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    Fixed. By looking at the manufacturers parts...

    Fixed. By looking at the manufacturers parts list, there are separate part numbers for 50 and 60Hz drive pulleys and belts. On dismantling further, the drive pulley was stamped with "60". I got a...
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    I didn't look too closely at the original device...

    I didn't look too closely at the original device - it sounded terrible so I just replaced it. LM3876s are hard to get hold of so I bought it from a random ebay dude. (There may be a clue here). The...
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    Fixed it. You need to sink 0.5mA from the Mute...

    Fixed it. You need to sink 0.5mA from the Mute input for the LM3876 to work. This is pulled down to -31V with a 30K resistor. Which should give about 0.9mA. Someone out their on the internet noted...
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    Sorry for delay. Finally catching up with some of...

    Sorry for delay. Finally catching up with some of the stuff on the shelf:

    Resistor values are 47K and 2K2 which are both about double the data sheet values so should give the right gain. Cap is...
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    Nasty Sounding Tech 21 Trademark 60

    Tech 21 Trademark 60

    It came to me with a melted power transformer, and the LM3876 power amp detached from its heatsink.

    Replaced transformer with something about the right size and reattached...
  6. Thread: Limit LMA 800

    by Ted
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    Limit LMA 800

    Bit of a long shot.

    Looks like an 80s 2 channel PA amp. No schematic visible on the interweb....
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    52799

    52799
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    This is a B15R 90s (maybe even early 00s). I...

    This is a B15R 90s (maybe even early 00s).
    I seem to remember you could use EL34s in them with a biasing switch.

    They didn't sound much like a B15N
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    Caver PT 1250 SMPS Caps

    I've got a bunch of Carver PT1250s here. They have a primitive SMPS which uses a pair of series 2700uF 200V Sprague electrolytics for high-side smoothing. (this is the 230V model BTW). These have...
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    Still didn't quite believe it until I unpicked...

    Still didn't quite believe it until I unpicked the winding wires from the tags and swapped the sense of one of the primary windings. Stuck it back in the amp and all was well. Someone has QA issues.
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    It was a Dagnall, bought direct from Marshall.

    It was a Dagnall, bought direct from Marshall.
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    It wasn't the same problem - the problem was that...

    It wasn't the same problem - the problem was that it sounded terrible - weak and distorted. I hadn't looked at the original transformer with the scope. It was a couple of days between trying it out...
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    OK. Got it. I'd already changed output...

    OK. Got it. I'd already changed output transformers before I started poking around with the scope - I'd injected 2VAC into the transformer secondary and the voltage on the primaries looked a little...
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    The effect is still there in half power mode

    The effect is still there in half power mode
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    Feedback disconnected. No Effect

    Feedback disconnected.
    No Effect
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    Changed the filter caps. No effect.

    Changed the filter caps. No effect.
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    The centre tap IS the centre tap. 44/42 ohms to...

    The centre tap IS the centre tap. 44/42 ohms to plates 86 ohms plate to plate.

    I'm beginning to suspect that I'm doing something stupid. WOuldn't be the first time. But I need to work out what.
    ...
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    Both Grids: 51907 One Grid and Speaker: 51908

    Both Grids:
    51907

    One Grid and Speaker:
    51908
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    Good advice here. Thanks. Just waiting for a...

    Good advice here. Thanks.

    Just waiting for a 2nd scope probe to be delivered....
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    JCM900 Frequency Doubling Weirdness

    50W JCM900 Combo.

    I'm injecting a 1000Hz signal. I'm getting a 2000Hz signal at the speaker. The scope says the signal on each EL34 grid is 1000Hz.
    All grounds have been cleaned, different sets...
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    Thanks. I've messaged him.

    Thanks. I've messaged him.
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    Mate. I wouldn't buy a plectrum from them.

    Mate. I wouldn't buy a plectrum from them.
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    Please do

    Please do
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    Cant see anything. Do you have a link?

    Cant see anything. Do you have a link?
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    AC30 Top Boost Module

    Anyone know of anywhere in the UK that sells repro AC30 Top Boost modules for retro-fitting to non-TB amps?
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    I'd considered that. So. I tried a phone with...

    I'd considered that.

    So. I tried a phone with a tone generator into an Inter-M 500W PA amp into the 55V winding of a 200VA with the 110V winding driving the power. It kind of works. But it loses...
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    Ampex 60Hz Tape Transports

    I have here an Ampex 601-2 that a friend brought back from the US. It one of those old Ampex suitcase 1/4" reel recorders. The electronics needed attention, but I could handle that (putting the...
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    This is a great book - has layouts, schematics...

    This is a great book - has layouts, schematics and component values for every conceivable flavour of vintage AC30.

    http://www.thevoxac30guide.com

    And yes. Those black or brown Hunts capacitors...
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