Hartke Systems Model 2000 Bass Amplifier - protection circuit

[xtian]

Signal arrives at the power board but does not exit. It has a protection circuit made of IC301 (7317) and relay RY301 (I marked these in yellow on the schematic attached). I know these relays go bad--I replaced one in another bass head some time ago. I need help with a testing approach. How should I test IC301 to determine good/bad? And how to test the relay?

Confused by possible error in schematic (see area in red)--seems like there should be a dot there to indicate a connection...otherwise output jacks aren't connected to anything! However, in probing, I measure a direct connection between what is shown as the ground end of R336 and the right-hand end of inductor/resistor R334.

[52 Bill]

How should I test IC301 to determine good/bad? And how to test the relay?

The first thing to test on the IC are the voltages at the power supply, voltage on the input and output pins. The relay has a coil that should show a resistance. There is an anti-spike diode wired in parallel to it that will change the readings, but it should still be readable with your meter.

Confused by possible error in schematic (see area in red)--seems like there should be a dot there to indicate a connection...otherwise output jacks aren't connected to anything!

Agree, the red dot area lines should be connected.

However, in probing, I measure a direct connection between what is shown as the ground end of R336 and the right-hand end of inductor/resistor R334.

This would show a direct short of the power amp output to ground. What reading do you get on the other side of R334?

[Enzo]

So does the power amp work? We need to find out if maybe the whole relay thing is just doing its job.

Yes, there is a dot at the red spot. So with signal applied to the power amp, is there a large output signal at the red dot? Any DC offset there will cause the relay to remain off.

Relays can fail, ANY part in ANY amp can fail. But relay failure is not really common. And that 7317 IC is also pretty reliable. In my experience, the problems with that circuit - assuming the amp itself works - are usually caused by bad small electrolytics there. Especially if they are near something warm. 1uf C318 is a small negative supply for referencing the IC. If that cap fails, we are left with a pulsing DC which will leave the relay triggered off. 470uf C318 is a timing cap. It prevents the IC from tripping on peaks, the problem needs to persist long enough for that cap to charge up through the 820k resistor. If the cap is bad, there is no wait. I don't remember what C319 is for, but at 0.47uf, it also seemed susceptible. I don't usually see the other two caps causing trouble.

[xtian]

Thanks, guys. Enzo, your theory looks good. I am seeing a strong signal at the red junction I marked (using my o-scope), but with a lot of chatter. With no signal input applied, I read in the neighborhood of half a volt DC, but it's a moving target.

The 50v power rails are also a moving target. They vary from 38 to 45 volts, slowly, with about a 30 second cycle.

I have not lifted the board yet, so I'm having to hunt for places to measure the pin voltages of the 7317. I can't say my results are reliable, but here's what I found:

pin / voltage
1 -0.71
2 -0.01
3 0.18
4 0
5 -0.76
6 45.0
7 1.50
8 0.03
9 2.7

Next steps?

(Thanks, guys...I'm a tube amp vet, but a solid state amateur.)

[xtian]

Different perspective: these amps sell for under $100 on eBay! Would be stupid to spend two hours of my time researching and repairing. Would it make any sense to bypass the protection circuit?

[The Dude]

Only if you don't mind buying speakers, too!

[J M Fahey]

Different perspective: these amps sell for under $100 on eBay! Would be stupid to spend two hours of my time researching and repairing. Would it make any sense to bypass the protection circuit?

Since you mentioned that, bypassing the protection circuit would be the stupid action of the day.

I find the 30 second rail dance from 38 to 45V weird, in principle can't even imagine what might cause that, find a measurement error much more possible.

Remeasure with a multimeter (not the scope) :
1) across C321
2) across C322 (multimeter probes straight on the capacitor legs)
3) end to end, from nominal +50 to nominal -50V

Also measure voltage from union C321/C322 to amp chassis.
I suspect you have bad grounding somewhere.

If you confirm grounding is right, check for DC presence at the output rail ,the protection IC (as Enzo noticed) may simply be doing its job.

Statistics show that bad power transistors are many times more possible than bad relays.

[52 Bill]

The supply voltage to the IC are off, pins 9 and 5. The negative voltage is missing and the positive seems low.

[J M Fahey]

Agree in principle, will try to look at other IC pins:

pin / voltage
1 -0.71
2 -0.01
3 0.18
4 0
5 -0.76
6 45.0
7 1.50
8 0.03
9 2.7

1) Pin 6 drives directly the relay, whose "other end" goes to +50V rail (please confirm actual voltage there).
Since pin 6 lies at +45V , the relay coil is getting 5V (or less).
If it's 12 or 24V (please confirm) it won't turn on or enable the speaker out jacks.
So that seems to rule out a bad relay.

2) at first I was also surprised at the very low supply voltage, only +2.7V at pin 9 .... but reading the TA7317 datasheet I saw that :
a) it's not much of "an IC" but nothing more than an an array of a few transistors, which work either open or saturated so for that simple task it's enough.
2) it has built in voltage regulation, 4 series diodes in parallel with the rest of the IC so, yup, 4 diode drops account for 2.7V .... amazing.
So that also looks normal or reasonable.

3) pin 2 is a "DC trigger or detector" and will trigger the protection with >(+/-) 650 or 700 mV (one diode drop) DC present.
Since you have -0.01V there, NOW we know you don't have DC on the main speaker out.
In fact value measured at Pin 2 must be multiplied by 820k/27k= 30X meaning it will trigger with actual +/-20V DC !!!!!!!
Scary
Not *that* tight a protection, if you ask me.
I wouldn't allow more than 5 or 6V DC at the speaker terminals, at any time
So DC is not triggering the protection.

4) pin 3 which is the other DC trigger/detector input is being used here, coupled to a thermistor, as a thermal protector .
Since it shows only +0.18V , below the 650mV trigger level, it is not shutting the output down .

5) so far I see nothing triggering the protection
Let's continue checking pin voltages.

6) pin 1 is "Power switch on/off detector" , goes to +2.7V through 47K and to rectified -50V through 100K
Should be negative but just -0.71V seems very low.
Please check voltage at D308 cathode or C318 - leg, you should have some -50V there.
****EDIT: please re-confirm that you have minus 0.71V on pin 1 .
If you have + 0.71V there, the negative reference is missing, the transistor is saturated and it never comes off the initial 4 seconds speaker mute..

OK, still too sleepy, will continue later.

So far don't think the relay is bad , and we might either have a bad TA7316 *or* it gets confusing data, we'll see.

[xtian]

Apologies for pulling the rug, gentlemen. I gave a two hour estimate to my client, and, because there is a used unit on GC for $179 and recent sales on eBay of under $100, he decided not to repair this one, and took it home.

Really appreciate your help. Come visit us at ampgarage.com for your tube amp questions!