Hi. I have a Grundig 97 a.m/f.m tube radio, which works fine on all bands exept ukw. I have tried the usual ploy of replacing ECC85, and EABC80 tubes, and also various capacitor and resistor replacements, but to no avail. I know that the ukw band did work once, but since changing mains transformer, this band seems to be inoperative. I was recently in contact with someone on the U.K Radio Forum, who also has a model 97 set with the same problem. Can anyone give some technical advice, please? Best regards Mike Horne (U.K)
#1. Can you hear on the FM-Band any band hiss (white noise, back ground noise)? #2. When you hear such noise, has it is the same level as on a free position (no station tuned in) on the AM-Band? Louder? Very weak? No noise? # 3. Are the band switch contacts (key set contacts) ok? Contact spray used?
Please take also notice of my safety notes on top of this page. Looking for your reply.
Regards Rainer
Geändert von / modified by Wumpus am / at 18.Jan.2006 21:46
Hi Rainer. Many thanks for your reply. The U.K.W button is pressed down ok, and there is no hiss or noise to be heard from the loudspeaker. I have not tried contact spray yet, nor tested the switch contacts for continuity. Many thanks
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s of the f.m seRE: Grundig 97 repair assistance wanted , 20 Jan. 2006 19:39
Hi Rainer. Many thanks for the message. I can not hear any noise from the loudspeaker when the set is switched to u.k.w, when the input to the EABC80 grid is touched- the medium, short & l.w bands give very good response though. I should hear a strong buzz when the grid of this tube, or the wiper of the vol control is touched. The ECC85 does get quite hot, and the voltage across the wirewound resistor (I presume that this is the cathode resistor for the ECC85 tube ?) is about 1.5 volts. Yes, I can handle measurements- I have a good digital multimeter. The problem in this radio is of the compactness of the underchassis, and trying to cross-reference the components to that of the circuit. I might try replacing the electrolytic cap in the ukw section near the EABC80 tube (positive lead to chassis), and see if this has any effect. I will let you know my results Regards Mike
Hello Mike, I see you are informed about the very powerful finger touch method for testing the grids of the valves. You have a schematic of the set? I have here ony a schematic for 97a. When you interested, I will send here more questions:
#1. On my schematic for the 97a I found the valves ECC85, ECH81, EF89, EABC80, EL84, EM84 and your set?
#2. Finger touch test to the two FM-diodes of the EABC80 in position UKW of the key set. Result? hum-noise?
#3. IF-stage valve EF89. Voltage value of anode and screen grid. Results?
#4. Mixer-Oscillator stage valve ECH81. Voltage value of anode and screen grids of the mixer?
#5. UKW (FM) tuner section. ECC85. Voltage values on both anodes (hard to measure, I know). You can remove the valve and take contact with the probe of the meter instrument to the valve socket. Results?
Looking for the results of your expedition into the compactness of the underchassis and into the tuner of the patient.
Hi Rainer. Many thanks for your reply. The results of the tests of touching the two anodes of the f.m section of the EABC80 tube, amounted to no noise or feedback (with ukw button pressed down). The voltage on the following tubes was also measured ECC85 Anode one (Pin one ) = 170volts Anode two (Pin 6) = 207 volts EF89 Anode (Pin seven) = 189 volts EF89 Screen Grid (Pin eight) = 85 volts ECH81 Heptode Anode (Pin six) = 177 volts ECH81 Screen Grid (Pin one) = 80volts I did notice a change in a resistor that feeds h.t to the first I.F block, that someone in the past had installed a 4k7 resistor. On the circuit diagram that I was given, it was shown as 1k. It is joined at the other end (1st I.F block) with a 27k resistor that feeds the Screen Grid of the tube ECH81. I might try changing this resistor to what the schematic says it should be. The h.t at the rectifier output is 259 volts, and that on the smoothing electrolytic capacitor is 207 volts. The anode of the EL84 is 243 volts Regards
Hello Mike, ok. Sorry, but now we come to a limit. For further measurements and tests some more equipment is necessary. At best is a modulated (FM) rf. signal generator or at least a combined rf - af signal injector. Such injectors offers also a little bit of FM among the AM modulation for testing the FM signal line of the set.
Temporary summery of your measurements: All voltages are in the green range. In general the IF and AF section is in operation, but there can be an fault in the FM demodulator stage or in the FM part of the IF stage or in the FM tuner stage. But we will not forget to clean the contacts of the key set.
Without the above listed equipment there only a little chance for fixing the problem. What to do now?
With an coupled FM-modulated rf signal (10.7 MHZ) on the control grids of the if stages (perhaps the mixer valve is used as first FM IF amplifier) you have to examine each IF stage, including the demodulator section.
When the IF section will work, the FM tuner ist the source of your problems.
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