| Author | Message | ||
| Barry Banks (banksy76) Username: banksy76 Registered: 06-2013 |
Hi there A while ago my 212 HD 130 combo (solidstate inverter circuit) started to produce some slight distortion when playing lower notes, even at the lowest of volumes, but this issue was somewhat intermittent. I checked speakers etc and even changed the tubes but the problem remained and worsened. Now I am getting nothing but extreme distortion on all inputs on each of the channels. I made a couple of posts a couple of months in another thread seeking assistance on biasing this amp and I explained that when taking a reading with the multimeter I wasn't obtaining any changes when adjusting the bias pot. I was advised to first try changing the tubes which I did but the issue remained. Now I am thinking that with no adjustment possible on the bias pot and the amp consistently producing distortion, could it be that the capacitors that control/regulate the bias have failed? Would this result in the problems that I have detailed above? Any assistance/advice on this matter would be greatly appreciated. Many thanks in advance. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
OK-you changed the tubes and have NO adjustment on the bias control of a SS driver? Sounds like you have driver problems then. Has the amp been re-capped at ANY time recently? It sure wouldn't hurt but you need to check the signal as it enters the driver and after. Without any signal check, I'd bet on driver transistors bad if it won't adjust. Are the voltages right per schem at the exits of the PS? | ||
| Barry Banks (banksy76) Username: banksy76 Registered: 06-2013 |
Hi Mike, and thanks for the reply. That is correct, when I adjust the bias pot there are no changes to the reading of the multimeter across that 3.9 Ohm resistor. I have recently acquired this amp and it appears that no re-capping has taken place recently, perhaps never? I appreciate your help but you may have to bear with me somewhat at I have limited experience/knowledge of electronic but have successfully re-capped other pieces of vintage audio equipment in my studio. What are the other pots on the board for? | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Should be only one pot on the little board in the back and one on the big board in the middle. The one on the little board is for the driver and the one on the big board is for the trem. | ||
| Barry Banks (banksy76) Username: banksy76 Registered: 06-2013 |
It appears that I have 3 pots in total, as can be seen here in this photo: http://s374.photobucket.com/user/banksy7 6_bucket/media/MUSIC%20MAN%20212%20HD%20 130%20capacitors/DSC_0015_zps63f6729f.jp g.html Please could you clarify and confirm for me what each of these pots do. Many thanks. | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
Hi Barry, I think your amp has been converted from the old tube phase inverter to the newer style at some point. The original trimmer for the negative bias voltage for the 12AX7 tube is on the main pcb whereas the trimmer for the positive bias voltage for the driver transistors is on the new pcb between the two heatsinks. Sincerely, Lars Verholt | ||
| Barry Banks (banksy76) Username: banksy76 Registered: 06-2013 |
Thanks Lars, that would make perfect sense. So in this scenario, does the original trimmer/pot on the right hand side of the main board have any affect at all or is this now totally redundant due to the conversion? | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
Hi Barry, that trimmer has no effect. It is no longer in the circuit. Sincerely, Lars Verholt | ||
| Barry Banks (banksy76) Username: banksy76 Registered: 06-2013 |
Thank you Lars for confirming that. I did assume that to be the case but just wanted to be 100% sure. And with this type of converted circuit board, it is still the 3.9 Ohm emitter resisters (orange, white, gold,gold) that are to the left of the bias trim pot in the photo that I am taking a reading of with the multi meter, yes? | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
Hi Barry, yes that is correct. To check your bias circuit you can measure the voltage at the middle terminal of the bias trimmer with respect to ground. It should read around 1V give/take. Sincerely, Lars Verholt | ||
| Barry Banks (banksy76) Username: banksy76 Registered: 06-2013 |
I have been persevering with the bias control again, at first with no success, but after making a few extreme turns in both directions it seems to have sprung back to life! With an instrument plugged in, the distortion I have described increased as the pot was adjusted to a lower setting and was eliminated on higher settings. I then unplugged the instrument and returned the bias pot to exactly when it was originally and took a reading of 25mv on both of the 3.9Ohm resisters, plugged the guitar in and the distortion problem seem to be resolved. Strange, as the bias pot is at the same position as when the problem was present, perhaps a dirty bias pot and poor connection was causing improper biasing and not allowing appropriate adjusts to be made? Regarding the bias circuit that you describe, measuring the voltage at the middle terminal of the bias pot to ground, I am obtaining a reading of 1.35V, does that sound ok? | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
AH-a CONVERTED amp. THAT makes more sense. Any positive voltage that, when applied, that gives you a nice reading across the emitter resistors is right. It can vary but you are in the right ball park Probably WAS a dirty pot. I guess we all like a little dirty pot once in a while!(KIDDING) Mike. | ||
| Barry Banks (banksy76) Username: banksy76 Registered: 06-2013 |
I have been running the amp in and performing checks on it during the last hour and so far, so good. The readings are remaining consistent, bias appears stable, and no signs of unwanted distortion. I shall keep my fingers crossed and continue running checks tomorrow. So there are 3 versions of the 212-HD 130 circuit board/s in existence? The original version that was produced with the tube phase inverter section. The 2nd version that was produced with the solid state circuit board. And the the 3rd type which are original versions later converted to solid state. I apologies for having digressed from the main topic of capacitors but even though it seems that the problem that I had was not due to failing capacitors after all, I will probably re-cap the amp anyway as it needs to be done. So having established that I have a 'converted' amp, are there any of the capacitors that wont need replacing due to them being on a redundant section that is no longer in the circuit? As can be seen in the photos at the link below, there are 9 capacitors in total on the boards (excluding the large filter caps underneath) : 2 (blue) located just behind the power supply rated at 150mf / 50v 2 (silver) located by the now redundant tube bias pot, rated 150mf / 50v 5 (silver) on the updated solid state board that appear to be rated 25uf / 25v http://s374.photobucket.com/user/banksy7 6_bucket/library/MUSIC%20MAN%20212%20HD% 20130%20capacitors?sort=6&page=1 | ||
| Christophe (vignau) Username: vignau Registered: 06-2011 |
You have exactly the same boards I have on my 210HD130. So 3 trimpots means converted (3rd type)????????????? So if Lars is right I have a converted as well. Barry, your caps have never been changed... They are original. You will hear a huge difference in sound after changing all of them, don't forgot the one of the filter section . | ||
| Barry Banks (banksy76) Username: banksy76 Registered: 06-2013 |
Yes, looking forward to getting my amp recapped. As explained in my previous message, I'm currently trying to establish if it is necessary to replace all the electrolytic caps on the converted boards such as ours, or if some of the caps serve no purpose due to no longer being in the circuit as a result of the modification. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
They are not a THIRD type, just a conversion. Some were done at the factory service center, some were done in the field. I've had both. The caps on the driver board are ALL inportant too. I've never sat down and picked and figured what got taken out of the circuit since there are very few on the main board. You still need to replace them all, just to be sure. The little ones don't cost much. | ||
| Barry Banks (banksy76) Username: banksy76 Registered: 06-2013 |
Thanks for confirming that Mike, I shall ensure that I replace all the electrolytic capacitors. When I said 3 types of board, I meant that upon opening up a 212HD 130, one would find 1 of 3 scenarios: 1. An original stock 'tube' version that was produced with the tube phase inverter section. This would feature 2 trim pots on the main board for the tremolo and tube bias. 2. An original stock 'solid state' version that has 1 trim pot on the solid state circuit board for bias, and a 2nd trim pot on the main board for tremolo. 3. A 'conversion' (such as mine) that will feature 3 trim pots. 1 being the bias pot on solid state board that was installed in the conversion, the 2nd being the tremolo pot on the main board, and the 3rd being the now redundant original tube bias pot, also on the main board. Regarding those smaller capacitors on the driver board, am I reading it right (please see my link above for photos) that they are 25mf / 25v? It appears to me to be the case but I have read a number of other posts that claim that the 212HD 130's uses 20mf / 25v. What is your opinion on this? Many thanks. Barry | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
20uF or 25uF accomplishes the same thing. As coupling caps they are spec'ed to be non-dominant (i.e. not do much else than block DC) and for decoupling the bias voltage there is no practical difference between 20uF and 25uF. Cheers, Lars Verholt | ||
| Barry Banks (banksy76) Username: banksy76 Registered: 06-2013 |
I'm having trouble sourcing capacitors of the appropriate values here in the UK. Any fellow UK residents here have any suggestions? | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
You might try here- http://www.tubeampdoctor.com/en/shop_Cap acitors/ |
Tue, 03/25/2014 - 09:54
#1
