| Author | Message | ||
     Paul |
Hi, I've just bought myself an HD-150, w/ the DB-4 driver board, and the power is nowhere close to where it should be at. The loss of power is across both channels, and the only common component in the pre-amp that I found is IC-3, which I changed (and cleaned the leads with no result). Here is a list of other things I have already tried: 1) Put in new tubes, and biased across the cathode resistors for 25 mV. 2) B+ measured at 725 V, input to the driver section at 42.5 V. 3) Cleaned and changed the IC's. Side question: are lf353 and lm1458 interchangeable? The ic's on the board didn't match the specifications in the schematic. 4) Voltage measured after all four screen resistor was 363 V. At this point, I'm a little stumped, so any help would be great. Thanks, Paul | ||
| Paul |
Captain's log, stardate unknown: Still stumped, the crew has disappeared without an answer. Scotty, the chief engineer has not yet answered my 'urgent request for help'. Will I be marooned on this desert planet forever? Here is a few more things that I have tried: 1) Replaced the phase inverters with 2N6844 2) The screen resistors had drifted, replaced those too. 3) checked cathode resistors. 4) plugged amp into another speaker cabinet to check speakers. But still have a power loss and can not leave planet. But must try. Last resort will be checking the individual components for any damage or severe drifting. If anybody hears this message, please reply. Thank you so much, Paul, Captain of the HD-150 MusicMan amp. | ||
| Paul |
PS: The amp sounds great. There is no distortion, crackles, pops, or *any* problem besides the power loss. It's just not running anywhere close to what the output should be. If I had to guess, I'd say it's around 30 to 40 watts. The really odd thing about this though is that the high voltage setting sounds slightly weaker that the low one. Thanks, Paul | ||
| Terry |
Paul You did ues 2N6488 didn't you. | ||
| Paul |
Hi Terry, Yep. Used two 2N6488, that I checked to see whether they were matched only after I installed them. Paul | ||
| Paul |
Captains log, later star date: Slowly troubleshooting component by component what's wrong with the MM HD-150 shuttle. Fortunately I was able to leave this planet with the help of my faithful Fender Twin Reverb. Even though this planet has shown tremendous archeological artifacts, very rich in knowledge and know-how, I have heard nothing from a current civilization. Just a short communication from a life form that wanted to reassure itself that my brain fart was really a brain fart, or maybe the word 'ues' has a different meaning in its language. All the same there was absolutely no added value to my query. Hopefully when I come back here to visit this planet again, this civilization will be alive and thriving again. Happy Holidays Everyone, Paul | ||
| terry |
Well Captain, It does sound strange that it sounds so good, but not a lot of power. What are the numbers on transformers? Could be a bad speaker. Check the wire out of the back of amp going to speaker, a lot of them are bad. You will need an ohm meter. Did you rebuild the driver board and check the diodes? You did set the bias on the 3.9 ohm resistor, didn't you? You are using 6L6 tubes? Did you rebuild the capacitors on top of the tube sockets? If this does not work, you may need to put it in the "transporter" and send it to me. GOOD LUCK!! | ||
| Paul |
Thanks Terry for the answer. That is currently what I am doing, is checking the driver board component by component. As I mentioned in my first post, everything else checked out. The 3Kv capacitors on top of the output tube sockets are just snubbers. They are there, in parallel with the diodes, as a safety measure to protect the output tubes against transient voltage spikes. Even if they were faulty, it wouldn't affect the amount of power the amp has. Ciao, Paul | ||
| Valley Point |
Hello, I have been working on a 2475 chassis HD-150 212 and I have the EXACT same problem. I rebuilt the ENTIRE DB-4 driver board and I'm only getting about 75 watts before clip at full power (through a 4 ohm bench load, with 4 ohms selected on the output tranny). It should be 150W, right? The board has the 750k feedback resistor for the op-amp and the 75k going to the output trannny. I have put in a new matched quad of 6L6GCs in the amp, and it has been rebiased. My print shows a JE1692 as the phase inverters. I crossed them with a pair of matched NTE196 transistors. The power select switch seems to have little change on the output. All else looks (and sounds) fine! Any suggestions? TIA, John | ||
| Valley Point |
Hi again, My typo...it has a 75k feedback resistor on the op amp and a 750k going to the transformer. Sorry 'bout that, John | ||
| Steve Kennedy |
Did you guys check the plate supply? I have a tired old HD-130 that only puts out 85 watts because the tired capacitors in the supply are only allowing about 650Vdc of plate voltage instead of the 700+ Vdc it should be. Steve | ||
| Valley Point |
Hi Steve, I've put in a fresh pair of Sprague 100µF/450VDC Filter Caps in the power supply section and I'm getting well over the 700VDC. I believe I MAY have found the problem, but, until I solder in the part, I'm afraid to "jinx" the fix ;-). Needless to say, I'll post up as soon as give it a test run, one way or another! :-) Thanks for the reply! John | ||
| Valley Point |
Hi everyone, I have good news...I fixed the problem in my HD-150. Hopefully this may come in handy for anyone who has this "power loss" problem. I went through the entire power amp and DB-4 circuits, searching for possible problems. First off, supply voltage was fine (710 VDC). Secondly, the bias was adjusted correctly. The entire driver board was rebuilt, with all the correct components or suitable cross-referenced replacements. But, while back-tracking, to the other supply voltages, I noticed that the zener diode (D15, 1N4748), which is supposed to feed 22VDC to the 220ohm control grid resistors (going to pin 5), had no voltage. I lifted the diode to verify that it was, indeed, faulty. I replaced this diode (along with R87, 1.5kohm, 1W feed resistor). I powered up, I measured 22VDC and, when I sent signal through, SHAZAM!!, I'm running at 140W, before clip, into a 4-ohm bench load. Paul, check that zener diode. It may be your problem as well. Steve and Terry, thanks for checking in and leaving some responses. It's nice to see that forums like this still exist on the web. Kudos gentlemen!!!! Peace, John | ||
| Paul |
Thanks John for sharing you're finds. I measured the voltage on pin 5, and it was 21.3 VDC. Once again, as stated in my first post, all voltages checked out. At this point I'm suspecting a bad output transformer. that would be a bit pricey, so I've but this amp on the back burner. I have many more amps, and whole lot less free time. Thanks Steve for administrating this site. I really do enjoy it. Paul |