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DMcMahon · 18th Apr 2021, 10:46 am

Been checking out a TK 14 I purchased fairly recently together with a TK 23, the TK 14 was said to be working and the TK 23 not working.

Opened it up to visually check inside before powering up. Both internal 1 1/4" glass fuses were quite badly corroded so cleaned them up. These fuses are marked in tenths of amp, i.e. the mains input fuse is "4/10A" (i.e. 400mA / 0.4A) and the HT fuse is "1/10A" (i.e. 100mA / 0.1A). It has been a long time since I have seen fuses marked in tenths.

It has a short 3 core mains cable (with 3 pin plug) that goes out through a hole in the rear panel. This hole I think would been the mounting hole for the mini Bulgin mains chassis plug that some TKs of this vintage have. I probably will refit a mini Bulgin chassis plug to tidy it up.

Powered up OK and all DC voltages check out good.

Found there was no Record bias getting to the record head and the record bias at the input of C19 bias pre-set trimmer was around 900V peak to peak, I say around because the bias waveform amplitude was too large to see fully on my scope using a x10 probe (mental note that I need to buy a x 100 probe). The bias at the output of C19 was 740V.

Traced problem to a yellow wire that connects back to the record head connections being disconnected at the junction of C19/R13. With wire refitted the bias at the head is now 480V p-p (still around 900V at C19 input).

480V p-p calculates to 170V rms, the record bias spec (measured across the head with a valve voltmeter) is 155V +/- 15% so 170 is in spec.

The erase head voltage is 230V p-p which is 81 V rms, spec is 70V rms adjustable by R37.

While checking the bias oscillator signals, kept intermittently loosing the signals. Eventually traced the issue to a tiny crack in the PCB that was causing 2 tiny breaks in 2 tracks. First PCB photo shows the crack which is hardly visible, the second photo using flash makes the crack look like a track cutting across the 2 actual solder tracks. Repaired the 2 tracks with solder bridges and put some super glue on the top of the PCB crack.

Two mechanical issue noted so far. The Temporary Stop (Pause) key does not latch down, unlike the TK 23 I cannot see any latching mechanism but the TK 14 User Manual indicates that it should latch down.

The Fast Forward key is difficult to get it to stay latched down, think all the various associated mechanism's for it just need a good strip, clean and re-lube.

David

18th Apr 2021, 10:46 am	#1
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,604	Grundig TK 14 Check Out Been checking out a TK 14 I purchased fairly recently together with a TK 23, the TK 14 was said to be working and the TK 23 not working. Opened it up to visually check inside before powering up. Both internal 1 1/4" glass fuses were quite badly corroded so cleaned them up. These fuses are marked in tenths of amp, i.e. the mains input fuse is "4/10A" (i.e. 400mA / 0.4A) and the HT fuse is "1/10A" (i.e. 100mA / 0.1A). It has been a long time since I have seen fuses marked in tenths. It has a short 3 core mains cable (with 3 pin plug) that goes out through a hole in the rear panel. This hole I think would been the mounting hole for the mini Bulgin mains chassis plug that some TKs of this vintage have. I probably will refit a mini Bulgin chassis plug to tidy it up. Powered up OK and all DC voltages check out good. Found there was no Record bias getting to the record head and the record bias at the input of C19 bias pre-set trimmer was around 900V peak to peak, I say around because the bias waveform amplitude was too large to see fully on my scope using a x10 probe (mental note that I need to buy a x 100 probe). The bias at the output of C19 was 740V. Traced problem to a yellow wire that connects back to the record head connections being disconnected at the junction of C19/R13. With wire refitted the bias at the head is now 480V p-p (still around 900V at C19 input). 480V p-p calculates to 170V rms, the record bias spec (measured across the head with a valve voltmeter) is 155V +/- 15% so 170 is in spec. The erase head voltage is 230V p-p which is 81 V rms, spec is 70V rms adjustable by R37. While checking the bias oscillator signals, kept intermittently loosing the signals. Eventually traced the issue to a tiny crack in the PCB that was causing 2 tiny breaks in 2 tracks. First PCB photo shows the crack which is hardly visible, the second photo using flash makes the crack look like a track cutting across the 2 actual solder tracks. Repaired the 2 tracks with solder bridges and put some super glue on the top of the PCB crack. Two mechanical issue noted so far. The Temporary Stop (Pause) key does not latch down, unlike the TK 23 I cannot see any latching mechanism but the TK 14 User Manual indicates that it should latch down. The Fast Forward key is difficult to get it to stay latched down, think all the various associated mechanism's for it just need a good strip, clean and re-lube. David Attached Thumbnails Last edited by DMcMahon; 18th Apr 2021 at 10:51 am.