Home Download Useful Links Contact Us Our Policy Search Bloggs

Leak Stereo 20
 

 

LEAK Stereo Twenty Repair/Refurbishment

Our full service and Modification includes:

Safety check, drill and fit a HT fuse* and a fused/switched IEC block for safety. We now fit a dual IEC outlet in place of the obsolete 2-pin sockets. Perform fault diagnosis. Replace all the out of tolerance and physically damaged resistors with modern high tolerance types. Replace the full set of small signal electrolytics and dual smoothing blocks. Replace the full set of paper couplers and decouplers with HV polypropylene capacitors. Check valve biasing and recommend what valves, if any,  need replacing. Faulty valves will be replaced at cost if requested. Full performance check and adjustment.  24 hour soak test/retest.  

190.00 (inclusive of labour!  

If the output or mains transformers are faulty we can replaced them at cost plus 20.00 fitting charge.

The images below show a typical full service and safety modification that will cost just 190.00 + return carriage if applicable.

 

Top view showing the finished amplifier.

   

 

Part schematic showing added fuse and GZ34 protection diodes.

The schematic above shows the position of the HT fuse  and GZ34 rectifier valve 'protection' diode mods. (Shown by red dots).  The diodes help preserve the life of the GZ34 particularly if it is a modern reproduction type.

The images upper left and left, show the rear of the Stereo 20 with the  modified connectors. In place of the round pin Bulgin connector a switched/fused IEC block is fitted. The upper left image  has professional speaker terminal posts, twin IEC outlets and an internal HT fuse fitted. On the lower image the HT fuse is external and fitted just above the switch cable.  The fuse is 315mA medium blow type.

 

 

We replace all the original resistors with high quality 1% metal film types. These offer several advantages. The most important being tolerance and stability. The closer tolerances allow the ideal and original operating points to be more easily obtained by careful choice of valves. Also  the noise floor will be reduced slightly.  The full set of Visconol -X  grid coupling/decoupling capacitors are replaced with 1000V and 600V  polypropylene capacitors. Rob prefers polypropylene to oil filled due to their superior temperature v leakage resistance  and lower distortion figure despite being below audible detection.

Unfortunately  in the majority of  S20's and Varislopes  we service, the Vx caps are far from ideal. Surprisingly,  the value and breakdown voltage is usually still within tolerance but the leakage resistance versus temperature performance is far from satisfactory. The increasing leakage with temperature, has the effect of adding a small positive dc bias to the following stage. The leakage over rides the automatic grid bias and changes the operating point. The effect of this is increased distortion and possible over heating of the output stage. Our policy is to replace all the coupling and decoupling caps with polypropylene types. We also choose large high voltage types to keep the physical size similar to the original capacitors.

The finished amplifier is tested to ensure it conforms to or exceeds the original specification. Including frequency response, output power, and distortion.  Finally an electrical safety check is performed (PAT).

Note: There is a lot of nonsense on the Internet regarding the audible difference between the original Vx  , modern polypropylene (PP) and oil filled (OF) capacitors. This is made worse by some companies 'cashing in' on the inflated price of  paper-in-oil audiophile caps. See the blog on this here. We normally fit high quality polypropylene replacements. However if you prefer paper-in-oil types or any other audiophile type we will happily fit them at cost but please don't expect to hear any difference.** The subtle differences in the inherent distortion between good quality polypropylene and oil filled coupling capacitors is lower than the noise floor of a stereo 20. In other words, it is too small to be measured when fitted in a Stereo 20 and way below aural perception. The tiny distortion inherent in all capacitors is often greater with some makes of OF than PP or PE capacitors and vice versa. The important point to bear in mind is the relative amount of distortion added by a capacitor is miniscule, compared to the sum of distortions from the music source, the loudspeakers, your own aural system, and  rest of the amplifier itself. The capacitor make  and whether it is oil filled or polypropylene is the least thing to blame if someone is unhappy with the sound of their system.

SAFETY:
Compared to today's electrical safety standards, the Stereo 20 in its original form is inherently unsafe. To improve the electrical safety we fit a new IEC mains block with combined fuse and on-off switch. We also fit an internal or external HT fuse. In the event of an output valve going anode /cathode short the fuse will blow.   The stereo 20 is often used in its open chassis form and placed on a shelf or table. This is very unsafe practice. The chassis must be screwed firmly to a metal bottom panel with small rubber feet.

We can fit a bottom cover and feet when we service your Stereo 20 for 15.00 extra.

 

*We now fit an internal HT fuse and holder rather that the rear chassis fuse. The fuse  is added to effect a safer alternative to the original green wire wound resistor over-heating, and de-soldering itself.  Under short or near short circuit HT fault conditions, e.g. an EL84 internal electrode s/c, the fuse blows.

** Some paper in oil capacitors on sale exhibit a substantial leakage resistance that becomes very significant as the amplifier temperature increases. This will have a marked effect on the DC operating point of the pre and output valves. This results in distortion and  can lead to overheating of the output valves.

 

 

Copyright 2009-15 Amplabs
Last modified: Monday, 27. April 2015 12:57