Ampeg V-6B Transistor Substitutions

[evil79]

So I've got an Ampeg V-6B on the bench, and I need to replace the 2 driver transistors, and they are impossible to find. The schematic says NA under part substitution. I can't find ANY info online about their specs. I've pretty much exhausted all known resources. Even contacted Ampeg, and they are even more clueless. I talked to another local tech today and he recommended that I mod the circuit to accept current production transistors. This is out of my out of my area of "expertise". I know there was a similar thread somewhere online about this exact thing, but it stumped everyone. Enzo, you even chimed in on it. You're pretty much the jedi master with this stuff, so i'm hoping to hear from ya. The transistors in question are STA8236 (Q302-npn) and STA0202 (Q303-pnp). Btw, when I first opened it up, D210 & R216 were blown to bits. I'll try to post a schematic and some pics in a minute. Anyway, if anyone has any insight for me, I'd love to hear it. Thanks!

[evil79]

Here's the schematic and the transistors I need to replace:



http://i1223.photobucket.com/albums/...ps46d52f83.jpg
http://i1223.photobucket.com/albums/...psd422e81e.jpg

I took a picture of the schematic thats inside the amp instead of posting one from a website, because they're all so grainy online its too hard to read. I zoomed in on the last one to give you guys a better look at Q302, Q303, D210, & R216, since those were the trouble spots. Fyi, I've replaced all electrolytics, all 8 power transistors, a few diodes and resistors, and a few of the smaller transistors hoping that it would solve the problem since I can't find the drivers. Now, when I play it, it works, but only at very low volume. Turn it up slightly and it distorts and makes loud "coughing" noises. Same noises happen when I tap or jolt the amp at any volume. Solid State is such a pain to work on.

[flyingdutchman]

Those old TO-66 metal can types are just about impossible to find these days, but modern TO-220 types can be substituted without problems.
Use the MJE15030 (NPN), MJE15031 (PNP), just be sure you get the original ON and not some chinese knock-off.

[flyingdutchman]

Oops. Didn't see your edit before I posted.

Noises when you tap or jolt the amp point to bad solder joints or intermittent plug connectors. Make sure to get this sorted out first.
Tap the circuit board(s) all over very gently with the handle of a small screwdriver or other suitable (insulating!) implement. With some luck you can pinpoint the area where the bad connection lurks. Inspect all solder joints in the area for defects, if a joint looks suspect, resolder it. If necessary, remove excess old solder from the joint.
Since this is an old, pre-ROHS amp, use 60/40 Sn/Pb solder rather than the ROHS-approved lead-free alloy.

[evil79]

Thanks flyingdutchman!! That's what the local tech suggested as well. He just didn't know which ones were ok to use since there aren't any specs for the old ones. He said to make sure I get matched ones. So here are my questions (sorry if they are stupid): Are you positive those parts will sub for the old ones? What do you mean by "ON"? How do I make sure they're matched? Transistors are sorta my kryptonite. I know how to test them with a diode tester, but that's pretty much it. I'm understanding them more as I work on SS amps, so hopefully I'll learn something from you guys.

[evil79]

Yeah, I've poked around all over the place and haven't heard anything, and even resoldered a bunch of joints for good measure. All I use is 60/40 or 63/37. Lead free sucks. Anyway, I should probably keep poking because you're probably right. Hell, it's a small enough board that I should just hit everything with fresh solder. Its for a close friend, so I don't mind being super thorough.

[flyingdutchman]

"ON" stands for "ON Semiconductors", which is the semiconductor manufacturing division spun off by Motorola. Motorola, and hence now ON, are the original manufacturers of the MJE150xx series.

You do not necessarily need matched transistors in most solid state amps. If you run a bank of power devices in parallel you'd want them to be more or less in the same ballpark but that's about it. If a power amp will not work correctly unless you use matched complements for drivers, it means you're dealing with a very badly designed amp.
How you match transistors depends completely on which parameters you'd want to match. Long story.

[flyingdutchman]

Other things to watch for when amps make noises when jolted: Bad control pots, bad switches, loose fuse holders, faston (spade) connectors and so forth.

[evil79]

That's interesting about the loose fuse. That's worth looking into. Here's a pic of the original fuse before I replaced it. It was a bitch getting it off, so I clipped the leads and soldered a new leaded fuse to the old lead. It's definitely soldered to the leads, but I wonder if It's bouncing around. Hmmmm. I'll get back to you on that one. And thanks again man. You're being incredibly helpful. I really appreciate it.

[evil79]

Ok, so you got me thinking. Aside from the fuse, the filter caps might be suspect. The original ones were massive. New replacement caps are like half the size, and there are holes in the side of the chassis to clamp them in that are way too big for the new caps. Finding a way to secure them in the big holes was a challenge, and they might not be as secure as I hoped. I'll show you.


And here's the new fuse I put in:

The caps are only held in with one screw. I bent the clamps so that they press firmly against the chassis, holding the caps in place. That's the best I could do. I thought about it a long time, tried rigging it several other ways, even looked for extra space elsewhere that I could somehow secure them to, and nothing else really worked except the way I have it now. This problem takes creativity. If i had a way to do it, I would cut some metal with smaller holes in them to mount over the old ones, but that didn't seem practical. Anyway, do you think the way I have the fuse and caps are problematic? And if you have some clever tricks for mounting those caps, I'm all ears.

[52 Bill]

The old TO-66 case transistors are hard to find now. The TO-220 transistors will fit into the same sockets or pc boards by cutting off the center Collector lead and bending down the Base and Emitter leads.

Did you test the transistors that you are trying to replace? Have you tested the other transistors in the power amp?

[flyingdutchman]

The internal fuse shouldn't be a problem as long as the solder joints are OK. Do check the panel fuse holder for loose contacts though.

As for the PSU caps, I wouldn't leave them "hanging" like that. Get out your metalworking tools and some alu sheet and make an adapter plate with smaller holes such that the caps are properly supported.
Consider that the cap on the negative side of the supply has -Vcc on the can, if the insulating sleeve gets damaged for some reason you might get a nice short against the chassis.

The thing I usually do if I have to replace PSU caps, given that chassis mount types are increasingly hard to find, I make a simple PCB to hold the caps. Sometimes the new caps are small enough that they fit inside the chassis, otherwise I just have them poke through the chassis holes for the old ones. I admit though, that this solution is practical only if you're set up to make your own PCBs.

[52 Bill]

The thing I usually do if I have to replace PSU caps, given that chassis mount types are increasingly hard to find, I make a simple PCB to hold the caps. Sometimes the new caps are small enough that they fit inside the chassis, otherwise I just have them poke through the chassis holes for the old ones. I admit though, that this solution is practical only if you're set up to make your own PCBs.

I agree, but you could just use blank perfboard to make adapter plates that the new caps can mount on.

If you have a larger hardware store or industrial supplier around, you might find standard flat washers that can be modified to work as an adapter plate. 2-2 1/2" diameter washers are fairly common.

[evil79]

Thanks everyone for helping me out. Flyingdutchman, I tested those transistors, and they're ok. I must have done it wrong the first time. I poked around and found the source of the distortion. It was a wire connected to one of the transistors in question. So that's fixed, but I'm still getting a loud popping sound, either after a few seconds after warm up, or after touching random wires. So it's gotta be a solder joint or wire. I've replaced several wires and touched up lots of solder joints that seem to make noise when I touch them, but everytime I think I found the problem, it still comes back. I'll just have to keep poking around. I'll let you guys know. And the caps are actually held down pretty damn good.

[olddawg]

Thanks everyone for helping me out. Flyingdutchman, I tested those transistors, and they're ok. I must have done it wrong the first time. I poked around and found the source of the distortion. It was a wire connected to one of the transistors in question. So that's fixed, but I'm still getting a loud popping sound, either after a few seconds after warm up, or after touching random wires. So it's gotta be a solder joint or wire. I've replaced several wires and touched up lots of solder joints that seem to make noise when I touch them, but everytime I think I found the problem, it still comes back. I'll just have to keep poking around. I'll let you guys know. And the caps are actually held down pretty damn good.

Lots of things can cause pops. I couldn't read the schematics but if it has a difamp input its beta matched pair of transistors could have drifted. Small capacitors could do it and also could any component with physically intermittent connections internally. Do you have a scope? I would try a heat gun and some freeze spray to isolate the problem. If you tickle a difamp pair and there is a problem it will let you know, lol.