Hartke 3500 distortion in preamp

[nagruv5150]

First timer, so thanks in advance for your help. After searching the forum, I can't find any other post with info on the issues I'm finding, so here it is. I have been given a non-working hartke 3500 (non-ha3500 or 3500a) and trying to resurrect it. I was able to get it to produce sound after replacing the 12ax7 tube, but the sound was heavily distorted. i did find 3 leaking caps and have since replaced them, but still getting distortion, even if I use the effects send to another known good amp. Basically, I know my problem has to be in the preamp section, but the only schematic I can find is for the HA3500, which obviously has a few different parts. I suspect an IC is passing DC voltage. With no instrument connected, the tube pre all the way down, I can max the SS pre and it sounds like a harley's exhaust note, and the rate increases when I back off on the pot. This is independent of the tube pre position. I can only find a schematic for the HA3500 or 3500a, of which mine is not. Any help is greatly appreciated.

[Enzo]

OK, so it is distorted on tube or solid state both? Since you can turn either down with a knob.

The preamp runs on +/-15v I think. so are both those rails up to voltage and clean?

ANy of the op amps have substantial DC on an output pin? The amp has if I recall, compression and EQin/out. Turn off as many features as possible. ANy help?

Around the tube socket are a number of smallish film caps. They tend to sit up a little higher off the circuit board than teh other stuff. ANy of them have a broken off leg? Push on each to see.

CALL Samson in NY and ask for the drawings.

[oc disorder]

Hi nagruv5150 and welcome to the forum.
Well I'll try to give a little help while you are waiting...
As you have no doubt read on this forum cracked/broken solder joins are the first thing to look for.
As you have replaced some caps I guess you have already had the pre board out and checked for that.
From what I can recall I have seen two that had a faulty NJM2068DD.
What I do is replace the existing IC with a turned pin IC socket and then plug the replacement into that.
I guess as you can hear a Harley there is some instability in that area.
As its common to both SS and valve inputs if it is an IC it would have to be one early in the chain or at the end where they combine.
My hunch is at the beginning as the pot "interacts" with the fault.
Not sure which diagram is required there are a lot of variations so be prepared for just an approximation.

A sig gen and oscilloscope would be handy for you but perhaps you could start by checking around the IC for any higher voltages apart from the supply
going in at pin 4 (-15v)and pin 8 (+15v).

Check the supply too of course.

[nagruv5150]

Enzo,
It is distorted with either or both preamps engaged. I did turn the compression all the way down, but it does it whether comp is on or off. I do actually have +15v, but where I should get -15v, my meter shows +.789v. That could just be my stupidity as its the first time I've attempted something like this.

IC101 (2068DD)
pin 1: 2.222vDC
2: 2.222vDC
3: 1.532vDC
4. .789vDC
5. 3.5mvDC
6. 14.17vDC
7. 14.16vDC
8. 15.11vDC
Across pins 1 and 7, the meter doesn't stay in one place on AC, but I see 30vAC come up about 2x a second.

I did check on the caps around the valve socket and they are in good order. I also previously cleaned up any solder joints that looked back or had gunk in them. I called samson, but they had already closed today. I'll call and have them email the proper drawings in the morning.

oc disorder,
I had the same thought pattern as trying to see what the first op amp (IC107 5532DD) after the stages combine. IC103 looks to be in the SS section and pins 1 and 7 stay in the mV range in terms of DC output.

[Enzo]

NOTHING works right with bad power supply. There is little point in looking for anything else until you get your power supply restored. And I suspect the other issues will solve themselves when you do.

Pins 1 and 7 are outputs, so if you are trying to measure them while the noise signal is present, the meter will be trying to settle on the noise signal.

DO ALL the ICs have a missing -15v? or just 101? If all, then look for the -15v regulator in the power supply and find out what is happening there. Basic simple supply. If it is just one or two ICs, follow the traces to see where the supply rail comes from and find out where the circuit is interrupted.

The 5000/7000 is the same preamp board as far as I know. The extra crossover parts are stuffed for the stereo models, while the mono 3500 won't have that. And of course the stereo models has a larger power supply for the power amps.

[nagruv5150]

ITS FIXED!!! Enzo, you were right. I was missing the -15v output on all IC's, but it turned out to be a bad solder joint at IC402. I traced it through the entire board and got a connection everywhere I should have....Then it dawned on me to check the joint from pin 3 (-15v output) to the board. The pin was cut too short, so I desoldered the whole IC, moved it down, then resoldered and it worked! Thanks so much for the help!!

[Stoffie1974]

Hartke 3500

Hi there,
I also have a HARTKE 7000 (it's the dual power amp version) and i recommend you to resolder ALL the boards !! (and yes also the power amp PCBs)
I had several on my bench and they ALL suffer from this.
Yes i know, it takes time, but it's worth it, it took me 4 hours !!
Those "made in china" solders are VERY bad and when gigging around ... they WILL fail !!
Just try to bend a component 3 times while looking at the solder on the other side ... it breaks !
If it's not this pin today, it will be another next day/month/year, and it mostly happens while your on stage.
And you may hope that it's not one of the closed loop solders on the power amp board, or you'll be replacing several power transistors & drivers. $$$
Don't take my advice the wrong way, those HARTKE amps are very good, but they ALL need "retouching".

[Ronsonic]

That preamp board is famous for bad solders and the regulator ICs are the worst offenders as you discovered. Reflow everything on that board, then grab a tube of RTV silicone and use it to stick the caps down and build a small bridge between the regulator ICs and the caps behind them. Doesn't take much but a bit helps tremendously in making it so you don't have to take that thing apart again.

Note that the thing becomes a noisy mess if that bracket the pots are mounted to is loose, so snug the pots down and make sure it's well mounted.

[pacAir]

Hartke 5000 Distortion on peaks...

Just as a note: I recently repaired a Hartke 5000 whose symptom was that it sounded OK if the bass was played smoothly and lightly with a thumb but distorted when plucked, slapped or played vigorously with a pick!

After doing all the normal power supply checks (OK) and signal tracing (mostly OK) I came across a buffer stage with an inordinate amount of DC offset on its input. When traced it appeared that the DC bias was being injected into the circuit but I could not easily identify the source.

I started to isolate sections from the preamp and when I disconnected the DC supply that is fed to the EQ slide-pots (to light up the slider knob LEDs) the problem disappeared! The slide-pot assembly itself had developed an internal short from the LED + supply to the audio buss so all downstream electronics were being DC biased which was causing a directional headroom problem.

The owner didn't want to spend $40 for a new slide-pot assembly so we disconnected the DC wire from the connector and labeled it so it could be restored in the future. Problem averted at the cost of no LEDs on the graphic equalizer knobs.

A weird problem but it could happen to someone else too and it shows that not all Hartke problems are due to bad solder jobs. Sometimes it's just a defective part.

The HA5000 is an awesome sounding reliable amp otherwise!


Steve