Download Armstrong 521/525 output transistor replacement

Armstrong 521/525 output transistor replacement
This modification will allow you to keep your Armstrong 521 or 525 running if the original AL102's fail as mine have now done. A silicon pnp device such as a
MJ2955 will suffice ( it is a better spec than the old Germanium AL102 ). A few resistors have to be replaced on the power amplifier printed circuit card, these are
in the bias circuitry for the output transistors. Note, both output transistors have to be replaced for MJ2955's even if only one AL102 has failed ( it is not advisable
mix and match as the output symetry will not be as good ). The original output transistors run with a standing current of approximately 150mA, with the resistor
values given the replacement pair should also mirror this, if not, tweak R1281 and R1278 slighly until it is within plus or minus 20mA of this figure. Here are the
component replacement values ; change R1281 and R1278 from 18K to 11.6K ( place a 33K in parallel with the 18K ).
Change R1277 and R1279 from 750R to 675R ( place a 6K8 in parallel with the 750R ). Remove R1283 and R1282 ( a
51R ) and replace with a couple of 100R. Once modified check standing current in output pair by measuring the voltage
across R1284 or R1285 ( a 0.47R ) and use Ohms law to calculate the current, you are looking for roughly 0.07 volts
across either of these resistors. On the circuit diagram I have, R1279 is labeled as 279 and on the component layout
drawing it looks like 219 !! Pictures are included to help identify the relevent components. Be sure to use some heatsink
compound on the replacement transistors. The second photo shows the rear of the printed circuit card with the additional
resistors tagged on. The third photo shows the component side with the two screw drivers pointing at R1283 and R1282.
Note, these modifications are for the A15 power amplifier version, having never seen a A14 version I am not sure if this
mod would work for the A14, however from studying the circuit diagrams this mod should hold true for the A14
providing the total resistance for R1281, R1278, R1277 and 1279 ( i.e. using additional resistors in parallel with the existing ones ) is equal to the figures given for
the A15 board, also providing driver transistors Tr26 and Tr26 are silicon units as in the A15 card ( mine has BSV44's for the drivers - Tr34, Tr36 on the A15 card
which are silicon ). Change R1283 and 1282 for 100R as per the A15 mod.
Note; I also carried out this modification on 521 ( does not have the tuner ) but some HF instability and slight hum
was noticed. A 10nF capacitor from the collector of the top output transistor ( i.e. the one with full supply volts on
its collector ) to chassiss ground cured this. Mount the cap on the solder tag on the transistor collector connection
and the nearest chassiss ground point.
Armstrong 521/525 repair/modification
I have a couple of these old but loved tuner amplifiers, one was bought recently to use in the kitchen so my wife Ann could
listen to radio or CD's while cooking etc. The other is used as a sound system for my new telly and as a ordinary stereo
driving a couple of tannoy mercury wall mounted speakers. Sadly, a lot of these amplifiers are discarded due to the
unavailability of the germanium output transistors. When working, these are a nice sounding amplifier, a lot nicer than a
modern midi system as used in our lounge. Both of these units had non functioning tuners ( it is an FM only tuner ) and both
had identical component failiure - the self oscillating mixer transistor, a AF115. The photos show the offending component I don't have a circuit diagram for the tuner so am unable to give a component board number but the screwdriver is pointing at
the duff transistor. A AF139 was used as a replacement in the two tuners being as i have a few of them in the junk box. A
few days later the series pass regulator failed ( base collector open circuit ) in one of the amps so a MJ2955 was fitted, no
circuit modification is required. As a point of interest the original transistor is the same as the amplifier output transistors - AL102's. You will not be able to find
any of these ancient transistors now so if your amplifier fails due to these, a modern PNP silicon unit will have to be fitted ( MJ2955 ) and the circuit modified
accordingly ( a fairly straight forward thing to do, mainly bias currents and Si junction drops will have to be accounted for ). All of my amplifiers still have the
original germanium AL102's, when these fail MJ2955's will be fitted and the circuit modified accordingly - watch this space! Here is an interesting link to a site
with some history of these amps.
Barry Zarucki M0DGQ