2SB755 Transistor Data?

[bob p]

I've got a dead SS stereo HiFi amp (Carver M-500t) that look like its triggering its overcurrent protection circuit. The relay triggers at power-up. The voltage rail is sitting pretty at 74VDC, just like its supposed to.

I'm presuming that the relay indicates triggering of the overcurrent protection circuit because the amp has got bad power transistors, but I haven't found the transistor pinout data yet to do the confirmatory testing. The amp uses a compliment of 2SD845 and 2SB755 transistors by Toshiba. I've found a data sheet in English for the 2SD845, but I have only been able to find a data sheet in Japanese for the 2SB755. Unfortunately Japanese is Greek to me, and I'm still looking for the pinout information for the 2SB755.

According to the ordering page at MCM it looks like the replacements would be NTE92 for the 2SD845 and NTE93 for the 2SB755. I'm not sure if the pinouts would be the same, so I just thought I'd ask.

Thanks.

[Enzo]

Stay away from that NTE stuff unless there is just nothing else. Just because transistor so and so in their line is the closest one to what you want, it doesn't MAKE it what you want. And whatever you do NEVER mix NTE and other brands. All NTE or all Toshiba, not both, if they are in parallel.

If the parts are compliments - as in the push and the pull of an output - then the specs for one are the same as the other pretty much. They do the same job, just opposite polarity.

2SAxxx and 2SBxxx are PNP, while 2SCxxx and 2SDxxx are NPN. But pinouts do not care about polarity. All TO220 are the same, All TO3 are the same etc. Your TB-34 pinouts would all be the same. NTE pinouts will be like anyone else's.

2SD845 - 150v, 12A, 120W Looks like a TB-34 package maybe
2SB755 - same specs and is even listed as the complment to the D845.

MCM selector guide in the catalog shows several TB-34 options, some are larger than others, so check the dimansions.

All the three leg power transistors are BCE across the front. No two legs should be shorted together regardless of pinout. Pull it for testing. Remember there are generally reverse diodes across them in circuit and low value resistors too.

MCM sells transistor data manuals, and they are in Japanese, but a line like this is easy to interpret:

2SD845, 150V, 12A, 120W

Column headings are English letters like Vceo, and Ic (Voltage C to E, and collector current)

[bob p]

You're obviously a lot better at this than I am. I can read the english data sheet, but I can't make heads or tails out of the Japanese one.

2SD845, English

2SB755, Japanese

[bob p]

Stay away from that NTE stuff unless there is just nothing else. Just because transistor so and so in their line is the closest one to what you want, it doesn't MAKE it what you want. And whatever you do NEVER mix NTE and other brands. All NTE or all Toshiba, not both, if they are in parallel.

Thanks, you answered a question that I was going to ask about mixing brands. Right now MCM doesn't have the Toshibas, and I don't know where to find them. I'd rather not replace all of the transistors if I don't have to, as the amp takes 8 of them and they're about $15 each. I still haven't confirmed model number for the case type.

All the three leg power transistors are BCE across the front. No two legs should be shorted together regardless of pinout. Pull it for testing. Remember there are generally reverse diodes across them in circuit and low value resistors too.

Before you suggested pulling them, here's what I found:

Diode Test:
B-E: 23mV
B-C: 428mV
other 4 connections unshorted

Z Test:
B-E: 73R
B-C: 32K
other 4 about 500K

all 8 transistors measure the same.


I have obtained the Carver Service Manual for the M-500t. It is only 17 pages long, and I am missing two pages of exploded diagrams. I have the calibration specification page, the schematics and the parts list. Unlike the M-1.5 manual which has a lot of good troubleshooting data in it, this manual has none. Pretty disappointing, actually.

there's no question that the big relay in the center of the amplifier board is tripping a few seconds after power-on. I'm just not sure why:

The voltage supply is normal (74 VDC) at the test point.
The bias test points look like they're being taken across the two ballast resistors on each side of the push/pull compliment of power transistors. Although the difference between L/R isn't very far off, the offset for the two channels seems to be significant.

Is this really a problem with the power transistors, or could the protection circuit be being engaged by something else? Unfortunately the documentation on this amp doesn't list the conditions that can activate the protection circuit.

I will be hosting a copy of the calibration sheet here, as soon as my ISP fixes a server problem.

[Enzo]

Looking at the B755 sheet, I see this:
1. The part number of course.
2. Vceo is 150v. They write is as negative since it is PNP
3. The thing will work up to 20Mhz, which will probably cover the average audio signal.
4. ??? 2SD845??? I already know it is the complement. I can't think of any reasons to show a part of the opposite polarity unless it was the complement.
5. The dimensions. While the mounting holes might vary with the overall size, the three legs will tend to be about the same on all the large power xstrs.

Now that was just the top of the page. Moving down I see a box with the basic specs:
Vceo - 150v
Ic - 12A
Power - 120w
I don't know exactly what the japanese is to the left of those, but I bet it just says the same thing. It is all the same data you'd find on an English sheet. In any case, that is about all you need to know to sub the part.

The graphs? The axes are labelled in english characters. Look at similar curves in an english data sheet as a guide. But I look at the lower right one for example and I see a SOA - safe operating area - graph like on any power semi.

[Enzo]

Look at TB34 for the case. Look in MCM selector guide page for other TB34s. SOme like the 2773 might be larger in size, so check their data sheet for size comparison.

Look at the D845 as the push, and the B755 as the pull. When I say don't mix the parts, I mean on a side. Don't mix the D845s, and don't mix the B755s. All 845 or all NTE something. Same on the 755 side. However, it would be OK to use all NTE on the pull and all D845 on the push. It is the ones in parallel I worry about, not the ones across from each other. The parallel ones have to share current throughout the waveform, but the ones on opposite sides do not, they alternate conduction.

If they are not shorted CE, then they may well be OK. That is a low resistance BE, but in circuits that is common. Look in the schematic - how much resistance is there across that junction?

Usually all I need is a schematic. The troubleshooting CONCEPTS from the 1.5 ought to apply here. This one has less stuff. SO read the 1.5 procedures and try to adapt them.

CArvers tend to be full of protections. CErtainly it could be other than the outputs.

Carvers in my experience short out rectifiers a lot more than other brands. DOn't know why. I remember a bulletin replacing 3A diodes with 6A diodes at one point.

In the seconds before it trips, do the output terminals go to DC?

Are both polarity of all rails present? at least initially?

Protections usually fall into a couple camps. Too much current, or too much temperature. No way a manual could list all the possible reasons for either condition

[bob p]

Thanks for the tips Enzo. I always appreciate the time that you spend answering questions, and you may just have me reading Japanese before this is all over.

Usually all I need is a schematic. The troubleshooting CONCEPTS from the 1.5 ought to apply here. This one has less stuff. SO read the 1.5 procedures and try to adapt them.

yes, that is what i've been trying to do -- adapt the 1.5 troubleshooting guidlelines to the 500. yes, the idea that you had mentioned long ago about finding any Carver manual that is close is better than nothing. in this case, the 1.5 manual might actually be better for the 500 than the 500 manual!

in addition to re-checking the power transistors, i also checked the other smaller transistors that are mounted on the heatsink, and i got similar results with no shorts.

regarding the PSU, i also remember hearing something about Carver having replaced 3A diodes with 6A diodes at some point, as the 3A diodes had trouble standing up to the kind of abuse that happens in the Carver power supply. come to think of it, maybe it was the triacs that were underrated. i'm fairly certain that i ran across this tidbit at diyaudio.com.

CArvers tend to be full of protections. CErtainly it could be other than the outputs.
...
In the seconds before it trips, do the output terminals go to DC?

Are both polarity of all rails present? at least initially?

during my initial run-through i did check for DC voltages in the right ballpark on all of the PSU caps. each pair of caps had equal magnitude and opposite polarity, as you'd expect from 3 sets of bipolar rails. that's all the PSU testing that I did. i just took a quick measurement with a DMM to verify that nothing was grossly wrong, I didn't look at the quality of the DC with a scope. then I checked the amp board for the proper voltage at its test point.

you know, i didn't think to check for the presence of DC on the outputs at startup. good idea. i'll look into that.

Protections usually fall into a couple camps. Too much current, or too much temperature. No way a manual could list all the possible reasons for either condition

because this problem starts immediately with a cold amp, i had dismissed the idea of a temp related problem. i guess it could be temp related if the sensors have failed, and are sending an inappropriate shutdown signal. i hadn't thought of that until now.

overcurrent could be a problem related either to PSU problems or output transistor problems. i haven't found any glaring issues yet. anything else i should be thinking about?

thanks.

[bob p]

If they are not shorted CE, then they may well be OK. That is a low resistance BE, but in circuits that is common. Look in the schematic - how much resistance is there across that junction?

here's a picture:

[Enzo]

OK, looking at Q119 or Q121, the path from B to E is through a 4.7 ohm R157, then down through 68 ohm R151, and then back up through 0.22 ohm R171.

That all adds up to 72.92 ohms (4.7 + 68 + 0.22 = 72.92)

ANd that is darn close to the 73 ohms you got.

Overcurrent could be caused by SOOOO many things.

Are you operating without a load? An amp can spit out DC, but until there is a load to suck it up, it won't pop a fuse most of the time. Get rid of any loads.

If the outputs are OK, and they seem so, then consider both sides might be turning on at the same time. A loss of bias string. The bias circuit keeps the opposing bases pulled in together so the outputs don't turn on hard. If it opens they go to max.

I'd go down the row of rectifier diodes and just check them all for shorted.

Ah. The 500 has a speaker relay. SO if you are looking for DC on the channel outs, look at those 0.22 ohm ballasts. They are inboard of the relay.

Q109 is the bias xstr. You already checked the outs and drivers. Q111,113 are teh predrivers.

It would be real odd to have both channel with the same failure, but either one can shut down teh speaker relay.

This amp does not resemble the 1.5 very much, I see.

I have a bulletin, write me for it, it might apply

tmenzo at msn dot com


Holy crap, the lower channel - the right - is drawn upside down!

In any case, my spider sense is tingling power supply now..

[bob p]

Are you operating without a load? An amp can spit out DC, but until there is a load to suck it up, it won't pop a fuse most of the time. Get rid of any loads.

no load.

If the outputs are OK, and they seem so, then consider both sides might be turning on at the same time. A loss of bias string. The bias circuit keeps the opposing bases pulled in together so the outputs don't turn on hard. If it opens they go to max.

I wrote the observed bias voltages down on a copy of the calibration page from the manual. that's the image that i linked to at the bottom of post # 4 above.

I'd go down the row of rectifier diodes and just check them all for shorted.

diode tests on my DMM don't display any shorts (no beeps).

Ah. The 500 has a speaker relay. SO if you are looking for DC on the channel outs, look at those 0.22 ohm ballasts. They are inboard of the relay.

5 to 15 mV DC on the ballast resistors. I don't think that counts, does it?

Q109 is the bias xstr. You already checked the outs and drivers. Q111,113 are teh predrivers.

i ran quick tests on the transistors with the chirping diode test on my DMM. Q109 and Q110 (bias) are mounted on the heatsink. They were previously tested when i tested the output transistors.

Q111 and Q113, as well as Q112 and Q114 pass the chirping diode test. no beeps.

I also tested Q101, Q103, Q105, Q107 and Q103, Q104, Q106, Q108. No chirps.

Are the in-circuit chirping diode tests adequate?

[bob p]

I'm still trying to determine where the protection circuit is receiving the activation signal. Unfortunately, this amp's service manual provides no list of triggering events, or where to check the signals that correspond to them. That makes troubleshooting difficult.

I'd be happy if I could conclusively map the problem to either the PSU board or the amplifier board. The jumper wires that go from the PSU board to the amplifier board seem to be the regulated supply rails. To try to get a better idea of what's going on in the PSU, I've taken voltage measurements on some of the rail supply transistors in the PSU. All measurements referenced to ground:

Q401 to Q404:
B = +/- 75 VDC
C = +/- 42 VDC
E = +/- 75 VDC

Q405 to Q408:
B = +/- 42 VDC
C = +/- 75 VDC
E = +/- 75 VDC

Am I correct in assuming that this means that the PSU is working properly?

Throwing out a red herring, both power meters have burned out lights. The lamps are 8VDC 0.25A bulbs. It looks like they're fed off of a separate low voltage transformer. Measured voltages are switchable between 7.5 VAC (dim setting) and about 9 VAC (bright setting). I'm guessing that the burned out lamps are due to two bulb failures, though if the little transformer is shorted I guess that would cause a current drain where everything else checks out OK. I guess its reasonable to consider that the two problems could be related, rather than isolated and ignoring the lamps probem. Unfortunately that little xformer is hard to get to, and I can't find my right angle screwdriver to de-mount the little transformer.

I hope your spider sense is still tingling. Do you have any other ideas?

[Enzo]

The meter lights are bottom right corner of schematic. The switch just selects a tap off that little transformer. Likely not involved.

Let me review to see if we are on the same page.

When power is applied, the relay energizes right away? Or after a moment? ANd stays eneergized? That would be normal.

If the relay pulls in and then drops right back out, THAT would be protect mode. The relay disconnects the speaker load when trouble occurs.

Whatever it is doing, it WILL sit there with power on, right? No fuses blowing?

ANd under that condition, there is no DC on the output bus as measured at the 0.22 ballasts? Yes, a few millivolts can be ignored. I am worried about finding 40 volts there.

The center leg of the power xstrs is the collector. At idle what voltage is on those? SHould be 40v, not 74.

The 74v rail should only get to the transistors during music. IC301 is a quad comparator. The inputs are tied to the output buses. WHen the IC301 detects an output signal getting near a 40v rail, it then turns on Q301-304 to control Q401-404, which in turn control Q405-408. And those four switch the 74v rails onto the output transistor collectors.

So your readings worry me. Q405-408 look to me like they should have 74v on their collectors, but not the emitters. The emitters should sit at 40v until the signal peak tells them otherwise. In fact the schematic shows 37v at those emitters.

If one of them is stuck on, I might think it was bad, but if all four are stuck on, I'd look first at IC301

The relay runs off the +40. On powers speakers, off cuts speakers out. Q207 turns it on. There are two thermal breakers, top of drawing, that will turn it off it the heatsinks get hot. When excess current flows through the output, V drop across R171 turns on limiter Q129, which shunts the base drive to Q111 using D101. But it also pulls down D105, which - follow the line up and over - works Q201. That makes Q206, Q205 turn off Q207 and the relay drops out.

Also the output bus at the left end of L101 is sampled thhrough R201. That is fed to Q202, and to Q203 via inverter Q204. So if the signal is large in either polarity on or the other of those paths will kill Q203, adn from there see above discussion. SInce we don't want the realy to simply drop out on peaks, but do drop out on extended DC, they added C202 to delay things. C202 will keep Q203 on through a peak, but if DC sits there, C202 will "time out" and then Q203 will drop out the relay.

SO that is what works the protect relay.

[bob p]

Let me review to see if we are on the same page.

When power is applied, the relay energizes right away? Or after a moment? ANd stays eneergized? That would be normal.

When I turn on the power, the amp remains silent for 2-3 seconds, then the relay clicks ONCE. I guess that its energizing and connecting the transistors to the speaker terminals, and not disconnecting as I had thought previously. This would suggest that the protection circuit is not engaging. My bad.

Whatever it is doing, it WILL sit there with power on, right? No fuses blowing?

yes

ANd under that condition, there is no DC on the output bus as measured at the 0.22 ballasts? Yes, a few millivolts can be ignored. I am worried about finding 40 volts there.

Yes, we have no bananas. Er, I mean No DC.

The center leg of the power xstrs is the collector. At idle what voltage is on those? SHould be 40v, not 74.

The 74v rail should only get to the transistors during music. IC301 is a quad comparator. The inputs are tied to the output buses. WHen the IC301 detects an output signal getting near a 40v rail, it then turns on Q301-304 to control Q401-404, which in turn control Q405-408. And those four switch the 74v rails onto the output transistor collectors.

So your readings worry me. Q405-408 look to me like they should have 74v on their collectors, but not the emitters. The emitters should sit at 40v until the signal peak tells them otherwise. In fact the schematic shows 37v at those emitters.

This is where I think we get off of the same page. WhenI think of power transistors, I think of the output transistors on the heat sink, Q120-Q126, not the transistors on the PSU board Q401-Q404 and Q405-Q408.

Looking back at my previous post I did find a typo that might be throwing you for a loop. sorry about that. Here are the correct voltages for the transistors on the PSU board:

Q401 to Q404:
B = +/- 75 VDC
C = +/- 42 VDC
E = +/- 75 VDC

Q405 to Q408:
B = +/- 42 VDC
C = +/- 75 VDC
E = +/- 42 VDC <-- typo was here.

The relay runs off the +40. On powers speakers, off cuts speakers out. Q207 turns it on. There are two thermal breakers, top of drawing, that will turn it off it the heatsinks get hot. When excess current flows through the output, V drop across R171 turns on limiter Q129, which shunts the base drive to Q111 using D101. But it also pulls down D105, which - follow the line up and over - works Q201. That makes Q206, Q205 turn off Q207 and the relay drops out.

Also the output bus at the left end of L101 is sampled thhrough R201. That is fed to Q202, and to Q203 via inverter Q204. So if the signal is large in either polarity on or the other of those paths will kill Q203, adn from there see above discussion. SInce we don't want the realy to simply drop out on peaks, but do drop out on extended DC, they added C202 to delay things. C202 will keep Q203 on through a peak, but if DC sits there, C202 will "time out" and then Q203 will drop out the relay.

I have taken measurements referenced to ground on Q201-Q207 to try to get an idea what is going on with the protection circuit. Here are the numbers. I am reading the pinouts from Left to Right, looking at the flat side of the transistor with the numbering. As I understand the situation, a TO-92 is supposed to be E-B-C going left to right while looking at the flat side, though some transistors in the TO-92 case aren't pinned out that way.

All readings are in VDC
Trans    E       B       C
Q201   43.1    -0.59   43.2
Q202    0.7mV  12.4     6.6mV
Q203   12.4     0.7mV   5.78
Q204    6.8mV   5.78    0.7mV
Q205   11.8    43.4     4.61
Q206    0.1mV   4.55   -0.58
Q207   3.72    35.0     4.29

[Enzo]

then the relay clicks ONCE. I guess that its energizing and connecting the transistors to the speaker terminals

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.

This is where I think we get off of the same page. WhenI think of power transistors, I think of the output transistors on the heat sink, Q120-Q126, not the transistors on the PSU board Q401-Q404 and Q405-Q408.

No, I meant the outputs Q119, 121, etc should onl;y have 40 v on the collector.

The rest of that discussion was of the voltage switching circuits that control whether 40 or 74 volts in on those collectors.

At some point earlier you said the power rails were sitting there at 74v. That is OK for the 74v supplies - note there are two sets of +/-74v - but NOT for the outputs. That concerns me. So I gues the question is what did you mean when you said that?

But I still want to get on the same page further. 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.

If there is continuity, then all the relay and protection stuff is not the problem.

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?

SO if the relay stays on and its contacts work, what exactly is the symptom you are trying to cure? before we spend more time on the protection, what's wrong. And leave the power rail voltage for a moment.

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?

Only AMerican TO92s are EBC, Asian ones are ECB, and that means all the 2S numbers. Your amp is all asian numbers, so all teh TO92s are ECB.

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.

In the amp, this is linear, but in that protection, it turns a relay on and off, nothing linear about that. The xstrs run in saturation mode - all the way on or all the way off. They act like a switch. Look at Q207. The top end of the relay coil is wired to +40v. If we ground the other end, the relay turns on. Q207 when on conducts E-C, which grounds the relay through R212, and it turns on. So Q207 is a switch. When off, there is the 40v of the coil on teh C and ground on the E. R211 is just a pull down to make sure the Q207 turns of when control volts are absent. SO if we pull the base of Q207 towards its collector - bring a + voltage there - it will turn on.

In this circuit, some of them tirn on each other. FOr example, how do I get +40 to the base of Q207? Through R208 way up there. WHy is that resistor up at the top like that? Who knows. ANd how do we get 40v into that resistor? Q205. If Q205 turns on, the 40v at its collector will also be on its emitter, and that emitter is wired through R208 to the base of Q207.

Left alone, R205 will pull the base of Q205 up to the 40v. That is all it takes to keep Q205 on, and as above discussed, if Q205 is on, the current from teh 4-v rail will flow through it to the base of Q207, which turns on and pulls in the relay.

Q202, 203, 206 are three ways to turn OFF the relay. See my discussion in an earluer post.

If you think of these as switches, and bringing the bases to the collectors turns the switches on, it might be easier to intuit the thing. I find that easier than looking at it in terms of actual volts and cirrent. TO me it is a logic circuit, so I tend to think highs and lows in my head - offs and ons.

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.

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?

[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.

Yes, I understand that. But things are different today. Read on.

No, I meant the outputs Q119, 121, etc should onl;y have 40 v on the collector.

The rest of that discussion was of the voltage switching circuits that control whether 40 or 74 volts in on those collectors.

All of the amplifier's output stage transistors have 40-ish VDC on the collectors (center pin). I don't remember the exact value, but its something like 40-43 VDC.

At some point earlier you said the power rails were sitting there at 74v. That is OK for the 74v supplies - note there are two sets of +/-74v - but NOT for the outputs. That concerns me. So I gues the question is what did you mean when you said that?

Test point side of R178 measures 74 VDC, as specified on the "Alignment Procedures" page of the service manual, page 3.

But I still want to get on the same page further. 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.

This is where things have changed. For the past week on powering up the amp the relay would click ON after a 2-3 second delay. It appeared to stay energized, in that I never heard any subsequent clicks.

I took voltage measurements on Q201-Q207 last night to post the numbers for you, and today when I power up the amp there is NO RELAY CLICK. Its DEAD silent. Now that I think of it, when I was taking the BCE measurements on some of those 200-series transistors last night, my probe tip slipped and probably shorted an EC or CB junction on one of the transistors. This event was accompanied by a click, and now the relay isn't moving. Oops. Looks like I just made things worse.

more to follow...