Grundig TK-20 Recording problem

[MeanDumpsterCat]

Hello!

So I finally got a cable sorted out for my Grundig TK 20 which allows me to record.

I made a few recordings and they were perfect but after a while, the recordings started to get weaker and now it cannot even erase the tape. This happened over the course of about 20 minutes. I have already cleaned the heads.

Does anyone know where the fault may be? My first guess would be bad capacitors as I know this machine has probably not been serviced recently.

[SteveCG]

Alright then, have you cleaned the heads again? - That is, in case the tape you have used is shedding its oxide.

[MeanDumpsterCat]

Yep, it is still the same regardless of which tape I use

[SteveCG]

Can the recorder erase previous recordings if the tape-recorder is switched on from cold - ie, not have been used for a couple of hours or so? The favourite culprit, leaky wax capacitors, leak more when they are warm.

[MeanDumpsterCat]

Yeah, I saw another post here about that.
I shall power on the machine tomorrow morning and report back here with the results. I hope it is just capacitors and nothing too severe. But I should point out that playback is fine. Excellent, in fact

[vinrads]

As Steve said replace all those capacitors .Mick.

[DOFFERY]

As stated change all the caps you can, especially in the metal osc. can, this may well be a minor burn out.

Colin.

[MeanDumpsterCat]

Thanks! I will. Can these be replaced with standard electrolytic capacitors of the same rating?

[Marconi_MPT4]

Originally Posted by MeanDumpsterCat

I made a few recordings and they were perfect but after a while, the recordings started to get weaker and now it cannot even erase the tape. This happened over the course of about 20 minutes. I have already cleaned the heads..

..But I should point out that playback is fine. Excellent, in fact
Does anyone know where the fault may be?

Hi,
If the heads are clean and previously recorded tapes are not being erased together with low recorded signal level, I would direct attention to the bias erase oscillator circuit as it is common to both fault conditions. On this model it is a self contained dedicated circuit which simplifies things. Attached snippet.

In record mode, the anode voltage of EL95 should be 260V ±20%. There are a few capacitors but perhaps also check the 300Ω cathode bias pot as it is not unknown for these to develop bad contacts. Is the valve seated correctly and no intermittent holder contacts? If an oscilloscope or AC millivoltmeter is available you should be able to observe the oscillator output across erase head connections. You may even have a faulty EL95.

Rich

[MeanDumpsterCat]

Thanks for your advice. Is there an easy way to test for a faulty EL95. I just have a sneaking suspicion that this fault may have come about from a slight overloading on the recording input. Keeping in mind the only useful bit of electrical testing equipment I currently have is a digital multimeter.

[Lucien Nunes]

Don't worry, you will not have damaged anything by overloading the input, certainly not the oscillator. If you have not replaced the critical capacitors yet (C3,4,5), do so now! They are the prime suspects, much more so than the EL95. Pay attention to the voltage rating of caps in the oscillator circuit - ensure your replacements are at least equal to those originally fitted.

Can these be replaced with standard electrolytic capacitors of the same rating

The electrolytics can, but none of the capacitors in this section of the circuit is electrolytic. While you are at it, do the important ones in the replay circuit even though the sound quality might be fine. The audio output stage could be under stress due to leaking coupling caps, putting the output valve and transformer at risk of failure.

[TonyDuell]

You could try swaping the EL95 with the one in the audio output stage and seeing if the machine will still play back correctly through the internal speaker.

I would expect the AC votlage across the erase head (if the machine is working properly) in record mode to be 10's of volts. But it will also be 10's of kHz (above audio frequency range) and a lot of cheap digital multimeters have AC ranges designed for mains and nothing more. It can't hurt to connect your multimeter on an AC volts range across the erase head, but it may not tell you much.

[MeanDumpsterCat]

Originally Posted by Lucien Nunes

If you have not replaced the critical capacitors yet (C3,4,5), do so now! They are the prime suspects, much more so than the EL95

Thanks! Are the C3,4 and 5 caps in the cylindrical can or are they contained within one of the circuits containing an EL95? Sorry if it sounds like a silly question but I usually only work with equipment that is on a PCB so it's a little confusing for me.

[DOFFERY]

Any results DumpsterCat.?

Colin.