Help for Marshall PA 6215 SRX series

[Zzmegi]

    Hello. Last week buy this PA and during cleaning and inspection notice broken 1 reverb spring, scratchy Gain pots and slightly burning traces on preamp plate under Zener diodes ZD1, ZD2(bzy88c15v) and around resistors R25, R26 (unknown Ohms, 5w). My question is anybody have a schematic diagram for this Power Amplifier or maybe knows the values of these two resistors?
Already fixed snapped reverb spring connection, buy new ZD´s, but without data resistors my hands are ties to complete fixing.  This is a 6-chanell mixer/pa 150W head unit.
Thank´s in advance.

[Jazz P Bass]

Here is a snip of the 8008 power supply.
it uses 1K/ 1 watt dropping resistors.
I am not certain how your supply is configured.

[g1]

What do those resistors measure?  If the unit is running ok, they are probably close to correct and you should be able to guess the closest standard value.

[J M Fahey]

You have an old school (70s vintage) PA head.

Power supply must be very similar to what Jazz P Bass posted, only allowing for higher current, we have to feed 5-6 preamps + Master + Reverb.

So instead of 1k resistors I would expect 2 x 470 or even 330 ohm, in an extreme maybe 2 x 220 ohms.

They will run HOT while dropping from 40-45V main rails to 15-16V for preamps.

It is _common_ for similar supplies to overheat and even slightly toast nearby PCB.

In some Fender amps they run so hot that solder melts and resistors are pulled out on impact, go figure.

Personally I mount such resistors with 1" long legs, to minimize PCB heating and let air flow around them.

For the record, measure DC voltages at each resistor ends, then turn amp OFF, let caps discharge fully, and measure resistance.

It should be around the range I suggest.

Please post results here; many ask for help, solve their problem and dissappear 

[Zzmegi]

  ,,Dry˝ measured resistors on PCB, no powered (without turning amp ON, DC measuring, amp OFF, as you suggested) : R25-726 Ohms  R26-675 Ohms. R25 leads to ZD1, R26 leads to ZD2 and here(R26 to ZD2)are darker burnt traces.

[J M Fahey]

Ok, 726 and 675 ohm resistors make me think 680 ohm nominal value, +/-10% which is a normal component rating.
Dropping from 40-45V to 15V would make each drop almost 2W each.

Which makes for a more than "warm" resistor.

[Zzmegi]

  So, what You suggest to little bit cooling dawn, except putting higher legs?

[g1]

No matter what, you have to dump that voltage, and that makes heat.  A resistor that is physically larger will spread the heat out more, but it is still there.  An aluminum style resistor mounted on the chassis for heatsinking will transfer the heat to the chassis.

[Zzmegi]

  Quick report...pull out resistors from PCB and read true values from the under...680R, so J M Fahey was right!
  Remove old solder, inspect for trace cracks, clean everything around melting spots and install new ZD's and new blocky ceramic resistors at 1" from PCB surface.
   All pots and input sockets do with lubricate contact cleaner...head box cleaned and reverb fixed in the days before...put it together and turn ON...hear almost nothing, only currency streaming thru the unit. 
   No more screchy Gain pots,reverb works,Master Volume smoothly increase the power.
   Try with el. guitar every channel, run like a new! 
   Thnx to everyone for advices.
   Best regards from Belgrade, Serbia!🙋