Need Help with circuit Diagnosis (CRATE CR-112)

[Vokzel]

Hello, I am fairly new to circuits and amps, and I have an old SLM amp id like to fix. It’s my dad’s and it never really worked to well. The pots are dirty so turning them would often lead to no sound what so ever. I decided to take a look at it the circuit and try to mess with it a little but alas, I am stuck. I will admit I messed up, I forgot the order the wires were connected to the PCB, and while putting it back together I shorted the fuse. I then tried to replace the fuse but I read the amp-age wrong and ended up using one with too high of a rating. Anywho, i now have the correct fuse in, however when i turn on the amp, it makes a loud humming sound with no signal when i plug in a cable. It also gets warm around the heatsink area, specifically the GE 2n 3402 transistor (which i could not find on the schematic below). Un plugging the reverb tank from the pcb also does not do anything, the noise is still there. Any help would be greatly appreciated, thank you.
Crate CR-112 2.PDF

[DrGonz78]

If you hear a hum that can be DC voltage on the speaker. It will sit there until it burns up your speaker. So remove speaker and assume it is going to blow fuses. So research light bulb limiter to avoid blowing more fuses. You must have a digital multimeter since you are going to need one. Check for DC voltage across to speaker terminals and do not connect a speaker until it’s basically zero. Your best bet is a 40 watt lightbulb limiter plugged into the unit to make voltage checks. Q3-Q6 are all questionable. I guess if the unit is not blowing fuses it doesn’t really need a light bulb limiter. But you said it blew with higher amperage rated fuse. Why did it not blow with a lower amperage rated fuse now in it?

[Vokzel]

Replying to DrGonz,
I may have mis-spoke, to be honest I’m not entirely sure what the fuse i tried was rated, it did not say on it. All i know is that it caused something to happen.

[Jon Snell]

Replying to DrGonz,
I may have mis-spoke, to be honest I’m not entirely sure what the fuse i tried was rated, it did not say on it. All i know is that it caused something to happen.

All fuses have a voltage, current rating and type stamped on them. Any quality fuse will have the safety stamp as well.

There is one fuse; the mains input fuse. That will only blow in extreme fault conditions. The value is critical!
I would suspect the output transistors; Tip 120 and 125. They fail at the slightest hint of an overload.

C31 blocks DC to the speaker, so no damage to the speaker.

In the USA the fuse is a 1A T 32.5mm or 1 1/4". Elsewhere on 220/250 volts 500mA T 250v.

[DrGonz78]

Yeah I thought that was the purpose of C31. But I always see those caps in series so I wasn’t sure. And if there is any DC voltage at the speaker it could be a leaky C31. But it sounds like you need to test those transistors.

[g1]

It also gets warm around the heatsink area, specifically the GE 2n 3402 transistor (which i could not find on the schematic below).

The 2N3402 is a sub for the 2N5088 shown in Q4 position. It could be that it was changed for later production units. Q5 and Q6 also look to be better spec'd parts.
Did you adjust the trim-pot beside the TIP120? That could make the output devices and heatsink run hot.

[Vokzel]

The 2N3402 is a sub for the 2N5088 shown in Q4 position. It could be that it was changed for later production units. Q5 and Q6 also look to be better spec'd parts.
Did you adjust the trim-pot beside the TIP120? That could make the output devices and heatsink run hot.

I just checked an old photo i took of the circuit board before i messed with anything, the trim-pot is in the exact same position. Im going to check all the transistors touching the heat sink when i can, but from what I remember the 2n got significantly warmer than either of the TIP Transistors. Im not sure if that fully means anything, and i guess we wont know for sure until its been tested. Thank you all for your feedback, i will be sure to reply back and let you know what happens.

[g1]

The Q4 is mounted on the heatsink to sense the heat and compensate the bias, not because it gets hot. It should not be doing much work. So if it does feel like it is generating heat it is likely bad or something around it is causing it to heat up.

[Vokzel]

It was the TIP 125, i checked it in multiple different ways and each time, it gave an incorrect response to what people usually say it should

[Vokzel]

Hello again, I replaced the TIP125 with another TIP125 but now it doesn't even produce sound. I checked the power going into the circuit with a voltage meter and it showed it was getting 120 volts but nothing was happing through the speaker. The light also did not turn on. I'm not sure what the issue could be, if it is that the specific TIP125 model had to be the same or not, the one I used to replace the broken one had different model numbers and it was not made by RCA. It could also be that I need to get better at soldering.

[g1]

Did you check the fuse?
Did you have to remove some of the wires to replace the part? Are you sure the wires were put back in the right place?
If the light does not come on it likely means the main power supply is not working. There should be somewhere around 40VDC across the large yellow capacitor near the fuse.