evolvement in chronological order

How Huldra 10 amplification compares to TA-300, TR-200, TR-1000, TR-1010, TR-1020, TR-1040, TR-1055

 More Tandberg related stuff:


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