Latest Acquisition - Grundig TK 14

[DMcMahon]

Yes that would certainly make sense. I have not yet seen any GRMs but have seen quite a few GBMs.

[DMcMahon]

Had a quick check of the Bias Oscillator to determine why Erase and Record are not working.

The output pentode valve ECL86 is a shared valve and becomes the bias oscillator in Record mode. First monitored the anode output at pin 6/C20.

Initially no bias signal but after a few taps, started getting a good signal but intermittent, then noticed a little sparking at top end of C14, looks like a loose/dry soldered joint.

Top end of C14 is the output transformer speaker output and also goes to the output DIN. Do not see any obvious connection between C14 and the bias oscillator.

Will first make good the solder joint and retest.

[Techman]

You were probably using a scope to check the oscillator, but a little word of warning from bitter experience when using a standard DVM around the output stage when there's this type of intermittent fault. I destroyed a DVM while working on a tape recorder about a year ago with an intermittent output stage fault. Meter was set on the 600 volt DC range, but due to the high induced voltages from the transformer caused by the inductance being interrupted by the fault condition, the meter was destroyed in seconds! It was a cheap meter, but was an old favourite and I still haven't got over the loss

[DMcMahon]

Yes was using a scope. Thank you for the heads up, I can well imagine that scenario.

With my recent experience with the faulty EL42 output pentode on my TK 5, I measured AC noise spikes at the anode output (connecting to the output transformer) of well above 600 volts peak to peak, I think occasionally > 1,000V

[DMcMahon]

Repaired the dry soldered joint at C14, now getting repeatable good bias signal. Not convinced the previous intermittent bias signal had anything to do with C14, maybe more likely bad contact on the Record/Play switch. Would like to Servisol spray the switch contacts but difficult as it is covered/enclosed with a metal cover.

Very healthy and clean/stable 57kHz 500 volt p-p bias signal at the ECL86 anode going to the bias oscillator transformer.

Measuring at the bias transformer wrt to ground :-

40V P-P bias signal going to the bias amplitude adjustment R37 (via C21).

Large 240V p-p going out to the Erase head, seems large, need to check next at the Erase head.

Extremely large around 1,000V p-p going out to the Record head (via C19 frequency trimmer). This seems far too large ? need to check next at C19 and the Record head.

[DMcMahon]

Measured the signals at the heads. Same 240V p-p at erase head and 500V p-p at Record head (1,000V into C19 trimmer dropped down to 500).

As before seemed too high. Spec for erase voltage is 70V rms and record bias 155V rms, I must remeasure using my FET VOM.

By Peak to Peak calculation conversion to rms, 240 = 85 and 500 = 177, so my results are somewhat high, but not as high as I had originally assumed.

The heads have good continuity resistance.

I now need to recheck recording & erase to see if they still do not work.

[DMcMahon]

I see at Post 11, I said recording worked using GDM 18 microphone but at Post 15 I said recording did not work using GDM 11 microphone.

I need to double check my facts in case I am getting mixed up/confusing myself

Note - GDM 11 was the one in question if it was a Condenser mic (should not be, by the by D (Dynamic) designation letter).

[DMcMahon]

Retested record using 2 different Grundig GDM18 mics. Get good record signal on the EM84 Magic Eye but no sound heard.

On playback of that recording the previous recording is erased but no new recording, so need to go back inside and do some scope monitoring to see what is happening to the record signal. Playback of old recordings still works OK.

[DMcMahon]

By scope check am getting what appears to be a typical/normal record audio signal arriving at the record head superimposed on the much larger record bias.

But still no playback of new recordings, playback of old recordings are fine. Have tried several tapes, all the same. So more investigation required.

[TonyDuell]

Is the other side of the head being correctly earthed in record mode (contacts L23, L24 on the circuit diagram in the manufacturers manual)?

[DMcMahon]

Yes Tony I checked that previously when I checked out the bias and erase connections.

Yesterday while scope probing the live side of the record head I also scope probed the other side and it was grounded, i.e. if it was not grounded, then would have seen the bias signal on that side of the head as well as the live side.

Tape contact is good, head continuity is good and the head has been thoroughly cleaned. I see at Post 11 where I said recording worked with one microphone, that I then had only checked record level from the Magic Eye, i.e. had not done a recording to tape. The Magic Eye is still responding normally.

So currently a bit of a mystery. Intend to have a look at recording signal on one of my TK 23s (basically 4 track version of the TK 14) for comparison to see if I am missing something.