Many will remember the monthly stories of the servicemen's difficult jobs, as told in the pages of Electronics Australia magazine.
I was engaged to repair a custom crafted Hi-Fi stereo audio amplifier. Norh model SE 18
Naked type construction, with chrome plated chassis and very heavy!
The unit uses a pair of EL34/6CA7 parallel'd output valves in a well proven "single-ended" design, capable of 18 watt RMS output/channel.
The power and speaker output transformers were of a large physical size - probably more for appearance sake, than design criteria.
Applying the { 2.5 s l h t} formula with the transformer dimensions gave a rating of 400 VA (Watts)
A core transforming 18 watt of audio is then well under-run!
The amp ceased to function after the owner (and the amplifier) had returned from a lengthy overseas employment posting.
Upon inspection the O/P valve screen resistors were well "burnt-up"
Simple replacement just meant they burned too!
Voltage measurement indicated NIL plate voltage at the anodes.
Therfore any electron flow thru the valve came to the screen grid, which over-rated the screen resistors.
Continuity check showed that both (L & R channel) speaker transformer primaries were O/C
Further inspection revealed that both the transformers had been damaged by intrusion of a spike/screwdriver etc!
When informed of this; the owner suggested it was most-likely caused by South-American customs officers looking for contra-band when being shipped back from Venezuela to Australia!
One EL34 O/P valve had a control grid-screen grid short, as well (caused by nil Anode voltage; due to O/C tfmr)
To effect repair; both transformers were dis-assembled, turns counted and re-wound.
The transformer was of an advanced design where both primary and secondary windings were in thirds and interleaved - 6 seperate windings in total (see schematic). Quite a job to rewind and terminate!
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The interleaved windings concept is now well known; first appearing in the famous "Williamson" design Amplifier, published in Wireless World magazine in 1947. Such transformers produce lower levels of audio distortion than more conventional designs. The "Williamson" type circuit set the standard in audio amps' for many years to come. |
Further Reference:
1) Original Article (Wireless World: April 1947)
2) The Williamson Amplifier by Peter Lankshear.
Electronics Australia Mag' July 1990, pg 150
3) The Williamson Amplifier Revisited. by B.v.d. Kerk.
ELEKTOR magazine 9/97 (Audio Supplement)
The failed EL34 was replaced, and the transformers re-assembled (after checking polarities of all the respective transformer windings).
The amplifier is now working well again!