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Old 16th April 2013, 08:54 AM   #1
Atilla is offline Atilla  Norway
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Post Fixing an old Tandberg Huldra 6 Radio

Here is an old, old radio: Huldra 6-22 Radio Tandberg Radio; Oslo, build 1961/1962, 1 p

A friend of mine has one of these in his apartment and was wondering if it is usable. After a day of fiddling around, we found out that it is, mostly. The speakers were intact, the power supply worked and with the exception of one magic-eye indicator tube, everything powered up ok.

The radio tuner works, quite well in fact, and so does the output module. However it distorts, except on the lowest volume (and it might still do that but we just can't hear it well). It's meant to put out about 3W of power, but this distortion sounds more like an overdriven transistor amp and persists when the volume is lowered.

So my question is - how do I start debugging this thing? I guess almost all caps will need replacement, they're all insanely old. Especially the paper coupling caps. Some electrolytic caps are in solid metal cases, but 50 years is a lot of time. Caps seem to be standard values, so replacing them should be easy. I guess nothing can happen to the tubes themselves, but if needed, there are still replacements available. The output module is separate from the rest of the radio, so taking it out for repairs is Ok.

The schematic for the radio is available here: Tandberg Huldra 6 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics . It's quite complicated and I don't exactly understand how everything is wired. The 'GRAM' and 'BND' are the external inputs, we've tested them and they work. The weird circles labeled AI, AII, BI, BII are the tone control and volume controls, as far as I understand it and CI and CII are output selection - you can plug up to 4 speakers and select which output you want.

As far as I can see all heaters are powered by 6.2V AC, with the output module having its own separate winding, the Y label. I don't think the unfiltered heaters are a cause of the distortion though. The output module is based on an ECL86 tube, quite an odd thing overall, but fairly simple. Except for the fact that it's quite hard to see where every connection actually goes.

Any ideas and hints are welcome, we'd love to get this working better.
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Old 16th April 2013, 10:20 AM   #2
DF96 is offline DF96  England
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Don't change small value caps in the RF section, unless you can show by fault-tracing that they require it. They are probably OK and changing things here is as likely to introduce problems as solve them.

Check/replace electrolytics and any paper caps with high voltage across them, especially the one which usually feeds the output stage grid. Check DC voltages, but remember that 20% tolerance is OK and the original values were not obtained with a 10M DMM.
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Old 16th April 2013, 10:48 AM   #3
Atilla is offline Atilla  Norway
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I won't touch the RF tuning section at all. It does work fine, at least for FM. I'll mostly concern myself with the output section and maybe the tone controls. But the wiring around them is a scary mess, so not touching those unless strictly necessary.

From the schematic - I guess C 102/105 (cathode bypass), 97/99 (supply filtering) 98/100, 91/93, - output stage grids. That's probably what I'm looking for. I also noticed a few more voltages marked on the schematic now, so I'll take a note to carefully measure them. If there's anything worse than 20% I'll report.
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Old 16th April 2013, 03:21 PM   #4
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By 61 paper caps were a well understood technology. Tandberg was a premium brand here, I wouldn't expect them to be buying the **** grade cap typically found in a Western Auto or Airline appliance. Neither will a Tandberg be bounced around like a guitar amp. I would only change paper caps if changing the electrolytic caps didn't fix it. The wax seals on the paper caps (GI brand) in my 1961 dynakit equipment keep them pretty stable, except the one that the kit builder burned the case. Changing paper to polyester caps made my PAS2 preamp way too trebly. It wasn't until the tenth cap changed that I found the burned case one that was causing the channel to be too low in output.
Fifty year old electrolytic caps, are sure to be bad, if not today, then tomorrow. Except for extremely rare epoxy sealed e-caps they were sealed with rubber which deteriorates running or sitting still. Multiple threads on here about reforming e-caps are in my opinion, snake oil. Doesn't do a thing for the rubber seal or the amount of water contained. I re-e-capped a 197? Reader's Digest transistor FM + shortwave radio in 2010; there was a great improvement in sensitivity, selectivity, tone balance, and volume without any alignment at all. The electrolytics will be tall cans with circles squares and triangles for the sections, or cardboard covered cans with a plus on one end. To fit long life radial lead caps (>3000 hours) you may need to install terminal strips like the ones sold by tubesandmore.com(AZ), triodeelectronic.com(IL), or VT4c.com in china. A little coil of extra 600v rated wire on the same order is also a good idea.
If you still have problems after re-e-capping, look up your tubes on the internet and make sure the voltages on the plate and any high voltage grid are high enough - paper dielectric once it shorts out is shot, it turns to conductive carbon. Use one hand measuring voltage and alligator clip the negative of the meter to chassis or analog ground. Current across your heart can stop it. Don't wear jewelry. Don't work high voltage alone. Measure any metal under the chassis at < 25 VDC to chassis before your touch it.
Have fun. Sixties tandbergs and grundmans looked really cool, the ones they sold over here. My father always bought Sears or Montgomery Wards, which weren't worth rebuilding usually. He lucked out on the Reader's Digest radio, for some reason, it is fairly competent.
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Last edited by indianajo; 16th April 2013 at 03:38 PM.
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Old 16th April 2013, 03:43 PM   #5
DF96 is offline DF96  England
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My experience is that old paper caps often fail due to water ingress and so become leaky. A common symptom is the output valve running very hot becaue its grid is being pulled up.

On the other hand, old electrolytics can often be fine or just need reforming if the radio has not been used for many years. Reforming won't, of course, deal with electrolyte loss but it will deal with oxide layer degradation due to lack of polarising voltage.

Of course, my experience is with UK built radios. Elsewhere things might be different.
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Old 16th April 2013, 04:19 PM   #6
Atilla is offline Atilla  Norway
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That is all great advice, thank you. I'll start with some measurements and I'll check out how to unmount the output module so I can go trough the caps.

And I'll stay clear of the high voltage, I've zapped myself one time too many already.
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Old 16th April 2013, 04:58 PM   #7
mvd is offline mvd  Germany
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Quote:
Originally Posted by DF96 View Post
My experience is that old paper caps often fail due to water ingress and so become leaky. A common symptom is the output valve running very hot becaue its grid is being pulled up.
Yes, that's my experience too. Minimum Task for every old radio is to change the coupling-cap at the grid of the output-tube.

Quote:
Originally Posted by DF96 View Post
On the other hand, old electrolytics can often be fine or just need reforming if the radio has not been used for many years.
The oldest electrolytic capacitors that i have in use, are working in a Tandberg Solvsuper 2 from 1938.
A 16F/450V and a 24F/450V filter-cap in the power-supply.

Best regards
Michael
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Old 16th April 2013, 05:09 PM   #8
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Quote:
Originally Posted by DF96 View Post
My experience is that old paper caps often fail due to water ingress and so become leaky. A common symptom is the output valve running very hot becaue its grid is being pulled up.
On the other hand, old electrolytics can often be fine or just need reforming if the radio has not been used for many years. Reforming won't, of course, deal with electrolyte loss but it will deal with oxide layer degradation due to lack of polarising voltage.
Of course, my experience is with UK built radios. Elsewhere things might be different.
I run things I rebuild a lot, so the electrolytics get warm. I don't try them out and put them on the shelf. Even if a device is not my primary, it runs 16 hours a day some days. So cracked rubber seals will leak. I've probably put 250 hours on my 1967 Hammond organ since 2010 when I re-e-capped it for example- 71new electrolytic caps- every one made it sound better or made a function work that was previously dead. Hammond was a premium brand, these were not cheap caps.
On paper caps and resistors, the quality of the coating matters. there were superb wax coated paper caps like the GI black ones in my dynakit equipment, there were middle of the road ones wax coated caps like the beigh Mallory tonewheel caps in Hammond organs until 1964 that typically go 10-25% off in 40 years. There were really ****y uncoated caps that were just paper and aluminum rolled up, like those found in an Airline television I worked on. People that work on tube guitar amps in the US are adamant about changing out paper caps. Maybe there weren't any quality paper caps used in store bought guitar amps because musicians were always short of money. The sound sold a guitar amp. I bought a used dynakit preamp and amp aged 19 because the college was using SC35's 18 hours a day and they were holding up- a seriously premium brand. I've probably run mine 15000-20000 hours.
Resistors, people that have Danish built Hammonds have a lot more trouble with high carbon comp resistance after 40 years than people with American built Hammonds. The resistors look the same, but Allen Bradley and Sprague US coatings seem to be more effective. Perhaps the fact that the production lines sold to the US Army and Navy had something to do with the quality of the paint. 1/4 watt carbon comp resistors look just like RCR07g military spec resistors, for example. I checked every resistor in my 1961 build Dynakit preamp and amp in 2011, they were all within tolerance. My Hammond H182, the only resistors that were high in 2010 looked burned with bubbles in the coating.
So anyway, I've never worked on a German radio; check the tube voltages for proper for a reading on what quality of paper cap they used. The electrolytic caps, if you don't intend to actually use it warm, reforming might be okay for a few hours. If your house in Europe is as cold as the inn I stayed in Vernheim was in September 1982, maybe your radio will never warm up. I was warmer in my tent in the swarzforst. My USA houses go 80 deg F at the thermostat 3 months a year, warmer in odd corners. UK organ repairmen are a lot less proactive about replacing electrolytic caps than I, although I've convinced AndyG of Sussex, perhaps, that taking thirty year old e-caps out, measuring them, and putting them back is not the thing to do worldwide.
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Last edited by indianajo; 16th April 2013 at 05:30 PM.
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Old 5th April 2014, 08:31 PM   #9
borges is offline borges  Norway
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Hi guys,

I just picked up my Huldra 6-22 today. The 2nd hand store had tested it OK and it plays fine with FM and external input (bnd=tape).

It sounds like at least one speaker element has come loose, and there is a bit of (probably) 50Hz hum.

Ill probably start with the e-caps and look for new elements for it. Im glad I found this thread!

Cheers,
Brge

Last edited by borges; 5th April 2014 at 08:45 PM.
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