Workingman's 15 blew a fuse when turned on. (help me troubleshoot?)

[Stratapastor]

I bought this from a kid in town who needed cash along with a very sad looking squire bass. When I plugged it in and turned it on it blew the 8A fuse between the plug and the first internal hot wire. it was a hard soldered fuse and i'll be replacing that tomorrow but i'm not sure where I should start trouble shooting... or even how to trouble shoot some of the components on an amp. Also, before the fuse blew there was a good deal of humming.

I'm "ok" with a soldering iron but not a pro. I only paid $125 for the guitar and amp so i'm willing to risk bricking it so long as I'm taking that risk with as much information as possible. Thanks in advance for your help.

[DrGonz78]

First things first disconnect the speaker and do not connect the speaker until the amp has been repaired. Do you own or have access to a multimeter to measure resistance and voltages? Something is drawing excess amount of current and is blowing the fuse. You said you heard a humming and that is probably DC voltage hitting your speaker before it blew the fuse, hopefully the speaker is not blown too. When you reinstall the fuse do not simply turn on the amp or you might just blow another fuse and, well, your right back where you started. Instead research and build a light bulb limiter (a.k.a. Dim bulb tester and Poor Man's Variac) which will allow you to power the amp on with out blowing fuses. The light bulb limiter will light the bulb up bright if the amp is pulling excessive current which will save your new fuse from blowing. When you first power it on via light bulb limiter do not have a speaker connected. Here is a schematic for that amp and hopefully it matches up to yours. Do your research and approach the amp repair using troubleshooting techniques offered through correspondence on this site. Do not just pull parts and replace without justification.

Light Bulb Limiter

The problem is probably due to shorted output transistors or a shorted bridge rectifier, but it could be other issues beyond that too.

[Stratapastor]

Thank you very much for the reply.

I do have a multi-meter. I'm going out of town today for a funeral but should be back tonight. My intention is take the meter to my office (where the guitar is) when I get home and start by testing the transistors and the bridge rectifier. I am pretty sure I've identified those but I may take a picture mspaint a circle around what I'm testing and post it here. I'll also build a LB Limiter to take with me. That amp has a switch that can eliminate the speaker and output via headphone jack... any reason I couldn't just use that mode while chasing the short? Also, instead of replacing the soldered in fuse with another is there anything wrong with replacing it with soldering in either an 8A breaker or replacable fuse holder? There seems to be plenty of room and this would also eliminate the need to heat shrink the fuse. (it was in clear HS when I removed it)

Also, am I correct in my understanding that the schematic you shared does not show the AC power in but actually starts somewhere after the transformer? The only fuse I see in that diagram is on the board itself.

[52 Bill]

Welcome to the place.

Also, instead of replacing the soldered in fuse with another is there anything wrong with replacing it with soldering in either an 8A breaker or replacable fuse holder? There seems to be plenty of room and this would also eliminate the need to heat shrink the fuse. (it was in clear HS when I removed it)

The internal fuse is there as a safety device to prevent the user from replacing the main fuse with one of the wrong value, which I will guess was done on your amp.

On the back panel there should be a fuse holder with a removable cap that should have a 4 amp slow blow fuse in it. If this has been replaced with a 20 amp fuse or piece of foil, the internal 8 amp fuse will blow instead. When operated correctly, the internal fuse should never need to be replaced, therefore I recommend that you replace it with the original solder in type fuse.

The fuse on the pc board is the speaker fuse. It is there to help protect the speaker from damage when there is a problem with the power amp.

If you build the limiter, you can replace the outside fuse with the correct value one and temporarily bypass the blown 8 amp internal. That way you can order a new one when you order the other replacement parts that you will need. You will probably not find a pigtail fuse in any local stores.

[Stratapastor]

Welcome to the place.

The internal fuse is there as a safety device to prevent the user from replacing the main fuse with one of the wrong value, which I will guess was done on your amp.

On the back panel there should be a fuse holder with a removable cap that should have a 4 amp slow blow fuse in it. If this has been replaced with a 20 amp fuse or piece of foil, the internal 8 amp fuse will blow instead. When operated correctly, the internal fuse should never need to be replaced, therefore I recommend that you replace it with the original solder in type fuse.

The fuse on the pc board is the speaker fuse. It is there to help protect the speaker from damage when there is a problem with the power amp.

If you build the limiter, you can replace the outside fuse with the correct value one and temporarily bypass the blown 8 amp internal. That way you can order a new one when you order the other replacement parts that you will need. You will probably not find a pigtail fuse in any local stores.

The slow blow was in place but the 8A still blew first. That said, I got antsy tonight and put an 8 amp in that I already had and it blew the slow blow... so hopefully tomorrow I'll be able to actually trouble shoot it with my fancy new slow blow fuse and light/power source. I noticed tonight that the cord that came with it does not have a ground prong... i'm wondering if that might be part of the issue.

In positive news. I was able to replace the pot on the bass that came with the amp and it seems to play pretty well. With a little bit of set up it will be a solid guitar.

[The Dude]

You should certainly replace the plug or power cord for your own safety, but not having a ground wouldn't make the amp blow fuses.

[Stratapastor]

First things first disconnect the speaker and do not connect the speaker until the amp has been repaired. Do you own or have access to a multimeter to measure resistance and voltages? Something is drawing excess amount of current and is blowing the fuse. You said you heard a humming and that is probably DC voltage hitting your speaker before it blew the fuse, hopefully the speaker is not blown too. When you reinstall the fuse do not simply turn on the amp or you might just blow another fuse and, well, your right back where you started. Instead research and build a light bulb limiter (a.k.a. Dim bulb tester and Poor Man's Variac) which will allow you to power the amp on with out blowing fuses. The light bulb limiter will light the bulb up bright if the amp is pulling excessive current which will save your new fuse from blowing. When you first power it on via light bulb limiter do not have a speaker connected. Here is a schematic for that amp and hopefully it matches up to yours. Do your research and approach the amp repair using troubleshooting techniques offered through correspondence on this site. Do not just pull parts and replace without justification.

Light Bulb Limiter

The problem is probably due to shorted output transistors or a shorted bridge rectifier, but it could be other issues beyond that too.

today I got the multi meter out and poked around... Assuming I was testing them correctly. all of the transistors seemed ok. The bridge rectifier just after the transformer is good. I was about to give up but then decided to test every diode and I found that D10 is measuring 83 both directions using diode test function on the multi meter. Any chance that this could be causing the issue i'm experiencing? If i'm understanding correctly that should only have resistance in one direction... but i'm learning as I go.

[The Dude]

Edited: D10 is paralleled with a 100 ohm resistor R41. You are measuring the resistor also when you measure in circuit. It's not likely the problem.

[DrGonz78]

What sorts of measurements are you getting on Q5? Also, please post your actual measurements on Q3-4 and Q9-10 for second opinions. You can measure the transistors in Diode check mode and even resistance mode if you don't have a diode checking function.

[Stratapastor]

What sorts of measurements are you getting on Q5? Also, please post your actual measurements on Q3-4 and Q9-10 for second opinions. You can measure the transistors in Diode check mode and even resistance mode if you don't have a diode checking function.

I'll go back and check. Should have measurements in 10.

Would you be willing to video conference?

[J M Fahey]

The slow blow was in place but the 8A still blew first.

You mean it was already blown.

You said so in post #1.

[Stratapastor]

You mean it was already blown.

You said so in post #1.

The 8 amp blew after I turned it on the first time. I blew the slow blow last night after cleaning it up a bit and hoping that it was just a shorted wire... both are replaced and i'm currently chasing the short.


Measurements of Transistors are as follows. Measuring with negative on the center post and positive on the outside leg. CL, CR, and then the outside posts measured either way because I don't know which is right.
Q5 644, 1, 565/1
Q3 585, 724, 271/1
Q4 484, 589, 84/84
Q9 1, 1, 713/1 (if I put the pos on the center it goes to 700,700)
Q10 583, 546, 84/84

[DrGonz78]

Okay lets makes more sense out of these readings. First off each transistor can be PNP or NPN and each transistor can have different pin arrangements BCE: Base, Collector, and Emitter. To figure out leg of the transistor is B,C, or E we will rely on the datasheet to show us the pin out of the chip. I have attached those on this posting for you. Do you have a Continuity tester function or Diode testing setting on your multimeter? I am also referring a few web pages about testing transistors too. Just think of a transistor as a couple of diodes (B to C) or (B to E) and you measure the diode that is between these junctions. Now it might be PNP or NPN and then you just have a difference in the polarity of your meter probes when testing. Read this entire page and think about everything I have typed already.
Electrical Measurements Part 3 | RSR Electronix Express

So find the base leg and measure positive or negative lead (depending on PNP or NPN) to Emitter or Collector looking out especially for shorts. A continuity tester will beep between legs to show a shorted connection.
Another page to look at is this silly looking thing here>>>
Transistor Testing

Also, beware that sometimes you have to remove a transistor from the circuit to be able to get an accurate reading.

[DrGonz78]

BTW looking at the measurements made on your last post Q9 seems a little suspect to me and that would be the first transistor I would pull to test out of circuit.

[Stratapastor]

Just FYI. I'm swamped with work right now. I'm hoping I can shake free for a few hours tomorrow. Just don't want you to think I've given up. I've got those links open on my phone so that I can read when I get a second.
My multi-meter is a levetron It has a diode test mode but I have a high frequency hearing impediment and I don't know if I always hear it over the Air conditioner.

Thank you again for your help. I'll keep you updated. If I cant' get to it tomorrow then it will likely be at least next weekend before I can touch it again because I'll be out of town all week.