another Hartke Overheating Post

[Lee_ranaldo]

Hey i have this Hartke A70 Combo for ages but now on this unit started overheating in the heatsink section

Previously this amp was driven hard and had to replace 2 shorted power transistores (B688 and D718) and also replaced the TIPXX's Transistors and B631 just in case.

The Transistor mounted in the heatsink was faulty since a poor assembly (has 2 wires that connects it into the board)

With no load the Power Supply Delivers almost 28-29 Vac Rms on Both outputs and almost 38-39 Volts DC on both rails..

I've checked every transistor on the output stage, al diodes, replaced the filter caps everything. The regulated supply for the op-amps works fine.

There is a noticeable hum (ripple) since the output stage is driving more current.

The overheating starts even at low volumes and it keeps burning fuses when i turn it on (seems to be a fast transient)

I'll includhe the schematic of it in PDF format just in case someone has the same issue.

THanks in advance!

[gbono]

C36,C34,C42,C46,C47,C60 and C61 are all okay?

[Enzo]

I am confused I guess by your choice of terms.

Power supply. Both outputs? Are you saying you have about 38VDC both positive and negative for the power amp stage? Are they clean and stable DC? And was the AC voltage reference to the transformer?

+/-38VDC looks reasonable to me.

Now what exactly are we trying to cure? Overheating? Or is there more? Does it sound OK?

replace 2 shorted power transistores (B688 and D718) and also replaced the TIPXX's Transistors and B631 just in case.

Did you replace them with the same types? Or did you use some NTE replacement types? You seem to have replaced Q11,12,5,9, and Q7. Right? If you are going to replace the whole output stage, you should include the important bias transistor Q6.

The Transistor mounted in the heatsink was faulty since a poor assembly (has 2 wires that connects it into the board)

Does this refer to Q6? Very important this transistor be right, and in GOOD thermal contact with the heat sink.

Work WITHOUT a speaker load for now. Is there DC voltage on the speaker wires or on the output jack? Does it still get hot with no speaker load?

Upper left of power amp page, 15v zener diode D5 - is there 15vDC across that diode?

Over by the output jack is 10 ohm R67. Is that resistor OK?

If the unit works and there is no DC or at least very little DC on the output and the thing is hot, then it probably has a bias problem. I prefer to connect the unit to a current metered outlet to monitor mains current draw. You can connect an AC ammeter in series with the mains in some way for this.

Q6 is central to the bias. The driver transistors Q11, Q5 should have ROUGHLY +1vDC and -1vDC on their bases. About 2v apart. Q6 maintains this 2v. It will probably be less than 2v in practice. To reduce the spread and cool the amp, I might reduce the value of R46 a little. That assumes a good Q6.


Also, since you replaced burnt output transistors, check all the various resistors associated with them.

[Lee_ranaldo]

yes sry i was busy at the time i posted it

i checked all caps for short circuits in the power amp section and no shorts or whatsoever..

also i checked every resistor on that stage including the ones that goes to the limiter section (R126 & R125). no resistors seems to be out of value but i will replace the emiter resistors R56 & R64 anyways.

Q6 transistor is ok , i've replaced the termal paste everytime i installed it into the heatsink, but, this transistor is mounted on a separate PCB and the quality standards of this amp are very low. I had to resolder the wire because it was giving intermitent connections.

The replacements for the transistors are the original ones
that means

2SB631 for Q7
TIP41C & 42C for Q5 and Q11
2SD718 for Q12
2SB688 for Q9

everything is well connected , Q6 in particular, i triple checked those before turning the amp on again.

I did some test measurements , refered to ground (chassis), and i got almost 2 volts on the bases of each power transistor, for that i measured this voltage on R61 and R54.

The only transistor i haven't replaced are the 2SD600 since i can't find it here.

Every diode seems to work ok according to the multimeter checking foward voltages. Every capacitor was tested and i've replaced the power supply caps because i always do that when the output stage has been on short circuit.


I will post all the voltages i've measured with a drawing of the output stage. As long as i remember every resistor R71 , R68 , C62 and C71 are ok..

[Lee_ranaldo]

I was doing measurements again.

Re-checked the 10 ohm resistors R67 & R71 and they're ok.

I've attached the measurements i did today and for my surprize the V+ and V- voltage drops to 22.5 volts dc and when i had this board without the power transistors Q12 and Q9 the power supply was delivering 38 VDC totally stable. Also i have about 1,3 VDC measured on the output jack

i'm gonna replace the 2SD600 transistors with this ones

2SD669
2SD809

It overheats without an speaker connection too

I suspect the diodes D6 & D7 and also the transistors Q4 and Q10 since the Q6 is giving 0,95 volts the colector and emiter .

[Lee_ranaldo]

FIXED:.

Hartke Quality Control Seems to be seriously defective.

The Q6 transistors that controls the bias is mounted in a separate PCB . This Q6 is screwed onto the heatsink to control the bias depending of the temperature of the casing of Q6.

The thing is i was having good readings on emiter and colector (0,95 volts) on Q6 but no control voltage was applied to the rest of the circuit.

Simple.. this board is connecter with a pair of molex conectors to the motherboard and that was faulty. I've replaced the whole cable with a new one,(i've also soldered this directly to both pcb's) replaced the diodes D6 & D7 and zener diode (D5) just in case, cleaned the termal paste,applied new one and installed new fuses.

Now i have +/- 38.5 on Q12 and Q9 collectors
and the voltage of R61 and R54 dropped to 0,38 Volts. I Have 0,95 volts dc on each base of Q5 and Q11 so it means that is pretty fine. No hum ot whatsoever on the background.

i Got no DC voltage on the speaker output either (just 0,00 according to the meter) No overheat with low power even at lower volumes.

So i think it's fixed . I'll keep it running 24 hours straight to check any possible faults

Thanks all for your suggestions and help!

[Enzo]

It is running hot all by itself, so that means it is drawing a lot of current. SO I don;t doubt that the V rails are pulled down.

I am not sure how you can get 0.94v on either end of Q6 yet have 2.6v on the driver bases that connect directly to it.

Are those negative voltages around Q5?

Try this. Q6 is off by itself with two wires running back to the main board, right? SHort those two wires together with a clip lead. That results in the base of Q11 shorted to the base of Q5. Does that make the amp cool off? Operating that way will result in crossover distortion, but all we want to know at this point is what is causing the excess current. We can worry about distortion later.

And if that does cool it off, then that further points to Q6.

[Lee_ranaldo]

Thanks Enzo..
My guess is one of the leads that goes to the board with Q6 was short. I was monitoring the colector voltage of the Q12 and Q9 transistors when i moved that wire and jumped from 22.8 volts to 38.5 volts dc.

Anyways the amp is working normally now.. no over heating!

it does not have DC offset on the output
the supply rails stays withing values (around +/- 38 volts dc)

Q5 Base Voltage is - 0,964 volts DC
Colector Voltage is - 38,3 volts DC

Q6 Emiter Voltage is - 0,969 Volts DC
Colector Voltage is 0,972 volts DC

Now just the heatsink gets hot after 1 hour of moderate use
i usually pump all the bass frecuencies up when i'm testing amps (it draws more current).

No problems !

Base voltage stays withing 0,93 - 0,96 vdc depending how hot is the heatsink.

2 hours running and no problems.. i've tested all the inputs so all seems to be ok. (passive, active , power amp in and CD-input..)

Cheers!

[miguelcornelio]

Hello brother Lee_renaldo, I have two problems with the bass amp Hartke A70: too hot sink amplifier, equalizer distorts. Please could you send me the complete diagram? my email is macspirocorp@hotmail.com. Many thanks in advance

[Lee_ranaldo]

Schematic sent to your email Miguel.

I remember there is a small PCB attached to the heatsick with a transistor. If you got problems there (coax wires not properly connected) the power transistor will draw as much current they can causing a loud background HUM. That also caused a WAY TOO HOT heatsink when i had that problem years ago.

Check the transistors and the coax wires are good and there are no problems.

If the ecualizer distorts is has to be a problem of one of the OP-AMPS for sure. OP-AMPS die in such a way they just distort

[miguelcornelio]

schemtic A70

Hi lee, although I have not received your mail with the schematic, I could send it again please?. Although not solve the probem, check the small PCB and apparently presents no problem, I hope the schematic help me especially in the equalizer. Again I leave my email: macspirocorp@hotmail.com