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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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Carlsbro Showman parametric/1 oscillating amp , 1990
1 channel good the other one even with a 27 ohm load oscillates at 65
percent of mains and above, bursts peaking at 40 KHz dropping back to much lower f, extinguishing and restarting over 5 seconds or so. power amp probably much like Carlsbro S600M Anyone have any leads to schematic? uses 4x MJ15024 , 2x SM2177A, 2x SM2178A each ch predriver DC levels (prior to oscillation) certainly different to the good ch but no obvious reason other than rain got into the amp A lamp will go in each of the +/- rails before I fiddle again ignore any apparent email address appearing with this posting |
#2
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Carlsbro Showman parametric/1 oscillating amp , 1990
Removed the triac speaker prot cct just in case a reaction with that, same
result. Checked ESR of all cap types. Upto +/-40V rails , below oscillation, all DC agree near enough on both channels all TO3,TO220 and TO92, zeners. Go up to about +/-45V and the 6V, 5W bulbs start glowing that I added into the supply rails , alternately swapping dim and bright pairs very odd. Next I'll disconnect 1 of each complementary pairing of MJ15024 and see what happens. Any other suggestions? Next stage is sketching out the cct |
#3
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Carlsbro Showman parametric/1 oscillating amp , 1990
On Wed, 16 Jun 2010 12:32:24 +0100, N_Cook wrote:
Removed the triac speaker prot cct just in case a reaction with that, same result. Checked ESR of all cap types. Upto +/-40V rails , below oscillation, all DC agree near enough on both channels all TO3,TO220 and TO92, zeners. Go up to about +/-45V and the 6V, 5W bulbs start glowing that I added into the supply rails , alternately swapping dim and bright pairs very odd. Next I'll disconnect 1 of each complementary pairing of MJ15024 and see what happens. Any other suggestions? Next stage is sketching out the cct Are you posting from a cellphone? Do you have an internet service provider charging you by the byte? |
#4
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Carlsbro Showman parametric/1 oscillating amp , 1990
On 6/16/2010 1:13 PM AZ Nomad spake thus:
On Wed, 16 Jun 2010 12:32:24 +0100, N_Cook wrote: Removed the triac speaker prot cct just in case a reaction with that, same result. Checked ESR of all cap types. Upto +/-40V rails , below oscillation, all DC agree near enough on both channels all TO3,TO220 and TO92, zeners. Go up to about +/-45V and the 6V, 5W bulbs start glowing that I added into the supply rails , alternately swapping dim and bright pairs very odd. Next I'll disconnect 1 of each complementary pairing of MJ15024 and see what happens. Any other suggestions? Next stage is sketching out the cct Are you posting from a cellphone? Do you have an internet service provider charging you by the byte? Nah, he just uses this public forum to post badly-formatted, cryptic notes to himself for future reference, apparently. -- The fashion in killing has an insouciant, flirty style this spring, with the flaunting of well-defined muscle, wrapped in flags. - Comment from an article on Antiwar.com (http://antiwar.com) |
#5
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Carlsbro Showman parametric/1 oscillating amp , 1990
Still oscillatated with one pair and also with the other pair. Checking over
the board again there was no continuity between line level i/p ground and the pa ground. An innocuous 1/4W resistor the same body colour as all the others but overlooked at 2.2 ohm bands was o/c linking the grounds. No discolouration except I managed to originally see the third band as yellow , not gold. No overheating on the adjascent pcb , so presumably a fusible resistor. In hindsight the slow "hunting" effect of the osc giving alternate loading to main + and - rails should have suggested a ground or zero problem. |
#6
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Carlsbro Showman parametric/1 oscillating amp , 1990
The "good channel" also had an o/c 2.2R but presumably because full o/c no
oscillation there. The failed one in the oscillating channel had 3.7K across the ends so presumably not fusible resistors. Must have been a very high current surge to knock out normal MO resistors without any sign of overheating. |
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