Echowars: Ripple Current

[EchoWars]

C903 is the filter for the audio signal at the amp output so that normal use doesn't trip the protection. Wouldn't hurt to replace it, but...

1. First things first. Get the regulated voltages correct. Check the voltages on D703 and D704 and replace them if necessary....check R705 and R706 while you're at it. TR901 and TR902 should output a voltage close to the zener voltage. If not, the zener is fooked, or the dropping resistor (705/706) is. Until you have proper voltages supplied to all circuitry, ALL BETS ARE OFF!

2. Check the voltage at TP901 (the one side of R904 that is 'away' from the base of TR901). Needs to be low...like less than 100mV. It probably is.

Do these and we'll see.......

 

[Kevin's Rack]

C903 is the filter for the audio signal at the amp output so that normal use doesn't trip the protection. Wouldn't hurt to replace it, but...

I kinda had a feeling that's what it was for, due to the fact that it was non-polarized, so this is along the path that sends DC which in turn trips TR901 if DC climbs too high, resulting in subsequent shutdown

1. First things first. Get the regulated voltages correct. Check the voltages on D703 and D704 and replace them if necessary....check R705 and R706 while you're at it. TR901 and TR902 should output a voltage close to the zener voltage. If not, the zener is fooked, or the dropping resistor (705/706) is. Until you have proper voltages supplied to all circuitry, ALL BETS ARE OFF!

Kinda like having a good foundation to a house, similar to good rail voltage. What I am looking for as my zener voltage, the specified 56.1v and -57.6v?

2. Check the voltage at TP901 (the one side of R904 that is 'away' from the base of TR901). Needs to be low...like less than 100mV. It probably is.

I find it unbelievable how they put in 2 test points and expect you to figure out what the voltages should be...

Do these and we'll see.......

Ok, I'm off to work now, when I get home tonight...

 

[EchoWars]

Kinda like having a good foundation to a house, similar to good rail voltage. What I am looking for as my zener voltage, the specified 56.1v and -57.6v?

Nononono....that's the input voltage...the zener provides a reference for the transistor at the base, and the output (at the emitter) must therfore be about .6 - .7V less than that. The scat is marked a little wierd...there is a voltage labled '36.9' that doesn't seem right...if that's supposedly the voltage on the base of TR701, then the emitter should be about 36V, but instead is marked '31.4'. ??

Check the part number of the zener...sometimes the number will give a hint of the voltage. Also, you can't get zeners for every little voltage increment. The readily available sizes relevant to this circuit are 30V, 33V, and 36V, with 33V being the likely candidate here.

I find it unbelievable how they put in 2 test points and expect you to figure out what the voltages should be...

That's why people get paid to do this stuff...

 

[Kevin's Rack]

After what you said about the Zener diodes I realized what you meant, they are rated at the voltage they are supposed to maintain, and it turns out the parts list alludes to the correct zener voltages, when I took a reading of D703 (across it of course) I came out with 46.1v, hmmmm, that's nowhere near the specified 36v, so therefore the diode must have failed and/or it's dissipating resistor (R705) must have failed as well. Ahhhh, it all makes sense now, I can finally see the light, I just needed someone to point out to me what I completely ignored... :twak: I feel lke Jake Blues when he was in the church and had the epiphany to put the band back together (LOL). Looks like I have to take out 80,000 things again, but that's ok because now I know how to do it and what I don't even need to remove. Thanks a lot echo!! :thmbsp:

 

[Kevin's Rack]

I should also mention D704 was working correctly, gave me a reading of 31.1v, very close to the 30v zener voltage, but I'll replace it anyway while I'm in there.

 

[EchoWars]

You won't find a 32V zener...they don't exist. If it's working, then I'd leave it.

when I took a reading of D703 (across it of course) I came out with 46.1v,

The 22K limiting resistor may have given up...check it.

 

[Kevin's Rack]

You won't find a 32V zener...they don't exist. If it's working, then I'd leave it. The 22K limiting resistor may have given up...check it.

Actually it's a 30v zener, I guess I must have typed it in wrong...

 

[EchoWars]

Can't be, or the output voltage would be around 29.4V or so. The emitter chews up about .6 to .7V.

 

[Kevin's Rack]

I know about the .7v being chewed up, that's the standard bias voltage of a P-N junction, so I know you're correct there, I'm just going by what the parts list is giving me. According to the parts list, the zener diode (D704) is a 30v. I'm not trying to argue with you, I'm so happy you are able to help me, I am endlessly grateful, I hope I don't seem like I'm being argumentive (If I am I'm really sorry). By the way, I just removed R705, and got 22K on the dot, even checked R703 for good measure, got 2.2ohms exactly. Looks like it's time for a new diode (the 36v zener, D703).

Once again, thanks so much!!

 

[Kevin's Rack]

OK, so what's the verdict, I'm going diode shopping, should I spring for the 500mW diodes (I think so) or the 1W diodes(not so sure)?

 

[EchoWars]

You can calculate.... :scratch2:

The originals are almost certainly 500mW (they may even be 300mW). With a 22K dropping resistor, the zener current is gonna be very very low. In that case, a 1W zener won't regulate worth a chit as it won't have enough current.

 

[Kevin's Rack]

I think in addition to D703, C708 also failed. Prior to my recent re-assembly, the problem was intermittent. So I bought a crap load of resistors, capacitors and diodes. I plan on rebuilding the regulator circuit: all capacitors will be new (even ceramics) zener diodes, resistors, and transistors (already done). Just thought I'd mention this...

 

[Kevin's Rack]

2. Check the voltage at TP901 (the one side of R904 that is 'away' from the base of TR901). Needs to be low...like less than 100mV. It probably is.

Do these and we'll see.......

Varied: Generally gave a reading of about 57-58mV. Good sign I would assume...

 

[EchoWars]

Yep...

 

[Kevin's Rack]

Parts should arrive Monday (9/12). Maybe cranking that very same night?? I'll let you know...

 

[Kevin's Rack]

Well Echo, looks like I need a little more advice. I replaced the zeners (along with a lot of other stuff) and I still got the same voltages as before. I might add that the zener voltages are now correct. What I am not finding??

 

[EchoWars]

If the zener voltage is correct, then the voltage on the base of TR701 and TR702 should equal the voltage of the zener. Then, if the transistor is conducting, the emitter should be about .7V less than the base. If not, you have dead transistor, busted traces, a lost ground wire, God knows...

 

[Kevin's Rack]

As if I need more problems

Well, I went to go take a voltage reading on TR 701. In doing that, I somehow shorted one of the leads to the ground and and caused the transistor to almost go on fire (It glowed very brightly) . So, In addition to a damaged transistor, I disintegrated 2 (Brand new) resistors, and probably the diode too. What the **** is wrong with me? It seems as though fate is against me. I lifted a few traces here and there, but fixed them with no problems. Should I just give up?? :lazer: :cry:

Why is nothing going right for me? I've lifted traces, drilled through circuit boards, arced traces, disintegrated components, why is everything fighting back at me??

 

[DanTana]

Sounds like you just need to slow down a little bit. Like the carpenter's saying, measure twice, cut once. You need to check twice (connections) and measure once. I've done really stupid things myself, usually from getting in a hurry. I like to try to go over everything before I apply any power. I burnt out a few transistors myself lately, but that was from the capacitor still holding a charge. I try to be more careful about that now too.

 

[EchoWars]

Relax..it's all a part of the learning process.

Failure is not an option. Repeat.