-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
Kenwood ts820 keeps blowing fuses
- Thread starter W1RLP
- Start date
Take the tubes out and put a new fuse in. See if it powers on without tubes. It should. If it blows fuse it's not tubes.
If you heard a loud "POP!" when you keyed the radio with the new finals, I'll be surprised if you don't find the plate choke exploded or burned, and a burned resistor or three under the socket of one or both final tubes. A tiny epoxy-coated RF choke that feeds the lower lug of the big plate choke typically gets croaked as well.
Turns out that 6146B tubes made in Owensboro by GE, or "6146W" types from that same factory have a stability problem in the Kenwood 520/820 radios. They spontaneously burst into high-level oscillation and draw all the current the power supply can deliver until the fuse trips. This surge of plate current is what overheats and damages the plate chokes and cathode resistors. Happens too fast for you to react and unkey in time to prevent damage. By the time that "POP!" sound reaches your ears it's already too late.
Never have found a cure for this. Tubes built by RCA and/or the Sylvania (says Phillips ECG) don't seem to exhibit this bad habit.
Otherwise perfectly-good government-surplus "6146W" tubes made in the 80s came from both vendors. The ones built by GE are toxic for this type radio.
Learned this lesson the old-fashioned way. Not a cheap one to learn, since the tubes typically suffer damage as well as the radio.
73
Turns out that 6146B tubes made in Owensboro by GE, or "6146W" types from that same factory have a stability problem in the Kenwood 520/820 radios. They spontaneously burst into high-level oscillation and draw all the current the power supply can deliver until the fuse trips. This surge of plate current is what overheats and damages the plate chokes and cathode resistors. Happens too fast for you to react and unkey in time to prevent damage. By the time that "POP!" sound reaches your ears it's already too late.
Never have found a cure for this. Tubes built by RCA and/or the Sylvania (says Phillips ECG) don't seem to exhibit this bad habit.
Otherwise perfectly-good government-surplus "6146W" tubes made in the 80s came from both vendors. The ones built by GE are toxic for this type radio.
Learned this lesson the old-fashioned way. Not a cheap one to learn, since the tubes typically suffer damage as well as the radio.
73
My TS-820S still has the factory original Matsushita 2001 finals. They still make rated power easily too.
There is more than one possible reason for only one of the final tubes showing color.
You still haven't expressed an opinion about who may have made this set of final tubes. It's a vital make-or-break detail.
The fuse trips because the final tubes are pulling a dangerous level of current from the power supply.
Remember that any time this kind of tube gets hot enough for the gray plate structure inside to show visible red light you have already stressed this tube well beyond its rated limits. This stress can blow out the tube completely, or simply cause it to become weak, and deliver less than the rated RF power.
Could be the tube that stays dark is already blown out and won't draw current. This would keep it from getting hot.
Could be that only the tube that's getting hot enough to show color has gone bad. Drawing excess current caused by a fault inside the tube.
Could be that they're both shot to the devil.
Time for a tube tester. Just be sure that the final tubes were not fabricated by GE. The name printed on them may or may not reflect who actually made the tube, and this is what counts.
I would expect an accurate tube tester to show that both these tubes are now defective.
I would not recommend trying another set of final tubes in this radio until the reason for this meltdown has been determined. No good reason to toast another pair of tubes, if the radio turns out to have a defect causing this fault.
There is usually some collateral damage in the final-tube compartment when this happens, especially resistors connected to the lugs on the tube sockets. New tubes won't work if there are blown parts under the sockets.
There are other faults in the radio that can cause the tubes to overheat even if you don't key the mike. The tubes are placed in standby while receiving and should not draw any current until the radio is placed in transmit mode, with a switch on the front panel, by keying the mike, or closing the morse-code key in CW mode. A negative DC voltage applied to the grids of the final tubes is what shuts them off in receive mode. A fault in the radio's power supply (like a bad 40-plus year-old filter cap) can cause the final tubes to activate and draw current from the power supply without keying the radio. Testing for this failure gets done with the final tubes removed.
And with a LOT of respect for the high voltages in that section of the radio.
73
You still haven't expressed an opinion about who may have made this set of final tubes. It's a vital make-or-break detail.
The fuse trips because the final tubes are pulling a dangerous level of current from the power supply.
Remember that any time this kind of tube gets hot enough for the gray plate structure inside to show visible red light you have already stressed this tube well beyond its rated limits. This stress can blow out the tube completely, or simply cause it to become weak, and deliver less than the rated RF power.
Could be the tube that stays dark is already blown out and won't draw current. This would keep it from getting hot.
Could be that only the tube that's getting hot enough to show color has gone bad. Drawing excess current caused by a fault inside the tube.
Could be that they're both shot to the devil.
Time for a tube tester. Just be sure that the final tubes were not fabricated by GE. The name printed on them may or may not reflect who actually made the tube, and this is what counts.
I would expect an accurate tube tester to show that both these tubes are now defective.
I would not recommend trying another set of final tubes in this radio until the reason for this meltdown has been determined. No good reason to toast another pair of tubes, if the radio turns out to have a defect causing this fault.
There is usually some collateral damage in the final-tube compartment when this happens, especially resistors connected to the lugs on the tube sockets. New tubes won't work if there are blown parts under the sockets.
There are other faults in the radio that can cause the tubes to overheat even if you don't key the mike. The tubes are placed in standby while receiving and should not draw any current until the radio is placed in transmit mode, with a switch on the front panel, by keying the mike, or closing the morse-code key in CW mode. A negative DC voltage applied to the grids of the final tubes is what shuts them off in receive mode. A fault in the radio's power supply (like a bad 40-plus year-old filter cap) can cause the final tubes to activate and draw current from the power supply without keying the radio. Testing for this failure gets done with the final tubes removed.
And with a LOT of respect for the high voltages in that section of the radio.
73