Playmaster setup procedure

[PeteMcK]

I’m resurrecting an old Electronics Australia ‘Playmaster’ Twin 40W kit amp from Dick Smith I found on the side of the road. Sorry don’t have an electronic version of the schematic I can post.

In the set up procedure, there are 2 links which are left open. These remove the HT supply from the front end of the amp & the Vas. The idea is to wire 100 ohm resistors across fuses to the output complimentary pairs, and set the bias current such that the voltage is 200mV across these resistors. Then you solder the links and put the fuses in.

Is it necessary to open the links? The ETI 380 used the same setup procedure without links, and it seems that the Vbe stage would only have current available from the bases of the driver transistors of the output complimentary pair with the links open.

Two other questions – I’m using 2n2773s instead of 2n3055s as output transistors, Will these be OK without any other changes?
& the Vas, a BD140, has no Miller comp cap, should I put one in?

Thanks

[sakis]

2n3773 is fine .....
if you are assembling a complete kit make sure that you trush all elctrolytics ...they are probably very dead after all these years

as about the idle the best way is to assembly the all thing ,use a multiturn and nomatter what technique you decide to use make sure that you built the power and the idle of the amp slowly noticing in between for anything weird ( weird will be that idle is not stable and after a certain point idle goes nuts which is a good sine of oscilation )

you may skip the links and use a multimeter ( millivolt ranged) placed on the emmiter resistor of any of the output transistors and use ohms law to calculate idle

then if transistors are located ""far away"'' from the pcb make sure you dont use twisted cables ...and especially do not twist the base cable with emmiter and collector

finally as about the miller cap depents if your amp uses miller caps on the drivers ...

if so there is more than one approach for this

some use 100 pf arround the drivers ( or more ) and very small in the vas area 10-20pf

others use 68-120 pf in the vas and nothing arround the drivers or so

this procedure will requiere a scope and generally a too small cap or no cap at all in the vas transistor will result a superior performance in sound but may be an unstable amplifier

also you may notice that miller cap will effect sonics if too big

finally make sure that the cap used there is a good quality cap
example :
one of my amps managed to stay stable with 100pf ceramic but sonics were not perfect replaced that with 68pf styroflex ( silver micas is also supreme ) and results were better same stability but better sonics

(use of ceramics there is not recomended )

regards sakis

[PeteMcK]

Thanks Sakis, thanks for the reminder about the electros, and the hint about not twisting wires, I was wondering about that.
It makes more sense to me setting the bias with the amp operating as it normally would, so I'll be brave & try that...:-)
Re the miller cap, actually I think in this amp they do it somewhere else, there's a cap from the Vbe multiplier/ccs for the Vas to ground, that must be it.
cheers

[sakis]

post a schematic ..... even a hand drawn this will help a lot

[PeteMcK]

Sakis, schematic attached
I wired 100 ohm 1/4 w resistors across the fuses, when I turned the power on, the smoke evaporated from two on 1 channel; They recommended 1 W resistors in the article, and the 'troubleshooting' procedure they gave says to up these to 5W if they smoke
Haven't gone any further yet.
the smoked 1/4 W resistors still measure about 100 ohms, so I guess the current wasn't TOO high...

[sakis]

this is the most classic quasi schematic ...this one and many variations existed from ages ago

a topology like that one will have a margin to set the idle fro 10-100 ma so curent flow is not enough to cause this

you may try the same procedure by turning the trimer to the xact oposite side to reduce the idle but to me its more than obvious that you may have some other error there

you may also try with bigger resistors since this is too small ...i am too lazy to do the math and find out which is the curent that is possible to cross the 100R@1/4w do that ....