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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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Crate BV120H guitar amp, probably 1991
History , there was a smell of burning followed by cutting out.
Next time powered up high pitched squeel. Now on powering up very muted output like half-wave and sometimes a burst of high frequency squeel but all at low level. Nothing obviously burnt or over-heated inside. Stable HT voltage. 4x 6L6 Same distorted sound on the line out. Before testing the valves etc, anyone have any pointers ? -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
#2
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Crate BV120H guitar amp, probably 1991
"N Cook" wrote in message ... History , there was a smell of burning followed by cutting out. Next time powered up high pitched squeel. Now on powering up very muted output like half-wave and sometimes a burst of high frequency squeel but all at low level. Nothing obviously burnt or over-heated inside. Stable HT voltage. 4x 6L6 Same distorted sound on the line out. Before testing the valves etc, anyone have any pointers ? -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ And you can't find where the burning took place, either by visual inspection or smell? (I see your statement above.) You think maybe someone let the smoke out of the output transformer? Desolder the leads and test it. |
#3
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Crate BV120H guitar amp, probably 1991
Phil S. wrote in message
... "N Cook" wrote in message ... History , there was a smell of burning followed by cutting out. Next time powered up high pitched squeel. Now on powering up very muted output like half-wave and sometimes a burst of high frequency squeel but all at low level. Nothing obviously burnt or over-heated inside. Stable HT voltage. 4x 6L6 Same distorted sound on the line out. Before testing the valves etc, anyone have any pointers ? -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ And you can't find where the burning took place, either by visual inspection or smell? (I see your statement above.) You think maybe someone let the smoke out of the output transformer? Desolder the leads and test it. THe owner smelled burning, I have'nt so far. Resistance checks on the matching transformer seem ok. All anode voltages seem ok. Not the slightest sign of overheating on any resistor. It all looks perfect inside except the pcb polyester is covered in marginal scratches, like left by a dvm probe tip. But they are all over parts of the main board and go under the comps, never seen anything like it, presumably cosmetic damage at manufacture. I've not met optocouplers in a valve amp before - anyone know details of their construction used by Crate, 4 lead, not DIL packs ? Will check through the preamp tomorrow -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
#4
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Crate BV120H guitar amp, probably 1991
N Cook wrote:
History , there was a smell of burning followed by cutting out. Next time powered up high pitched squeel. instability narrows the options. Now on powering up very muted output like half-wave I cant think of anything that might be described by both those terms, so dont know what you mean. and sometimes a burst of high frequency squeel but all at low level. Nothing obviously burnt or over-heated inside. Stable HT voltage. 4x 6L6 Same distorted sound on the line out. after the preamp? If so that gets the power amp out of jail free. Before testing the valves etc, anyone have any pointers ? I'd just start by using a probe to listen to the audio at each stage thru the chain, see where it goes wrong. Would need to remove the preamp's nfb path before doing this. I once had something like this that IIRC ended up being to do with dirt on a spark gap... but memory is less than sure, and it doesnt sound like youve got a power amp problem so this presumably wouldnt apply. NT |
#5
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Crate BV120H guitar amp, probably 1991
wrote in message
ups.com... N Cook wrote: History , there was a smell of burning followed by cutting out. Next time powered up high pitched squeel. instability narrows the options. Now on powering up very muted output like half-wave I cant think of anything that might be described by both those terms, so dont know what you mean. and sometimes a burst of high frequency squeel but all at low level. Nothing obviously burnt or over-heated inside. Stable HT voltage. 4x 6L6 Same distorted sound on the line out. after the preamp? If so that gets the power amp out of jail free. Before testing the valves etc, anyone have any pointers ? I'd just start by using a probe to listen to the audio at each stage thru the chain, see where it goes wrong. Would need to remove the preamp's nfb path before doing this. I once had something like this that IIRC ended up being to do with dirt on a spark gap... but memory is less than sure, and it doesnt sound like youve got a power amp problem so this presumably wouldnt apply. NT I decided to have another look. The distorted and lower level signal than I would expect was at the input grids side of the push-pull splitter/driver valve so problem is presumably further back to V1,2 or 3 or op-amps -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
#6
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Crate BV120H guitar amp, probably 1991
On Tue, 12 Dec 2006 15:11:03 -0000, "N Cook" wrote:
Phil S. wrote in message ... "N Cook" wrote in message ... History , there was a smell of burning followed by cutting out. Next time powered up high pitched squeel. Now on powering up very muted output like half-wave and sometimes a burst of high frequency squeel but all at low level. Nothing obviously burnt or over-heated inside. Stable HT voltage. 4x 6L6 Same distorted sound on the line out. Before testing the valves etc, anyone have any pointers ? -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ And you can't find where the burning took place, either by visual inspection or smell? (I see your statement above.) You think maybe someone let the smoke out of the output transformer? Desolder the leads and test it. THe owner smelled burning, I have'nt so far. Resistance checks on the matching transformer seem ok. All anode voltages seem ok. Not the slightest sign of overheating on any resistor. It all looks perfect inside except the pcb polyester is covered in marginal scratches, like left by a dvm probe tip. But they are all over parts of the main board and go under the comps, never seen anything like it, presumably cosmetic damage at manufacture. I've not met optocouplers in a valve amp before - anyone know details of their construction used by Crate, 4 lead, not DIL packs ? Will check through the preamp tomorrow Opto's aren't uncommon in guitar amps, usually used in channel switching circuits or as a mute if there's a relay doing the switching. It's a Crate, so the first suspect is solders and mechanicals. My usual first step is to run signal and hit it with a fist, listen for any changes. You mention checking voltages, did you go through the whole amp or just the output section. Output section, did you check the screen supply and their resistors. Very common source of a burning smell. Ron |
#7
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Crate BV120H guitar amp, probably 1991
N Cook wrote:
History , there was a smell of burning followed by cutting out. Next time powered up high pitched squeel. Now on powering up very muted output like half-wave and sometimes a burst of high frequency squeel but all at low level. Nothing obviously burnt or over-heated inside. Stable HT voltage. 4x 6L6 Same distorted sound on the line out. Before testing the valves etc, anyone have any pointers ? -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ high pitch squeal usually indicates dying preamp tube, but should have no odor... ty |
#8
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Crate BV120H guitar amp, probably 1991
ty wrote in message
... N Cook wrote: History , there was a smell of burning followed by cutting out. Next time powered up high pitched squeel. Now on powering up very muted output like half-wave and sometimes a burst of high frequency squeel but all at low level. Nothing obviously burnt or over-heated inside. Stable HT voltage. 4x 6L6 Same distorted sound on the line out. Before testing the valves etc, anyone have any pointers ? -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ high pitch squeal usually indicates dying preamp tube, but should have no odor... ty I've not seen the track side of the main board but i'd expect to see trace of overheating of a copper track through the polyester/glass -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://home.graffiti.net/diverse:graffiti.net/ |
#9
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Crate BV120H guitar amp, probably 1991
followup see new heading
16 ohm amp -- 4 ohm speaker block (ex speakers) |
#10
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Crate BV120H guitar amp, probably 1991
That could cause some damage to your amp since your speakers have less
resistance than what your amp is rated to handle. "N Cook" wrote in message ... followup see new heading 16 ohm amp -- 4 ohm speaker block (ex speakers) |
#11
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Crate BV120H guitar amp, probably 1991
Michael Kennedy schrieb:
That could cause some damage to your amp since your speakers have less resistance than what your amp is rated to handle. "N Cook" wrote in message ... followup see new heading 16 ohm amp -- 4 ohm speaker block (ex speakers) Michael, pssstt, it´s a tube amp.... Jochen |
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