Ferguson 3T18 boombox (no audio)

[David B]

Hi, I have a very nice early 80's boombox that recently stopped working, it's not completely dead, it powers on, the lights still work, the tape goes round ff, rw etc.. The speakers still move and make the normal popping sound when switched on and off and there is still a low hum from the speakers when powered up (also normal for this boombox) but no other audio, no radio sounds or radio frequency numbers showing on the LED display, no music from the tape or line in. This was the state of affairs when I bought this BB on Ebay a few years ago and I had it professionally repaired and it worked fine until recently, my partner told me she was listening to it and there was a loud buzzing noise for about a second then nothing. Unfortunately the chap who repaired this last time no longer does repairs and I have no repair shop near me and i,m wary of sending it in the post (a repair shop in London has quoted me £150 to repair it, which seems a little steep if it turns out to be a minor repair, this does include return postage though) and what if it goes wrong again? this would turn into one expensive boombox?! I have had a go at repair by replacing a capacitor (it was one that was replaced in the previous repair by the Technician, (please see yellow arrow on the 2nd photo below, these photos were taken before the Tech repaired it) funny thing was this had a ceramic capacitor there before the repair but it was replaced with an electrolytic for a good reason I suspect, I was hoping that this capacitor had failed again and replacing it would fix the problem ( it is a 1000uf electrolytic, I replaced it with a 50v one instead of the 25v one, thinking the 50v would last longer (hope this was OK) anyway it did not fix the problem.
Anyway unless I can find someone to fix this at a reasonable price I would like to fix it myself BUT I have very little electronics knowledge or equipment, basically a cheap soldering iron and multimeter. I am hoping someone will look at my photos and read my description of the fault and say oh it's probably such and such transistor or that capacitor or chip on that board. And all I have to do is replace it. Or is this wishful thinking?
Edit I forgot to say there is another Electrolytic on the back of one of the boards (orange arrow 1st photo), it might be hiding in the photo or perhaps it was added when it was repaired by the Tech.

 

[kellys_eye]

What a cracking piece of kit!

Piece of the proverbial to fix too (if you know how of course ).

Check your PM's

 

[David B]

What a cracking piece of kit!

Piece of the proverbial to fix too (if you know how of course ).

Check your PM's

It is indeed a nice boombox that sounds as good as it looks (well it did). I am reluctant to send it in the post in case it gets damaged or lost but the alternative might mean me messing it up anyway trying to fix it, I already drilled the holes larger to fit the 50v capacitor (in place of the 25v one). Afterwards I thought "hang on this isn't a £5 radio from a charity shop!" unless it's a simple fix I may take you up on your offer.

 

[David B]

I believe this boombox is a variant of this http://www.wikiboombox.com/GE 3-5286A
It does have the same Rohm BA536 amplifier chip, I,ve looked.

 

[kellys_eye]

I note you've tried via the Vintage Radio Restoration site to get it fixed...??? Was this the route you found didn't end satisfactorily?

 

[David B]

I note you've tried via the Vintage Radio Restoration site to get it fixed...??? Was this the route you found didn't end satisfactorily?

Yes, I had one offer to look at it but he advised against sending it in the post. He said he could pick it up as he very occasionally passes nearby. I gave him my details but he never replied again. So I thought I would try here instead.

 

[kellys_eye]

Yes, I understand the concern for safe transport and given my location with respect to you it's not surprising. That said, I'm happy to send telescope equipment by post - the key to successful transport is to pack it such that you could literally drop-kick the box down your driveway and expect the contents to survive.

Any less is inviting disaster.

A good 'bale' of bubble wrap and double-boxing should be ok. There's not a lot 'loose' inside one of those to start a breakage sequence - keeping it away from the box inner walls is the key.

 

[David B]

Yes, I understand the concern for safe transport and given my location with respect to you it's not surprising. That said, I'm happy to send telescope equipment by post - the key to successful transport is to pack it such that you could literally drop-kick the box down your driveway and expect the contents to survive.

Any less is inviting disaster.

A good 'bale' of bubble wrap and double-boxing should be ok. There's not a lot 'loose' inside one of those to start a breakage sequence - keeping it away from the box inner walls is the key.

Thanks, I may wait a day or so and see what other replies I get, but if general opinion seems this is not an easy fix for a novice like me then I guess I have nothing to lose if I send it to you to look at, I will PM you in the next couple of days if I decide to send it to you to look at.

 

[kellys_eye]

On a very basic level, try using an insulated 'poking stick' and press around the pins of the volume control and any connectors joining the volume control pcb and the main amplifier pcb.

There may be a simple fracture of a track cutting the audio path.

 

[David B]

On a very basic level, try using an insulated 'poking stick' and press around the pins of the volume control and any connectors joining the volume control pcb and the main amplifier pcb.

There may be a simple fracture of a track cutting the audio path.

OK I will try this, also could I try shorting out the connections on the volume pot with a resistor to see if there is a problem with the pot or could this cause damage?

 

[kellys_eye]

Using bare wire you can probe the volume control wipers and you should hear a response (crackle or hum) that varied with the volume control - this would (potentially) prove the amplifier to be working.

 

[David B]

Using bare wire you can probe the volume control wipers and you should hear a response (crackle or hum) that varied with the volume control - this would (potentially) prove the amplifier to be working.

By this do you mean pushing a wire into the gaps on the pot (the same place where I would squirt contact cleaner) rather than bridging the contact legs with the wire, I hope I'm making sense.

 

[kellys_eye]

Not between the contacts - actually touch the contacts. The 'wiper' contact should, if you hold a bare piece of wire against it, induce a loud hum to the amplifier. Try all the contacts - some will be 'earth' and some will be the 'wrong end' but one of them will make it hum.

 

[David B]

Not between the contacts - actually touch the contacts. The 'wiper' contact should, if you hold a bare piece of wire against it, induce a loud hum to the amplifier. Try all the contacts - some will be 'earth' and some will be the 'wrong end' but one of them will make it hum.

I was just wondering to save me taking the BB apart again could I simply plug a lead into the aux socket, switch the BB to aux and short out the other end between my finger and thumb and see if this induces a loud hum?
Also my partner said she was no where near the BB when it suddenly went wrong which makes me doubt there is a connection problem on the volume pot as it was working perfectly before it's sudden demise?

 

[kellys_eye]

Yes, these test I refer to are just 'get the basics out of the way' test and your idea to use the AUX input should achieve (almost) the same result.
It does sound as if the fault is more involved though but, hey, all the simplest checks have to be done first anyway!

 

[David B]

Yes, these test I refer to are just 'get the basics out of the way' test and your idea to use the AUX input should achieve (almost) the same result.
It does sound as if the fault is more involved though but, hey, all the simplest checks have to be done first anyway!

I tried the aux test and there was no extra hum when I put my finger over the other end of the lead (which reminds me, my son was doing this exact same thing a while back when the BB was working making it hum just like the hum makes when plugging an electric guitar into an amp when it's on, could this have done any harm at all?). When I just turned the volume up past 7 the background hum got louder, not sure what that means? Is it still worth me taking the front and back off to get to the pot? If you think the problem lies elsewhere I think I might as well send it off to be looked at.

 

[kellys_eye]

Can you find an audio source to actually apply to the AUX input? Many MP3 players have an audio-out socket (3.5mm stereo jack) that could provide such a signal - nothing quite like using the right input after all....

It does still sound as if the audio amp has blown for some reason or other (unless the test described makes a difference).

 

[David B]

Can you find an audio source to actually apply to the AUX input? Many MP3 players have an audio-out socket (3.5mm stereo jack) that could provide such a signal - nothing quite like using the right input after all....

It does still sound as if the audio amp has blown for some reason or other (unless the test described makes a difference).

Still nothing, I think I,m out of my depth here and will send it to you to have a look at. I don't know if it means anything but I did notice the transformer was quite hot when I moved the BB to clean the switches and pots a few months ago, it was very close to a wall so it could have been inadequate ventilation or perhaps a sign something was not right?