A friend dropped by an old (early 80s?) solid state amp that was blowing fuses. I confirmed that and then he told me that the volume was not as loud before the fuse popping. So I thought it would be an easy fix. I started with the output transistors and found 2 out of 6 bad. I ordered a new full set and put them in with new isolators. Still blowing fuses. I hooked it up to the variac and only got about 5V AC applied before the speaker started humming loudly. I'm guessing that something is shorted but I have no idea what or where to start. Any help would be appreciated. I have tools and am willing to learn. I have attached a couple of pics. Thanks in advance.
First GET RID OF THE SPEAKER, until the amp no longer draws current, makes no DC on the outs, and is stable. Just removing the load may stop the current draw. If your amp has say 45v on the output, it can sit there doing that without a load, but soon as you connect a a load to it, the LOAD will draw a ton of current.
I you replaced all the outputs, then certainly check ALL the resistors associated with them. They often burn open after transistor failure, and that leaves the amp not working too. ANY time outputs fail, I generally replace the drivers, even if they check OK.
And don;t overlook the obvious, a shorted rectifier in the power supply.
If you no longer find any shorted transistors and the thing draws current anyway, then look for open bias circuits.
Hey Enzo,
I finally got back to this project after I got some parts in. Thanks for the pointers.
Here's where I am with it. I ditched the speaker. Still the same. I checked the rectifier. Checks OK but had another so swapped it out. No change. Reading +41.25/-41.25...seems about right since all caps are rated at 50V. I replaced the TIP41 and TIP42 transistors. No change. I pulled all the output transistors and checked again. All good. Any more ideas? I don't really know where to go from here. Not sure how to go about checking the bias circuit. This thing is hand built and not very easy to work with. Wires everywhere :(
I am confused what "no change" means. I can assume it means it is still not working but... I hope you are not hooking the speaker up each time to check if the amp is passing signal. Perhaps you are scoping the output? We need to understand your testing procedures.
Lets measure for DC voltage on the speaker output and check that first thing. Best regards.
Get Methodical.
A Limiting Lamp (http://www.ssguitar.com/index.php?topic=2093.0) is a must first.
When you can get some voltage in there, what is each supply, and the output half-rail?
Post three voltages.
Leave speaker off until advised otherwise.
1) it's a steel guitar amp, so meant to be loud and clean.
2) semi-homebuilt by a steel guitar player in the 70's so NO schematic no factory backup no nothing.
You are on your own.
3) 50V caps point to +/-40V (or thereabouts) rails are reasonable
4) 6 large ceramic resistors (3 on each end of PCB) hint at 6 power transistors, maybe all NPN in a quasi complementary circuit (most probable) or 3+3 NPN+PNP
5) blown power transistors often blow corresponding drivers ... which you have not replaced or even measured yet, so it's not surprising the amp still malfunctions.
6) lacking a schematic or even a layout, start labelling parts, at least most important ones.
a) take a picture of the heatsink showing all 6 transistors, then on Photoshop or similar label each transistor as: Q1 - Q2 - ..... Q6 and on some free or unimportant area of the picture identify them as: Q1: 2Nxxxx and so on or Q1 to Q3 xxxx ; Q4 to Q6 yyyy or if all the same: Q1 to Q6:zzzz .
b) do the same with the PCB , the picture you already uploaded will do, label all transistors and identify them somewhere, also label all large ceramic resistors.
c) the heatsink *should* have some thermal tracking devices attached to it, either some diodes or a transistor, glued or clamped to it , post some closeup picture of that, but showing, say, a couple inches around.
7) start by building a lamp bulb current limiter, plug the amp there, disconnect speaker or any load.
Turn amp on, measure +/- rails and DCV at speaker out rail.
Get datasheets for all transistors, you need to be certain at least of the pinout, hand draw some pin identification diagram (B , C , E) and stick it to the wall.
Just curious, what speakers does it have?
How are they connected?
8) that said, I wouldn't be very surprised if it turned out to be a homemade clone of Peavey Nashville 400 , in which case that schematic would help.
It might also be a clone of a Sho Bud amp, the one which started it all, when steelers noticed that even Fender Twins weren't loud and clean enough.
ali-ooop!
Hi Roly, DrGonzo78, J M Fahey,
Thanks for chiming in guys. I do not have a limiting lamp but I will get right on building one before I do anything else.
I don't have a scope but I think I could borrow an old one if you think it would help. I have not hooked up the speaker since Enzo advised not to.
JM you are right on most everything you said. Pretty perceptive ;)
1) It is a steel guitar amp. Came from a friend that plays steel and he got it from another steel player.
2) Built by Heilman electronics in Nashville. Serial NO 15. I think may be this guy http://bb.steelguitarforum.com/viewtopic.php?p=1084881&sid=50bc43207f3365895f900757367ece01 (http://bb.steelguitarforum.com/viewtopic.php?p=1084881&sid=50bc43207f3365895f900757367ece01)
3) All caps are rated at 25V or 50V.
4) There are 6 power transistors 3 NPN + 3PNP. Mounted in sockets on large heatsink under chassis. Will post pics later.
5) There are 2 transistors on each side with heatsinks and diodes. (drivers???). I did replace those.
6) I will work on getting part numbers and pics and post everything I can soon.
7)Will build a limiter ASAP.
The combo currently has a JBL Professional 2220B speaker 16 Ohm wired to a 1/4" speaker plug. I doubt it is the original. I will get some pics of the cab, speaker, etc soon.
Thanks to all of you guys for helping me with this little project.
1. - Limiter Lamp.
2. - please don't just start replacing stuff on spec. Once we can get some voltage on it safely we can diagnose exactly what is wrong, and fix that. It's quicker, cheaper, and more reliable than the "scattergun" technique (which basically doesn't work anyway, and only makes component shops rich and guitarists poor)
3. - are you able to compare and confirm that the circuit I posted is (close enough to) the reality to work from as a reference?
1) Will build limiter asap!
2) Not doing anything until I get limiter and can post some voltages from amp.
3) The schematic you posted is similar but there are several voltage differences. I don't know enough to be sure it will work as a reference.
google " Heilman Electronics Nashville, Tn. "
He seems to be active today (or short ago so as to still be listed as an active business) and the first link gives Phone and Address .
I would phone, explain the situation and ask for a schematic.
Even a handdrawn one , scanned or in a readable picture will be better than nothing.
If they are hesitant about revealing trade secrets, tell them no worry, keep the preamp hidden , just the power amp will be a great help.
As they say around here: "you already have a NO, any other answer can only be same or better ":)
I got me a limiter and got some voltage through this thing.
From the main caps to ground is +14.2/-14.2.
There is +13.2V DC on the speaker out :(
Quote from: kennymc75From the main caps to ground is +14.2/-14.2.
There is +13.2V DC on the speaker out :(
Cheer up, that's progress. ;)
Then it looks like one (or more) of the transistors on the upper (+ve) side is shorted.
At this point what I normally do is isolate (or remove) each of the output transistors in turn and test them out of circuit. It is wise to do the same with the driver transistors since they can also be damaged by an output transistor failure. You may have more than one dead device and you need to find them all.
OK looks like my last post was incorrect. I screwed up because I'm a NOOB. I didn't have the output transistors installed when I powered it up and measured voltage. After I put them back in things changed drastically. The current limiting lamp is bright and the voltages are way different. +2.17V/-2.17V with 1.37V on the speaker out. I have checked all of the output transistors again and show good. I have checked all of the transistors on the board and they test good. Not sure what I'm missing.
Quote from: kennymc75I didn't have the output transistors installed when I powered it up and measured voltage.
Ah. Not tragic, but not a great idea either.
Quote from: kennymc75After I put them back in things changed drastically. The current limiting lamp is bright and the voltages are way different. +2.17V/-2.17V with 1.37V on the speaker out. I have checked all of the output transistors again and show good.
If the first bit is right then the second bit almost certainly isn't - the bright lamp and these voltages strongly suggest that you have at least one and possibly two shorted output transistors.
A good transistor will test as a diode between Base and Emitter, and between Base and Collector, and open circuit between Collector and Emitter. With an NPN transistor it will appear to have two diodes with their anodes connected to the Base; with PNP, with their cathodes to the Base. In all cases the Collector-Emitter will appear open. A good diode will measure between 500 and 700mV on a DMM diode test range, more or less is suspect.
(http://www.ozvalveamps.org/repairs/solidstateamprepair/trtest1.gif)
(http://www.ozvalveamps.org/repairs/solidstateamprepair/tranlead.gif)
Hello, all. I'm a new member and I believe I own this amp now (don't know why I did this to myself). Hoping Kennymc75 replies or someone may be able to help me w/ contacting him (don't see a PM option).
Thanks!
E
He only made the six posts in this thread and has not been here for over two years. To contact people, click on their name and options are on the profile page.
Or start a new thread for your amp, and specify what the issues are.
Hey Ethernaut ,
That is indeed the same amp. John Sawyer loaned it to another aspiring steel player and it died on him. He brought it to me to see if I could get it working again. At the time I had no idea what I was doing hence seeking help on the forum. The problem that I figure is a short in the amp section. I really didn't have any experience with amps at the time but I poked around on it for a while before I gave up. There should have been some new output transistors in the back of the amp. Some other parts blew off the board at one point and I hope that I replaced them with the proper ones. Anyway since then I have had much better luck with amps and have learned quite a lot. If I had known then what I know now I may have got it going again. Anyway I hope I put it back together well enough that you can figure it out. Good luck :)