Marlborro (Sound Works) Model SRA920 made in the USA

[kevtronic]

I was given a Marlborro (Sound Works) Model SRA920, made in the USA, 6 channel PA head by a buddy for parts. I hate to strip it for parts if I can save it, but it needs the power transistors replaced and I can't find any info on them at all. Actually, I can't seem to find much info on Marlborro amps/heads at all, let alone schematics, parts lists, etc. Which brings me here. The Power transistors are NPN TO-3 and have the following markings: S, Marlborro M18-325, 8210. The M18 may actully be M78, as I can't tell if it's a 1 or 7. Any thoughts on how to determine which power transistors I should by to replace them?

Thanks...

[Enzo]

And you know they are bad? Clearly that is a house number and so will be meaningless. Unless you can find it in an NTE crossreference. 8210 is a date code, the part was made in the 10th week of 1982.

You can count how many of them are on a channel output, and you can measure the power rails they operate between. For example 60v rails means the supply is 120v across, so any transistors must be higher than that, so 180 or 250v parts make sense. Does the PA head have power output specs printed by the speaker jacks? That tells us how much voltage to expect across the specified load at full out, and we can infer the current from that.

You want your outputs to be the same, not a mixed collection. So if there are say six of them, and you have four bad ones, replace all six, so they will all be whatever we wind up using. To me the MJ15024 works in almost all places in this application.

[kevtronic]

And you know they are bad? Clearly that is a house number and so will be meaningless. Unless you can find it in an NTE crossreference. 8210 is a date code, the part was made in the 10th week of 1982.

You can count how many of them are on a channel output, and you can measure the power rails they operate between. For example 60v rails means the supply is 120v across, so any transistors must be higher than that, so 180 or 250v parts make sense. Does the PA head have power output specs printed by the speaker jacks? That tells us how much voltage to expect across the specified load at full out, and we can infer the current from that.

You want your outputs to be the same, not a mixed collection. So if there are say six of them, and you have four bad ones, replace all six, so they will all be whatever we wind up using. To me the MJ15024 works in almost all places in this application.

I tried the NTE cross reference and even called DigiKey with no luck. There are four power transistors and one of them is shorted. Unfortunately there are no power output specs printed by the speaker jacks. I just got the head yesterday and gave it a quick check up and it has about 28Vdc on the speaker outputs. So, I took a closer look and came to the conclusion I had a bad power transistor. So I pulled/tested them and found one part bad. I tried to find the parts by part number with no luck, which led me here. I'll measure the power rails today and see what I get. I feel the same away about the MJ15024's and actually picked four up last night at $3.75 a piece. Can't hurt to have them even if I don't use them for this project. Thanks for the info/tips...

I went in and measure the voltage that would be going to the power transistors themselves (the power transistors themselves are still removed) and I get: B-C = 43.2vdc, E-C = 43.7vdc, B-E = 0.46vdc. The MJ15024's might be over kill, but do you think they'll work properly or should I drop to MJ15003's or ? This is my first time replacing power transistors with out knowing the specific part...

Also, the power transformer is feeding about 63v into the pa with about 42v across the filter capacitors...

Thanks again for the help...

[Jazz P Bass]

To be sure that you caught it all, the amp can be tested with the bad transistor removed.

Albeit at a much lower output wattage.

If you have the MJ15024 transistors, I would not hesitate to install them.

[Enzo]

I specifically stock MJ15003 because some circuits used them, and I did warranty repairs. On my own, if I stocked MJ15024, I would likely not bother to also stock the MJ15003. You have about 80v across the circuit, so the difference between a 140v part and a 250v part is irrelevant. And either one has plenty of current capability. If you already had MJ15003 in stock, then sure, don't order the other. But if you are ordering, well really pick either one, but I like the MJ15024 in general.


Much as I dislike them, NTE would be more likely to have crosses for OEM numbers like yours. Digikey would have no idea in the world about OEM numbers.

When you have a blown output transistor, always check its emitter ballast. That would be some fractional ohm resistor like 0.22 ohm or something, typically 5w. Look for opens. Also, check the driver, a smaller transistor wired to the base of the bad output.

[kevtronic]

I replaced all the original power transistors with the MJ15024's. It's been running for a good while with no issues. So, hopefully it's fixed. Thanks for the help as always...