Hi there,
I just replaced all capacitors in the power supply stage to get rid of an annoying hum. So for so good, hum dissappeared so that's great. However after trying to rebias the amp, one of the trimpot hoods came off... (Barely touched it
)
However, this is where i need some help. As you can see on the attached pictures, there are two trimpots. One which is a 1KOhm trimpot, and the other one which is broken is a 200 Ohm trimpot. Since the quality of the trimpots is terrible I would like to replace them all (2 on both sides). The 200 Ohm is the trimpot for biassing, so no problem there.
But what I do not know is how I can find the correct trimpoint for the 1KOhm trimmer. How do I do that?
Since I am not able to upload more than one picture via an Ipad, I will upload some pictures in consecutive posts.
Furthermore I would like to get in contact with people who have done some upgrading on this Amp especially in the output stage. So If you have any experience in doing so, pleas throw me a message
Thanks to all for your help
I just replaced all capacitors in the power supply stage to get rid of an annoying hum. So for so good, hum dissappeared so that's great. However after trying to rebias the amp, one of the trimpot hoods came off... (Barely touched it
)However, this is where i need some help. As you can see on the attached pictures, there are two trimpots. One which is a 1KOhm trimpot, and the other one which is broken is a 200 Ohm trimpot. Since the quality of the trimpots is terrible I would like to replace them all (2 on both sides). The 200 Ohm is the trimpot for biassing, so no problem there.
But what I do not know is how I can find the correct trimpoint for the 1KOhm trimmer. How do I do that?
Since I am not able to upload more than one picture via an Ipad, I will upload some pictures in consecutive posts.
Furthermore I would like to get in contact with people who have done some upgrading on this Amp especially in the output stage. So If you have any experience in doing so, pleas throw me a message
Thanks to all for your help
Attachments
Ahh now I get it: How to set them correct after replacing them.
In the servicemanual there should be a proper description.
Can you find it in the file that you have taken these images from?
Otherwise it is something like:
1) Set the 200ohm voltage offset in middle position, and the 1K bias pot in maximum resistance setting.
2) Adjust the 1k bias pot for optimum Idle-current through the output transistors (measure voltage over R414 and R427 )
Do this procedure with extreme caution! Use very small and slow adjustments, wait a couple of minutes for the idle current to settle, then readjust again. Do this until the heatsink has reached a decent temperature and check again.
3) Finally adjust the 200 ohm offset pot for minimum DC-voltage on the output, with a shortcircuit INPUT RCA-plug.
In the servicemanual there should be a proper description.
Can you find it in the file that you have taken these images from?
Otherwise it is something like:
1) Set the 200ohm voltage offset in middle position, and the 1K bias pot in maximum resistance setting.
2) Adjust the 1k bias pot for optimum Idle-current through the output transistors (measure voltage over R414 and R427 )
Do this procedure with extreme caution! Use very small and slow adjustments, wait a couple of minutes for the idle current to settle, then readjust again. Do this until the heatsink has reached a decent temperature and check again.
3) Finally adjust the 200 ohm offset pot for minimum DC-voltage on the output, with a shortcircuit INPUT RCA-plug.
No...that is wrong! The schematic which is widely available from Parasound HCA2200 doesn't show any 200 ohm trimpot.
I was assuming it was one of the classic John Curl setups with the 200ohms trimpot in the sources of the input JFETs ...sorrry for that.
Can you upload some closeup pictures of the PCB?
I was assuming it was one of the classic John Curl setups with the 200ohms trimpot in the sources of the input JFETs ...sorrry for that.
Can you upload some closeup pictures of the PCB?
Update:
Exchanged the 2 trimpots on either sides with bourns (3296) 1kOhm and 200ohms. And adjusted the 1kOhm trimpot to get the amp on both sides on approx. 20 mV. Never touched the 200 Ohm pots.
Temperature on both heatsinks are in balance and both go up to 50 degrees after approx. 2 hrs.
I do have one problem though. The left channel does not start up (at cold start) at all. Right channel comes up immediately whereas the left channel doesn not do anything. It seems
that if I yank up the input volume now and again in the first minute, then suddenly the left channel kicks in and after it has kicked in once; all is fine. No problem after that whatsoever...
Could it be that this is due to the fact that I am only looking at the 20 mV (voltage) and not at mA (current?). Could someone explain if this could be the case what I should do in order to adjust the DC current settings?
Or do you think that the fact that the left channel kicks in late is due to another cause? If so what do you guys (and clever girls) think is the consecutive way of approach?
Exchanged the 2 trimpots on either sides with bourns (3296) 1kOhm and 200ohms. And adjusted the 1kOhm trimpot to get the amp on both sides on approx. 20 mV. Never touched the 200 Ohm pots.
Temperature on both heatsinks are in balance and both go up to 50 degrees after approx. 2 hrs.
I do have one problem though. The left channel does not start up (at cold start) at all. Right channel comes up immediately whereas the left channel doesn not do anything. It seems
that if I yank up the input volume now and again in the first minute, then suddenly the left channel kicks in and after it has kicked in once; all is fine. No problem after that whatsoever...
Could it be that this is due to the fact that I am only looking at the 20 mV (voltage) and not at mA (current?). Could someone explain if this could be the case what I should do in order to adjust the DC current settings?
Or do you think that the fact that the left channel kicks in late is due to another cause? If so what do you guys (and clever girls) think is the consecutive way of approach?
Hello to all, I am refurbishing a HCA 2200 MKII and I also found 2 trim pots and 1 transistor 2SC2240, the schematic have only one trim pot and 2 transistors, can any one help with the bias settings regarding the 2 trim pots.
Regards
Regards
An externally hosted image should be here but it was not working when we last tested it.
https://1drv.ms/i/s!AsKVQS7H2G1lmHa7JP-Xz9Ah6KJtI send an email to Tony @ Parasound, He is a great guy, his respond below:
Hi,
Sorry for the confusion, one pot is a coarse bias adjustment(1K ohm) and the other fine bias adjustment(200 ohms).
Allow amp to warm up for 30 minutes, bias levels should be 20mV +/- 2mV.
Best regards,
Tony
I worked on another Parasound amp here a while ago, and the bias pots were different from each other in that one increased while turning counter clockwise, and the other increased in the clockwise direction. It was kind of entertaining that they were going for a true symmetrical layout, but really was just annoying.
Yes, they are full of cheep parts, and also sound really nice with high quality parts added.
Yes, they are full of cheep parts, and also sound really nice with high quality parts added.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.