2SB755 Transistor Data?

[bob p]

SO what is actually wrong with this amp? The original pst alluded to it was in protect, but if that is not the case, what's up?

2.0V, 1 KHz sine wave in, nothing out. Both channels. Meter lights out.

[bob p]

Ohm meter between one of the 0,22 ohm and the speaker hot terminal on the rear panel. If the relay is on, then there will be continuity.
...
If the relay clicks on at power up, then a lot of the amp must be working. Verify the relay is staying on like it should. That means two things, first check the contacts as I described above, using continuity from the speaker terminals to the output bus at the ballasts. Obviously this must be done with the amp running or the relay can't be ON.

With the amp powered on, here are the Z measurements between the ballast resistors and the speaker terminals:

L+ 8k
L- 0
R+ 0
R- 8k

If no continuity there, then either the relay is not energized or its contacts inside have failed or the armature is bound up. Here is a tip: Relays can be hard to get at the terminals on a pc board, but ther will always be a diode across the coil - D203 - and nearby. SO I just measyre voltage across the diode. It is wired in reverse, so if there is no voltage acros it the relay is not energized. and if there is voltage acros it, then it IS. I would guess it is a 24v relay?

Yes, its a 24VDC relay. The potential across D203 is 8.3 VDC.

No output? Both channels? Then what do they share other than power. There is IC101. APply signal. Is it there on pins 1 and 7 - the ICs output pins? Is ther power to the chip, pins 4 and 8, I'd guess +/-15V?

Now things get interesting:

[B]IC101 pin measurements:[/B]
PIN   Idle      2V Sine
1      0          0.08 VAC
2      0          2.0  VAC
3      0          2.0  VAC
4    -14 VDC     -14   VDC
5      0           0
6      0           0
7      0           0
8    +14 VDC     +14  VDC

It looks like we have two problems: a) no output on the left channel at Pin 1, and no input at the right channel at Pin 6. Pins 5 & 6 show continuity. In addition, the Right channel input signal is present on the input side of R102 but not on the output side of R102. This suggests that the Right channel signal is being passed to ground somewhere after R102 -- either at IC 101 or perhaps at C104. The Left channel signal goes into IC 101 but doesn't come out.

Maybe signal tracing isn't such a bad place to start after all.

I can't wrap my head around your numbers for the Q2xx stuff at the moment, I'm tired. But the circuit, while oddly drawn is really pretty strait forward. Here is a way to look at the stuff. A transistor can be thought of as a valve, with the main current flow from E to C. The more you pull the base towards the collector voltage wise, the more it turns on, - the harder it will conduct.
...
Q207 and friends are never halfway on. You don't want the relay sitting there with less than enoght cirrent to pull in. All that would do would be heat it up.

I re-checked the measurements at Q201-Q207:

All readings are in VDC
Trans    E       C       B
Q201   43.4    -0.59   43.4
Q202    0.2mV   4.6     6.1mV
Q203    4.6     0.2mV   3.0
Q204    6.2mV   3.0     0.2mV
Q205    4.6    43.5     4.6
Q206    0.1mV   4.6    -0.58
Q207    3.72   35.0     4.29

Notice that the values of Q202-C and Q203-E have changed from 12.4V to 4.6V. Note that the voltage on the E and C of Q205 has dropped also.

[bob p]

is the NJM072DE just another name for a TL072 ?

[Enzo]

Yes, TL072, plain old vanilla.

It does sound like your probe slip might have burnt something. When the 3 second pause and then click was happening, that was normal. Now the relay won't come in, probably because a channel is now blown. Recheck for DC on the output busses at the ballasts. If those outputs are still sitting near zero, then the relay circuit itself was damaged.

One thing to know with op amps is that it is often difficult to measure a signal at the input pins. Look up "virtual ground" with respect to op amps. In most cases when I find I can see signal at an input pin, it is because the chip is bad. The signal will appear at the other end of an input resistor though. SO if it is there, then it should bo on the output pin too. If not, I suspect the IC. SO in my head, actually, the side with the 2v at the input and zero out is the suspicious one, but I'd have a new IC in there just to get past this step. Parts are cheaper than labor.

I think we chased a wild goose for a while. The relay was working all along, and your 40v on all the output collectors was OK. Make sure they are still OK - rail on the collector, and more or less zero on the emitters.

Oh wait, you were probing teh 200 xstrs. OK, do drop checks on them and see who croaked. SInce their whole job is controlling the speaker relay, a dead one will screw things up.

Meanwwhile, even with the relay off, the amp channel will still show their output on the output bus, it just won't get to the speaker terminals. Scope for it.

The same voltage at B and E of Q205 is very suspicious.

[bob p]

Q205 looks like its shorted B-E. Other Q201-Q207 transistors look good.

There is no DC on the output ballast resistors. OT drop checks look normal.

Here's what I've observed tracing a 1 KHz sine wave input to the RCA jacks:

- Pins 1 and 7 of IC 101 show an ugly little triangular wave (?)
- The ballast resistors show 60+ VAC peak-peak sine waves on both L+R channels!

The amp appears to be working. It looks like the problem is that the relay circuit is inappropriately engaged, which takes us back to the wild geese. I still can't explain why the relay might have been malfunctioning before. At least we've now got Q205 to blame it on.

It looks like we need to turn our attention back to the relay circuit. I guess that the first order of business is to find a new Q205.

[Enzo]

I think, correct me if I am wrong, that initially you misinterpreted the relay. It was doing its normal delayed speaker on thing, but you thought it was snapping into protect.

Now after a probe mishap, the relay IS staying in protect due to the shorted xstr. SO a new 2SC1328 it seems. Is there a C1328 in there now? It is 50v 50ma, so most anything will work there. From my shelf I'd grab a 2SC1815, or even a 2SC945 (The C945 is a very common type, sorta the asian equivalent of a 2N3904) Others I see like 2SC1740 or C2240 should work. You have any NPN asian types? Got a dead board to steal one from?

Get the relay running again, then we can see how those both channels running respond to a load.

[bob p]

I may well have initially misinterpreted the relay's action. But it appears that the amplifier section of the amp has always been working. A load will tell us for sure. If the amp is working, then the problem has always been in the protection circuit, no?

I don't have anything to steal parts from. If what I need is a common type, maybe I'll get lucky and I can find something suitable at Radio Shack instead of having to place an order.

Do you think that Q203, Q205 and Q207 should all be replaced? If the actual parts are used only for switching and their characteristics are non-critical, it would help to know what alternative selections I could use for each of these parts before I go out shopping.

thanks again.

[Enzo]

Radio shack probably only has US types - the legs will be in the wrong order. Though they might have a C945. I highly recommend using an asian type for replacement - or I suppose you could use an NTE thing in this case. Chances are you only burnt the one part.

No dead consumer products in the garage?

Note D201 across the Q205 EB, lift it to make sure the xstr is the shorted thing and not that diode. Most likely the xstr though.

If you short E-C on Q205, the relay should energize. If it doesn't, Q207 is bad

[bob p]

Hi. I'm back after some unexpected downtime.

I don't have any dead consumer electronics in the garage (no garage) so I'll just have to order parts. I've tried to source replacement parts, but I've had some trouble.

Q203: 2SA722. I can't find this one. The closes hit that I can find is at MCM, where it is out of stock. They recommend 2SA1127 and 2SA991 as substitutes. Both of them are out of stock as well.

Q205: 2SC1328. Out of stock as well. I have found a 2SC1815 at MCM, which I think is an appropriate substitute.

Q207: 2SC1509. In stock at MCM.

D201: DS442. This one is also out of stock at MCM and nobody else's website seems to recognize the number. I can't find a cross reference. This is the diode that spans the speaker relay.

Its a bit of a problem that I can't seem to find either the part or an equivalent part cross reference at places like Mouser, Allied, and Digikey. When I can find the part at MCM, it always seems to be out of stock with no anticipated availability.

Any ideas?

[Enzo]

First the diode. Across the relay coil, it is just a snubber, anything will work. I'd just grab a 1N4007 from your drawer. Is yours bad? Lift it to test, it is parallel to the coil resistance in circuit.

The Q2xx xstrs in this are just switching xstrs. I recommended the C1815 as a sub for the C1328. Most any general purpose type will work as long as the voltage and current minimums are met for the original part.

The 2SA722 is simply a PNP versi and one set of speaker terminals.

[bob p]

First the diode. Across the relay coil, it is just a snubber, anything will work. I'd just grab a 1N4007 from your drawer. Is yours bad? Lift it to test, it is parallel to the coil resistance in circuit.

i have to admit that i'm still not up to digging around in the unit, so i'm just trying to put a parts order together so that when i feel better i'll have all of the parts on-hand. i figure i might as well do something constructive during my downtime. its good to know that a garden variety 1n4007 will work.

The Q2xx xstrs in this are just switching xstrs. I recommended the C1815 as a sub for the C1328. Most any general purpose type will work as long as the voltage and current minimums are met for the original part.

good to know. now i've got a sub for Q205.

The 2SA722 is simply a PNP versi and one set of speaker terminals.

i think your message got mangled. the last part isn't clear.

thanks again for your help.

[Enzo]

What do you mean isn't clear?

No actually a couple paragraphs are missing there

The 2SA722 is simply a PNP version of the C1328, so here I might use a 2SA1015. MCM selector guide offers a 2SA733 as a general purpose PNP 60v 100ma.

Transistors are extremely easy to sub, you can be rather liberal with alternatives. It is not like a tube amp where a 12AX7 can be replaced by something else, but the gain will be way off.

I have both the M-500 and M-500t books. The only difference I can see is the M-500 has two sets of speaker terminals and a blinking LED when the relay is off. The t version has no blinky and one set of output terminals.


I originally went on a bit about picking subs, but this is the gist of it.

[bob p]

parts are on their way from the supplier. maybe next week...

[bob p]

First the diode. Across the relay coil, it is just a snubber, anything will work. I'd just grab a 1N4007 from your drawer. Is yours bad? Lift it to test, it is parallel to the coil resistance in circuit.

i lifted the diode to test it, and it was the electronic equivalent of a jumper wire. i replaced it with a 1N4007.

i haven't powered-up the amp to test any of the other components yet -- i just thought i'd check in with you to see if you had any recommendations now that we know that the diode was bad. would you like another set of measurements of the Q200 series transistors with the amp at idle?

[bob p]

Following replacement of the shorted diode, the relay resumed its "one click" behavior 3 seconds after switching the power ON. I turned the amp on and off a few times and the relay clicking behavior seemed to be normal, so I took some measurements.

I observed the following measurements. All readings are in VDC referenced to ground:

Trans    E        C        B
Q201    43        0       43
Q202     0       12.4      0 
Q203    12.4      0        5.7
Q204     0        5.7      0
Q205    12       43.5     12
Q206     0        4.6      0
Q207     3.7     36        3.6

Thinking that I was lucky, I thought I'd signal trace through the amp. With the power off, I hooked up a power resistor to the Left channel outputs and a sine wave generator to the left channel input. I switched the power on and there was no click from the relay. I never bothered with signal tracing. I rechecked the Q200 voltages and got these results:

Trans    E        C        B
Q201    43        0       43
Q202     0        4.4      0 
Q203     4.6      0        3.3
Q204     0        3.1      0
Q205     4.0     43.5      4.0
Q206     0        4.5      0
Q207     3.7     36        3.6

Notice that the voltages dropped at Q202, Q202, Q203 and Q204. I have no idea why the voltages dropped or why the relay stopped clicking. I'll have to try to intuit may way through the switching logic to see what the heck is going on. got an idea?