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Old 6th Sep 2017, 9:50 am   #1
Join Date: Aug 2014
Location: Featherstone, West Yorkshire, UK.
Posts: 141
Default Dynacord

Not really vintage, they are only around 10 years old.
In a recent post regarding musical instrument amplifiers, a member mentioned Dynacord, asking are they still around?

Most certainly yes.

Dynacord are now part of the mighty Bosch conglomerate. We have a constant “trickle” of these units coming in for repair. Specifically, the Power Mate 600, 1200 and 1600, and the compact variant, the MP7, and there are also rack mounted power amplifiers, ranging up to the P3000 – a behemoth; 48 T03 output devices, and a back breaking 33kg to lift onto the bench – 2kW per channel into 2. All use the same amplifier topology, just vary the quantities of output devices, and the supply voltages/currents.

Few technicians seem to want to touch these. The Teutonic amplifier design has very efficient over current, over voltage and over temperature protection, which makes it quite daunting at first sight.

Typically, the unit will arrive with a note from the owner “stuck in protect”, but I often have them with notes from other service departments such as replaced XYZ parts, it blew up again.

I start by checking the output devices to see which ones have been destroyed. Then, working VERY CAREFULLY back, follow the trail of open circuit resistors as far as the fault current has gone – it is a hefty current, and it will go a long way, taking a lot of resistors and semiconductors along the way.

Take note. Trying to check the Vbe multiplier (2SC4793) with a DMM is pointless. It may have a 500k leak when it is cold, but as soon as it warms up, the quiescent current rockets up, and the output stage goes bang again. Only an AVO will tell the truth when checking semiconductors.

I usually find a handful of small resistors along with the relevant 0R22 5W dual emitter resistors and quite a few semiconductors have expired.

Replace all the faulty parts. The service data states a lot of the resistors are non-flammable types, but I have seen many which have clearly burst into flames; Replace them with non-flammable types.

When completed, place a meter between the output stage centre point and ground, and a meter across the quiescent current test points, turn down the quiescent current bias potentiometer, and connect the amplifier to a variac – NO LOAD YET.

Bring the supply voltage up slowly with the variac, watching for any jump in the mains input current or the bias current, and any deviation on the output dc offset of more than 5 to 10mV. If anything out of the ordinary is noted, STOP.

Find the faulty component(s) you missed, and start again.

When you can take the mains input to 230V, quickly check the output quiescent current, and adjust as required to give the figure stated in the service manual, but note it must be done whilst the amplifier is still quite cool.

Now input a sinusoid, and look at the output with an oscilloscope. If you have a nice clean sinusoid, you can finally connect your test loudspeakers. Check the fan speeds at the various heat sink temperatures, check the over current/over voltage protection, and if all is well, enjoy some music whilst you soak test it, and write out the bill.

The parts are not expensive; even the output devices are only a couple of pounds each.

I have attached a copy of the Power Mate 600 output stage. All of the service manuals can be found on the web, various qualities from pristine to unreadable. It is pointless contacting Dynacord/Bosch direct, they will not reply.


[B]It is smoke which allows electronic components to work.
If you do something to make the smoke come out of them, the unit will stop working.
When you have put in new components, full of fresh smoke, the unit will work again.
Attached Files
File Type: pdf Dynacord PowerMate 600 output.pdf (367.5 KB, 91 views)
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