Grundig TK18 plays with loud buzz / howling

[Bryson10]

Hi there. I'm new to this site. Just got a Grundig TK18 which plays but with a loud buzz/ howling from the speaker. At min vol setting you can just hear the noise, but it quickly gets to an unbearable level as the vol knob is turned up - you can actually feel the vibration through the casing getting worse to the point where you expect the machine to go up in smoke! I have another slightly newer TK23L machine (working) and swapped out the valves, but no change. Visually everything looks fine with no obvious burnt out capacitors etc.
I am a mechanical engineer by trade and have a reasonable basic electrical understanding. I can solder and use a multimeter etc. Any suggestions appreciated.

[Andy - G8MNM]

All capacitors will be suspect particularly if they are wax covered 'waxies' Also the main electrolytic) smoothing and reservoir capacitor(s) likely housed in an aluminium can.
There is a capacitor well known on this and other forums as 'that capacitor' this is the coupling capacitor to the output valve, if this is leaky (likely if it is a waxie) then it can cause the output valve and output transformer to fail due to excessive current flow through the valve.
The hum could be due to the main smoothing capacitor which would need replacing.
Otherwise the buzz and howling is possibly being caused by feedback due to other faulty capacitors.
Also dirty connections in plugs and sockets and valve holders can cause these problems - particularly when earthing to screened cables has failed
It would be a good idea to get hold of the schematic diagram for this machine to aid fault finding.
I am sure that other forum members with particular experience with tape recorder repair will be along to help you out.

Good luck

Andy

[Steve_P]

Out info on the TK18 is here:

http://www.service-data.com/product.php/2610/3452

Sounds like dirty connections and things loose inside. If the buzz is not varied by the volume control, then it's probably the smoothers. Also check the diode as well.

Any old capacitors on these need to be replaced, but please do one at a time.

Cheers,

Steve P.

[Bryson10]

Thanks Andy and Steve for your replies. The buzz is is variable with the vol control so I guess that rules out it being the smoothers? This is a bit of a new venture for me so I'll do a bit of research on waxies first and study a schematic. The logical approach would be to start with the most common suspect ones - I'd be interested in knowing some more about 'that capacitor' and where it is. I also forgot to mention in my original post that I tried recording and got nothing at all - no new recording, it didn't even erase anything. Don't know if this is linked or a separate stand alone problem (challenging opportunity) to overcome.

[Steve_P]

Looking at the circuit above, C22 is a good example of THAT capacitor. So is C8.

Has the wiring to the heads been interfered with? Is the switching OK.

Cheers,

Steve P.

[yestertech]

As I recall my 18L/23L had a similar problem. More thank likely the triple section electrolytic in the PSU assembly is the culprit ( probably a HUNTS ) This is a separate sub chassis to the main PCB and can be removed to work on it. There's an additional decoupling resistor fitted between sections of said electrolytic ( 4k7 ? ) which may well be open, starving parts of the circuit of HT.
I think there's enough room in that area to wire in replacement parts. If all 3 sections are defective, you're better off removing the whole can to make some room for new parts.

Andy

[Bryson10]

Thanks for further advice Steve P. The heads and wiring look fine, all original soldered joints etc (as is the whole machine for that matter). I have identified C22 and C8 on the board so will probably start with these first. They are WIMA, axial type. I'm going to show my ignorance here - I just had a look on Maplin site and there are many to choose from. Can anyone recommend the most suitable replacement type? Is it just a case of matching capacitance and voltage values?

Regarding the advise from Andy - thanks. I did have a look at the triple electrolytic in the PSU assy. I was pleased enough that I managed to find it, but that is about my skill limit at the moment. Looks a bit of a tricky task to open up and sub in new parts so I'll leave it for now and have a go with the individual easy to get to ones on the main board to start with. The 4.7K decoupling resistor checked out OK by the way.

This is hard work. I'm learning how to read wiring diagrams and read resistor codes etc. as we go. Still, the machine is a scrapper as it is, so I might as well have a go and hopefully learn something in the process

[Steve_P]

There are now preferred values, so pick the nearest one. I go for the 400 or 630 ones listed here...

http://www.cricklewoodelectronics.co...me.php?cat=151

The other board - well I haven't got a TK18 apart in front of me right now, but I'd go for the smoothing capacitors but no lesser voltage than 450v.

Cheers,

Steve P.

[DOFFERY]

There are many potential problems as mentioned above, however might I suggest you check the "humdinger" preset (R 61)just forward of the osc. coil on the pcb, just try moving this control a little back/forward.

Colin.

[Bryson10]

Tried the 'humdinger'. It was at the min setting when I found it and felt like the wiper was not making a firm contact. Turned it through 10 degrees or so but hasn't make any difference. Thanks for suggestion anyway. I'll get some capacitors bought soon and start and swap out. Will let you know how it goes.

Bit of background info - this is my third TK I've aquired in as many months! Started off by needing a machine to play some long lost familiy recordings that surfaced. Got a TK23 and TK23L locally on freecycle but neither worked. The 23 had blown main fuse as motor locked up with a collapsed bearing. Managed to put bearing back together and crimp it all in place - machine works fine now.
The other 23L worked mechanically but no amp at all. Found blown fuse 120mA - replaced and works fine.
I bought the TK18 as I wanted a 2 track machine, mainly to play period pre recorded tapes. Bit disappointed to discover it doesn't have the magic eye - has less character and I wouldn't want to record with the auto level. Still would like to fix it up though and get some use out of it.

[DOFFERY]

Sorry to persist but move the humdinger from end to end you will not harm anything, nothing to lose.

Colin.

[Bryson10]

I tried turning humdinger through entire range but afraid no difference. Thanks anyway Colin and I don't mind you being persistent....all suggestions are welcome.

A further question, does anyone know what the metal plate is for on underside of circuit board? Does it have some shielding or heat dispersal purpose or is it just put there just to make life harder when servicing?! Also noticed it does run very close to the underside of joints etc at EF86.

[DOFFERY]

It's for shielding purposes Bryson, any luck with the machine?

Colin.

[Bryson10]

I ordered some capacitors today - hoping to get to work on it at the weekend - will let you know how I get on. Thanks

[Bryson10]

Here's a progress update on the TK18.
Just spent a couple of hours swapping out the 9 capacitors I bought but afraid has not made a jot of difference I replaced one by one in the following order - C22, C8,C3,C4,C13,C11,C9,C1 and C19. There was no practical theory behind the swap out choice other than when I ordered them I started with the suggested 'that capacitors' and then picked others at random or that visually looked a bit grotty to make the order up to a decent quantity! I don't have any means of testing capacitors so am at a bit of a quandry now as to what to do next. I could carry on get some more and replace the rest, but I've already exceeded the value of the machine with the ones I just fitted! However, it's one of those - 'I've started now so don't want to give up' situations. I think my next step will be to try and swap out the the triple section electrolytic in the PSU assembly - temporarily robbing from my TK23 machine if the same.

[Steve_P]

Where in West Yorkshire are you?

Cheers,

Steve P.

[Bryson10]

FYI I'm in near Wakefield Steve. The more I read up on this the more I think it must be one or more of the caps in the triple electrolytic (and yes - it was suggested before by Andy ).These are known as the smoothers..am I correct?...smoothing the ripple out of the AC?. The buzz I'm getting is a really nasty variety - it's not just a tonal thing - it's more of an acoustic feedback sound. It is present at zero vol setting but ramps up exponentially as the vol is increased - Playing a tape is out of the question, although you can just hear the audio at low settings, it's totally drowned out after a 1/4 turn!

I did start and try swap the tripple elc out of my TK23L. Got all the wires off but couldn't get enough heat in to melt the big lumps of solder on twisted tabs so gave up and wired it all back again.

So I've decided I'm going to take out the original tripple can and replace with 3 separate caps (50,50 and 100 uf). I read somewhere about 'running in' new filter caps using a variac - gradually winding up the voltage over the first few mins. Is this neccesery in my case or is it more relevent to larger caps?

[Steve_P]

Yes, it probably is that smoothing can, as we call them. Some like to call the first capacitor in the can the 'reservoir capacitor' as this is correct, but most engineers use the term 'Smoothing Can'.

I tend to use Cricklewood Electronics.

If you get three capacitors, then make sure that they can handle the voltage! Seeing as they don't have a 100uF Can (They do a 200uF, 40uF and 40uF but 200 is too high so it's a no no) then I would use 2 of these:

http://www.cricklewoodelectronics.co...cat=159&page=2

And one of these....

http://www.cricklewoodelectronics.co...cat=159&page=2

They are polarised, so they have to go the right way round. Tie the negatives together and this goes to earth. The positives go where the old capacitor did. Looking at the diagram should make it clear.

Cheers,

Steve P.

[Bryson10]

Finally some progress The tripple electrolytic smoother - I've replaced the two 50uf electrolytics and it's made a heap of difference. The loud buzz has completely gone and you can actually hear what's on the tape now However, there are still some strange high pitch whistles and rustling noises as you vary the volume especially at the high end.

The reason I only changed the two 50uf's and not the 100 - I'm a tight yorkshireman and the 100 seems hard to get in 400 / 450 volts. Only place I can buy in quantities of 1 seems to be Cricklewood (thanks Steve) but this means £3.50 postage on top. My local Maplins had the 50's in stock at 89p each. My gut feel now is seeing as the 50's made such a difference I better had do the 100 as well - I'll look to source one locally. Also need to try recording etc..will keep you posted. A really big thank you to all have helped with this

[Steve_P]

The 100uF one is the most important. Why not put two 50uF ones in parallel - that'll make up your 100uF.

Cheers,

Steve P.