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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Default 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

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Default 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


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Default 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?
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Default 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.


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Default 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.




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Default 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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