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Thread: Carver FTM-25

  1. #1
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    Carver FTM-25

    This amp came in with one channel sounding distorted. Found that the test points in the power amp Left channel were off. There should be +/-3.2v on the bases of the pre-drivers Q207/209. I resoldered the heatsink mounted Vbe multiplier - Q205 and the issue was fixed. Gave back to customer. He came back, same issue with those voltage test points. I resoldered the entire board around the bias and driver section of the amp, was fixed, gave back to him. He came back, same issue. I put it on my bench and the voltages are wrong again. I poked and prodded, checked voltages again, now it's working again. So, at this point I'm thinking I need to replace the smaller electrolytics in the circuit, ala C205 which is across the Vbe transistor. And any other similar electrolytics.

    I don't want to do this without first really finding the issue, but I've failed at that up till now. I have tried tapping, pushing, freeze spray etc... none of which seem to change the condition. Only powering down, waiting, powering up seem to change things from working to faulty.

    How would y'all go about this? Advice please. I fear replacing these caps, it then working fine, giving it back to him, and then having a very angry customer on my hands. Y'all know how it is, I just want to fix his gear for him in the end, but having to bring it back 3 times is quite frustrating.
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  2. #2
    Supporting Member The Dude's Avatar
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    I don't have any tips. It sounds like you've tried most everything. The only thing I might try is ESR testing the caps, if you have a meter. Sure you can replace them, but as you said, then you won't know for sure if you fixed something. If the caps are drying out, an ESR test might show that. I feel your pain!
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  3. #3
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    I have an esr meter Ill see if it reveals anything. Thanks!

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    Quote Originally Posted by lowell View Post
    I poked and prodded, checked voltages again, now it's working again. So, at this point I'm thinking I need to replace the smaller electrolytics in the circuit, ala C205 which is across the Vbe transistor. And any other similar electrolytics.
    Lowell, you know that this is the worst method of fixing amps and this was stated here many times by Enzo and other guys (including me). It's simple because of the fact that after replacing this capacitor, you wouldn't be able to tell whether the amp is now fixed, or not. Another question is: how much would you charge the customer if you don't know whether the amp is fixed.
    For me, you haven't tried any basic test procedures that should be applied in such a case. If you have 0 Volts on the capacitor (you wrote that "test points were off" - I assume it means 0V), I would keep the amp in "failed" state and measure surrounding voltages. If you have power supply rails and 0 Volts on some capacitor in the middle of the circuit, then somewhere in between there is some interesting voltage that could help you to solve the problem.
    For example voltages on Q201, D201, D203 and similar. Why haven't you done this? This would bring you much closer to the solution of the problem. "Poking and prodding" is not a good method in this case. You can use it but only if afterwards you identify the problem. Otherwise, you are left without any knowledge about the failure. You also leave us without any reasonable info about the problem but, on the other hand, you expect advice. My advice is: do it properly - meaning do more measurements.
    Apart from this: the only reason related to C205 that there is 0 Volts across the cap is that it is shorted. And shorted capacitor can be verify without desoldering it (in circuit). Have you tried this? If the capacitor is not shorted, I would search for the problem elsewhere. This could be for example cracked track (to be verified with magnifying glass).

    Mark

  5. #5
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    Markus thanks for the reply. I have tested voltages and attempted to track down the issue. Problem is, after testing a few points....I mean a few, the problem mysteriously goes away. Hence the poking and prodding to to get the problem to RETURN.

    The first time I fixed it the voltages earlier in circuit seemed off... at the Vbe mult transistor. So I resoldered it as my meter showed no continuity from it to the board. I hope this clairifies things. I'm poking and prodding to UPSET the circuit. Not to fix it.

    Those test points both read -1.3v when the fault happens.

  6. #6
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    Quote Originally Posted by lowell View Post
    Those test points both read -1.3v when the fault happens.
    OK, now I understand. But you only wrote that "the test points were off" (whatever this means) and now you say that both test points were -1.3 V. This is not the same. You also did some measurements but you didn't tell us about it. You realize, that this make diagnosis much more difficult, don't you? The more we know, the better we can help you.

    Mark

  7. #7
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    I realize Mark. The trouble is, that everytime I start measuring I'll get one or two measurments, then the thing suddenly starts working right. At that point, I cannot get the FAULT condition to happen. It's very frustrating and seems that only a complete power down and hours later does it do it again. I'm convinced it's electrolytic related because nothing seems to be loose and solder joints don't seem to be the issue. I'm gonna toss it on the bench right now and see how many measurements I can get.

    I do recall testing both Vbe transistors as well as the current source transistors and all surring resistors and diodes. All tested fine with the meter. Again, all this points to a leaky electrolytic. Let me see if I can get some more solid details for you and I'll post them.

  8. #8
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    Well now we really have something to troubleshoot. Borrowing a friends meter, it doesn't have a clear marking on the selector knob. I accidentally had it on ma instead of vdc (they're 180deg on the turn dial). Blew the fuse and now the thing is blowing fuses. I ended up shorting Q209 (pnp driver) emitter to gound, so that transistor was a dead short. Replaced it with MPSA92 after swapping the base/collector holes. So it's still drawing too much current, and I'm using the bulb limiter.

    I've checked all the outputs, no shorts. Voltage are all really low unless I lift the Re ballast resistors R247/249. I've had them lifted for troubleshooting as the voltages seem good and also symmetrical with them lifted.

    Now, I have +5v at BOTH Q201/203 collectors. Q203 should be a low negative voltage. I've disconnected Q205/237 and C205. Still +5v in both spots. Q201 has Vbe of 0v. I replaced it thinking it had a b/e short or internal open circuit. No change. So I then questioned how this could be. The only way it could be was if Q103 in the preamp wasn't on. Sure enough no voltage across R125.

    Q103 has a Vbe of -.8v. (using bulb limiter so voltages are low)
    Q103 Ve = 0v.

    I've tested all the resistors in the Base biasing string. All test fine.
    +/-15v are reading +/-3.3v (seems low because of bulb but still symmetrical)

    IC101 has these voltages:
    1 -1.5v (here's my problem, but why and how?)
    2 0v
    3 0v
    4 -3.3v
    5 0v
    6 0v
    7 280mv
    8 +3.3v

    Please help!

  9. #9
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    I haven't looked at all the voltages you listed, but it's clear that you shorted some transistors. Keep checking.
    The voltages on IC101 pins #4 and #8 should be -/+15V (instead of 3.3V). This may be due to bulb limiter but I would check it.

    Mark
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  10. #10
    g1
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    Also, with your supply voltages so low with bulb, I don't think you should expect the IC to behave properly, so the DC offset at pin 1 may just be due to the low supply.
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    Quote Originally Posted by Justin Thomas View Post
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  11. #11
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    Thanks for your help guys. Yeah I realize I should ditch the bulb with the ballasts lifted since it's not in over-current. I'll report back.

  12. #12
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    Ok so with ballasts disconnected I have:

    IC101
    1 -14v
    2 +5v

    With pins 1/2 disconnected and R111 lifted I have these voltages on the solder pads:

    1 +32mv
    2 +5.5v

    The only place I see thats DC connected to pin2 is via R113 which is the 0v point between both halves of the PA. So...there is offset. +5v of offset. This could only be from the limiter transistors as I see it. Q301 is either on more than Q303...or Q303 isn't on enough.

    I also see only 88mv across R289 on Q203 NPN collector. The other working PA has 300mv across that resistor.

    Q203 tests fine with a meter. There is 750mv across D205/207. There is 930mv across D201/203.

    The positive side of the PA seems to be on more. Not EXACTLY sure why. Please, suggestions!

  13. #13
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    Please ignore my previous post as everything has changed.

    Ok I lifted D309/307 and saw smoke. Turns out that was a really bad idea. I meant to lift D305/307 to take the limiters out of circuit. R243/245 are burning up and smoking. I reconnected the diodes and replaced the resistors and Q215 as it was shorted in and out of circuit. I used a KSC1845 for Q215 as it was the closest I had. I swapped the B/C pins.

    R243/245 are still smoking. Using variac at 22V and 100w bulb limiter. I have to take these measurements fast cause the resistors keep smoking.

    Admittedly....I seem to be making this worse the more I work on it.

    Q207
    Ve -11v
    Q209
    Ve -16.5v
    Q201
    Vc -10v
    Vb +21.6v
    R209 has 62mv across it
    Q203
    Vc -18v
    Vb -19v
    R211 has 2.4v across it

    Seems Q203 is on WAY MORE than Q201

    So tracing backwards the Rc of Q103/105 in the preamp.

    Q103
    R125 has 150mv across it
    Q105
    R127 has 800mv across it

    I lifted IC101 pin1 AND R111.
    The junction of R117/119 is at -170mv.

    +/-15v rails (with variac at 22v and 100w bulb) are at +4.3 and -4.7.

    I have a million questions but I think I'm just gonna leave this here and see if anyone has any insight. I'm not feeling too competent at this point.

  14. #14
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    In such a situation. What do you experts do? Would you replace the output transistors with resistors? Would you disconnect the preamp and use resistors to "pull down/up" the bases of the 2 PA input transistors?

    I feel I'm not dividing and conquering enough and am chasing my tail.

    Finally, any references or books on advanced repair techniques for direct coupled transistor amplifiers?

    I understand how transistors work. And THINK I understand this amp... but it's the approach, and isolating smaller sections of the amp for troubleshooting that's throwing me for a loop.

  15. #15
    Supporting Member Jazz P Bass's Avatar
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    I would have to wonder why R243 & R245 are getting so hot.

    Where is the current path?
    Are they not they part of the limiter circuit?
    If so, you can remove the limiter transistors for now.

    As to inserting resistors instead of the output transistors, I have always used a 1K/1 watt resistor from Base to Emitter.
    For testing, you only need to add one per rail if the transistors are in parallel.

    (aside: I will not take in anything made by Bob Carver!)
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  16. #16
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    Thanks Jazz. So I can leave the outputs in and ADD a 1k 1watt in parallel with b/e? Or should I remove one of them?

    I have the output ballast resistors lifted. Could this be why those resistors are burning up? There is no current through the output bases right now. This could be my misunderstanding with solid state vs tubes.

  17. #17
    Supporting Member Jazz P Bass's Avatar
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    The idea is to remove all of the outputs & simply bridge the B/E junction of each rail to complete the circuit.

    Mind you, this is only for testing.
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  18. #18
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    So to be super clear. Disconnect Base, Emitter, and Collector of each output transistor. Then solder in a 1k/1w resistor from Base to Emitter. ? (ignoring any paralleled devices)

    I think that's what you are saying.

    Finally, am I right in that this is the error in my thinking? Current needs to flow from Base to Emitter in order for the previous stage to work correctly? So if the output bases are conducting, the bias resistors for the Vbe multiplier for the outputs would not be attempting (and failing) to carry the current that would otherwise be going thru the output's bases?

  19. #19
    Supporting Member Jazz P Bass's Avatar
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    Not too sure what is going on with the amp.

    If you are afraid of blowing the outputs, taking them out of the circuit & inserting a 1K/ 1 W resistor B to E will take awy that fear.

    The resistor simply completes the circuit.

    For sure I would remove the limiting transistors.
    They shouldn't even come in to play until you are at full power.
    It does seem that they may be the reason for the overtemp in the two resistors that you mentioned.

  20. #20
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    Ok I pulled the limiters completely.
    Disconnected all 3 outputs on each side. Added 1k/1w B/E resistors. All voltages listed are in reference to ground with no speaker connected.
    With bulb limiter and variac I was then able to bring up the voltage to where I could take some measurements.
    I ended up finding that R209 (Re) for Q201 was open. Replaced it.
    Then found that it was smoking again, this led me to find that Q237 had a short. Replaced it.
    OK, NOW WE'RE GETTING SOMEWHERE
    I now bring the variac up to about 18v and have bipolar supplies throughout the PA.

    Q201
    C +5v
    Q203
    C -5v
    Q207
    E +4.4v
    Q209
    E -4.7v
    Q211
    E +4v
    Q215
    E -4v
    Q217
    B (will not go above 200mv)
    Q219
    B (will rise to -3 volts and more at which point I turn the dial back down)

    So the Base of Q219 is going too high.
    R243 and R245 are still smoking.

    TO SUM IT UP - I have bipolar voltages throughout, albeit HIGH voltages. I turned VR201 bias pot as cold as I could get it and the voltages posted are as cold as can be.

    At this point I've checked everything I could think would cause this. My guess is that Q205 may not be conducting enough...or at all.

    Maybe at this point I should reinstall the outputs? I'd think the base voltage could not exceed ~700mv if they were installed. Or should the base voltages be in spec wirh the 1k resistors installed?
    Last edited by lowell; 02-14-2017 at 08:38 AM.

  21. #21
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    Can I put a 100ohm 2watt resistor acrosa the Vbe and see if that fixes it? I have a feeling the vbe isnt conducting regardless of my meter saying it's ok. I figure there's no more than about 100ma thru it when working and I need about +/-4v at the current mirrors' collectors.

    I'm guessing a little at the math. But if the current mirrors are around 100ma, and the rail is +90v that means the collectors should be at .6% of 90 = 4v.

    4v/100ma = 40ohms. But it is from rail to rail so 80ohms or 100ohms and 8v is more like it.

    8v X .1 = .8watt so a 2w will suffice.

    Am I crazy? Or onto something?

  22. #22
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    Ok no one is into this thread haha. Understandable cause I feel like I lost everyone with my post updates. ANYWHO -

    I replaced the Vbe Q205 (already replaced a shorted Q237) and the AMP IS WORKING now... with the 1k/1w resistors in.

    I just put the outputs back in and things are skewed a bit. The base of Q219 PNP output is at +100mv. Q217 is on point at ~500mv. I traced this all back to the collectors of the current mirrors. They are about 1 diode drop difference.

    My QUESTION: Do these mirrors have to be complementary pairs? I checked their Vbe and the rail voltages. Both identical. I'm thinking Q201 is pulling more current. I replaced both of them with suitable subs, but need to know if they should be complementary pairs. THANKS!

  23. #23
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    Quote Originally Posted by lowell View Post
    Ok no one is into this thread haha. Understandable cause I feel like I lost everyone with my post updates. ANYWHO -
    My suggestion is that you post your questions in a different way. When you list 20 voltages (and the voltages change all the time because you measured them incorrectly, or shorted the amp), it is to difficult (at least for me) to follow what is the actual problem. I have to open the schematic, search for every component you listed and try to verify whether the voltage is correct or not. Instead of such approach, I suggest that you make a copy of the schematic and list the voltages directly on the schematic. And attach this schematic as image. This can be done with almost any graphic program, even as simple as MS Paint.
    Quote Originally Posted by lowell View Post
    I replaced the Vbe Q205 (already replaced a shorted Q237) and the AMP IS WORKING now... with the 1k/1w resistors in.
    The question is how it is possible that you haven't look at Q205 until now .
    Quote Originally Posted by lowell View Post
    I just put the outputs back in and things are skewed a bit. The base of Q219 PNP output is at +100mv. Q217 is on point at ~500mv. I traced this all back to the collectors of the current mirrors. They are about 1 diode drop difference.
    My QUESTION: Do these mirrors have to be complementary pairs? I checked their Vbe and the rail voltages. Both identical. I'm thinking Q201 is pulling more current. I replaced both of them with suitable subs, but need to know if they should be complementary pairs.
    You haven't mentioned what current mirrors you are talking about . Do you mean Q201 and Q203? They are not current mirrors - they are current sources (which is not the same as current mirrors). If you have 1 diode drop difference, I would immediately look at D201, D203 (D202, D204) - is one of them shorted?

    Mark

  24. #24
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    Mark,

    Thanks for the reply.

    My main question is unanswered. Should the current sources be complementary pairs?

    I believe I had stated already that Q205 measured correctly all along. Only after replacing it did the bipolar voltages come back.

    D201/203 are not shorted. D202/204 are in the other "working" channel. But D205/207 are for the negative rail current source and are also not shorted.

    Thanks!
    I agree about posting the schematic with voltages.

  25. #25
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    Also, can you suggest some references for advanced troubleshooting of direct coupled power amplifiers?

  26. #26
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    Quote Originally Posted by lowell View Post
    My main question is unanswered. Should the current sources be complementary pairs?
    I haven't answer to this question because you asked about current mirrors and you didn't specify which transistors you were talking about. If you want to find out the answer, go to wikipedia: https://en.wikipedia.org/wiki/Current_source and read it. However, I think that the question is irrelevant because you have differences like 100mV and 500mV. I don't think that such a difference can be caused by unmatched transistors. The current provided by current source depends on voltage drop on the diodes, and on emitter resistor. Are you sure that the diodes are OK and the resistor is OK? Basic test would be measuring the voltage drop on each diode and each emitter resistors. Are they the same?
    I cannot recommend and book on advance troubleshooting of such amps but I think that required knowledge is available for free on the internet (eg. on wikipedia). You already know that you have to identify functional blocks of the amp, understand how each of them works and verify whether actual behavior is the same as expected. And if not, find out why.

    Mark

  27. #27
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    In my previous post I stated that the current "sources" had identical and good Vbe and rail voltages. It wasn't so clearly stated though I admit And to be clear I'm saying that yes those diodes are all reading good and have good diode drops.
    Do these mirrors have to be complementary pairs? I checked their Vbe and the rail voltages. Both identical
    Yes the Re resistors for them both have identical voltage drops across them too and the resistors are good.

    I just discovered that Q215 is reading suspect. I lifted it's biasing resistors R243/245 and still read very low. Pulled it from the board and still very low. I had already replaced this so it seems during troubleshooting that I did AFTER that repair it has fried again. Replaced it and now the output bases look good!

    I brought it up to full voltage without the limiter and voltages are good.

    I am playing music through it right now and it sounds good too.

    Thanks for your help with this Mark. As is always the case on challenging repairs I learned a lot on this one. Mostly that transistors are most likely suspects when all surrounding components SEEM to be reading correctly on the meter. Also Jazz's suggestion with the 1k/1w resistors from B/E was a big help!

    I have a few other power amps that have stumped me thus far and I feel more competent so will be returning to them and will most likely have some more threads on similar repairs. I will try to be more clear with my posts. I think it'll be easier to be clear with my posts when I have more confidence in what I'm actually measuring and what my results SHOULD be.
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  28. #28
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    The service manual specifies the biasing voltage with "no input signal." It does not specify a load. Should a load be connected when biasing? I ask because the bias changes when a load is connected.

    Also what are your processes for testing a power amp after replacing transistors and checking bias? Monitoring transistor temps with infrared gun? What temp is acceptable at what wattage?

    I'm aware of the deg C/W ratings. The 2SA1302 has Tj 150deg max rating. I have it running at 36 watts on just the left channel and my gun reads about 56degC at the hottest spot I can find.

    56deg - 25deg = 31deg

    So it seems to be raising in temp less than 1deg per watt. Does this approach work? And does this look normal?
    Last edited by lowell; 02-24-2017 at 04:04 AM.

  29. #29
    Supporting Member The Dude's Avatar
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    If it doesn't specify, I would bias with a load. It's normally specified.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  30. #30
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    Hmmm... the +Vb of the outputs doesn't go DOWN if I turn the bias pot WITH a load. The -Vb DOES go lower. In any case I'm convinced it's fine. The -Vb is a hair lower on both channels at about -450mv. I've run the thing at about 36watts at 4 ohms and the temp seemed to go up < 1deg per watt. From what I could find and have read online this seems ok to me. Ofcourse if I were to run the thing at 200watts that would not be good so now I'm wondering.

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