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Old 3rd Nov 2021, 10:14 am   #8
Join Date: Aug 2007
Location: Walsall Wood, Aldridge, Walsall, UK.
Posts: 2,245
Default Re: Grundig TK27 - Low audio output


C.A. Quarrington's "Radio and Television", (1963 edition) Volume 2, Chapter 1, Power Supply and Decoupling", gives typical performance curves for a metal rectifier with a capacitor–input filter, and a good full wave bridge unit should give a d.c. output of approximately 20% more than the r.m.s. a.c. input voltage – for example, the Elizabethan LZ29/LZ29L quotes "225V" as the h.t. secondary voltage from the mains transformer and "270V" for the rectifier output, and there are several Ekco and Murphy circuits using metal bridge rectifier units that also agree with this!

The grid–bias voltage of an EL95 is about –8.5V to –9.5V for 250V on anode and screen with respect to cathode which agrees with Grundig's Figure!

Lack of volume in a tape recorder, should lead initially to a very careful examination of the R/P head, as deposits bad enough to severely restrict volume and treble output can be present without you actually being able to see them, so use a "chamois leather" or microfibre cleaning cloth and IPA to make sure the head is scrupulously clean first!

The next step is to check, against the circuit diagram, all the contacts of the record/playback changeover switch, looking for any high–resistance ones – even an ohm or two at the input due to tarnish, oxidation, etc., end can seriously impair volume because of the very low signal voltages handled!

Thirdly, are the anode and screen voltages of Rö 1/Rö 2 (EF86) correct? The low cathode current taken by this valve can lead to "cathode poisoning", which results in a chemical degredatlon of the emissive cathode coating – either replace the valve or temporarily shunt a 10k 2W resistor across R103/R203 (anode) and a 47k resistor across R104/R204 (screen–grid) and run the recorder for a couple of hours without the EL95s, this should help clear any cathode poisoning that may have developed in the EF86 valves!

If nothing comes to light from the above, the electrolytic cathode decoupling capacitors C111/C211 and C12 should be checked for partial or complete open– circuits, whilst lack of volume and poor bass indicates partially open–circuit anode–to–grid coupling capacitors! (C3, C110/C210 and C112/C212)

Very poor volume and bad distortion can be caused be s/c turns in the primary of the output transformer, but a d.c. resistance comparison with one in the other channel should pick this fault up!

Chris Williams
It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!

Last edited by Chris55000; 3rd Nov 2021 at 10:28 am.
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