Marantz MM500 output relay repair

My good old Marantz MM500 multi channel power amplifier has been in my use for quite some years. It’s a surround power amp with 5 channels, well it’s actually only 4 real channels as the 2 surround speakers are using the same channel and are just connected in series. The 2 channels for the main speakers are based on a discrete implementation, whereas the center and surround channel is based on a STK401-110 amplifier module.

I have actually had to repair the amp before, as the STK401-110 module blew, and I had to change it.

The amp has been used for a long time powering a Peerless 12″ XLS ported subwoofer, and has therefore been drive pretty hard ….. have seen the red peak indicators on the amp blink a lot of times while watching movies.

First time I was using the back channel for driving the sub … that’s when it at some point blew …. STK401-110 not really meant for drawing a lot of Amps.

After that I switched to using one of the main channels. Over time this has taken it’s toll on the output relay. I’m currently using it go driving my new small PC speakers. Actually only the tweeters, through a DSP. Some times the sound in the right channel disappears, and I have come tot he conclusion thet it’s the output relay that is done.

The amp uses two Takamisawa VB 24MBU-510 24Vdc relays. These are no longer to be found but a suitable replacement type is the Fujitsu FTR-F4GAK024T-KW they have almost identical data and will hopefully work fine for a lot of years.

It took me ca 1 hour to do the change. I used an Air Soldering station like this one to desolder the relays, as they have 4 pins in one end which is otherwise a bit hard to keep warm at the same time. The Air Soldering station is a relative cheap tool, and is handy in many cases when doing repairs, not leas working with chips.

There is quite some screws to take out, but keep the output transistors mounted on the heatsink, same for the amp module, pcb and heatsink mounted together can easily be taken out in one piece.

It I had no problems and it all came together again and worked first time.

Did it change the sound … I actually think it did (but not sure) …seems to be more “hole through” …. but main thing is that it works all the time, and not just half the time … success 🙂

The Service Manual can be found on the HiFi Engine web site.

Leave a Reply