Re: 7904A restoration questions

Mark Vincent


I restored one of these earlier this year. I upped the capacitance of the condensers and used high temp. low ESR and long life ones. I restuffed the two main screw cans with 1200mfd 250V LGR types. I changed the 150000 ohm 1W bleeder across the 1200mfd condensers to 100000 ohm 5W. I wanted a faster bleeding due to additional capacitance and the resistor is runs cooler than the original since it is larger in wattage. Some of the resistors were underrated in wattage so those were increased in wattage. ALL the 9/9,1V zeners will be open in any plug-in or mainframe. Look at the schematic for these. I used the 1,3W BZXB85 types. Those are 2% tolerance. The higher power of these from the original 1/2W will not hurt. The higher power means the Z of the zener is lower. The 1mfd coupling/bypass on the rear facing board between the hypcons had high ESR so I replaced them with film types. These 1mfd condensers bypass the 9/9,1V zeners. Check the diodes in the d-c restoration circuit. Those can be leaky. The 22meg carbon resistors will very likely be high, about 30meg. I replaced mine with 1/2W 1% types. When these are high, the brightness is high then will drop as the unit warms up. I put good quality heatsink compound on the devices that have heatsinks. I did add a few TO39/5 heatsinks on some of the transistors because these did run hot. I did take all the knobs off and oiled the set screws and shafts. A steel set screw in aluminum to another metal means electrolysis making set screws hard to get out or seize. I did put in an NTC, CL70, to limit inrush current from the mains when turned on. Pull the transistors and ICs out enough to move back and forth to make sure the contact is good. I have seen ones that were "open circuit" until it/they were moved a couple of times in their sockets. I added a resistor in series with the green pilot lamp to make it last longer. I like my pilot lamps in things to be bright enough to know the piece is on while running on a lower voltage, e.g. 5V on a 47/1847 lamp, 3,5-4V on the 683 lamps Tek. used. Look out for the 151-0367-00 transistors. These likely will be bad from C to E as a diode. The ones that have a blue face will not be bad. Check the others. This number is used in a number of plug-ins and mainframes. I used KSP10BU as replacements. Someone else suggested a couple of other numbers are as good as replacements. What I said is what I have done. You do what you want with your item. No criticism or preaching is intended you must do these things. I am picky about my things and what parts I get because of my experience and research. I will never claim to be "the" expert or perfect. I am happy to see you got an item you wanted, it works and will give you the service you desire. It is nice to read of others getting what they want and getting it working again. A great sense of joy comes from making something not working to working again whatever the item is. There are many great nice people on this group that will be happy to help you and give advice. I have learned from them and am grateful to them.


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