Help finding fault with old bass amp that blows fuses

[permanentrecordstudios]

Bias transistor can be mje340 npn or bd139 both can be mounted to the heatplate..

When powering the amplifier up are you doing so with unloaded.

Looking at the circuit dig..them transistors just in front of the bias tr are parts of the amps short circuit limiters and have these been tested..

It is a basic amplifier and there are ways to isolated it if need be. I have had devices leak under load thus making the unit do crazy stuff.

Much appreciated.
I seem to have a 5th transistor C3298B mounted to the heatplate I'm guessing this is the bias transistor Tr122 (none of the transistors on my board are marked as Tr120 or 121, etc., and this one is slightly different that what's shown on the schems [C1953]...whereas I can match Tr120-121 & Tr123-124 via their specs on the bottom of the schems).

I haven't tested any other transistors, as I was hoping to avoid having to desolder & resolder everything (the amp is old and the board is showing signs of wear).
I may test Tr122...I think I just found specs for this here....

"When powering the amplifier up are you doing so with unloaded."
I'm mainly waiting on fuses at this point. I'm in a rural area, and these aren't easy to find in this area, so I'm just waiting on delivery of more. And the times they have blown, I don't recall if a load (the speaker) was attached or not.
But every time it has blown it has been on power up. There have been times, between blown fuses, when the amp didn't blow on power up, and I was able to play thru it for extended periods....only to have the fuses blow on next power up.

 

[Wayne Phillips]

It's a transistor amp and will work with or with a load.. them small transistors can be tested in circuit. You may use a test speaker to save burning up the amps own speaker or a dummy load resistor. 10 ohm 10w and if it gets hot with no signal applied then pull the plug!

Yes that transistor is part of the bias servo and thermal tracks the output devices.

If you have two meters connect these in place of the on board fuses in current mode and this will show how much idle current is being pulled..

 

[permanentrecordstudios]

How do you test the transistors in circuit? Voltage drop? Resistance? I have transistor slots on my meter and have just been removing them & testing em that way.
I can't do a voltage drop test right now because I'm out of fuses, but if a resistance test across B-C-E in circuit is accurate I can just do that. I think I've read that a resistance test alone across leads is not necessarily sufficient...but I don't know enough.
Between B-E (in circuit) on Tr122 shows 0.612 kOhm
No continuity between any other leads on TR122.

Sadly, I only have one meter.

 

[Wayne Phillips]

3.15 amp fuse will be ok.. or tack a thin wire on the blown fuse.. tr122 is a npn device to92 transistor.

Use your meter on diode test as your testing the base to collector and base to emitter pins.

 

[Bluejets]

To quote Kellys-eye again, is the fuse blown to smithereens or merely melted apart? I did not see where you answered that.

 

[permanentrecordstudios]

To quote Kellys-eye again, is the fuse blown to smithereens or merely melted apart? I did not see where you answered that.

Both. Twice they've been destroyed (cracked glass) & twice just looks like a mild melt.

For what it's worth, I have one fuse left, placed that in F2 whilst checking the current across F3 with my meter and got:
34.8v 0.035a.
Same reading vice-versa with the fuse in F3 whilst the meter in F2.

No surging detected. Load (speaker) is attached. Nothing plugged into input though.

 

[Wayne Phillips]

Remove the speaker untill the amp is working as you don't want to damage the speaker.

Have you checked the speaker? A 8 ohm would read 6.8..

 

[permanentrecordstudios]

3.15 amp fuse will be ok.. or tack a thin wire on the blown fuse.. tr122 is a npn device to92 transistor.

Use your meter on diode test as your testing the base to collector and base to emitter pins.

Ok, thanks.
I get a voltage drop across B-C of .047v (.543 --> .496)
I get a voltage drop across B-E of .236v (.543 -->.307)
I get those same reading regardless which direction the leads are connected (+ to B, - to C; versus - to B + to C).
These are voltage drops as read by the meter testing function...with the transistor in circuit....and not based on circuit voltage btw.

Per Fluke:
Diode test analysis

• A good forward-based diode displays a voltage drop ranging from 0.5 to 0.8 volts for the most commonly used silicon diodes. Some germanium diodes have a voltage drop ranging from 0.2 to 0.3 V.
• The multimeter displays OL when a good diode is reverse-biased. The OL reading indicates the diode is functioning as an open switch.
• A bad (opened) diode does not allow current to flow in either direction. A multimeter will display OL in both directions when the diode is opened.
• A shorted diode has the same voltage drop reading (approximately 0.4 V) in both directions.

So I have the same voltage drop in both directions, but of only .2 to .047 v. Does that mean bad transistor?

Also, if it matters from a previous reply above:
For what it's worth, I have one fuse left, placed that in F2 whilst checking the current across F3 with my meter and got:
34.8v 0.035a.
Same reading vice-versa with the fuse in F3 whilst the meter in F2.

No surging detected. Load (speaker) is attached. Nothing plugged into input though.

 

[permanentrecordstudios]

Remove the speaker untill the amp is working as you don't want to damage the speaker.

Have you checked the speaker? A 8 ohm would read 6.8..

It's reading like 4.7 ohms (it's an 8 ohm speaker, but I've read the static ohm reading really just reads the voice coil resistance, not the true working resistance across the speaker range (compensated for varying ohms across different frequencies). I've read the static voice coil resistance, in general, should read 5-7 ohms...so mine may seem a little low, but the fact it's a 15" sub may account for that?
Plus the amp can handle 4 ohm speakers.

As per my previous post, with one fuse inserted and my meter in the other fuse slot, I took readings with the speaker plugged in & unplugged both and nothing changed in volts or amp draw.

I've checked for shorts in the speaker leads (jiggled & bent em) but show no shorts.
Unless my speaker is somehow shorting internally, intermittently. Not sure if that's possible.

 

[duke37]

I get a voltage drop across B-C of .047v (.543 --> .496)
I get a voltage drop across B-E of .236v (.543 -->.307)

Is this Tr122 you are talking about?

As I asked before, measure the voltage across Tr122 i.e. collector to emitter. The output transistors need about 0.6V to turn them on so there should be about 1.2V across Tr122. If this voltage rises due to a bad connection or a bad potentiometer, then the output current will rise to excess.

Does the output sit at about midway between the power lines?

If the fuses blow on switch on, I would suspect the reservoir capacitors. You could connect them with thin wire to act as a fuse.

 

[Wayne Phillips]

This amplifier speaker protection relay ic circuit will act fast if there is any offset above 1v at the o/p.

Your readings look normal volts and current wise..now with a mains 240v 60w bulb limiter in the primary side...does it stay bright at power on with speaker load removed?


There will be a volts drop due to the limier in place, and double check the underside pcb for any solder over laps between tracks.


I will relook at the circuit dig..

 

[permanentrecordstudios]

Is this Tr122 you are talking about?

As I asked before, measure the voltage across Tr122 i.e. collector to emitter. The output transistors need about 0.6V to turn them on so there should be about 1.2V across Tr122. If this voltage rises due to a bad connection or a bad potentiometer, then the output current will rise to excess.

Does the output sit at about midway between the power lines?

If the fuses blow on switch on, I would suspect the reservoir capacitors. You could connect them with thin wire to act as a fuse.

Yes, this is Tr122.
I can't test the voltage across Tr122 because I'm currently out of fuses, and I don't live in a place where I can easily get them, so I'm waiting on shipping. Why I'm testing it with this diode function. I think I'm just gonna pull it & use my transistor tester function. But I will test across Tr122 once I get a new supply of fuses.

"Does the output sit at about midway between the power lines?"
I'm not sure what you're asking. Which output & which power lines are you talking about? The speaker output? The speaker output sits on a pcb to the right of the power cord ins. The leads going to the speaker don't go near the power.

"If the fuses blow on switch on, I would suspect the reservoir capacitors."
Yeah, problem is it doesn't happen all the time. I just powered it up to test the current & everything was fine (34v .035a going thru F2 &3). I powered it on & off several times (with one fuse in & my meter replacing the other fuse) & the one fuse didn't blow.

I'm gonna go yank Tr122 & test it.

Thanks again for the ideas & good suggestions.

 

[permanentrecordstudios]

This amplifier speaker protection relay ic circuit will act fast if there is any offset above 1v at the o/p.

Your readings look normal volts and current wise..now with a mains 240v 60w bulb limiter in the primary side...does it stay bright at power on with speaker load removed?


There will be a volts drop due to the limier in place, and double check the underside pcb for any solder over laps between tracks.


I will relook at the circuit dig..

"now with a mains 240v 60w bulb limiter in the primary side...does it stay bright at power on with speaker load removed?"
I'll let you know as soon as I get more fuses.

"and double check the underside pcb for any solder over laps between tracks."
There has been previous work done, but I've checked it all and it seems good.

I appreciate the continued suggestions.

 

[duke37]

I'm gonna go yank Tr122 & test it.

Tr122 is almost certainly all right. It has less than 2V across it and little current. It is more likely that there is a bad connection since the transistor is fixed to the heat sink which expands and contracts with temperature.

You have two power lines, one positive and one negative, going to the collectors of the output transistors. The output should be about midway between the power lines. From other posts, there is a protection circuit which only connects the speaker when this is so. I have not analysed this.

 

[permanentrecordstudios]

Tr122 is almost certainly all right. It has less than 2V across it and little current. It is more likely that there is a bad connection since the transistor is fixed to the heat sink which expands and contracts with temperature.

You have two power lines, one positive and one negative, going to the collectors of the output transistors. The output should be about midway between the power lines. From other posts, there is a protection circuit which only connects the speaker when this is so. I have not analysed this.

Sorry, I'm still confused. You're talking the speaker output? I now know what you're talking about regarding the power lines going to the transistors. Where is the protection circuit you're talking about?
There's a relay (Ry101) that connects the output to the speaker.

 

[duke37]

There is an IC111 which looks at the offset output volts and only connects the speaker when this has settled. It is fed through R210 and a smoothing circuit to reduce the audio signal. Shorting the output would enable a very high current to flow before the IC111 has time to react.

A transistor can avalanche when the temperature rises sufficiently for the transistor to lose control at high voltages. The higher the temperature, the greater the leakage, the greater the leakage the higher the temperature, the higher the temperature the greater the leakage - boom. Transistors are rated for a safe operating area, for example a 100V 100mA transistor will probably fail with 100V and 100mA, it can stand 100V at zero current or pass 100ma at close to zero volts. I do not know if the SOA changes with abuse but gain can be degraded.
The output transistors are operating with a high power dissipation and a heat sink is intended to control the temperature rise however, if the fuse is blowing instaneously then temperature is not likely to be an effect. Hence I would go for a bad solder joint on the Tr122 which controls the standing current or a bad reservoir capacitor which cannot stand the inrush current.

You could place three diodes in series across TR122 to limit the voltage and hence output current. If this stops the fuse blowing, you can look closer.

 

[Wayne Phillips]

There is short circuit limiter transistors in this amplifier..v limiter...

Look at the circuit and your see just in front of the bias servo..there are small signal transistors with there emitters connected to the output line.. if there is a short at the o/p end these limited the drive voltage..

 

[permanentrecordstudios]

There is an IC111 which looks at the offset output volts and only connects the speaker when this has settled. It is fed through R210 and a smoothing circuit to reduce the audio signal. Shorting the output would enable a very high current to flow before the IC111 has time to react.

A transistor can avalanche when the temperature rises sufficiently for the transistor to lose control at high voltages. The higher the temperature, the greater the leakage, the greater the leakage the higher the temperature, the higher the temperature the greater the leakage - boom. Transistors are rated for a safe operating area, for example a 100V 100mA transistor will probably fail with 100V and 100mA, it can stand 100V at zero current or pass 100ma at close to zero volts. I do not know if the SOA changes with abuse but gain can be degraded.
The output transistors are operating with a high power dissipation and a heat sink is intended to control the temperature rise however, if the fuse is blowing instaneously then temperature is not likely to be an effect. Hence I would go for a bad solder joint on the Tr122 which controls the standing current or a bad reservoir capacitor which cannot stand the inrush current.

You could place three diodes in series across TR122 to limit the voltage and hence output current. If this stops the fuse blowing, you can look closer.

Ok I have new fuses in, and the bulb limiter in place.
Per your earlier query, the voltage across Tr122 C-E is 2.240 volts.
EDIT - Sorry - I just adjusted the bias pot (it was way off) and the voltage between C-E changed to 2.437...if that matters.

 

[permanentrecordstudios]

There is short circuit limiter transistors in this amplifier..v limiter...

Look at the circuit and your see just in front of the bias servo..there are small signal transistors with there emitters connected to the output line.. if there is a short at the o/p end these limited the drive voltage..

I have new fuses in and the lamp limiter in place.
Per your earlier query, the lamp (120v100w) doesn't even light up (that I can see). Everything is working (power to the amp, fuses are still ok, etc).

I see the limiter transistors you're talking about...sadly this is another area where my amp differs from this schematic...so I'm trying to find the equivalent on my board.

Problem now is that everything is working fine (no shorts or anything causing excess current draw).

Is there anything I can test to check if there's a short in the output?

 

[Wayne Phillips]

If all is well and no sign of things over heating etc. Have you tried it with out blub limiter and no speaker hooked up?

Now if all is well use a different speaker instead of it's own bass speaker and power up with with the limiter in is it ok?

The big test is no limiter and power up, guitar
Amps can be tested with signal generators set at 1k of around 300mv applied to the input and watch the clean sign wave on a scope.

Now let's say an amplifier is 100w at 4 ohms
It's out ac volts will be v divided by load = current times volts out..

20v ÷4 = 5amp times 20v power 100w

At 8 ohms will be 50w..v÷r=I times v=p.