Hartke LH500 DC voltage on the output lowering down...

[prulup]

Hi everybody,

can anyone advise me with this, please...?

I have LH500 where the relay doesn't switch on because of the 3V DC on the R334 (speaker output). The strange thing is that the DC is going slowly down to cca 1,8V where it ends in about 15 minutes. I checked these:

AC = 2 x 64,1V, DC = +-87,4V, voltage across R330 /and similar/ is less than 1mV, adjusting SVR301 lowers the output DC voltage but there are still about 2V DC. R343 and C316 have the new correct values = 82K and 470u 25V. No AC or DC voltage on R350. Both fans are running. PWR transistors are cold. Capacitors seems OK. Soldering checked. I don't know....

Q304 and Q306 are hot. Q305 and Q307 are very hot.

Voltages (C-B-E) after 15 minutes:

Q304:
40,5V
86,4V
87V

Q305:
3V
86,4V
87V

Q306
-85,8V
-85,8V
-86,3V

there is cca 120V DC across R317 !

Q307
0,8V
-85,8V
-86,3V

Q308
3V
1,4V
0,8V

Q309
87V
3V
2,4V

Q310
-87V
0,8V
1,4V

THANKS!

[Jazz P Bass]

Can you post the schematic?

[Jazz P Bass]

Got it.

[Jazz P Bass]

Looking at the voltages it seems to me that Q310 is not working.
And possibly Q307 & Q306.

R317 should be like a balance point between the + & - supplies.
So with a full + voltage there, it appears that Q306 is not conducting.

[52 Bill]

I had one of those amps that had about 3 vdc on the output at start up and wouldn't come out of protect mode. The offset control couldn't swing it far enough to zero it out. Turned out to be the two differential input transistors. Once they were changed the control would center the output at zero volts.

Tested the old transistors and the hfe was down around 8-10 on both. Both of the new ones were around 325.

[prulup]

tHANKS, i ORDERED THE TRANSISTORS AND i WILL LET YOU KNOW!

[prulup]

Hi everybody, bad news. All the Q304, Q305... ...Q310 transistors works! I really don't know now. I checked also the diodes, they work.

[prulup]

HEY, I started to unsolder the input transistors and I found Q067 defective! I will post the results in a few hours

[prulup]

It is even weirder. The voltages on input transistors Q067 and Q301 changes on base and emiter. The base of Q067 (amplifier output) could be set to zero volts with the SWR301, but I have zero volts across 0.22R resistors. And SWR302 doesn't change it, it does nothing (but it is OK, I measured the resistance), it raises the B voltage of 1943 and 5200, but slightly.

I have signal on the R334 (oscilloscope) and there is usually zero volts DC on the output but there is only 3,8V on the relay diode (D310), so it doesn't switch on.

Voltages:

Q067:
B -0,1 (varies)
E 84,5
C -0,7 (varies)

Q301 (same as Q067)

Q308
B - 0,4
C 1
E -1

2SA 1943
B -0,43
C -87
E 0

[prulup]

...no one? :-(

[52 Bill]

The base of Q067 (amplifier output) could be set to zero volts with the SWR301, but I have zero volts across 0.22R resistors.

Use SVR301 to set the voltage on the speaker output to zero volts, not at the base of Q067. They are not the same thing.

[Jazz P Bass]

Agree, SVR301 is the dc offset adjust.
SVR302 is the bias adjust.

At any rate there is still something wrong with the amp.

To achive a balanced (zero volts) output, every part must do it's job.

I would start with a power off, power supply drained check of all of the transistors.

Any of the transistors before the output stage that are not operating properly can throw off the whole balancing act.