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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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I would like to ask for help once again with the repair of my own
personal old radio. I started this project almost a year ago, and it's been on the back burner since then. But I had a little free time and so I put it back on the bench. I would really like to see this radio working again. This is a Grundig "transistor 305", circa 1970 I think. The radio is intermittent. I have been working on this set on and off for almost the past year. I recently took some voltage measurements and there are some serious differences in base and emitter voltages in the last IF stage and the detector stage when this radio fails. I have tried many things, heat and cold, including substituting all the transistors in the RF and IF sections one at a time and have studied the schematic until I felt a migraine coming on. l think that by now I could almost draw it from memory, and I'm still coming up empty. I considered trying to inject a signal and/or signal tracing, however with the circuit voltages so far out of whack when it fails I think that approach would be futile. The base and emitter voltages in these three stages increase substantially when the radio quits. Some voltages are jumping from for example 1.2V up to 4.0 V. With conditions like this you would think that the cause would be obvious, and perhaps I've been staring at this this too long and it is. However the remedy has stubbornly eluded me thus far. The intermittent nature of the problem is what makes it so difficult. This is typical of what is going on: If the radio has been off for a long period of time it will typically work for several hours and then quit. In the beginning of this Quitting sequence, just after it first quits it will try to come back on intermittently for a short period of time, crackling etc, (as though something was intermittently breaking down). Eventually it will just remain silent. There was an electrolytic in the audio driver section, C57 a 100uf/3.0 volt with slightly high ESR. Replacing it improved the low frequency audio response, however the AF amplifier is not the problem though. You can still get a good audio signal from the volume control out when it fails. So the AF section seems to be unaffected. The voltages to that part of the circuit substantiate that as well. And the RF and oscillator circuit voltages during dead time seem to be fine as well. The problem when it occurs affects both AM and FM. The strange thing is that when powering this radio up again after a shut down, the time it remains on before it quits once again is directly proportional to the time that it has been off. For instance if you leave it off overnight it might remain on for a half hour or so the next morning before it quits again. Recently after trying it again after months of it sitting idle it played for about 3 hours before it quit. Then five minutes later when trying it again it quit almost immediately. The few other electrolytics in the affected areas check good on ESR and bridging them during dead time yielded no improvement either. I have the schematic and I have uploaded it to the site listed below. I know that there are technicians out there better at this than I am, and I would really appreciate it if someone could please take a look at the schematic along with the voltage readings I obtained during Go and No Go conditions and give me your opinions. With three stages affected I suspect that I'm looking for a common denominator but I'm just not sure. The initial voltage readings (on turn on with the unit working normally) will be listed in the left hand column. The voltage readings during a failed condition are listed in the right hand column. The major differences are marked with an asterisk*. I used to strap this radio to my bicycle when I was a kid and ride around The Bronx with it. It has a great sound. I've owned it since I was a teenager. I guess it's just a sentimental thing. Thanks for any assistance. Lenny http://dl.dropbox.com/u/65394789/Gru...0schematic.pdf Voltage readings Test point Radio working: Radio not working ------------------- -------------------- ------------------------- E. AF178 -1.10V -1.20V B. AF178 -1.50V -1.50V E. AF124 -1.10V -1.20V B. AF124 -1.32V -1.44V E. AF121 -0.85V -0.90V B. AF121 -1.20V -1.30V E. AF126 (1) -1.00V -3.70V * B. AF126 (1) -1.20V -4.00V * E. AF126 (II) -0.80V -3.70V * B. AF126 (II) -1.10V -3.60V * E. AC151 (I) - 0.90V -0.80V C. AC151 (I) --3.05V -2.95V E. AC151 (II) --0.66V -0.62V C. AC151 (II) --3.20V -3.10V Thanks once again for looking. Lenny |
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