Working on a Grunow 500 Chassis 5-A. Works Ok when first turned on. Some Stations come in clear with good volume level after a while of playing a hissing static begins to rise to a point that reception of that station is lost as well as all other stations weak or strong across the dial.Turning the radio off for a while helps but the problem occurs again. I don't think it's an antenna/weak reception problem where that one station is lost where other stations can be tuned in. Here all stations across the dial are lost. Any sugestions on where to start on this problem ? Thanks
TESTING: Lets have the set running initially and have the set tuned into one of those stations that you found later being "covered up with a hissing static ". Checking the audio aspect: Fully wait for the set to go into its fault mode , check the audio in the same manner, to then be able to ascertan if the fault is in the audio portion of the set, or any corruption coming in from the sets power supply. Go back to the AVC buss and check to see if THAT initial voltage reading has made any drastic change. You do have two 4 ufd low voltage electrolytics associated with the filtering of the sets bias supplies. Standing by for any findings . . . . |
:Sometimes tubes after warmed up for awhile can develop a short inside. If you do have a tube tested, leave the tube under test for at least the same time as the set goes bad. Especially the mixer/osc. IF and detector tubes.
:
: : : :  : : : : :Hi Ed: I wish I had evil toobies in this set it would be allot better to deal with. The chassis in this set is tight to work in and in replacing the two wet electrolytic's ontop of the chassis I had to place the new ones under the chassis blocking the 75 tube to make any reading the power power oytput tube is at the other end of the chassis and is more accesable. did test the AVC by placing the DC meter on the end of the volumn control an got no negative reading. I am going to do a test on the tubes that Warren suggested mixer/osc : : : :Sir John . . . . . : : :Excluding any "toobies" problems . . . . : : :With that problem that the set has, being potentially so all encompassing. : : :An initial question now . . . is it from the RF front end ?. or . . . is it originating in the audio section ? : : :What you might initially confirm if it is thermally related ? :You already mention the "warm up aspect" before it onsets, but is that seeming to be repetetive on its time period . . .30 min------2 hours-------3 hours ? : : :Let's initiate two analysis' by initially consulting the sets schematic and taking note of the lower detector plate of the #75 and its grabbing a handfull of healthy IF signal via the 100 pf cap from the other plate above it. : : :That RF then has its + nodes conducted off to ground via the diode plate and then only has the accumulative negative nodes remaining, and their flowing on down to the 500k/250k resistors junction ,where it picks up a static negative bias reference level via the 500k unit . However, the more important aspect is via the passage of that cluster of - nodes thru the 250k unit to reach the initial .075 ufd section of the block condenser. : : :That sums up the negative nodes into a smoothed out level DC negative voltage, of which strength, will be in accordance to the incoming IF level. : : :With a strong negative voltage present on well received stations and being proportionatively decreased with weaker signals. : : :The derived AVC voltage is then resistively isolated and capacitively bypassed to ground via the additional 250k res and the 0.1 ufd cap to then feed AVC bias to the low side of the inductor tied into the 1st grid circuit of the 6F7 stage. : : :Looking back to the #78 stage one sees that the full developed AVC voltage flows right up into the low side of the input IF transformer which is feeding into the 1st grid of the # 78 stage. : : : : :TESTING: : : : : :Lets have the set running initially and have the set tuned into one of those stations that you found later being "covered up with a hissing static ". : : :Monitor the negative voltage level that last mentioned location which is feeding up that AVC buss to the bottom of the 1st IF transformer. : : :Use either a DVM, VTVM or a VOM of 20K per volt sensitivity . . to minimize circuit load down at this chosen test point. : : :Tune right on the station and log down the present AVC voltage. : : : :Checking the audio aspect: : : :Ground the 1st grid cap of the # 75 with the short length of a screwdriver blade to the closest ground, run :the volume to max and observe the quietness for a few seconds. Run down the volume to normal listening level and unshort the 75. : :Fully wait for the set to go into its fault mode , check the audio in the same manner, to then be able to ascertan if the fault is in the audio portion of the set, or any corruption coming in from the sets power supply. : : : :Go back to the AVC buss and check to see if THAT initial voltage reading has made any drastic change. : : : :You do have two 4 ufd low voltage electrolytics associated with the filtering of the sets bias supplies. :BUT, usually they would improve deficient capacitive value with ambient heat onset, whereas paper capacitors typically will degrade or fault with thermal heat onset. : : : :Standing by for any findings . . . . : : : : : :73's de Edd : : :  : |
:
:
:
:
:
:
:
:
:
:
::Sometimes tubes after warmed up for awhile can develop a short inside. If you do have a tube tested, leave the tube under test for at least the same time as the set goes bad. Especially the mixer/osc. IF and detector tubes.
::
:
:: :: :: :: :: :: :: :: ::Hi Ed: : I wish I had evil toobies in this set it would be allot better to deal with. The chassis in this set is tight to work in and in replacing the two wet electrolytic's ontop of the chassis I had to place the new ones under the chassis blocking the 75 tube to make any reading the power power oytput tube is at the other end of the chassis and is more accesable. did test the AVC by placing the DC meter on the end of the volumn control an got no negative reading. I am going to do a test on the tubes that Warren suggested mixer/osc IF and detector. Any sugestions on working around this problem I would appreciate. Thanks John :: :: :: ::Sir John . . . . . :: :: ::Excluding any "toobies" problems . . . . :: :: ::With that problem that the set has, being potentially so all encompassing. :: :: ::An initial question now . . . is it from the RF front end ?. or . . . is it originating in the audio section ? :: :: ::What you might initially confirm if it is thermally related ? ::You already mention the "warm up aspect" before it onsets, but is that seeming to be repetetive on its time period . . .30 min------2 hours-------3 hours ? :: :: ::Let's initiate two analysis' by initially consulting the sets schematic and taking note of the lower detector plate of the #75 and its grabbing a handfull of healthy IF signal via the 100 pf cap from the other plate above it. :: :: ::That RF then has its + nodes conducted off to ground via the diode plate and then only has the accumulative negative nodes remaining, and their flowing on down to the 500k/250k resistors junction ,where it picks up a static negative bias reference level via the 500k unit . However, the more important aspect is via the passage of that cluster of - nodes thru the 250k unit to reach the initial .075 ufd section of the block condenser. :: :: ::That sums up the negative nodes into a smoothed out level DC negative voltage, of which strength, will be in accordance to the incoming IF level. :: :: ::With a strong negative voltage present on well received stations and being proportionatively decreased with weaker signals. :: :: ::The derived AVC voltage is then resistively isolated and capacitively bypassed to ground via the additional 250k res and the 0.1 ufd cap to then feed AVC bias to the low side of the inductor tied into the 1st grid circuit of the 6F7 stage. :: :: ::Looking back to the #78 stage one sees that the full developed AVC voltage flows right up into the low side of the input IF transformer which is feeding into the 1st grid of the # 78 stage. :: :: :: :: ::TESTING: :: :: :: :: ::Lets have the set running initially and have the set tuned into one of those stations that you found later being "covered up with a hissing static ". :: :: ::Monitor the negative voltage level that last mentioned location which is feeding up that AVC buss to the bottom of the 1st IF transformer. :: :: ::Use either a DVM, VTVM or a VOM of 20K per volt sensitivity . . to minimize circuit load down at this chosen test point. :: :: ::Tune right on the station and log down the present AVC voltage. :: :: :: ::Checking the audio aspect: :: :: ::Ground the 1st grid cap of the # 75 with the short length of a screwdriver blade to the closest ground, run ::the volume to max and observe the quietness for a few seconds. Run down the volume to normal listening level and unshort the 75. :: ::Fully wait for the set to go into its fault mode , check the audio in the same manner, to then be able to ascertan if the fault is in the audio portion of the set, or any corruption coming in from the sets power supply. :: :: :: ::Go back to the AVC buss and check to see if THAT initial voltage reading has made any drastic change. :: :: :: ::You do have two 4 ufd low voltage electrolytics associated with the filtering of the sets bias supplies. ::BUT, usually they would improve deficient capacitive value with ambient heat onset, whereas paper capacitors typically will degrade or fault with thermal heat onset. :: :: :: ::Standing by for any findings . . . . :: :: :: :: :: ::73's de Edd :: :: :: :: |
::
::
::
::
::
::
::
::
::
::
:::Sometimes tubes after warmed up for awhile can develop a short inside. If you do have a tube tested, leave the tube under test for at least the same time as the set goes bad. Especially the mixer/osc. IF and detector tubes.
:::
::
:
Replace these and see if this does not cure your static problem.
...Could also be caused by a cracked resistor.
...Or less likely due to some other arcing or a failed tube, as was mentioned.
T.