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Old 19th September 2008, 06:54 PM   #1
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Default Schematic request - Crate TD-70 Guitar Amp

Hi Everyone, I need some help .....

The tube preamp section of my Crate TD-70 guitar amp has recently stopped working right after I finally bought my first decent guitar!

The solid state power amp section is still working well so I'm using the 'line out' and 'line in' to acces the power amp directly.

I've tried different 12ax7 tubes and that's not the issue. It's wierd but I get no output from the preamp and also I can't change 'channel' from A (clean) to B (dirty). The light just stays on 'A' when I press the button or even use a pedal.

Anyway, I'm about to crack it open to have a look. I'm not too experienced. I know the safety rules though. I've got an oscilloscope and a multimeter.

Does anyone have a schematic for this? Has anyone ever encountered a similar problem?
I was hoping that the channel switching thing might be a giveaway to someone.

Best wishes,
Martin.
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Old 19th September 2008, 07:27 PM   #2
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It sounds like one of the voltage rails has gone.

On one website, someone complained about poor quality fuse holders while others told of intermittent operation.

I would look for a relay operated by the footswitch and the frontpanel switch


andy
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Old 19th September 2008, 07:59 PM   #3
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Cheers Andy. You're a star.

I've just ordered a schematic now. Seems like a good way to improve my knowledge of these things.

I'll open it tonight!

Thanks again,
Martin.
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Old 20th September 2008, 12:07 AM   #4
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Well .... I'm still waiting for the schematic to come through so I had a peek inside:

You can see the tube pre on the left hand side and the yellow wires that come directly from the transformer. There are two zener (I think?) diodes around the fuse there. The fuse is intact and no relays are used.

Click the image to open in full size.

Power supply, ac enters through yellow spades:

Click the image to open in full size.

Power amp shown just for fun:

Click the image to open in full size.
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Old 20th September 2008, 12:10 AM   #5
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Click the image to open in full size.
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Old 20th September 2008, 06:12 AM   #6
poynton is offline poynton  United Kingdom
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Yellow wires look like ac for the tube heaters. You checked the tube is glowing???

The power supply section is on the right of the main photo
with the big capacitors.

You need to multimeter probe (carefully) round the valve base with regard to ground.

Andy
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Old 20th September 2008, 03:57 PM   #7
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Yep, the tube is glowing fine.

I've now got the schematic and I'm beginning to panic about working near 300volts !

One of the 16v zeners measures short in circuit though. Maybe I should just try replacing that first and hope for the best?
I know I should take the diode out of circuit to test first though .... I just feel a little out of my depth with such high voltage.
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Old 20th September 2008, 04:25 PM   #8
poynton is offline poynton  United Kingdom
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What is the voltage across the zener?
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Old 20th September 2008, 05:21 PM   #9
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Quote:
Originally posted by poynton
What is the voltage across the zener?
I measured zero Volts across D24 when switched on. (Also 0 Ohms.)


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Old 20th September 2008, 05:34 PM   #10
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...... and the other diode of the pair (D23) shows 16VDC across it.


....... big 10watt resistors also check out fine .....
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