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Blown Fuse....old Carver amp....nobe willing to learn
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KMS



Joined: 10 Apr 2007
Posts: 8
Location: St Louis

                    
PostPosted: Tue Apr 10, 2007 11:46 pm    Post subject: Blown Fuse....old Carver amp....nobe willing to learn Reply with quote

I hope I don't burden anyone too much and thanks in advance for advice.
250 watt stereo carver amp.

I'm not sure where to start.

Symptoms, turn on amp....hum.....meter jumps to 30watt output, then a click sound and then a burnt smell.

Open amp and can see nothing burnt looking. Fuse is blown.....put in new fuse and same thing.

These are the symptoms I was told by the guy who gave it to me.

I know this is not enough info to help me, what else do I need to post to get some direction for fixing this?

I can solder, I can read schematics, the circuit boards in the amp have image of traces on the non clad side.

Fuse casing is broke, and bad solder blob on fuse casing lug (like someone worked on it that can't solder as good as I can....not a good sign I know). The guy that gave it to me said he broke the fuse casing trying to replace the fuse but did not solder anything.

I found a brass washer laying on the amplifier section of the circuit and it looks like it might have a slight burn mark (maybe) on one outside edge of the washer. The washer is a perfect match to a residual footprint on the cabinet where it looks like there is a washer missing between the fuse casing and the cabinet.

I am reluctant to install a new fuse casing and plug it in to see what happens, as I'm wondering if there is some testing I might do with a digital multi meter prior to that?
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zaphod7501



Joined: 02 Aug 2004
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Location: Peoria Illinois

                    
PostPosted: Wed Apr 11, 2007 8:08 am    Post subject: Reply with quote

First of all, every time a fuse was replaced, additional damage was probably inflicted.

First check: discharge the power supply voltages: You need to bleed off the main filter capacitors with a 1 or 2 watt resistor (500 -2000 ohms). Unplugged!

Locate the output transistors and measure for shorts. You will probably find 2 shorted. Remove the shorted transistors from the circuit. (unsolder or cut leads)

Try a fuse again. If you are lucky, it will come on but with one channel inoperative. If it comes on but stays in "Protect" then you have additional problems besides just shorted outputs.

Report back. There are too many troubleshooting paths to enumerate the next possible steps depending on the results of the initial tests.
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Last edited by zaphod7501 on Wed Apr 11, 2007 11:06 pm; edited 1 time in total
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KMS



Joined: 10 Apr 2007
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Location: St Louis

                    
PostPosted: Wed Apr 11, 2007 7:46 pm    Post subject: Reply with quote

I understand bleeding the filter capacitors.....and I have done that before on an older yet amp (no apparent capacity was left in those caps however).

I don't understand how to check the power supply voltage with the unit unplugged.
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zaphod7501



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PostPosted: Wed Apr 11, 2007 11:04 pm    Post subject: Reply with quote

The idea was that all testing needed to be done with the unit unplugged and the power supply discharged. You can't use an ohmmeter to check for shorted outputs if there is a charge left on the filters. In rereading I realise my comment was misleading. (a needed word was missing, soon to be edited for clarity in future readings)

You don't have any particular need to check voltages, basically check for shorted output transistors with the power supply fully discharged.
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KMS



Joined: 10 Apr 2007
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PostPosted: Thu Apr 12, 2007 6:57 pm    Post subject: Reply with quote

OK,

That makes sence too.

Like I said I hope not to be a burden.

I will check the main transistors for continuity.

I'll report back what I find.

Thanks!
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KMS



Joined: 10 Apr 2007
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PostPosted: Fri Apr 13, 2007 1:16 am    Post subject: Reply with quote

The Power Transistors are all Toshiba.

Two on left channel are shorted. I found a Chinese data sheet and these are PNP number 2SB755. So there are 4 PNP total and 4 NPN also. The NPN are 2SD845 and I found a English data sheet for those and if the pin-out is the same for the NPN as the PNP then I have a short from Collector to Base on both of the 2SB755 on the left side. Also 70 ohm across the other two tests on those same tranys that are shorted.

On the Right side that is not Shorted; the two 2SB755 have the same readings which are no continuity collector to base....no continuity collector to emitter....and 71ohm from base to emitter.

All four 2SD845 NPN have the same readings; no continuity from collector to emitter, 73 ohm from base to emitter, and no continuity from base to collector.

I cannot find a supplier that will sell the 2SB755 individually and NTE does not have an equivalent. The least quantity I found was 35 from a wholesaler.

Do you know of a supplier that has the 2SB755 individually or even a minimum order of $30 or less?

The amp is a Carver Magnetic Field Effect M500t.
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zaphod7501



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PostPosted: Fri Apr 13, 2007 8:14 am    Post subject: Reply with quote

You always replace outputs in pairs, never just one. In your case a "Pair" may consist of 2 NPN and 2 PNP. I am not specifically familiar with the amp, but if it is a stereo amp and not a multichannel (5.1 or Dolby Prologic) then you will need to replace 4 transistors.

MCM Electronics and Tritronics both list them but both are out of stock. MCM shows NTE92 and NTE93 as substitutes. NTE subs may work for a short time but are never considered to be acceptible substitues except in extreme emergencies.

Most shorts are emitter to collector. You need to remove the obviously shorted pair before you can get a good test on the other sets. Always treat outputs as pairs (NPN-PNP) never as single transistors.

Another part(s) to check are the large ceramic emitter resistor(s), usually in the .1 to .3 Ohm range.

I don't know if they are still in business, but "City Refrigeration" was a major electronic parts supplier in the St. Louis area (despite their odd name).

Four transistors per channel circuits were a workaround for a lack of truly high power component availability. They tend to very difficult to repair reliably since any mismatched part in the entire output stage will destroy all of the outputs quickly. The factory would always use a matched set of 4 transistors which will be pretty much impossible due to the age of the amp.
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KMS



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PostPosted: Fri Apr 13, 2007 7:31 pm    Post subject: Reply with quote

I was really wanting to learn about repair of this type of amps and you have taught me things I did not know but it looks like this is the end of this Carver amp.

I cannot justify the cost that it will take to replace all the Transistors and after what you said I do not want the amp unless I can drive it real hard and that would not be possible without a matched set.......how many Transistors would I have to buy to find all 4 NPN with the nearly the same hfe and all 4 PNP with the nearly same hfe and then more testing and parts would need to be replaced....Also I can now see black on the clad side of the output board under two of the big carbon resistors. I could blow one of my new transistors while completing the testing and be back at looking at a whole new set of transistors again. Na.......it ain't worth all that........It is worth about $400 in perfect operational condition (same model sold on eBay last night $375) but this Carver is never going back to that.

I could use as it the basis to build my own amp....it has a lot of good parts and the Power Supply looking to be in good condition and the casing and heat sinks are fantastic.

Thank you for your time and information....you have saved me from what I find to be a lost amp.
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fpreston



Joined: 11 Jan 2007
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PostPosted: Mon Apr 16, 2007 8:29 pm    Post subject: Amp repair Reply with quote

Just to add my 2 cents.

I haven't dealt much with true high fidelity stuff, but add a few comments based on what little I know about this stuff. It is true that for the best HiFi performance (lack of distortion, etc), you need a matched set of output transistors. Since this amp is basically a boat anchor now, you might get lucky with just a partial replacement. If the NPN or PNP pairs are in parallel, you need a pair pretty well matched for either to provide the maximum output. Another possibility is that you may be able to remove all shorted transistors and power up what is left to see if that is the only problem (may need an o'scope). For lower output power, you may be able to use a single NPN-PNP set on each channel, especially with having enough heat sink there for both pairs (keeping the remaining pair cooler). If this works, you will not be able to adequately drive extremely low impedance loads. Stick with 8 ohms or higher.

As you indicated, getting a clean power supply at high current is a big part of making a good amp. With some of the latest amplifier modules available, you could utilized that clean power supply.

Fred
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unclemiltie
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Joined: 21 Jan 2004
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PostPosted: Mon Apr 16, 2007 9:09 pm    Post subject: Reply with quote

1: given the description of what you have, you have a push-pull amp with one set of transistors taking the positive voltage swing and the other taking the negative. The reason you want to match the output transistors is to get the crossover behavior (where one turns off and the other on) as close as "same" as possible. You don't have to match from channel to channel, only within each channel. (it sounds like each channel on this amp has 4 output transistors, 2 of each type)

2: that said, if you're willing to live with a bit of crossover distortion or you are willing to waste a set of one side just to make sure the amp works, you don't have to buy a matched pair. Just buy the ones that are bad, put them in and fire it back up. You'll probably never hear the distortion anyway, but if you find that it works and want to get it back to as close as you can to the original specs, you can then buy a matched set (2 NPN, 2 PNP)


If I remember correctly these are High power FET amps, and this was a well respected piece back in its day. (I remember that Carver took on a challenge to match this amp against a much higher priced Mark Levinson ML9 in a shoot out and numerous experts weren't able to discern the difference)


Anyway, good luck.
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KMS



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PostPosted: Mon Apr 16, 2007 9:39 pm    Post subject: Reply with quote

I appreciate the help.....Thanks to all of you.

It is 4 Transistors on each channel 2 PNP/2 NPN.

Also the Power Supply is a dual supply providing isolated +/- power to each Channel.

And the two 2SD845 that must be connected to the two tranys that are blown also have slightly less impedance across base to emitter 65ohm. I didn't think that was important and reported all 4 2SD845 at 73ohm because it seems like a small difference but both of the 2SD845 on the blown side had the same 65ohm base to emitter readings.

I plan to use this for pro-sound.....and a little distortion would not be noticed, but I will be driving it hard. I have not yet planned on speakers but it is likely to become a sub-woofer amp hooked up to two 18" speakers......for which almost all powerful 18" speakers are ratted at 4 ohm. I could run two 4ohm speakers in series and wire them to the amp in mono.....the amp has a wiring diagram on the back panel for mono set up. That should rock.

I guess I will make an attempt to fix....I'm not sure.....I love working on stuff like this.....but it detracts from my music practice....and my band mates don't mind that as long as they get something worthwhile out of my slump.

It should be easy to switch out a few transistors and the black spots on the clad side of the board are no doubt resistors which I can easily get and replace......I should do that because the board is black....obvious overheat.

I don't have a scope, but I do have a solder suckeruper and a multimeter...and I have seen some free evaluation scopes (lite versions) on line as software).

Now back to the big question....where to find these transistors?

I will no doubt have to order them online but I can't find them unless I buy 30 of them at a time.....I'm not doing that.
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fpreston



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PostPosted: Mon Apr 16, 2007 9:58 pm    Post subject: Transistor info Reply with quote

One other thing: The best way to test transistors is with the diode function on a multimeter. This applies a low current voltage source to the diode junction. Each junction should measure about 0.5 volts drop when forward biased. With leads reversed, it will measure full open-circuit voltage (about 2 volts or full scale). Emitter to collector readings always will look like an open circuit being across two reversed diode junctions.

Fred

PS: Check out eBay:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=200003827544
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KMS



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PostPosted: Mon Apr 16, 2007 10:20 pm    Post subject: Reply with quote

Thanks....I tried eBay the other day and nothing.....but I was looking for the 2SB755 so It looks like I can get the 2SD845.....now all I need are the 2SB755. Ebay does not show any for keyword search 2SB755 Toshiba and neither do the stores from those two eBay auctions....I tried those same stores and about 50 others earlier in the week.

The 2SB755 is the rare one.

I will just have take out some time and try all the sites I can find.

Thank for you attention.
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fpreston



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PostPosted: Tue Apr 17, 2007 1:48 pm    Post subject: Transistor Reply with quote

http://www.bdent.com/search/part.jsp?partnum=2SB755
$9.33 each

http://www.partstore.com/Part/Denon%20Electronics/Denon/2720060001.aspx?s=froogle
$10.78 each

http://www.mcminone.com/product.asp?catalog_name=MCMProducts&product_id=2SB755&CMP=datafeeds&ATT=froogle
$15.68 (Lead time 67 days)

Fred
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jherrick
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PostPosted: Tue Apr 17, 2007 3:58 pm    Post subject: Reply with quote

Just be careful of counterfeits. I did a quick search on that transistor the other day (longer back than my history is set to keep) and found a couple of sites where people who were trying to replace that same transistor started to receive fakes. They looked real, and the numbers were printed on well, but when one guy cut them open to compare with the original it was quite different. Beware.
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