Juicy Music Peach II Preamp

[nre]

I have a Peach preamp sent in for repair, no 6.3VDC filament supply.  The filament circuit is shown below.  X7-X8 provide VAC to a bridge rectifier which, in conjunction with the 4700uF filter cap, provide VDC input to the LM350 regulator (TO-3) with output set to ~6.3VDC.  At power up there is no output from the regulator.  VAC is available from the transformer tap.  This apparently is a known issue with the amp and the fix is straightforward.




Removal of the PC board requires desolder of two LED terminals, all other connections to the board can be disconnected without desoldering.



PC board removed:

[nre]

The rectifier bridge showed distress, the case was bulged and cracked.  Pulled and prepped the board for an off-board replacement with a bit higher forward current capacity.

Underside of the bridge rectifier.



The outer case of the LM350 package is the device output.  Over time, operating the preamp causes this connection to fail.  The fasteners holding the entire assembly to the board were loose and they assembly would  wiggle a bit.  Here, I replaced the Nylon spacers, six locations, with threaded aluminum standoffs of the same height.  Mica spacer keeps the input and adjust terminals of the LM350 spaced relative to the heat-sink hole pattern.  The heat-sink and the LM350 case connect to the two solder pads to provide power to the filaments. 



Note the four fasteners hole locations that attach the regulator assembly float, making no contact to the preamp circuit.  The entire assembly connects electrically to the two large pads shown.  Check protection diodes.



The voltage regulator now works, a good thing given it's a NLA part.  10VDC applied to it from a bench power supply produces proper output voltage for filaments:



Bottom of assembly, new #6 Keps fasteners:

[nre]

Almost done with the Peach.  Replaced the on-board bridge rectifier with a more robust device.  To facilitate cooling it's mounted to the chassis.  On the unit I have here there are a few unused fasteners attached to the rear of the unit.  Replaced one faster with a #6 machine screw and attached the bridge to the aft section, interior of the amp.  13 VAC is applied to the bridge from a tap on the power supply transformer (a Heyboer).
The circuit provides 6.4VDC across the filaments of the preamp tubes.   





 
on-board

[nre]

Bright filaments, good sign (didn't do this when it came in).




And clean output.

[nre]

X7-X8 is the secondary tap that provides power to the filament curcuit.  At turn on, the curcuit pulls about 3200mA RMS (3.2A RMS) peak current.  Under steady state conditions the tap sources about 2900mA RMS (2.9A RMS).



The bridge rectifier mounted the aft chassis panel reaches about 90F after an hour or so of operation.  The leads used to connect the bridge rectifier to the chassis are #18 AWG tin-platted, stranded copper which has a little over 2X the current capacity for the application.

[nre]

This assembly will reach about 170F at equilibrium, measured at the top of one of the fasteners holding the device down.  The OEM nylon spacers would creep under these temperatures resulting is the output of the device to the PC board faulting.  The aluminum ones will be better able to handle the temperature.  A 6.3-6.5VDC potential should be measured at the test points.

[nre]

The unit came in with two broken RCA inputs which I removed. 





The RCA jack is NLA.

There are many configurations similar but I've yet to find a similar solder pad pattern.  One simple fix is to hang a female RCA "dongle" from each which will provide connectivity.  The alternate is to drill the chassis for panel jacks which I haven't done.  I may tape them both with Kapton and leave them.  Should the owner wish to add jacks the diffiult part, removing the OEM units, is done.