Marantz SR-50 receiver repair

[Stuey]

Hi people,

I've volunteered to have a look at a friend's Marantz SR-50 which blows the main fuse at the power PCB. She took it to a tech who repaired it - it then was OK for about a year, and stopped working again with the same problems. I know it is probably time for a new one, but she's a single mother and all that...you know the rest.

I can see where a complimentary pair of output transistors (A1263, C3180 Toshiba) have been replaced but only on one channel. Some common transistors have also been replaced.

I have limited knowledge about transistors, but I note that those output trans which weren't replaced are open circuit between the centre collector and the other legs, so need replacing. The other channel's output transistors are OK in this respect. I know I should replace all four if it comes to that as they may have been damaged.

I note also that the output legs (+,-) of the plastic package bridge rectifier are practically open circuit with respect to each other (both ways) showing ~ 2.5ohms between them. I assume this isn't correct and the device is kaput?

But to my primary question. One of the power supply caps (4700uF) has (easily seen) cracks around the solder on the board, so is probably intermittently connected. Could this have been the cause of the issues? I don't really think so, but I can't find anything else wrong (albeit within my limited knowledge).

Any other tips on what I can check on-board with a DMM?

Thanks

Mark

[KISS]

Hopefully your DMM has a diode test function use it.

If it doesn't try taking a two 1.5 V batteries and a 1K resistor in series and put a voltmeter across the combination. Measure the voltage across the bridge diodes. In one direction you should get around .6V. The other direction you should get the battery voltage. I haven't tried it, but it should work. 0-0.4 volts would be bad.

You can then check the B-E (diode behavior) and C-E (both open, I think, out of circuit) junctions of the transistors the same way. I normally use a transistor checker.

Warning, testing in circuit can sometimes give erroneous results.

The intermittant connections with the power supply caps can certainly cause problems, but you need to look at the fusable emitter resistors (usually <0.5 ohms and white, sometimes dual and any resitors <200 ohms near the outputs.

When the amp is functional, measure the voltage between the two bases of the output transistors (~1.2 V) with no load and also the voltage across the emitter resistors and we'll go from there.

The intermittant cap connections most likely blew the bridge recifier.

Variacs are very useful when troubleshooting amps.

[Stuey]

Thanks KISS.

My multimeter has a diode function test but I supposed it would be of no use in circuit.

The emitter resistors I assume are the three legged ones, white ceramic, near the output transistors. They are marked 0.18 ohms.

I've found more. The voltage regs (I'm assuming this is what they are - two three legged devices in the PS - I can't get a schematic) are also in a similar state to the PS caps. One is so loose you can push it back and forth...

I'll try and dig up some parts and see how we go.

Cheers

Mark

[KISS]

The diode test fill find a shorted transistor. So will an ohmmeter.

Resistance readings across transistors are meaningless.

The emitter resistors are the three-legged things.

If you have one good channel comaparisons are nice to do.

[Stuey]

Hi there again,

I now have the service manual...

I was considering testing in stages. Firstly, I was going to remove the output transistors (faulty), replace all other components I test to be faulty, resolder the vregs and ps caps and then test the voltage BEFORE replacing the large transistors.

Question - can this be done without problems? Or do the output trannies need to be replaced before powering up?

Thanks

Stu

[Leolabs]

Personally I would disconnect the power supply from the circuit and power up to see whether the PS is Ok or not,assume that all the faulty components in the PS section have been replaced.Then proceed on the other circuits.

[mightydub]

The diode test function on your DMM should tell you if the transistors are good or bad, in circuit. A good transistor will look like two diodes with the base as the common terminal - NPNs will look like the two anodes are connected at the base (i.e. 0.6V B->E and B->C, open circuit the other way) and PNPs will look like the two cathodes are connected at the base (0.6 V E->B and C->B, open circuit the other way.)

You can find the data sheets for the transistors on the web, always helps to know if the package is BCE or CBE. As mentioned previously, compare measurements on the good channel to the bad.
Also check the individual diodes in the PS bridge rectifier, and any trimmer potentiometers for setting the bias or DC level to make sure that the wiper isn't open or intermittent.

You should be able to power the amp up with the output transistors removed without any problems, though depending on the circuit, voltages you measure may not make sense (compared to what's noted in the service manual or the other channel) if any active devices are removed. Certainly there will be no output -> input feedback if the output drivers are removed. If you measure something that is way off from what you expect, study the schematic and ask yourself why. It may be because of removed devices, or because of a problem.

Once you have identified and replaced the bad components, then power the unit up with a 40-60W lightbulb in series with the AC. This will act as a current limiter and also give you a visual indication if something is wrong. The bulb should flash briefly as the PS caps are charged up, then settle down to a faint glow. If the bulb continues to glow brightly, switch off fast hopefully before any damage is done. I built a test power jig out of a junction box, wall outlet, switch, and lightbulb socket I had lying around in the garage. Having an old loudspeaker driver that you don't care about blowing up is also handy for testing.

I recently fixed an old receiver that had blown the output transistors, drivers, and the bias transistor in one channel so I have an idea of what you're facing. Once I had the amp up and running again and the bias adjusted according to the service manual, it still would produce random pops and bursts of static and shut down occasionally. I replaced all of the electrolytic caps in vain, what ultimately fixed it was reflowing every single solder connection in the problematic channel. Hit every connection with flux and a hot iron - no telling what time and corrosion might have done.

One thing that I found really helped was understanding the circuit. Being new to amplifier design I didn't get it at first, reading Rod Elliot's articles was a good way to understand the topology and design tradeoffs. He also has some articles on debugging for novices that may be helpful.

[Stuey]

Geez, thanks both of you - that's good information.

Leo, that's what I meant. I thought it was best to do, as there are a few dodgy solder joins in the PS. So if I get that working seemingly OK first it'd be best.

Mightydub, I had downloaded the datasheets for all the transistors already, and the service manual shows the CBE legs anyway, With your info and the previous advice in this thread I think I can make a start!

By the way, I've been trying for a while to get flux for the purpose you mention (and to aid removal of components) but I haven't a clue what form it comes in. Can you point me to a supplier, not necessarily so I can buy it from them but so I can see what it looks like! Liquid, paste, what? Quite embarrassing, but the electronics store here in Perth don't stock it.

Thanks

Stuey

[mightydub]

You should be able to find flux in liquid form at any decent electronics shop - if not, then just get a spool of rosin core solder and add a little to every connection, it should make it turn from dull grey to nice bright smooth silver. Then you can clean up with some flux remover and a small stiff brush.

Make sure you get "rosin" flux, do not use acid or paste type plumbing flux.

I have a little squeeze bottle of liquid pine resin flux that I filled up about 15 years ago from the 5 gallon bucket by the wave solder machine at work - this was in the good old days before the no-clean flux. Boards went over a bubble fountain of flux, then over the solder wave, then into the cleaner which sprayed and soaked the board in solvent - I think it was TCE - to get the junk off.

[Stuey]

Just an update; I finally finished this job today. Replaced the output transistors and another 6 transistors surrounding these, plus the bias trimpots which had shorted.

The PS caps had dry joints, the voltage regs were loose, there were a few burned transistors....bah!

Gee, what a very poor quality amp this is. The boards are atrociously soldered. There were dry joints all over the place. I resoldered at least 20 components before it would work satisfactorily, and you can see many more that are just not as well soldered as you'd expect. The amp is built in the Phillipines.

Then when the thing was playing, the display showed to be intermittent...more dry joints!

Anyway, all is working now.

Cheers

Stuey