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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 30th Jan 2021, 12:44 pm   #1
Xyience
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Default Grundig TK20 'motorboating'?

My TK20 is experiencing an audio issue which I have described as motorboating in the thread title, but the truth is I do not know the name or source of the problem.

Symptoms: Audio output through the built-in speaker is clean and distortion-free until I set the volume control past a certain volume. At that point the playback breaks up into a low-frequency, loud "duh duh duh duh" where the music can still just about be heard, albeit chopped up.
If I back off the volume the clean sound returns.
The volume level at which the oscillations start gets lower and lower over the course of a listening session.
Letting the deck cool down seem to reset the point of oscillation on the vol. control.

What I have 'fixed' so far: The deck was untouched when I received it. Sound was OK for a few weeks and then got quieter and more distorted. I then replaced all of the Wima toffee caps which all had surface crazing. The sound came back.
I have not touched any of the electrolytics or the rectifier.

Is this motorboating and how do I fix it?
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Old 30th Jan 2021, 12:52 pm   #2
Edward Huggins
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Default Re: Grundig TK20 'motorboating'?

This could be HT starvation. Maybe check the voltage DC voltage from the Bridge Rectifier.
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Old 30th Jan 2021, 1:34 pm   #3
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Default Re: Grundig TK20 'motorboating'?

That sounds exactly like my Ferrograph. The usual cause is the reservoir/filter capacitors (often a dual-type in the same metal can). If they're original they could well be past their sell-by date. As their impedance increases at low frequencies, increasing the current draw by driving the loudspeaker more causes feedback through the DC power supply circuit. Wikipedia describes it as a 'sub-audio oscillator'. In my case, I could see the fluctuations on the HT rail with an oscilloscope.

The problem was solved by replacing the reservoir/filter capacitor with a new one. The ratings of the original are no longer available (8F/16F) so I replaced it with a dual 16F from Cricklewood. Their selection is here.

So, as Edward Huggins says, check the DC conditions!
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Old 30th Jan 2021, 7:29 pm   #4
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Default Re: Grundig TK20 'motorboating'?

Personally have not experienced the Motorboating symptom.

I see there are some old Posts suggesting it could also be due to bad contacts on the Play/Record switch which I think for the TK20 possibly may be relay contacts.

David
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Old 30th Jan 2021, 11:07 pm   #5
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Default Re: Grundig TK20 'motorboating'?

Grundigs seem to be favourite for this. I had a TK145 and I was sure it was a dry joint but it turned out to be an Electrolytic at the time the recorder would have been just 5 years old. (we were still in the 1970's just)

Oh and I still have it must dig it out now unused in the last 40 years.

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Old 31st Jan 2021, 9:29 am   #6
Xyience
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Default Re: Grundig TK20 'motorboating'?

Thank you for all of the helpful responses.
I will check for sagging DC, see if the relays are clean and intact and probably replace the smoothing cap if I do not like what I find.

I have looked around the Cricklewood electronics website. Are those yellow axial caps worth it in terms of sound quality? They are a bit pricey but I am can jemmy open the wallet for future projects if its going to make a difference.

I lived in Willesden Green for 6 months but can't remember noticing the Electronics shop just down the road.
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Old 31st Jan 2021, 9:41 am   #7
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Default Re: Grundig TK20 'motorboating'?

I have found Cricklewood's range of "500V Dual Capacitor for Valve (Tube) Amplifiers, " to be very satisfactory for power supply applications.
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Old 31st Jan 2021, 9:54 am   #8
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Default Re: Grundig TK20 'motorboating'?

While you are in there checking out the reservoir/filter electrolytic can capacitors & HT supplies, certainly worth giving some of the contacts a clean.

On the underside of the main selector there are 3 contact sets, 2 for PB/Record, easy to give them a spray of electrical contact cleaner (and well exercise the contacts afterwards) if contacts really bad also easy to get to them to clean with fine abrasives.

The PB/Record relay (designated Q on the schematic) is not so easy. It is a plastic cased relay under the main selector towards the middle behind a big can capacitor. Its plastic case does have a hole in it where you could spray contact clean into but to actually mechanically clean the contacts would be major job, I think you would have to remove all the various cables to it, for it to be removed, so not very practical.

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Old 31st Jan 2021, 10:50 am   #9
Xyience
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Default Re: Grundig TK20 'motorboating'?

I have gently cleaned all contacts that I can physically get to, so a squirt of the good stuff into the cased relays is now on the to do list.

My previous comment on yellow axials was a bit misplaced. Independent electronic shops need my custom more than the huge suppliers. This isn't about saving a buck here or there. I have slapped my own wrist and apologise to Cricklewood - expect an order at some point soon.

I will not edit the original comment but consider the question as rhetorical.
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Old 31st Jan 2021, 11:00 am   #10
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Default Re: Grundig TK20 'motorboating'?

Quote:
Originally Posted by Xyience View Post
I have looked around the Cricklewood electronics website. Are those yellow axial caps worth it in terms of sound quality? They are a bit pricey but I am can jemmy open the wallet for future projects if its going to make a difference.
No 'difference', but they will be more reliable than buying on eBay. There's a lot of electronic stuff that has poor quality control, and those eBay deals that look too good to be true usually are (components being sold at far less than places like RS and Farnell).

An excellent introduction to the various capacitor types here.

Audio, as far as electrons are concerned, is very undemanding. You will hear no difference whatsoever if you take into account the manufacturer's ratings. See here.

The important thing for valve equipment when replacing filter and reservoir capacitors is the ripple current, as described in the link above. Most modern components are designed for switched-mode power supplies, so are expected to work at higher and higher frequencies, not deal with a ripple of charge/discharge at high voltage on a rectified supply.


Edit: crossed post with #9. This wasn't a dig at your post #6, Xyience - just you won't find much enthusiasm around here for the idea that capacitors sound better. Cricklewood are worthy of the hire and have been helpful on the 'phone too for me. I also find them good because of the more limited stock. There's not the ruminating on trying to work out why fifteen similar components are all different prices, as seen with the giant suppliers! However, no-one wants to pay over the odds and when I've not been paid I do have to put a hold on capacitors that cost 1 each!

Last edited by Uncle Bulgaria; 31st Jan 2021 at 11:18 am.
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Old 31st Jan 2021, 11:45 am   #11
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Default Re: Grundig TK20 'motorboating'?

Quote:
Originally Posted by Uncle Bulgaria View Post
Audio, as far as electrons are concerned, is very undemanding. You will hear no difference whatsoever if you take into account the manufacturer's ratings. See here.



A very useful, interesting and informative Thread link.

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Old 31st Jan 2021, 11:57 am   #12
Xyience
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Default Re: Grundig TK20 'motorboating'?

Uncle Bulgaria - no dig taken. I realised that my post #6 could have been seen as negative towards a trusted supplier. They do not need me questioning their quality and prices!!

I have been using good quality components from RS. These are often surface mount, so good value as well. If/when I move to the yellow axials this will be more to do with convenience in soldering and a neat job than a search for audio perfection. The forum discussion you linked in #10 ( and David in post #11) would have cured me of that fallacy anyway.

Facebook often subjects me to adverts for specialist guitar suppliers. The capacitors they sell have nice names, pretty graphics and steep prices.

Last edited by Xyience; 31st Jan 2021 at 11:58 am. Reason: Fair attribution of link sent
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Old 31st Jan 2021, 1:31 pm   #13
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Default Re: Grundig TK20 'motorboating'?

I had the same problem with my TK14.
Traced it to bad relay contacts, took out the relay, carefully cleaned the contacts using a small brush and servisol, fault cured.
John.
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Old 11th May 2021, 7:06 am   #14
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Default Re: Grundig TK20 'motorboating'?

One Post moved to new Thread here:

https://www.vintage-radio.net/forum/...40#post1373040

Cheers

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