Hartke LH500 - Fuse blows - Scary Voltages

[billbockay]

Hi everybody,

I'm currently trying to fix a Hartke LH500 for a friend.
The fuse blows right after the amp is powered on.
Below is the schematic of the PA section.



I've been searching accross the forum, and found this thread which is about the same failure : http://music-electronics-forum.com/t30702/

I have disconnected the power amp circuit and the amp correctly powers on.

However, the power supply voltages I read from the connector are hair-rising :

+90VDC > -164VDC
-90VDC > -336VDC

Checked the power supply section, rectifier and caps are okay.
Do you have any idea?

Thank you so much.

Bill

[Jazz P Bass]

Please indicate where you tested these 'hair raising' voltages.

I have attached the power supply schematic.

LH1000 v3 Power Supply.pdf

[km6xz]

Without any load on the high current/power supply, high voltages can developed as the capacitors keep building a charge. The red notes on the schematic were measured while the power amp and supply were connected? None of the voltages recorded show a radically wrong voltage, just 2 volts output offset. I am confused, does the fuse blow with the speaker disconnected or just with the speaker load in place?
Don't test the amp with any load until it tests as stable.
If those voltages are without a load, check the offset adjustment potentiometer at the base of the first diff amp transistor. Being open or misadjusted could account for a 2 volt offset.
Is the speaker protection relay clicking in normally?
What test instruments do you have to do these test? Scope, DMM, signal generator,??

[g1]

The voltages you reported in your post as "hair raising" are not the same as the ones you show that you measured on the schematic.
As Jazz stated, you need to describe where you measured the -164V and -336V, and also where you had your black probe referenced to.

[Enzo]

On your schematic you show those 90v rails as 80-something volts, which looks right on to me.

Your were blowing fuses until you disconnected the power amp, so the power amp is what is blowing fuses. Start by checking all the output transistors for shorts.

[The Dude]

Something looks wrong at Q306. Is it shorted?

BTW, welcome to the place!

[billbockay]

The voltages you reported in your post as "hair raising" are not the same as the ones you show that you measured on the schematic.
As Jazz stated, you need to describe where you measured the -164V and -336V, and also where you had your black probe referenced to.

Actually, the values added on the PA schematics are not the one I measured (I picked up this schematic from another thread).

On your schematic you show those 90v rails as 80-something volts, which looks right on to me.

Your were blowing fuses until you disconnected the power amp, so the power amp is what is blowing fuses. Start by checking all the output transistors for shorts.

Yes I think so, I have checked the output transistors using a DMM and it seems that all of the 8 transistors are burnt. Short between B-C and C-E junctions...

Something looks wrong at Q306. Is it shorted?

BTW, welcome to the place!

I don't get the role of Q306. Is it supposed to behave as a phase-shifter, driving the push-pull section?
Thanks, Mr. Lebowsky ;-)

--

BTW, sorry for my poor english, I am from france

Thanks again for your replies,
Bill

[52 Bill]

Actually, the values added on the PA schematics are not the one I measured (I picked up this schematic from another thread).

So we need to disregard all of the red notes on the schematic.

...I have checked the output transistors using a DMM and it seems that all of the 8 transistors are burnt. Short between B-C and C-E junctions...

You have tested them in circuit? If you have, then you should test out of circuit. The outputs are connected in parallel, so if one is shorted, they all will test as shorted.

[The Dude]

Actually, the values added on the PA schematics are not the one I measured (I picked up this schematic from another thread).

Good to know! I had assumed the voltages were measured by you. There I go making assumptions again.

Ignore what I said about Q306 then. Do as 52 Bill suggested and test the outputs out of circuit. Also check all of your emitter resistors (.22 5W).

You may also want to build a light bulb limiter right away so that you don't take out new outputs if there are other circuit problems. Search "light bulb limiter" on the forums for instructions and info.

[km6xz]

I asked about the red noted measurements because if those were right, a bit less obvious problem was blowing the fuse but if those were not your measurements, the most obvious cause would be 2 blown transistors, maybe more but surely at least two, one on the positive rail and one on the negative rail. If it is hard removing all the power transistors to test them just unsolder one leg of the emitter resistors and measure emitter to collector on each transistor. Some of those emitter resistors are likely burned open which is a good clue that the transistor it is connected to is shorted.
Remember to keep any load disconnected until the amp is repaired and deemed stable.