Sherwood AM-7040 No Left Channel

[KilgoreCemetery]

I've been working on this integrated amplifier with no left channel and it's turning in to a real head-scratcher. I know that when I first got it that the left channel fuse was blown (250V, 7A, ceramic, Slo blow), so I checked the MOSFETs and found one of the P-Channel, 2SJ162's was burnt. Checking the N-Channel, K1058's some of them were shorted. Not wanting to mess around, I replaced all 8 of the MOSFETs on the left channel. (I actually bought enough for both channels, but haven't gotten that far yet)

I wasn't able to find the exact same model, so I replaced:
2SJ162 with IRFP9240PBF
K1058 with IRFP240PBF

Also, I didn't have enough thermal paste, so I decided to try a Silicone Elastomer. It's an H48-6 and can be found on Digikey here. The only reason that I bring this up is because, after receiving the product and testing it out, I'm not sure if I'm supposed to still have some sort of isolator between the pad and the heat sink. Unfortunately, the phone numbers on the manufacturer's website are for Digikey, and their technicians didn't know either. This is one of those "untested guns in battles" type of situation.

Anyways, I've also checked all of the resistors on the boards with the MOSFETS. There are only four different ones total, and, aside from the 0.27Ohm ones, they were easy to check. My multimeter only displays ohms to one decimal point. Combine that with slight fluctuations between the contacts, and you get unreliable results. I did put a different resistor in line with my test probes to verify that none of the 0.27 Ohm resistors were open or closed however.

I also checked the single diodes on each board and they are good. So are the 1uF caps.

With the left channel unhooked, I get the proper voltage at the connects (+74.5V and -74.5V). However, if I hook up the left channel and try to power the unit on, it makes a buzzing, high current, sound and blows the fuse before I can even power it back off.

I do have the Service Manual/Schematic for this, but, so far, nothing seems out of the ordinary.

Does anyone here have experience with Silicone Elastomer that can verify whether or not I'm using it correctly? Also, any ideas as to what else to check would be incredibly helpful. I'd really like to see this amp working again!

 

[duke37]

Repeated fuze blowing will tell you nothing. Use a lamp limiter (a 40W bulb in series with the mains). so that a voltage can be applied without blowing the fuze. Compare the voltages on the good channel and bad channel. I would think that the output voltage should be near 0V, There may be an output capacitor but not likely.

Heat sink compound contains thermally conductive particles, the elastomer may not help much. If the MOSFETS have a live tag, then an isolation connection will be necessary. You could lift the transistors from the heat sink and try for a second or so. Do not let the transistors overheat.

Can you get a circuit diagram (schematic)?

 

[kellys_eye]

The devices look like fully plastic bodies therefore no insulation is required but heat compound should be used (sparingly) to cope with surface irregularities.

Of course, it doesn't help your cause that the pin outs differ.......!!!! Check the datasheets! Drain and Source are reversed on the replacement device!

 

[KilgoreCemetery]

I do have a schematic/Service Manual that I picked up from HiFiEngine, but the file was too big to upload here.

I'll try a 40W bulb once I get everything reassembled.

My understanding of this particular elastomer was that it is supposed to be highly thermally conductive and fill in tiny gaps. It wasn't until I went to install it, that I began to wonder if I needed something else to act as an electrical insulator. I'm still on the fence about it.

As for the transistors, the replacement ones do have metal backs, so I'll try lifting them temporarily to see if that helps.

 

[kellys_eye]

Of course, it doesn't help your cause that the pin outs differ.......!!!! Check the datasheets! Drain and Source are reversed on the replacement device!

 

[KilgoreCemetery]

Are they really?
Checking now

 

[KilgoreCemetery]

Ack!! Why would they do such a thing?

I mean, I assume that the data sheet I was able to find for the originals is accurate, but I guess I didn't realize that manufacturers swapped pins around

Thanks, kellys_eye. Good catch.

So I guess now my question is where am I going to find the CORRECT replacements...

 

[duke37]

You can connect in a different order with little loops of wire.

Give details of the four types you have quoted. Voltage, current and pinout.

 

[KilgoreCemetery]

I suppose, but if at all possible, I'd like to find proper replacements. Here's the info for the P-Channel. N-Channel will be posted soon as well.

The original P-Channel is a 2SJ162
Vdss = 160V
Drive = 10V
Continuous Drain = 7A (Ta)
Pinout: G S D


Replacement P-Channel is an IRFP9240

Vdss = 200V
Drive = 10V
Continuous Drain = 12A (Tc)
Pinout: G D S

 

[KilgoreCemetery]

After doing some research, I found that the original transistors are still available, but on ebay. I normally steer clear of websites like that for discrete components, but I've looked at over a dozen different P-Channel MOSFETs on Digikey, and they all have the same pinout. Only the 2SJ162 and 2SK1058 have the Drain and Source pins reversed.