Dynaccord Powermate 1000 MK1 Bias Problem

[styler173]

Hallo Guys,

I'm new in this Forum and I have a Problem with the Bias Setting on Powermate 84169H Amp.

Just I want to set the Bias L to 4mV but I have produced inadvertently an bridge with my meassuring neadle from Bias L+ to VCC+.
Now my Bias L is around 70mV can Anyone help me to fix the Problem?

Sorry for my bad english.

heres the Schematic from the AMP

Dynacord-PowerMate1000_1600-6.pdf

the Bias L+ is the CNSERV1-6 Connector

I hope anyone can help me as soon as possible


Thx for and have a nice evening

[J M Fahey]

Ouch!!!!
What a (needlessly) complex amplifier.
Very German, canīt understand the function of half of what I see there.

Worst of all, they claim it to be Class H but all I see is plain Class AB , power transistor collectors go straight to +/- 82 V rails, emitters go straight to speaker out rail, *where* is the dual voltage rail or tracking/switching rail voltage ???????

Unless the supply itself is modulated and tracks audio or switches when needed, because "high efficiency" means amp works with +/- 40V most of the time and switches 82V rails only on peaks.

I see nothing of that on this schematic, would love to see the power supply schematic which in this case is part of the power amp.

Back to biasing, that dreaded ultimeter tip slip is a mess because it causes random damage which is not what weīd expect normally.

That said, you got very lucky because you didnīt blow the main power section ... I hope.

The bias and actual power section (not the peripherals stuck around it) is simple: bias must supply "4 diodes" voltage because you have a chain of 4 BE junctions to forward bias, 2 above,2 below.

So you must be able to get between , say, 2000 mV and 2800 mV across Q327 ... do you?

A classic Enzo trick is to short bias transistor (Q327) CE, so bias vltge becomes 0; so voltage between CNSERV-6 and -7 becomes 0 too.

Amp will show crossover distortion but thatīs not bad *at this moment*, all we are trying to determine is whether power transistors respond to biasing or not, because problem might be some blown track or open resistor which does not allow proper bias voltage to reach where it should.

Again as Enzo often says, not all problems come from "bad parts" , many are due to "bad connections" ; in this case I can think vaporized thin tracks involved, because they were designed to carry low voltage and practically no current , not the Atomic Bomb power available straight from the +82V main rail.

So check bias voltage at the sourc and report.

In any case continuously control power transistor current (measure regularly voltage across emitter ballast resistors) toavoid cooking them while you re measuring something else.

Just in case, post power supply schematic also.

[styler173]

Hey J M Fahey,

thx for your quick answer I hope I have understand you right...

That what I must do is to brigde the C-E Conection from Q327 and meassure the Voltage over that? Musst be 2000mV - 2800mV.

So I have check the resistors and have found maybe an blow up resistor R374 its an 10ohms resistor Ihope thats the problem.

here is the schematic from the Powersupply



Thx for help that was it one small safety resistor

Bias with 4mV works thank you a lot I would reply when I have another mistake

[J M Fahey]

Hey J M Fahey,

thx for your quick answer I hope I have understand you right...

That what I must do is to brigde the C-E Conection from Q327 and meassure the Voltage over that? Musst be 2000mV - 2800mV.

Those are two different tests.

1) measure Q327 CE voltage, check that it responds to trimmer adjustment and that itīs somewhere between 2000 and 2800 mV.

2) supposing you can NOT get less than 40mV bias voltage across ballast resistors, then you short Q327 CE and recheck.
If still 40 mV at ballast resistors, then there is "some other" cause, to be determined.

Looks like you found it..



here is the schematic from the Powersupply



Please repost, maybe under other name , because it canīt be opened.

Worst case upload it to some server and post link here.

[styler173]

so another try here is the schematic

Dynacord-PowerMate1000_1600-8.pdf

[Jazz P Bass]

Here is the complete 1000-1600 schematic:

Dynacord PM1000 & 1600 powermix.zip