Hartke 3500 power amp module - any replacements?

[bagghist]

This Hartke 3500 power amp module keeping me awake nights, and not in a good way. First of all, I highly-appreciated earlier posts covering this, especially "Hartke Schematics; any solid state guys out there?" (Sept 2006). Like that poster, I never worked on solid state before either, just lots of tube DIY.

Got the amp (non-working) for free. Replaced fuse, unplugged power amp, and the preamp-out worked fine through another bass power amp. So I snagged a schematic (well, something close enough; attached). On the power amp board, I located several bad large transistors (Q310, Q316, Q317, & Q319) plus one little guy (Q311). All nearby resistors were ok, as were other components in the vicinity. Got replacements from MCM, and installed.

When I powered up (no variac available), something immediately smoked on the board (roundabout the Q311 neighborhood, but I was too busy cutting the power to pinpoint it). Couldn't figure out what was wrong, and decided in frustration to re-power up, and look for smoke (I know, stoooo-pid). And guess what happened? Well, the fuse did what fuses are supposed to do.

I'm ready to give up on this board, it had so many bad transistors that I probably should've replaced them all. Plus, I don't think I got to the root of the problem. Seems like I'm just chasing my tail or something. When I could be out there chasing someone else's tail.

Any suggestions and ideas would be appreciated.

*** Someone in that old thread (oc_disorder and maybe enzo too?) mentioned a replacement module. Does anyone know where to get these, and how much they go for? If they're exorbitant or unavailable... well at least I got a decent preamp for free.

Thanks in advance,
Bagghist

[SWEINC]

Replacement power amplifier module hartke 3500

Call Samson Technologies 800-372-6766. Samson owns Hartke. The module part # is 303001 and the price is $59.40.

[bagghist]

Re: Replacement power amplifier module hartke 3500

Ok, perfect. Thanks. It's a nice price too.

[olddawg]

OK...I don't know about replacement modules for this unit. I do know that they are usually more expensive than the amp is worth in most cases unless the amp is under warranty or a service contract and even then the manufactures usually save them for burned board replacements for authorized shops. This amp's problem shouldn't be that complex if the offset detection circuit isn't damaged that drives the speaker protection relay. Have you checked all 8 of those output transistors out of circuit? Personally I would loath to work on a big amp like this without a variac. It can get expensive blowing finals. If the board isn't charcoal you should be able to get it going. I would start by metering out every transistor and diode on it and every low value resistor, replace what is bad, then buy, beg borrow or steal a variac to run it up carefully. With the a protection relay you will have to monitor dc offset at the relay. It will not close until the power supply is correct.

[bagghist]

re: OK...I don't know about replacement modules

Thanks for the ideas, olddawg. I don't know, if replacement is $60, it sounds pretty ok to me. But it may feel different in the morning.

Re: checking all 8 transformers out-of-circuit: Yep, the first time around. i didn't recheck them after replacing the bad ones and getting the smoke signals. Instead, I sought wiser counsel.

Re: checking every component: No problem, unless each thing needs to be unsoldered first. Seems like resistors often check ok while still in-circuit. And some of those other parts are pretty damned small, I'm not sure I could work with them (if they require unsoldering) without spending days on this. (Then again, I HAVE spent days on it. That's part of the fun...)

I'm not very experienced at working with relays and circuit protection stuff, but that's good to know anyways.

As for the Variac, you have some variac good advice (heh!). In fact, I CAN probably borrow one - - and next time I will. Although for $8 I snagged an old Heathkit isolation transformer. I added a grounded outlet and power cable to it and it works well. It's not as flexible as a regular Variac, but it does go as low as 90 V, and its fused (2 or 3A I think). Is that low enough, or do I need even lower voltages such as offered by the Variac?

[olddawg]

Wow....I've never seen a large power module that cheap. I wouldn't blink at that..done. In my experince modules usually have part numbers but are unavailable if you try to order them. But I was never authorized for Hartke. If the power supply is good and the preamp works it's a no brainer.

[Enzo]

90v doesn't do it. The point of a variac is you can advance the mains voltage from zero, watching teh current. if the current starts to ramp up at 10 or 20v on hte mains, you back off now. Applying 90v is not much different from just flipping the thing on.

MAke a "light bulb limiter" and use it.

You can check every transistor on the module - I do - but when they burn up, they usually burn up resistors as well. For example that 150 - sometimes 180 - ohm one between the emitters of the drivers.

of course check all those 0.47 ohm 5w resistors, but also make sure the 4.7 ohm Zoble resistor is OK across the output.

Is the bias transistor OK and the diodes and resistors and trim pot around it?

[olddawg]

90v doesn't do it. The point of a variac is you can advance the mains voltage from zero, watching teh current. if the current starts to ramp up at 10 or 20v on hte mains, you back off now. Applying 90v is not much different from just flipping the thing on.

MAke a "light bulb limiter" and use it.

You can check every transistor on the module - I do - but when they burn up, they usually burn up resistors as well. For example that 150 - sometimes 180 - ohm one between the emitters of the drivers.

of course check all those 0.47 ohm 5w resistors, but also make sure the 4.7 ohm Zoble resistor is OK across the output.

Is the bias transistor OK and the diodes and resistors and trim pot around it?

Even a lightbulb limiter with a suicide cord is risky IMHE. +1 on the bias transistor and the trem pot. I used to always replace them as a matter of course with blown outputs on most large discrete amps. I got to many call backs if I didn't. Also cheap bias pots have killed many an amp. Make sure you do the bias adjustment.

[Enzo]

I rarely use the bulb myself, then again my variac sits right by my elbow. Assuming he won;t be running o0ut and getting one, the bulb is better than nothing.

If the outputs burnt up, it is a good idea to replace the drivers even if they test OK. And the bias circuit connects directly to them.

Most times in these basic bipolar power stages, you can just short dead across the bias transistor. That straps the driver bases together. This then is an extra cold bias "setting" and results in some crossover distortion, but it eliminates certain questions while hunting down larger fish.

Samson does sell the Hartke power module boards pretty reasonably - don;t recall now, but seems to me they came without the rows of outputs - just got the stuffed card. I think I even have a couple here in stock, I should dig one out and look.

Screwing around for an hour or two, versus a $60 board? Works for me.

[Alex R]

Yep they don't have the outputs, just rows of solder notches for their legs along the edges of the board.

That is a good price - the UK distributor would only sell me two and charged me £150. So I too have one in stock!

[olddawg]

I rarely use the bulb myself, then again my variac sits right by my elbow. Assuming he won;t be running o0ut and getting one, the bulb is better than nothing.
If the outputs burnt up, it is a good idea to replace the drivers even if they test OK. And the bias circuit connects directly to them.
Most times in these basic bipolar power stages, you can just short dead across the bias transistor. That straps the driver bases together. This then is an extra cold bias "setting" and results in some crossover distortion, but it eliminates certain questions while hunting down larger fish.
Samson does sell the Hartke power module boards pretty reasonably - don;t recall now, but seems to me they came without the rows of outputs - just got the stuffed card. I think I even have a couple here in stock, I should dig one out and look.
Screwing around for an hour or two, versus a $60 board? Works for me.

I haven't been on the bench except in my garage occassionally for about seven years, so some of what I say must be dated. I used to "take over" a lot of problem units from other techs. When they got stuck they invariably always tried to order the board. Many times I would get the unit with a parts box on top of it with a board in it with no parts at all on it, a naked board. Sony was famous for this. Anyway my bench variac had a current and volt meter as well as the ability to do AC leakage tests. The AC leakage test as a big deal in CA and required by law after every repair. It also gave the shop the opportunity to slap on an additional fee after the estimate along with a cleaners and solvents charge. When you repair thousands of units a year it adds up. The only techs I saw using the light bulb limiter setup were old TV techs working on a HT problem. But it is a useful tool if you have nothing else. Unfortunately, it's hard to run faster than an electron.

[bagghist]

Samson does sell the Hartke power module boards pretty reasonably - don;t recall now, but seems to me they came without the rows of outputs - just got the stuffed card. I think I even have a couple here in stock, I should dig one out and look.

Screwing around for an hour or two, versus a $60 board? Works for me.

I called Samson and they said it would be $90. They said that this is a populated board including all the output transistors (or so they say), and that all you need is your own heat sink.

Well, every man has his price. And... somehow when the cost went from $60 up to $90 (plus shipping), I decided to sleuth the problem out myself. And definitely borrow a buddy's Variac. <-- Thanks for that excellent advice, Enzo and olddawg.

And hey, the worst thing that could happen? Aside from, you know... death by electrocution. I'll learn a few solid-state basics. I may spend ninety bucks. Pretty good, considering that this amp was left for dead (free; used ones go for $250+). Then maybe I'll sell it. Or just enjoy the goodenough (for me) standalone hybrid preamp. Or turn it into a unique yet striking centerpiece / warming tray suitable for special occasions.

[olddawg]

I called Samson and they said it would be $90. They said that this is a populated board including all the output transistors (or so they say), and that all you need is your own heat sink.

Well, every man has his price. And... somehow when the cost went from $60 up to $90 (plus shipping), I decided to sleuth the problem out myself. And definitely borrow a buddy's Variac. <-- Thanks for that excellent advice, Enzo and olddawg.

And hey, the worst thing that could happen? Aside from, you know... death by electrocution. I'll learn a few solid-state basics. I may spend ninety bucks. Pretty good, considering that this amp was left for dead (free; used ones go for $250+). Then maybe I'll sell it. Or just enjoy the goodenough (for me) standalone hybrid preamp. Or turn it into a unique yet striking centerpiece / warming tray suitable for special occasions.

This really is not that complex of an amp. It's not like it's a MOSFET output with a switching power supply or something. It can get rather frustrating and expensive if you keep blowing output transistors. As long as the board isn't charcoal it should be resolvable. I would run it up with OTs completely removed. Take your time. Check everything. It doesn't have to be done in a day. BTW, has anyone worked on it before you? If so, make sure the resistor values are correct and that the caps, diodes, and transistors are in the correct orientation. Don't trust the board print (well trust but verify) if it has any. Use the schematic. Use the variac patiently.

[jrfrond]

If you want to treat it as a learning experience, that's cool. But I will tell you that DC-coupled amps are way different animals than AC-coupled tube circuits, and if you are unfamiliar with how to work on them, you can blow through more than $90 worth of components that may get toasted over and over again. If that amp were in my shop, I'd buy the module (and have). But then again, after 30+ years at this, I don't need to prove that I CAN fix it, I just need to expedite it with minimal wasted time. The ground rules change at the pro level.

[olddawg]

If you want to treat it as a learning experience, that's cool. But I will tell you that DC-coupled amps are way different animals than AC-coupled tube circuits, and if you are unfamiliar with how to work on them, you can blow through more than $90 worth of components that may get toasted over and over again. If that amp were in my shop, I'd buy the module (and have). But then again, after 30+ years at this, I don't need to prove that I CAN fix it, I just need to expedite it with minimal wasted time. The ground rules change at the pro level.

That's interesting. I almost never worked on tube stuff until relatively recently. This amp is pretty straight foreward except for the protection relay circuit. I always thought SS stuff was easier. I guess it all depends on your background. They had stopped teaching tube theory when I got my electronics degree. I cut my teeth on big discrete SS amps.