Stub Mandrel Posted August 17, 2019 Share Posted August 17, 2019 (edited) Old fashioned 1980's combo 150W. Symptoms - intermittent noise, occasional cutting out. Noticed Transformer running VERY hot. The noticed speakers are all slightly forward of their normal rest position when amp switched on. Otherwise, sounds normal with apparently good headroom but hard to test in living room... Conclusion: problem with the power amp bias. Possible dodgy transistor in bias circuit? I do have a schematic + test gear. First check, 8V DC offset across speakers. Played with pot VR410, may have been part of the problem dirty as it instantly improved, now between 4V and 4.3V but still making speakers jump forward and noisy (NB this shoudl be adjusted to minimise crossover distortion). Discovered C409 was partially desoldered and 10uF (47uF specified). Tried both a 10uF and a 47uF and noise greatly reduced but still bias issue. Thing is, VR410 looks to me to be for adjusting the quiescent current to eliminate crossover distortion rather than the bias... Very simple amp, but I'm struggling to see what might be the problem. Have been over resoldering to make sure no dry joints. This is issue 4 of the MPB2 board, my amp is issue 2 and only two mosfets fitted + fan (but places for the other 2) Edited August 17, 2019 by Stub Mandrel Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted August 17, 2019 Author Share Posted August 17, 2019 Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted August 17, 2019 Author Share Posted August 17, 2019 (edited) Talking to myself... but... Working on the principle that the collector and base voltages of the differential pairs should all be pretty much equal, the collector of TR204 is at -5.1V, 206 is at about -21V. [edit] This is supposed to be a 'current mirror'. I'm sure the collectors of TR203 and TR204 shouldn't be this far out of balance one is at -21.5V the other is at +5.0V. In an ideal world the collector of TR204 should be a couple of volts negative and I would expect TR203 to be somewhere near this with the amp quiescent. BUT the voltages across R208 and R211 are almost identical suggesting the current mirror is working.. Next looks the bases of TR201 and TR202 in the differential amplifier bit both at 0.4V. Reasonable enough, this suggests a base current of about 7 uA. Suspicion currently on match of R204 and R205 and TR203. But can anyone offer some advice? Edited August 17, 2019 by Stub Mandrel Quote Link to comment Share on other sites More sharing options...
Sparky Mark Posted August 17, 2019 Share Posted August 17, 2019 (edited) Agedhorse is probably the best qualified BCer I'm aware of that may (if he's inclined) be able to advise. He has designed a lot of bass amps including the latest Mesa range and has forgotten more than most ever learn. You could message him but I hope for your sake that your electronics theory is adequate as Agedhorse (Andy) doesn't take prisoners! Edited August 17, 2019 by Sparky Mark Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted August 18, 2019 Author Share Posted August 18, 2019 I've found some help on another forum. Looked like a mismatch in the differential pair. I pulled the transistors this morning and their gains are 80 and 64. Enough to give hifi buffs the heebeejeebies, not sure if its enough to give the symptoms I'm seeing but I'm going to see if I can find a better matched pair rated 100v. Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted August 18, 2019 Author Share Posted August 18, 2019 Yep, seems to be these transistors, if I swap them (1) the direction of the offset reverses and (2) the amp gets noisier. Swap them back and the noise goes and the offset reverses. Quote Link to comment Share on other sites More sharing options...
ahpook Posted August 18, 2019 Share Posted August 18, 2019 Looks like you're getting somewhere, that's good news. 1 Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted August 18, 2019 Author Share Posted August 18, 2019 1 hour ago, ahpook said: Looks like you're getting somewhere, that's good news. It was also beneficial to discover that one of the speakers was wired out of phase. This morning I moved the amp into my workshop which is better lit. After swapping the transistors not only did I notice the speaker I've been watching now moved in instead of out - so did two of the others but the third one moved out! 1 Quote Link to comment Share on other sites More sharing options...
ahpook Posted August 19, 2019 Share Posted August 19, 2019 Out of phase ? That is weird - is it possible someone's been in there already and made a bit of a botch of a repair ? Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted August 19, 2019 Author Share Posted August 19, 2019 People have been in it a few times over the last 30 years. A resistor has been replaced and the speakers have all be replaced. They used Eminence Alpha 10s - not the ideal choice for a tuned bass cab, when they could have got Beta 10As for the same or less. I suspect they didn't have a clue what they were doing and assumed 'different' would be 'better'. I do need to check they are phased to match my extension cab. Quote Link to comment Share on other sites More sharing options...
ahpook Posted August 19, 2019 Share Posted August 19, 2019 Well, at least you can look at all the extra stuff whilst it's opened up. Which combo is it btw ? Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted August 19, 2019 Author Share Posted August 19, 2019 59 minutes ago, ahpook said: Which combo is it btw ? A good question... It's a 1110 Combo, only proven by it being lightly scratched into the panel at the back. It has a GP11 preamp on the front and an issue 2 AH150 power amp at the back (only two FETS but with a fan - later issue 4 AH150s had four fets and no fan, the AH250 had four fets and a fan). Probably one of the oldest ones, not sure if Issue 1 or Issue 3 ever made it into the wild? Quote Link to comment Share on other sites More sharing options...
ahpook Posted August 19, 2019 Share Posted August 19, 2019 Oooh, that is an early one. Well worth resurrecting Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted August 19, 2019 Author Share Posted August 19, 2019 I've discovered the amp is running of +/-50V ac not 40V. I suspect it's an early 250W one rather than 150W. This could explain why there were no external extension cab sockets. I'm going to fit an extra pair of mosfets - the PCB is ready for them, I just need to drill the heatsink. This should give greater reliability and safety margin if running at 4 ohms. Also, I found a thread, in Dutch, about someone with the same problem, except their DC offset was only 127mV. Much discussion and debate until in the last post they replaced TR201/2 with a matched pair and the offset came down to 5mV, then replaced TR203/4 as well and it came down to 3mV. I have high hopes swapping these transistors will cure my problem... There should be a translated version of the thread here: Thread on Dutch forum Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted August 21, 2019 Author Share Posted August 21, 2019 The replacement transistors have come. And... the output offset voltage is now 0.22V, a bit higher than I hoped, but now no visible displacement of the speaker cones. I will replace the pair of BF422s and adjust the bias current when I fit the two extra FETS, but I need to wait for the mounting kits to arrive. Neil PS I've had some advice from Agedhorse and it seems my amp is fitted with a 200/300 into 4R PSU. Quote Link to comment Share on other sites More sharing options...
VTypeV4 Posted August 22, 2019 Share Posted August 22, 2019 Love an 'adventure' thread! Keep it coming. 😀 1 Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted August 22, 2019 Author Share Posted August 22, 2019 Gah! Searched eBay for TO3 mounting kit and bough the cheapest that came up - should have looked properly, I ended up with TO3P sets! None on CPC, but plenty on Farnell. Weird. Quote Link to comment Share on other sites More sharing options...
fleabag Posted August 22, 2019 Share Posted August 22, 2019 I was thinking of you maybe contacting Clive Button , one of the designers of trace amps back in the day, but i see you've had some good advice from Old Horsey. Then again, from my conversations with Clive when i was asking him about my GP11 Mk 5 combo, i believe he said he didnt design those models, or earlier, so maybe he came into the designs at the Mk 6, which i guess is no good to you either, if yours is an early model. Clive was helpful though in other ways. I was asking him about stuffing in a bigger amp and he said it wouldnt be problem, as the speakers in my 4x10 combo could take a lot more than the amps were rated at giving out Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted August 22, 2019 Author Share Posted August 22, 2019 1 hour ago, fleabag said: I was thinking of you maybe contacting Clive Button , one of the designers of trace amps back in the day, but i see you've had some good advice from Old Horsey. Then again, from my conversations with Clive when i was asking him about my GP11 Mk 5 combo, i believe he said he didnt design those models, or earlier, so maybe he came into the designs at the Mk 6, which i guess is no good to you either, if yours is an early model. Clive was helpful though in other ways. I was asking him about stuffing in a bigger amp and he said it wouldnt be problem, as the speakers in my 4x10 combo could take a lot more than the amps were rated at giving out According to the pedia of Wik (which tends to be unreliable or apocryphal on such details) he came in at MK6. I think I'm sorted now, just gagging to get the extra FETs fitted and test it out (especially as I now have some decent earplugs...). 1 Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted August 23, 2019 Author Share Posted August 23, 2019 Well this amp is the one that just keeps on giving. Removing the power amp board revealed the FETs have been replaced. Probably the consequence of running it off that high power PSU. Both fets had 'rubberised' insulators, one with compound on one without, but they looked like the work of the same person (the part numbers for both fets are written on the board in the same fat felt pen). Probably along time ago, as the original Hitachi fets, now obsolete, were used. I replaced all the insulators with mica ones and servisol heatsink compound - best to have them all matched. Also redid the compound on the heatsinking. End result, offset voltage still 0.2V, low enough to be acceptable. That's 1/100th of a watt wasted compared to 16 watts when I started, and there's now no visible displacement of the speaker cones. Most importantly, with parallel fets each one now has to handle ~75W instead of 150W, which is probably a good idea when they are individually rated 125W. Best of all, I've learned the amp inside out and if it DOES fry in the future I'm confident at worst I could rebuild a complete replacement amp from the ground up. 3 Quote Link to comment Share on other sites More sharing options...
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