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Old 14th Oct 2021, 6:48 am   #1
Jan Zodiac
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Join Date: Sep 2021
Location: Aabenraa, South Jutland, Denmark.
Posts: 219
Default Restoration of a Neutrofon Guldsegl 5308

Hello.

This thread will be about the restoration of a danish high end (large) table radio from 1953/54.

First a small introduction to the few danish "ultimate luxury" receivers that were produced in small numbers between approx. 1949 and 1954.
Since I wasn't around at that time, and only limited information is found on these special receivers, I can't really give a specific reason why the five largest/finest radio manufacturing companies, some time after WW2 started a challenge of producing the best and most advanced receiver possible.
It was probably a project of prestige and manifestation. All these recievers were produced in very limited numbers, and they were only produced after an order was placed. They were assembled by hand, off the usual assembly/production line.
This also made them very expensive to produce, and I heard from several older technicians, that they lost money on producing them, which makes sense, since these "ultimate" receivers were only around a few years, and then completely dropped out of the product listings.

The five manufacturers and their respective supermodels are as follows:

Eltra Skymaster 919 - produced from around 1949/50 and a few years into the 50's. First it was an AM only, but later a FM unit was offered as an option. Was offered as a radiogram also (Skymaster 929).

TO-R Imperial C8U - I think this was only produced in 1950 and maybe in 1951 also. AM only, and no later FM options. Was offered as a radiogram also (VERY rare).

Linnet & Laursen Primas 501 and 512 (1950 and 1951). Started as an AM only, and was later offered with optional FM unit. As for the Eltra Skymaster, the FM unit could be mounted in previous AM only models also. Offered as a table radio only. A very large one though. Primas 512 with FM unit had 21 valves!

Bang & Olufsen Grandessa 506, 507, 508 and 509 (1951 to 1954). 506/7 is the first introduced and 508/9 the "second generation". Big, bulky and heavy. All of them could be ordered with an FM unit, which more or less was an integrated part of the main chassis. The first generation (506/7) was offered as a console and radiogram also. The 508/9 as table radio only.

Neutrofon Guldsegl 5308 - This was produced in a very limited number, introduced in 1953 and then probably gone already by 1954. It was offered in 2 radiogram versions also. It is a fully integrated AM-FM receiver.

I will later on make a description on each individual model and their special features, but to limit the space a bit, I will focus on the Neutrofon Guldsegl 5308 in this thread.

The 5308 was probably the most well engineered of all the "big ones". On AM it features a tuned RF amp. stage, and two IF stages with additional windings for a 3 stage IF bandwith control. FM with two IF stages + discriminator, and AVC controlled RF amp. The complete tuner section has its own power supply. The secon PS feeds the AF section. This is an AC/DC set (here they were called "universal"), as most radios was around that time, given the fact that we had a lot of local DC power plants.
The AF section has input for xtal cartridge, or upon request, a transformer could be fitted for 1,5 and 50 Ohm magnetic cartridges (Fonofilm/Ortofon and Bang & Olufsen). Mine has the optional transformer.
The tone controls uses selectors instead of potentiometers. In fact that was very common here. Both bass and treble features two individual controls. One for boost/attenuation and one for crossover frequency (bass control) or sharp roll off (treble control).
The treble roll off control is attached to the bandwith selector as well, and also a steep slope record noise filter for worn records is operated by this.

The output stage features two UL41 in push-pull, with a large section wound "high fidelity" output transformer. I measured the output power to 9 watts right before clipping. It is indeed a nice amplifier section.

There are two 8" Peerless bass speakers and a 4" Peerless "tweeter" (probably a bit large to be called a tweeter). Crossover frequency is around 1kHz. These speakers are very sensitive and the radio is capable of playing very loud.

The extremely beautiful cabinet consists of so many individual parts, that it must have cost a fortune to produce. It was quite a challenge to take it all apart for polishing. Resonances are supressed by soft fibreboard, glued to the inside walls of the cabinet. I have never seen this on any radio before.

It was in general a challenge to restore, because of all the paper in oil capacitors used, as well as a few regular paper caps. All the electrolytics in both power supplies were fine though. The two selenium rectifiers are only kept for display. I use silicon diodes and a larger series resistor to keep the correct voltages.
I aligned both AM and FM section completely. The AM sensitivity was specified to 1uV (bandwith in "narrow" position of course), which was quite good for a domestic radio. I can confirm that it is indeed very sensitive.
FM section is quite sensitive for its time. It has low noise and it sounds clear and crisp.
There is actually some noise/hiss from the amplifier section itself, which is especially noticable on low volume, or if volume is turned all the way down. There is no hum though, even if it is an AC/DC set.

/Jan
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