Robmuso 21 Posted December 16, 2020 Share Posted December 16, 2020 Hi all, Firstly, I would like to thank the administrators of this site. What a great idea. I live near Brisbane in Australia and I have owned my Carver PM1201 since the 80's. I love this amp. In the 80's, I used it mainly for my on stage keyboard monitors and this amp was ultra clean and punchy and above all, very reliable and robust. As an electronics and audio enthusiast since my early teens, I have learnt how to repair most of my own gear. When I've inspected this amp inside and read the schematic, it is very well designed. This amp has given me many years of service with only minor problems. 1. The fan brushes wore out, so I contacted a company in the USA in 2004. I was told that the original fan motors were no longer available. If I am correct, I think they were 12 volt DC motors in series requiring a supply voltage of 24 volts.. I was able to purchase 2 replacement motors from them which from memory, I think were 6 volt rating. Very costly when freighted to Australia, but I still bought them. I later found out that Jaycar Electronics in Australia sells the same motors very cheap. I received the following instructions in an email for a modification to the fan circuit to use the replacement motors. From the PSU, there are 2 diodes D24 & D25. Where they connect is the point where the power for the fans is taken from about +36 volts. There is no filter cap at that point, but there is a filter cap C3 (470uf 35v) at the connection to the fans. From the +36 volt supply, there is a resistor R4 (6.8 ohm 5 watt wire wound) to reduce the voltage to the fans. For the new motors to match, this resistor must be replaced with a 12 ohm 5 watt resistor. Anyhow, the modification seemed to work, but now the fans turn very slowly at idle when the amp is first turned on. As the capacitors in this amp are getting old, I will soon be replacing all the electrolytic caps in this amp. I was also emailed a service bulletin on adjusting R34 on the PSU board for the correct calibration of 124 volts DC at the filter caps. And I have the info to correctly adjust the idle bias trim pot RP1 on each amplifier board. Anyone can email me at keysound@powerup.com.au if you need that info. Regardless of all the comments above, there is one thing about this amp that bothers me. In my opinion, the heatsinking for the power transistors in this amp seems to be inadequate. Cooling is heavily relying on fans. Even with no signal sitting idle, this amp gets very hot. I have tried to look at a way to fit 2 x 80mm computer fans as a replacement, but there is no room inside the case because of the fan board, and the power cord and fuse are also in the way. Even using some metalwork externally to house the fans at the back is not an option really. So I guess I will have to stay with the original design and sort this out If anyone has been successful with this issue, I would welcome any information. Thanks everyone and please stay safe. Robmuso 4 Link to comment Share on other sites More sharing options...
Dadvw 2,802 Posted December 16, 2020 Share Posted December 16, 2020 Hi @Robmuso Welcome to the site! Hopefully one of the members here can help you out with your issue. Glad to have you from such a long way from the US........grin Link to comment Share on other sites More sharing options...
Robmuso 21 Posted December 16, 2020 Author Share Posted December 16, 2020 Hi again, Thanks for the nice welcome. I also have a PM1200 that needs repairing. Apparently it's power transistors often failed in the past. I don't know the history of that but I will get it working again. The great help from this site is that I found the service manual for the PM1200 and downloaded it. A lot of circuitry in the PM1200 & PM1201 appears to be the same example, the PSU and the Led Ramp Display. However, the amplifier stages and parts are very different with the 2 models. I did find out that the original fan motors in both models are the same Carver part number 112-10003-00 and they are in fact 18 volt motors. The original motors have an armature shaft of about 51mm length with about 12.2mm external for the fan blade. The shaft is about 2.5mm diameter or maybe slightly less. I will be designing a transistor circuit for the 3 speeds and when it is done, I'll post the info here. Thanks, Robmuso 2 1 Link to comment Share on other sites More sharing options...
Sk1Bum 9,860 Posted December 16, 2020 Share Posted December 16, 2020 Hello Robmuso, and welcome to thecarversite.com. We're glad you're hear. There is a lot of information here, much technical know how, and a pretty good bunch of folks that are happy to help. I suggest you start a thread in the Pro-amplifier repair section here regarding your PM1201. There are several here who use the pro-amps, and hopefully one of them will chime in. Your issue may have already been solved, and checking out that area of the site may point you in the right direction. Once you get to Novice status, please consider uploading some pics of your gear in the Members Systems area. We all like pics of all kinds of audio gear. In the meantime, kick your shoes off, turn up the music, and welcome aboard. 1 Link to comment Share on other sites More sharing options...
johnrotten 1,070 Posted December 16, 2020 Share Posted December 16, 2020 Welcome!!! 1 Link to comment Share on other sites More sharing options...
Sk1Bum 9,860 Posted December 16, 2020 Share Posted December 16, 2020 Robmuso, there are several topics on site regarding the PM1200/1201 & PM1.5 fan motors. Here's the search link I used: https://thecarversite.com/search/?&q=pm fan motor&page=2&search_and_or=and&sortby=relevancy I hope that helps. 1 1 Link to comment Share on other sites More sharing options...
wrf 3,801 Posted December 17, 2020 Share Posted December 17, 2020 On 12/15/2020 at 11:00 PM, Robmuso said: Hi again, Thanks for the nice welcome. I also have a PM1200 that needs repairing. Apparently it's power transistors often failed in the past. I don't know the history of that but I will get it working again. The great help from this site is that I found the service manual for the PM1200 and downloaded it. A lot of circuitry in the PM1200 & PM1201 appears to be the same example, the PSU and the Led Ramp Display. However, the amplifier stages and parts are very different with the 2 models. I did find out that the original fan motors in both models are the same Carver part number 112-10003-00 and they are in fact 18 volt motors. The original motors have an armature shaft of about 51mm length with about 12.2mm external for the fan blade. The shaft is about 2.5mm diameter or maybe slightly less. I will be designing a transistor circuit for the 3 speeds and when it is done, I'll post the info here. Thanks, Robmuso Contact Greg. Send him a PM @Nahash5150. I know he has a bunch of fans -- if the fire didn't get them. 1 2 Link to comment Share on other sites More sharing options...
johnrotten 1,070 Posted December 19, 2020 Share Posted December 19, 2020 The location of the fans in a 1201 are mid board and have slots for the fan blades to spin freely. There are other functions of the board besides the fan, where as other amps have a dedicated fan board, The circuit boards also have traces on both sides. Making it difficult to engineer a relocation fix and tricky to implement. If you can get a new fan that would be the way to go. The speeds can be adjusted by swapping out some resistor values but again, those 2 sided boards are tricky to work on. Link to comment Share on other sites More sharing options...
Community Admin AndrewJohn 8,142 Posted December 19, 2020 Community Admin Share Posted December 19, 2020 Welcome to the site !! you are going to fit right in! follow-up on @wrf's thought no a correct fan! Link to comment Share on other sites More sharing options...
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