JoelG

Thanks, I'll forward this to the repair guys, hopefully it gives them a leg up

Yes, multi turn pots Thanks for the reply, I'll ask them to check.

Thanks, I'll forward this to the repair guys but if no luck try it when I am get it back. Yes, watts.. Yep , blue multi turn pot. I attached the schematic diagram, had troubles converting it on my phone. Thanks for your advice

Hi all, would very much appreciate some help with my amplifier. Marantz ma-7a class A monoblocks. This is the Japanese version (100 volts) of the music link ma-24a. One amplifier has has ongoing bias adjustment problems. Started out as sometime running nice and warm like the other one but occasionally running cold. This was originally random but started to get more frequent so I started to adjust the bias back to 180 MV, as per the working one. (Adjustment was very touchy, tiny turn big adjustment) It would stay set for a few days or a week then drop down to running at about 20 to 70 MV. The good one draws 90 Volts from the wall the other sometimes just 10. It still has sound and volume and a non audio person probably wouldn't even notice. I had the DC offset and bias pots replaced but it was still unstable. Last time I tried to adjust the bias it wouldn't go up at all. It's at a repair place ATM but they are at a loss? I've attached the schematic diagram. Any suggestions would be appreciated, thanks in advance. Link to schematic diagram https://www.diyaudio.com/community/threads/marantz-ma-24-the-quad-was-received-last-nite.198411/

Just thought I'd let you know how it went. Couldn't track down a service manual, not many of these made. But I took a reading off the "good amp" and adjusted the bias/idle to match then readjusted the DC offset and double checked the power consumption was equal. Both running at the same temperature now and sound much more present and responsive. Thanks again ?

Great, thanks! I'll I'll try to find out the specs and then give it a go. Much appreciate the info guys!

Yes, sorry I couldn't work out how to send a clear copy. Yes, I can only see a pot for DC adjustment inside as well. I guess being a class A bias isn't usually s problem or something that needs adjusting?

Hi, it's a ma7a, Japanese, runs on 105 volts.. it's basically the same as the European ma24a. I'll attach a schematic. Not sure how to check the bias? It's been running ok, without getting too hot but I'll try to pinpoint the problems asap.

Hi all, just looking to tap into the broad knowledge of you all and thanks in advance. I have a pair of 30w class A maranz mono blocks, one was running intermittently cool for awhile then eventually it just ran cold. I borrowed a power consumption meter and one amp was drawing 110watts and the cold one only 9. (It still had pretty good sound btw) anyway I took the lid off and contact cleaned the only pot I could see which were the DC offset potentiometer. The amp warmed up straight away but still is only now drawing about 75 to 90 watts.. seems unstable and although much closer and sounds much better not quite up to what it should be and while warm again still less than the other one. I've set both DC offsets to 0. But I guess the bias is still out on one? Any suggestions where to go from here?

Thanks for your reply, hope these photos help.. if needed let me know and Great! Actually Tesla was my thinking too after illiminating tungsram as my second best guess. Yes, no Telefunken diamond underneath. I wasn't sure if Phillips had a connection with Tesla but probably they owned it at some stage.. ? Thanks for your help, much appreciated! Oh.. it's labelled 6dj8 ecc88 bit you think it's a e88cc? That's good if it is, I believe they're a better tube..

Hi all Hope this is the right place to ask couldn't find a dedicated section. Underneath the tube in center moulded into glass one tube has has a backwsrdss 7 and the other one I have an 8 Both have a 2 imprinted between the pins Any help would be great and questions welcome. Thanks, Joel

yes, I've never heard of this before watching this video and you would really think it'd be something worth mentioning ?

I see, so without a load the operating point might be out. A load will just relatively change the relationship based on the starting point... And adjusting this bias will do the trick ?

Ok, I actually unplugged everything from the amp and ran them to eliminate any variables and one was hot one cold so not sure, does your diagrams depend on having a load , being the speakers or is that the amps internal bias ? Anyway I can see how it can be a variable load. Thanks, Joel

Ok, that's good to know... Hopefully it's an easy fix then. ? Thanks, Joel

Ok, I'm not very tech savvy but I kinda thought class A was just full on all the time so bias doesn't come into it? Happy to be wrong about this and if it's just a simple adjustment I will be stoked ?

Hi all, just after a good technician to repair one of my vintage marantz mono blocks. One amp is running way hotter than the other... Not sure if it's the hot one or cool one that's the problem though, as both sound fine and they always ran hot. Anyway any recommendations for a amp guru in Melbourne would be much appreciated. Joel

Hi Mike, good to get your message. I was referring to single ended inputs (unbalanced) rca. My amps are class A (30watts) push pull/ MOSFET output stage. They do run pretty hot.(good for cold winters) I'll do some research on single ended class A output stages, sounds interesting. The question originated from the attached video which stated some amps are able can convert a normal single ended signal(unbalanced) (rca) into a differential signal (balanced)using a twisted pair IC to achieve some sort of common mode noise reduction not dissimilar to true balanced gear.. anyway that what he thinks.. (unless I got it totally wrong) So yes atm just using single ended unbalanced to SE unbalanced. Here's a good article that explains. https://jlaudio.zendesk.com/hc/en-us/articles/209357588-Differential-Balanced-Inputs?mobile_site=true Thanks Joel

Thanks Mike, much appreciate you answer. Yes, sticking with single to single. It's all very interesting stuff, just started scratching the surface. Cheers, Joel

Hi, no, the particular hole was about single ended input(rca) connection that acted like a balanced input but requires a twisted pair IC to work. (Common mode noise reduction) he said that about 25% of amplifiers (usually high end)use this method but it has been overlooked for a long time in the industry. I did find a lot of corroborating evidence and many car amps seem to use twisted pairs to work with this type of input, but as usual also many contrary statements.

I see, well it's good to know, thanks for the information ? I watched something on YouTube and fell down a rabbit hole ?

Excellent, thank you very much for that. Twisted pair going in the dumpster ?

So apparently my marantz mono blocks have a differential input stage using rca type connection. At the bottom of the attachment there is a diagram of the differential input stage, I've also included the amps schematics. The music link sc-22 preamp that is this models (music link) preamp has two rca outputs, one in phase and one out of phase. Although as the power amp only has a single input I don't see how this is relevant but it might be? ( I don't own this preamp) Also the diagram shows the output coming from a BTL (bridged source) again probably irrelevant. From my limited research I will run a twisted pair rca IC in the hopes the opp amps in the differential stage work to create a differential signal from a single (I believe this is possible?), To achieve some common mode noise reduction. Anyway if anyone could help my understanding it would be much appreciated and hopefully somewhat interesting to you as well. Joel Marantz_MA-24_AllegroSound.pdf hfe_marantz_ma-24_schematic.pdf

One of the standouts of all I heard at the show. Soul speakers sounded the most original and fresh like their design.

From the seller and reading forums and sites on net.