Help for a rookie(amp. troubleshooting)

[edsin]

Hi,
I am looking for a guy who has lots of patience who can help me with troubleshooting a transistor based amp(power)-been working at it more than a month.I have the schematic.I have a little knowledge and would like to gain more.Been reading a lot (rod Elliots site,etc.),but have some basic/stupid questions.Would prefer it to be through email-it might take a bit long as i go to internet cafe every 2 days.One guy has already abandoned me.
Thanks(edsin333@yahoo.co.in)
Ed

[Netlist]

I’m sure you’ll find enough patient people here on the board.
Could you start with giving us some more info about the amp? Brand? Type? Can you post the schematic? What’s wrong with it? Are both channels broken? …

/Hugo

[Enzo]

I have beeen training techs for over thirty years, and if there is one thing that has shown me, it is that I am a patient man. Answer netlist's questions and we can start.

SOlid state power amps all tend to be similar under the skin. Even the real esoteric designs still have an underlying basis that is familiar. Hopefully you will learn something about amplifiers along the way, more than just what part is bad in this one.

The key to troubleshooting is to isolate the problem. Knowing where it is NOT is part of finding out where it IS.

Your amp either does something it should not, like blow fuses. Or it does not do something it should, like produce nice clean sound. SO we want to make sure the power supplies are working right, and we want to find out if the channels work, one bad or both bad. So describe the problem you are trying to solve, and also tell us what, if anything, is working OK on the amp.

[edsin]

Hi,
Thanks netlist n Enzo.Here goes:
I can't post the schematics here as file is too big-i dont know how to cut a PDF file. Please go to http://www.myownemail.com/ or http://www.myownemail.com/moe4/login/default.cfm(hope it works)
Login name:edsin
Password:edsin (my email add. is edsin@myownemail.com)
The file is attached as an attachment
.I have removed the FM boards and radio section board( not posted the sch. of those)-so there is a possibility the amp is not working due to this.
The power supplies +/- 36v(shown on sch.as 33v) are correct.Both channels r not working n i have isolated 1. When i connected source to pre-amp,there was no sound.Voltage on wire going from preamp to power amp is 0.8v ac(should it be ac only?)When i touch the multimeter leads to this wire(ac setting) ,i get a very faint sound in the speaker(8 Ohm),but when i touch it,there is no sound. I get the click of relay,i.e. it works.
The previous guy told me to start from Q701; dc voltage at output is 0v.Output transistors are removed for safety reasons-p.g.(previos guy) told me amp will work at low vol.Also output transistors are to be replaced by BD909 and 912 and drivers MPSU56 and other one with BD237 and 238.Also BC447A(this one seemed ok) is replaced by i forgot...maybe BC547?All this due to courtesy of p.g.
Connected a 100W series bulb-the filament glows very faint-so i don't think anything is short.All transistors i have removed and checked.
1.Voltages near LTP Q701 are OK(as shown- -0.6v,0v,-15v)
2.Vc at Q703 is 0.75v instead of 1.1v.Removed Q703.Testing with analogue multimeter shows OK.Should i change this?When i went to buy this.....is it (motorola)BD418 or BD41B......was not available.Is there a substitution for this?(could not find a ref. to this in google/angliac.co.uk).Is it possible the amp. won't work if voltage is 0.75v?
3.Vc at Q705 is -1.0v. This should be ok?My multimeter is not v. accurate.

What i would like is this-we will go each transistor by transistor.So u can help me by telling what are the likely voltages at each going to be?A short explanation why would be v. helpful.
Also i want to know-when can i say that power amp. MAY be OK. i.e. what are the voltages at what point should show?
Thanks
Ed

[Enzo]

At your link, I enter what you told us, but it says invalid.

[edsin]

Sorry(could not check yest. due to slow internet connection).
Loginname:edsin@myownemail.com
Password:edsin

[Netlist]

This is a part of the schematic of the relevant section of the amp. One channel only.

Here you'll find a good explanation of how to check transistors:
http://www.bcae1.com/ampfail.htm
Homepage: http://www.bcae1.com/
This is some basic stuff you should try to learn when doing repairs.

Hope this helps for now. I'll see what I can come up with tomorrow.

/Hugo

[Enzo]

You are operating the amp without the output transistors present. While th amp may well settle down and operate that way, it may also be part of why the voltage a Q703 is a bit off. If the output voltage wants to drift off a bit, the feedback will tell the amp to compensate. it does that by controlling 703.

721 and 723 are your output devices. 717 and 719 are the drivers. Lookat the drawing and note that the collector of 703 where that 1.3v ought to be is also the base of the positive side driver. Now look at the negative side driver. Its base should be a -1.3v.

Q707 is your bias transistor. Its job along with the resistors next to it is to keep the driver bases that steady 2.6 volts apart. If we were to short across 707, the amp would still work, but we would get crossover distortion. If we adjust 707 so it conducts too little, then the bases of the drivers can turn on more and both halves of the output would try to conduct at once. This would cause the amp to draw excess current and run very hot or even blow fuses. SOunds like that is not your problem.

Meanwhile 703's job is to tug that whole mess up and down with the music signal. Both drivers get the same signal, they just have it offset with a bit of DC to prevent crosover distortion.

The bases of the drivers are at the 1.3v away from the output line. Hopefully that is also zero with respect to ground. The B-E of each driver is roughly half a volt, so the bases of the output transistors out to be about half a volt from the output line.

What package are the output transistors? TO220? TO218? TO3? Other?

[edsin]

Hi,
Thanks.Output transistors are from ST Micro plastic package-i think it is TO-220.
447A is replaced by BC546.The original problem started due to shorting of voice coil of 1 speaker-i didn't know about this.
Q703 looks OK.Still i would like to know the replacement-if it is available easily,i might as well put the replacement(please give 3-4 choices if possible).
All diodes look OK-inline testing shows them conducting only in 1 direction.No resisters are black on this channel- so i assume they r ok. Inline testing them-shows no complete open.Checked some electrolytics (inline and 1-2 removed)-seemed to be OK.
1 0.47 Ohm emittor res. is replaced by 1 OHM res. as due to non-availablity(of wirewound),altho fusible res. r available.Should i go for the fusible one?I had removed 1 0.47 resistor(has become blackish) and measured it with a digital MM-it was showing 1.2 Ohm.Maybe this is the problem?
With this much info.-what r the logical conclusions possible ?(just for knowledge).
Ed

[Netlist]

BC546 is a suitable replacement for BC447.

BD418: BD520 (EBC), BD528 (EBC), BD388 (ECB) and BD830 (ECB).

/Hugo