Carver m1.5t (Attn: Anatech) - diyAudio
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Old 26th June 2006, 02:51 PM   #1
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Default Carver m1.5t (Attn: Anatech)

Re my earlier post about help for my Carver:

Just wanted to say many thanks to all that helped me out. The amp is up and running. I just have to change those 3 amp diodes to some 6 amp ones.
Anatech, you advised me about the top rails being at 124 VDC and you set them at 118 VDC. My supply is currently at 128 VDC! It makes me nervous! You said to adjust it using the thumbwheel pot on the power supply board. Before I touch it, can I assume CCW lowers the voltage?
Please advise me.
Thanks again.
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Old 27th June 2006, 03:05 PM   #2
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Hi oldsthunder,
I can't remember off hand which way increases the voltage. Just try a gentle movement with the meter clipped on so you have both hands free. The caps will survive a brief over voltage just fine. As the voltage increases you would also hear the mag coil growl a little.

So relax and keep your wits about you.

After you set the voltage, I'd like you to hang a 'scope on the low and medium voltage rails. Check positive and negative rails. I want to see the wave form, or you can describe it for me. The 6 amp diodes are recommended, so that is fine.

Cal,
Thanks for having the sharp eyes to see this!

-Chris
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Old 27th June 2006, 04:54 PM   #3
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Hi Anatech,
I have adjusted the pot (2Kohm) to the extent of its travel and have lowered the voltage to +/- 124.5 VDC.
The medium voltage is at +/- 78.7 VDC. The scope shows a sawtooth ripple of about 1% (0.07 vac).
The lower voltage is sitting at +/- 44.8 VDC, again with a ripple of about 1%.
The amp is now connected to a set of boxes and is extremely quiet.
I guess next is simply an endurance run after I change those diodes.
Thanks for your help.
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Old 27th June 2006, 05:04 PM   #4
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Hi oldsthunder,
I am interested in the shape of the waveform across the caps. On the leading edge, watch for a peak, or a "pip" that falls back down as if there is an inductor in series.

Due to the limited range of adjustment, I am thinking you have some resistors that have drifted out of tolerance. I would be checking those. If I'm am not mistaken, the voltages are sampled from each supply and averaged out. Check those resistors. If they are fine, then the optocoupler is suspect.

-Chris
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Old 27th June 2006, 07:14 PM   #5
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Hi Anatech
Increasing the sensitivity reveals the "pip" you refer to. If the ripple is .07v then this pip is almost .01. It seems a little bigger on the +/- 45 VDC supply. What is causing that? I took a look at the resistors you mentioned and can see a previous repair. There is a new 0.2 meg, 1/4 watt resistor going from one leg of the pot (not the wiper) to +124 VDC rail. The other side of the pot has 1 watt resistor, could be a 3.9k as may the one on the bottom. These have been so hot that the colour code is too faint. Also the foil pattern on the reverse side has seen enough current to melt the copper pattern. There is now a small jumper wire installed..
Check the photo and tell me what you think. I can replace the resistors if I can determine there value.
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File Type: jpg dave-2.jpg (54.6 KB, 817 views)
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Old 27th June 2006, 07:24 PM   #6
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Hi oldsthunder,
The "pip" gets larger as the caps deteriorate. I also check these waveforms while drawing current (output into a dummy load). Try to attach a picture on the waveform if you can (turn the flash off, camera on something to hold it steady).

This amp has been repaired before. Use solder flux and wick after removing most of the excess solder with a desoldering tool. A large DS-017 (or copy) works very well. I'll hunt around for some information. Those overheated resistors are very common. Always watch for these. Check the two small caps there as well.

-Chris
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Old 27th June 2006, 07:29 PM   #7
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It seems to me that the 124 volt rails got 200 K. 330 K to the mid rails (approx 35V) and the transistors came from rails close to 12 VDC. I always increased the resistor wattage and spaced them off the PCB a bit. Your pot should have about the same resistance in series, or around 2 K. There will be a 43 ohm resistor in series with the optocoupler (between the emitters of the two transistors).

-Chris
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Old 28th June 2006, 03:13 PM   #8
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Hi Chris,
Something new this morning! The power transformer seems to be buzzing. I isolated the sound with the help of a mechanic's stethoscope (screwdriver to the ear). It is barely audible but it is there. Any ideas. I will try to check all the resistors out of circuit.
Will also check online for a schematic or service manual.
Attached is a photo of the ripple + pip. Amplitude setting was 0.1 volts per cm.
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Old 28th June 2006, 06:47 PM   #9
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Hi oldsthunder,
The picture is great! I had hoped for something that clear.

The bad news is the the dual section filter caps are bad. They are the dark blue caps near the chassis soldered to the power supply board.

Hum is normal from the mag coil. Don't even begin to worry about it until the rest is working. You will understand later.

Anyway, those caps are N.L.A. You need to make up a PCB to hold separate caps.

-Chris
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Old 29th June 2006, 05:56 PM   #10
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Hi Anatech,
I did some searching at DigiKey and they have caps that will serve well as replacements. As you say, I will need to put together a board but at least I found some.
Thanks for everything.
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