Grundig TK18L distorted sound on record

[martin.m]

This early 1960s tape recorder played back perfectly but recordings sounded overloaded and a little distorted as though the recording level was too high. However this model uses an auto record level circuit so finding the fault was a little more difficult than usual. There is an excellent description of how the circuit works in the Trader sheet available from this website. In record mode audio signals over a certain level are fed to a "limiter" circuit which produces a negative voltage that is supplied to the grid of an EF83 valve in the record amp circuit, controlling it's gain. The limiter consists of an ECC81 double triode valve wired as a two stage amplifier. The first triode has it's cathode held at a positive voltage so that only signals on the grid that are over a pre set level are amplified. The second stage is a cathode follower and audio at the cathode (pin 3) is rectified to give a negative voltage proportional to the level of signal. This is used to turn down the gain of the EF83 record amp stage. There are no voltages around the ECC81 given on the Trader sheet. The ones on my circuit were taken from another working TK18. I found that the voltage on pin 3 (the output of the cathode follower) was 250 volts, far too high. The cause was an open circuit 8.2K resistor, circuit reference R17. Once a new component had been fitted recordings were excellent both with line in and microphone. Auto record level has a bad reputation but the TK18's works really well.

[ricard]

Thanks for the excellent description of the fault and it's cure, and the additional voltage readings which certainly will help future readers with this kind of fault.

Out of curiosity, what would you say makes the auto level control successful on this machine?

I'm assuming the auto level function simply reduces the gain quickly when there are peaks in the recording signal, and then slowly ramps it up. The trick I'm assuming is not to reduce it too quickly for very short peaks, and not ramp it up too quickly, both would cause a pumping effect.

[vinrads]

Nice bit of logical fault finding Martin . That one needs to go in my little black book , Mick.

[martin.m]

Thank you for your comments. Ricard, I am not a design engineer but I think the reason why the Grundig's auto record level works so well is that it does exactly what a human operator would do. It turns down the gain of the record amp to prevent over-recording and keeps it at that level. The negative rail charges up C6, which is 10uf and takes a long time to discharge as it feeds a high impedance circuit. According to the service information, the time constant is around 15 minutes for the line input and 3 minutes when the microphone input is switched in.

[ricard]

Those are fairly long time constants, but as you say that's probably why it's a successful implementation. I'm thinking the 15 vs 3 minutes corresponds to the "music" vs "speech" setting on other machines.

It looks like there is no mic/line switch on the front panel of the machine, is there a switch connected to the mic DIN socket?

Also, it looks like the contacts labeled st3 in the schematics reset the gain to the maximum level, presumably each time the machine is stopped so one is not potentially left with a low gain setting from a previous recording and have to wait several minutes for it to recover.

[DMcMahon]

Totally different circuitry of course but the time constants on my Uher 4000 Remote Monitor are 120 seconds for music and 12 seconds for speech, these are the times (adjustable) for the record signal to return to same level after a 20dB input level reduction when in ALC (Automatic Level Control) mode.

ALC recording seems to work well on the Uher.

David

[DMcMahon]

Of course 4000 Remote Monitor should have read 4000 Report Monitor.

[martin.m]

Ricard, the TK18 has a switch to select microphone input. The layout of controls is the same as for the TK14 series. According to the service information, contacts st3 are activated by the Start key so you are probably correct. The negative rail for the auto level circuit is shorted to earth in playback mode.

[ricard]

Thanks Martin! (I should have remembered that these small Grundigs had a microphone selector switch right on the panel).