popping at power on/off and random crackle after replacing blown caps

[prosenberg]

hi
this is my first post so go easy on me. I have a vintage receiver, a Ferguson 3400. It's nothing special, but it looks nice and it gives me way better audio than I get from my tv built in speakers. Recently one side went really quiet and swapping the speakers showed it was a problem in the receiver. Opening it up I found one 750uF cap that had exploded and one 400uF cap that was bulging at one end. Both on the power supply board. There were 4 other caps on the power supply board so I figured I'd replace them all.

when I went to do that job today I found that although I had thought all the 4 intact caps were 750uF, actually 3 were 400uF. So I didn't have the right quantities. So I replaced the bulging 400, the exploded 750 and the intact 750, leaving the 3 remaining intact 400s.

Anyway, now I have audio back in both speakers. However, 3 things concern me.

1, when I turn the receiver on/off I get a loud pop through both speakers, I'm sure that didn't used to happen, but to be honest I just left it on 24-7, so maybe I'm wrong.

2, I get random medium volume pops/crackles - this is not associated with touching any dial, the volume of these pops/crackles are exactly the same if I turn the volume to 0. A few hours after the repair they are significantly less frequent. Could this be forming of the new caps?

3. I have quite a lot of hum/hiss. With the volume on 4, sat on my sofa at the other side of the room I can hear it. The receiver always had quite a lot of this, but I had hoped the new caps would help.

my question are, do I need to worry about 1? What might be the cause of 2? Will replacing the remaining power supply caps help with 3?

Just as a point to note, I am in the market for spending a few 10s of £, not 100s.

Thanks for any advice.
Phil

 

[gadget73]

1) is there a cap across the power switch? If not, adding one or a resistor + capacitor network may help to suppress the arc that happens when the switch breaks.
2) are all the controls clean, and is the power switch clean? Could be the switch is flaking out on you. Arcs from the switch opening with lack of suppression will burn the contacts over time.

3) hiss might be noisy transistors, noisy resistors, or just bad design. hum could be power supply caps or bad wiring layout. Schematic and pics of the internals might help here.

 

[prosenberg]

I have just purchased the service guide - am I allowed to post it up here or do I get in trouble for making it available for free? Can I PM it to you gadget73?

So to update, the caps I replaced were actually on the "audio board" I had thought that the large caps and 4 diodes were the bridge and filters, but actually the what I had assumed were diodes were really transistors. Told you I was a nube, sorry!

The 750 uF capacitor I replaced was on the side that is giving the random crackles and pops. Some more googling and I see suggestions for bad transistors for this. It is clearly happening after the volume/balance controls as they have no impact at all on the noise. Each channel has 4 transistors, a D1569 that is labelled AF amp, a u3847/2 labelled audio driver and an AD161 and AD162 labelled complementary push pull output stage.

To answer your specific questions gadget73

1) No there is no cap across the power switch. Googling about arcing it seems that the suggestion is to put a 10 nF capacitor across the switch - however the switch in this system is double pole - should I bypass the neutral side of the switch or add a capacitor to both poles? I also realised that I get the same popping if I switch off the receiver by the mains socket.

2) I have cleaned the controls as thoroughly as I can with Servisol Super 10 switch cleaning lubricant. My local electronics store didn't sell Deoxit, so it was this or own-brand contact cleaner - I'm not sure how readily available Deoxit is in the UK.

3) As I said, if I can work out how to PM you the manual then I will. Or let me know if you think it's not bad form to upload it here. I'm going to guess that the first port of call will be replacing the transistors I mentioned above. I have found datasheets for all except the u3847/2, but I would welcome advice on how to spec modern replacements or whether I should find old new stock.

The power supply is a 4-diode bridge with a 4500 uF cap. There is a voltage stabilizing circuit, consisting of 4 transistors and a zener diode, but there is a 1 k pot on that board that has disintegrated. This pot sets the dc voltage so presumably this could be way out of spec (should be 25 V).

 

[slimecity]

I understand the caps you replaced were in the power supply? Generally the powersupply feeds both channels to the amp - there isn't really a special "side" to the PS that feeds a particular power amplifier section, unless your amp is special that is.

If you have both the phase and neutral being switched by the powerswitch then a suppressor cap will work by soldering it across the phase and neutral wires where they go into the switch.

random medium volume pops/crackles

Does this occur in one, or both channels?

Does it occur more when the unit gets warm?

Does it occur in all inputs, or only one?

 

[prosenberg]

I understand the caps you replaced were in the power supply? Generally the powersupply feeds both channels to the amp - there isn't really a special "side" to the PS that feeds a particular power amplifier section, unless your amp is special that is.

sorry, I got this wrong, the replaced caps were on what was labelled the audio board in the schematic, (I guess this is what's often called the power amp stage). The exploded cap was on the problem side, but I replaced the equivalent cap on both sides.

Does this occur in one, or both channels?

Does it occur more when the unit gets warm?

Does it occur in all inputs, or only one?

Just one channel, selecting different inputs makes no difference. It is the same regardless of volume and balance settings. It occurs even if no inputs are connected. I don't think warm up time makes any difference.

 

[slimecity]

Noise regardless of volume means there is a problem after the volume control - probably in the power stage.

If u havent already - find a service manual and identify how many transistors are in the power stage. You may have a failing small signal transistor. I would be looking for a small signal transistor in the power stage before the big output transistors.

 

[Hyperion]

Regarding DeOxit - availability in the UK.

http://www.audioaffair.co.uk/caig-labs-deoxit-d5-contact-cleaner-spray-142g

https://www.amazon.co.uk/s/ref=nb_s...s=aps&field-keywords=deoxit&rh=i:aps,k:deoxit

http://cpc.farnell.com/deoxit/d5s-6/contact-cleaner-5-oz-can/dp/SA03297

 

[prosenberg]

Noise regardless of volume means there is a problem after the volume control - probably in the power stage.

If u havent already - find a service manual and identify how many transistors are in the power stage. You may have a failing small signal transistor. I would be looking for a small signal transistor in the power stage before the big output transistors.

Service manual purchased - wasn't sure if I'd get in trouble for posting it up here, given people are making a living selling them.

Anyway the power stage has 4 transistors per channel. They are labelled in the service manual as: AF Amp D1569; Audio Driver U3847/2; complementary push-pull output stage AD162 and AD161. I would welcome any advice for where to source them or how to spec modern replacements.

 

[slimecity]

They are labelled in the service manual as: AF Amp D1569; Audio Driver U3847/2; complementary push-pull output stage AD162 and AD161

At this point - lets look at small signal transistors in the power amp section of your unit -BEFORE the outputs. What do you have listed as these in your SM, what are the part numbers?

You could also post a scan of that section here for us to see

 

[prosenberg]

At this point - lets look at small signal transistors in the power amp section of your unit -BEFORE the outputs. What do you have listed as these in your SM, what are the part numbers?

You could also post a scan of that section here for us to see

here is the page showing the preamp and power amp stage. Thoughts welcome.

 

[prosenberg]

sorry, the quality of that last image wasn't very good. Here is just the power amp stage zoomed in, but I had to do it in 2 parts.

 

[prosenberg]

I have been trying to find some information on the two transistors before the large output transistors as you suggested @slimecity. For the transistor D1569 labelled as AF amp I found this datasheet http://www.semicon-data.com/transistor/tc/d0/D1569.html, which is a silicone transistor, but the package is different, the one on the board is in a metal can. If that data sheet is correct then the closest I could find in terms of specification is http://uk.rs-online.com/web/p/products/7743681/.

I couldn't find any reference to the U3847/2 transistor. But I have found AD162 and AD161 NOS - these are germanium.

I really am totally lost when it comes to specifying transistors. Resistors and capacitors I can handle just about but in terms of active components I don't know what makes a good audio frequency vs driver transistor or how to deal with silicone vs germanium. Any help would be very welcome.

 

[slimecity]

Hmmmmm, you may have an issue here as germaniums shoud be replaced with germaniums. This thing must be quite old, yes?

I will let someone else chime in who has more experience with this sort of stuff.

 

[triode17]

I have been trying to find some information on the two transistors before the large output transistors as you suggested @slimecity. For the transistor D1569 labelled as AF amp I found this datasheet http://www.semicon-data.com/transistor/tc/d0/D1569.html, which is a silicone transistor, but the package is different, the one on the board is in a metal can. If that data sheet is correct then the closest I could find in terms of specification is http://uk.rs-online.com/web/p/products/7743681/.

I couldn't find any reference to the U3847/2 transistor. But I have found AD162 and AD161 NOS - these are germanium.

I really am totally lost when it comes to specifying transistors. Resistors and capacitors I can handle just about but in terms of active components I don't know what makes a good audio frequency vs driver transistor or how to deal with silicone vs germanium. Any help would be very welcome.

Germaniums!! These are known to cause failures as power devices. It's what killed off The Fisher, HH Scott and the likes in the late 60's when they switched over to SS amps. that used germanium power transistors. Also, if the blown caps. were in the power supply, did you check the rectifiers that drive them? Caps don't usually swell up and pop for no reason.

 

[prosenberg]

Okay, well thanks for the help @slimecity, I'll start a new post about vintage transistor replacement.