Carver m1.5t (Attn: Anatech)

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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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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
 
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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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
 
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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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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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
 
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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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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Hi oldsthunder,
You are on the right track now. Let me know how things work out. Carver did produce a replacement cap board, one board holds four caps. They became N. L. A. as well. It was cut to fit the contour of the chassis exactly. The pins fit in where the old mounting holes were for the original caps.

My cost was about $120 (retail was much higher). I have one left only that I was saving for the next job. If you are interested I'll sell it to you for what I paid for it plus whatever shipping is. Designing and making a PCB is a real pain in the other end of a tech. ;) My mail button works if you are interested.

-Chris
 
Those caps are a real pain on the PM/M1.5 amplifiers. I was having trouble getting them from Carver so I started to make my own. When you add up the price of the caps (I use 8), the circuit board and all the labor involved in getting the 16 mounting pins aligned you will know why they were so expensive from Carver. I won't be making any more for for another month or two, but if you are into making one yourself and want some photos then let me know. I still have a disassembled M1.5t with the new caps sitting on the bench.
Todd
 
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Hi Todd,
Eight caps. Interesting. I'll bet it's easier to source two caps in the proper voltage than one.

How close am I?

It's also a real pain to get it all to fit with the contour of the chassis in the area. Mind you, I really liked the sub board when it came out. It's just a big job getting there. I usually replace a couple problem resistors when I get in there.

Anyway, I'd love to see you solution. I have but one of those Carver cap. boards left.

-Chris
 
Hi Todd & Chris,
Todd thanks for the reply & info. I just recieved my order from Digikey (next day delivery--amazing!), and now have two new Panasonic caps, 4700 MFD, 200 VDC & four 2200 MFD , 100 VDC. Hopefully I will get them installed in the coming week. Chris, I'll let you know when they are in.
Dave
Slainte!
 
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Hi Dave,
I love Digikey. I wish they carried 100% of what I need. They are the best for shipping to Canada (Mouser fails big time there!).

Yank out those caps now and have a look at the solder joints and some resistors in the area. Now's the time while the area is clear.

-Chris
 
4 of those caps are 120uf HF, back when I was testing for ripple on the scope they really seemed to make a difference, so I kept them in the PB design. The other caps are 2 X3300@80v and 2 X 6800@50v, sourced from Digi-Key. I pay $20 each for the boards ($60 for 3) double sided FR4 glass epoxy. PCB file I have is "expresspcb" so it wont work for Gerber or other PCB programs, you will have to order directly from expresspcb only. Let me know if you require more information.
I hope the attached photo works..

Todd
 

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Hi Todd,
That sure is a forest of caps you have there! I'd like to see some more details for sure. The 120 uF caps get most of the pounding I bet with that power supply design. They go a long way to smooth out teh rising edge of the waveform. Is that what you found?

-Chris
 
Chris,

I found some old photos from some testing that was done a few year ago. Im not sure of the bias current setting during the measurement. The scope shows the low rail ripple, two DMMs show the low/mid ripple (ok, its just AC measurement on DC rails) and the third DMM shows the hi voltage rail in DC volts. I remember during one hi power/hi frequency test the little 120uf caps did get hot, never had any of them fail. Normal loud music don't really seem to change the temperature at all. I had (if I can find them) some thermal IR scans of a M1.5t running fairly hot, you can see the temperature differences in the transformers/resistors/etc. If your interested I'll see if I can find them and post them here.

Todd
 

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Hi Todd,
Never run these at high frequency / high power. Not even to the second rail. The commutators will lock up and the amp will go into thermal runaway. So use these for full range or bass only. Mids and highs only will kill them.

I'd be interested in the thermal pics. Just for curiosity.

On the pictures, see the "pip" on the leading edge of the waveform? That's my indicator for bad capacitors. You should look at it with a real 'scope and compare.
-Chris
 
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