Servicing Kreft Valve Radio
This valve radio was brought to me on 6th January, 2018 with the complaint that it was not picking up any stations, the band switch knob was broken and the dial chord had broken loose. As per my records, this set was brought to me on 20th June, 2014, with the complaint that Audio was weak. Then I had replaced around 20 components and aligned the IFT properly and gave it back in perfect working condition.
It did its revisit on 8th August, 2015 with multiple complaints. The main transformer was heating up and got rewound. EBF89 Valve was replaced. The Oscillator IFT was rewound. ECL82 Valve was replaced as I found it to be weak. So, this was making the revisit after a gap of around two and a half years!
After opening and cleaning it, I investigated the reasons for the failure. Noticed that B+ voltages were ok and the amplifier part was working. I gave signal to the aerial input socket and noticed that nothing was coming out. Gave the signal to the output IFT and got the signal. So, the defective stage was in the RF section. Checked the voltages at the pins of ECH81 Valve and found it to be ok. Replaced the valve with a working one. The set started working. But when I tried to retune the IFT feeding 455KHz signal, the ferrite core of one of the IFTs was not turning and the holding screw at the top was just going up and down, indicating that the ferrite core has come off it. So, removed the IFT, and opened it up. Fixed the core to its holding screw using less than one drop each of Fevibond and Superglue, after Fevibond dried. Allowed it to bond properly and put this back in its place. Re-fixed the IFT and retuned the Radio for proper peak signal. The Radio started working perfectly well. Tied the dial chord. Provided new knobs. Now here are the pictures of the Radio, along with the measurement of the ferrite core using Vernier caliper.
I took the measurement of the ferrite core for future reference to fetch it as a replacement, in case it gets broken. Sharing it for those of you who might require.
From the pictures, you can see the inside of the set very clearly. You can also see how the construction of the IFT was done those days. This is a very delicate thing to handle. You can also see the dial chord tied properly in its place. The magic Valve, which is a peak tuning indicator, is always an attraction in Valve Radios. The valve number generally used is EM84. In fact I like the flower type! In this Radio, ECH81 (Oscillator and mixer), EBF89 (inter IF amplifier), ECL82 (Audio pre-amp and power output together) are used. Originally it might have had EZ80 or 81 as rectifier. But that has been replaced with IN4007 diodes, as you can see from the bottom side of the set. This is a lovely set. Here, take a look at it after I replaced all the knobs. You can also see the defective valve and broken dial chord.
A job completed and added to my satisfaction bag!
(For more detailed explanation of RF and IF tuning and other Valve Radio related topics, kindly refer to my earlier articles.)
This article was prepared for you by Parasuraman Subramanian from India. He is 69 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.
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https://jestineyong.com/servicing-48-led-random-led-light/
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Henrique Jorge Guimarães Ulbrich
June 8, 2018 at 4:07 am
Dear Parasuraman, when you replace a rectifier tube by silicon diodes, there is a noticeable increase of the B+ voltage due to the difference in the characteristics of these components. The voltage drop in a rectifier tube is relatively high (I am not shure, but perhaps about 50 to 60 V). On its turn the voltage drop in the silicon diode is less than 1 V. Of course, this corresponds to an increase in the +B voltage in this proportion. Unless the power transformer is changed to compensate.
Parasuraman
June 8, 2018 at 9:35 am
I'm aware of this. The B+ is reduced by additional dropping resistors, which you can see at the bottom. I have ensured that the B+ is within the limits of the tubes. Another major disadvantage with the Silicon Diodes is that the B+ is present at the plates of the tubes, much ahead of these get ready. But I didn't put an EZ80 or EZ81 in its place, because of cost factor.
Steve
June 8, 2018 at 8:30 am
I really like the old tube radios and equipment.
Albert
June 8, 2018 at 9:26 am
Replacing the original EZ80 or 81 as rectifier tubes by 1N4007 diodes sounds very familiar. I once did that too although I hardly repaired any old tube receiver or amplifier afterwards. I also long ago disassembled a completely dead very old receiver that in those days didn't have permanent magnets on the loudspeakers. It used the noise surpression coil between both the positive e-cap poles of the DC voltage supply as elektromagnet.
alois
June 8, 2018 at 8:49 pm
hi parasuraman,am have this car radio for toyota.it was brought to me in that when its powered i plays for like 2mins then shuts down.i checked all the main culprits which i guessed could be the problem bt were ok.it uses TDA 7384 .to my level of electronics ,this is not direct short ,pls help am about to give up of which is very hurting to me leaving something not solved.
Parasuraman
June 9, 2018 at 10:46 pm
First and foremost in such cases is to find out what is getting abnormally hot. Is it some semiconductor or IC or offer components on the PCB. Next method is to see whetherthere is any voltage drop, if so where and then check the load. Another method after finding the thermally hot part is to see whether it becomes normal if we cool it off manually.
Robert Calk Jr.
June 8, 2018 at 11:16 pm
Good job, Parasuraman.
Humberto
June 14, 2018 at 2:39 am
Good job Parasuraman, I'm really fan of vacuum tubes.
Masse Andersson
September 3, 2021 at 7:58 pm
"Another major disadvantage with the Silicon Diodes is that the B+ is present at the plates of the tubes, much ahead of these get ready."
NTC resistor fixes this problem.