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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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#1
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Audiolab 8000A repair well half there
No known cause for one channel to catastrophically burn out -
literally a burnt hole in the board and loss of many components. At this stage (justified in the event ) replacing the expensive o/p 15 amp 2SA1216 and 2SC2922 with 25 Amp TIP36C and TIP35C; mid power trannies 2SA1668 and 2SC4382 with TIP41C and TIP42C. Replaced low power trannies and passives and tracks. Had to change biasing as driven too strong with original Rs but settled amplifying undistorted 1KHz, phones only, with variac at 60 per cent of mains. BUT Every 20 seconds or so a strong short duration power drain - monitoring on variac ammeter recovering each time without damage. Assuming high-frequency instability somewhere any ideas before I blow up another pair of TIP35/36s where to place some hf suppression ?. I was just about to isolate pre-amp stage but putting a DVM probe to i/p of this channel caused signal injection and permanent allway B-C-E s/c of both the o/p trannies. The emitters of the TIP35/36s are at rail volts in this amp. Seems to be mainly DC coupling in this amp so theoretically could be DC surges in the preamp as I didn't get around to a scope to monitor the excursions. electronic hints and repair briefs http://homepages.tcp.co.uk/~diverse Nigel,Diverse Devices,Southampton,England |
#2
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Audiolab 8000A repair well half there
I replaced the TIP35/36 and replaced both collector 10W 0.2 ohm droppers
with 1/3W 1 ohm resistors temporarily. Putting scope on o/p there is continuous low level 10mV or so 20KHz or so oscillation on this channel. Trying my old freezer spray trick to find any pitch variation a squirt in the biasing area caused full level oscillation. I had changed biasing ,R plus pot range, from 1.1K to 1.3K to 1.7 to 2.1K but not changed the trannies in this area. Could well be the original problem. Bringing my fingers close to components in this area presumably changes parasitic capacitance and noticeably changes the oscillation frequency. Before dropping the gain or changing all the trannies and caps in this area any ideas to localise problem better ? I tried card dividers around components and then squirting but no help. Putting a scope on this area causes full level, otherwise fatal, oscillation. |
#3
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Audiolab 8000A repair well half there
"Nigel Cook" bravely wrote to "All" (18 Mar 04 16:51:23)
--- on the heady topic of "Audiolab 8000A repair well half there" I'm inclined to think it is not a hf oscillation but rather a lf instability. It could be high esr supply caps problem in the power supply (regulator) letting the signal through or a dry electro in a preamp feedback stage. For example some input bootstrap circuits are prone to oscillate when there is a critical value of capacitance, not too much nor not too little and source resistance could affect it too. One thing for sure if it oscillates then there is a loop to be opened somewhere! NC From: "Nigel Cook" NC No known cause for one channel to catastrophically burn out - NC literally a burnt hole in the board and loss of many components. NC At this stage (justified in the event ) replacing the expensive o/p 15 NC amp 2SA1216 NC and 2SC2922 with 25 Amp TIP36C and TIP35C; mid power trannies 2SA1668 NC and 2SC4382 with TIP41C and TIP42C. Replaced low power trannies and NC passives and tracks. NC Had to change biasing as driven too strong with original Rs but NC settled amplifying NC undistorted 1KHz, phones only, with variac at 60 per cent of mains. NC BUT Every 20 seconds or so a strong short duration power drain - NC monitoring on variac ammeter recovering each time without damage. NC Assuming high-frequency instability somewhere any NC ideas before I blow up another pair of TIP35/36s where to place some NC hf suppression ?. NC I was just about to isolate pre-amp stage but putting a DVM probe to NC i/p of this channel caused signal injection and permanent allway B-C-E NC s/c of both the o/p trannies. NC The emitters of the TIP35/36s are at rail volts in this amp. NC Seems to be mainly DC coupling in this amp so theoretically could be NC DC surges in the preamp as I didn't get around to a scope to monitor NC the excursions. NC electronic hints and repair briefs NC http://homepages.tcp.co.uk/~diverse NC Nigel,Diverse Devices,Southampton,England |
#4
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Audiolab 8000A repair well half there
"Nigel Cook" wrote:
I replaced the TIP35/36 and replaced both collector 10W 0.2 ohm droppers with 1/3W 1 ohm resistors temporarily. Putting scope on o/p there is continuous low level 10mV or so 20KHz or so oscillation on this channel. Trying my old freezer spray trick to find any pitch variation a squirt in the biasing area caused full level oscillation. I had changed biasing ,R plus pot range, from 1.1K to 1.3K to 1.7 to 2.1K but not changed the trannies in this area. Could well be the original problem. Bringing my fingers close to components in this area presumably changes parasitic capacitance and noticeably changes the oscillation frequency. Before dropping the gain or changing all the trannies and caps in this area any ideas to localise problem better ? I tried card dividers around components and then squirting but no help. Putting a scope on this area causes full level, otherwise fatal, oscillation. First of all, you need to stop the oscillation. Try connecting a 470pF capacitor between collector and base of one or the two driver transistors. If this doesn't work, put another in parallel with the feedback resistor (the one that goes from the channel output to the input circuit) If the imput circuit is an Operational Amplifier, then try connecting another capacitor between its output pin and its negative input pin. Once the oscillation has stopped, you can find what else is wrong. --- Ing. Remberto Gomez-Meda http://ingemeda.tripod.com/ INGE - Ingenieria Electronica. Puerto Vallarta, Jalisco, Mexico. |
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