Hartke 3500 Mosfet

[McBarry]

Hi, I'm trying to trace a hum in the above amp.
It's permanently on once the relay kicks in, and is not altered by the master vol.
The preamp works fine, ie: all controls seem to do what they're supposed to.

My main problem is I can't find a schematic that relates to THIS amp.
There's a zillion for the main board of 4005182800 ( the HA-3500), but this board is 4005106900A, and the amp is only called Hartke Model 3500 MOSFET.

As such, none of the component part numbers match.
Would anyone have a schematic for this board/amp??

I'll email Samson US and the Australian importer, though the latter are often not very helpful..
Thx, Dave

[Jazz P Bass]

Have you checked the power rails?
The B+ & - should be relatively ripple free (Vac).
The opamp power rails should not have any ripple.

[McBarry]

Hmm, (all below via DMM - waiting new CRO lead...)
B+ steady 93vdc, B-steady -93vdc. B+ has 12.9mvac measured to chassis, B- has 11.1mvac.
LV rails steady 18.3vdc & -19.4vcd (7818 and 7819 regs), but there's ac at vs+ and vs- to m/volume op-amp (an RC5532)
Pin 8 (vs+) has 17.9vdc and 13.3mvac, pin 4 (vs-) gets -19.2vcd and 0.9vac.

The speaker output has -32mvdc and 75mvdc on it, which I'm guessing will account for the awful hummmmm.

[jrfrond]

You'll need to get in touch with Samson Technologies for that schematic. The MOSFET version was dumped around '98 due to reliability issues, in favor of a bipolar transistor design. The MOSFET versions would literally go up in flames, and you cannot get a MOSFET replacement PCB, only a bipolar retrofit. Therefore that is OLD documentation.

Offhand, it sounds like a bad filter cap or rectifier diode causing ripple on the power amp rails. If you are experienced, this is something you can trace back without a schematic.

The offset you describe does NOT sound like an issue, but COULD be indicative of one. Not enough info to go on here.

[McBarry]

Had a quick look at the LV regs..
7818 has 0.001vac at pin 3, but the 7918 has 0.110vac on pin3.
Am I right in thinking the 7918 needs to be replaced?

[Enzo]

Maybe, what does the input side of the 7918 look like? If the raw voltage supplied to it lacks filtration, the regulator can't clean it up on its own. Compare the 7918 input pin supply to the one for the 7818. Same except for polarity?

At one point you said there was -19v on an IC with 0.9vAC on that. Then later you were down to 0.1vAC on the -18. Which is it? Almost a whole volt of ripple on the 18v rails will for sure make hum.

Do we not have a scope?

[McBarry]

Enzo, I re-checked with my Fluke (alas, the CRO is still sans lead/probe). Earlier were with my Chinese DMM, but I trust the Fluke a lot more..
B+ = 89.7vcd with 0.03vac
B- = 90.7vdc with 0.02vac
7818: 18.38vdc. 0.3vac on input pin1, 0.001vac on output pin3
7918: -19.2vdc. 0.27vac on input pin2, 0.101vac on putput pin3
Based on above I'm wondering about/suspecting C180, if the regs don't limit any ac.
IC113a (master vol op/amp) gets 0.050vac to pin4.

[MarkusBass]

The MOSFET version was dumped around '98 due to reliability issues, in favor of a bipolar transistor design. The MOSFET versions would literally go up in flames, and you cannot get a MOSFET replacement PCB, only a bipolar retrofit.

Another reason why it was dumped can be the fact that it used lateral MOSFETs (most probably 2SK135/2SJ... from Hitachi) which are not manufactured any more and replacements are expensive. Also DBX integrated circuits are not manufacured any more (at least to my knowledge).
The power amp seems to be typical. Maybe they didn't use flameproof resistors in the output section and this was the reason for flames.

Mark

[Gtr_tech]

I wrapped up on a 7000 mosfet yesterday. One output section was bad, and it was a rather polite failure. Several FETs on both rails were shorted, but the drivers survived. No evidence of flames, smoke, etc. Replaced all the FETs and .47/5w Rs, set the idle current, and it took right off.....

FWIW, the customer said they were running a Moog thru it when it failed. I suspect they were gettin' loud with it and one good filter sweep into the subsonic range freaked it out. I had to explain to him that analog synths can and will chuck out anything you tell them to....well into the subsonic range....and pwr amps draw mega current when asked to do this.

[Ronsonic]

Enzo, I re-checked with my Fluke (alas, the CRO is still sans lead/probe). Earlier were with my Chinese DMM, but I trust the Fluke a lot more..
B+ = 89.7vcd with 0.03vac
B- = 90.7vdc with 0.02vac
7818: 18.38vdc. 0.3vac on input pin1, 0.001vac on output pin3
7918: -19.2vdc. 0.27vac on input pin2, 0.101vac on putput pin3
Based on above I'm wondering about/suspecting C180, if the regs don't limit any ac.
IC113a (master vol op/amp) gets 0.050vac to pin4.

An 18V regulator with 18V on the input is a problem. Check the cap and rectifiers that supply that.

My first guess on your problem is something in the front end. Those amps usually need resoldering on the preamp board. The pots MUST be secured tightly to that bar going across and the regulators have a bad habit of breaking their legs at the board. I'd pursue the LV supply and those things first.

[Enzo]

And that long row of connector pins along the back edge of the preamp board - mostly transformer connections to the board. I often find those male pins need solder.

[McBarry]

Sorted.
replaced the 7918, resulting neg rail 0.001ac, and amp quiet as a mouse.
Thx to all for input, appreciated!
DB

[dmeek]

Link is dead. Does anyone have the 3500 Mosfet schematic to re-post?

Thanks
Doug

[nickb]

Link is dead. Does anyone have the 3500 Mosfet schematic to re-post?

Thanks
Doug

HA3500 Power Amp

[dmeek]

Thanks nickb but that is the transistor version. I just found a copy of the mosfet version over here:

http://music-electronics-forum.com/t34880/