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Hello, I a new to this forum and I would appreciate any help you may give me regarding a problem I have with the above amplifier.
I have acquired a Dynaco 400 stereo amplifier and it has some problems. Upon pulling off the top cage cover and before plugging it in to my dim bulb tester I noticed it had a transistor that was hanging off the PC-29 board on the very top by one metal leg only. I looked at the service manual and it was labeled Q6 on page 39. Upon searching for the part number I could not find it listed. Then I noticed on the schematics of the board in small print it stated "PC-29 Add 100 to all component numbers". I searched again and on page 23 found the number Q106 a transistor MPSU56 and the part number 567043.
Am I correct in assuming this is the correct transistor to replace the one that is hanging off? There are no numbers on the one in the amp. I really can't go any further until I replace this part. I also plan on re-capping it.
Follow Ups:
You can, of course, assume nothing about an unmarked part that is just "hanging there." If you do some basic testing on the transistor and can verify that it is not failed and is a PNP type, your chances are better that it is the right one.Q106 is part of the relay driver, half of a Darlington pair with Q105. In normal operation the relay driver grounds the low end of the relay RY301, allowing current to flow through the relay coil, and thus closes the relay to connect the power amplifier to the loudspeakers. If there is excessive DC sensed at the output of the power amplifier, the relay driver circuit senses this and shuts off, thus opening the relay and disconnecting the loudspeakers to prevent damage. The threshold for relay operation in early Dynaco Stereo 400 production was ±0.5 volt at the output of the power amplifier. Later it was increased to reduce premature triggering of the relay circuit at high output levels.
I would suggest checking the amplifier WITHOUT speakers connected, first by the Preliminary Tests on Page 27 of the Dynaco Stereo 400 manual. If it passes those tests, continue to "No Sound Output" on the same page. If you get it powered up, especially check the DC voltage at eyelets 10 and 11 on PC-29. These are the centerlines of the complementary-symmetry output stage (eyelet 10 right channel, eyelet 11 left channel). If both are not close to zero volts DC, you have another problem besides that "hanging" Q106, and it's probably in the power amplifier.
I am thinking that someone may have messed with that Q106 because the relay wasn't closing. But the ACTUAL reason for the relay not closing to begin with would more likely be an output stage failure.
GP49
Edits: 02/21/12 02/21/12 02/21/12 02/21/12 02/21/12
Thanks for the followup. That helps a lot.
I am also assuming that the B+/B- fuses are not blown. Most Stereo 400s I've seen were equipped with ceramic fuses that have to be checked with an ohmmeter.
If they are already blown, you probably already have shorted output transistors and very likely, damage on the driver boards. I'd power up such an amp through a VariAC but you're going to power it up through a light bulb to absorb excess current, as a precaution.
Good luck!
GP49
I did check all the fuses and they are OK including the large ceramic ones on the top of the units between the filter caps. I am going to power it up as soon as I get the part installed I was referring to in my first post and plan on using a dim bulb tester.
I am going to order that part today along with some other items I am in short supply of.
Intact fuses: a good sign, especially if they are still the original ceramic type. Most of the time if a non-Dynaco technician replaced the fuses, they'd use glass ones because that was what was usually kept in stock.
GP49
I built one of these in 1975 from a kit and later bought 2 others, all of which need some form of attention. So far, all of them have 6-ampere AGX type fuses on top of the relay plate. I have yet to encounter one with ceramic fuses installed.
Mark
Could be one of those "whatever we have in the parts warehouse" kind of things that Dynaco was famous for. I've had three of those, too. Two of them, I fixed for "owners" who never reclaimed them. After looking at them for too long, with no responses from registered letters sent to the owners, I took them home and used them myself. After about ten more years I sold them off and as far as I know, they are still running. The one I still have, and which drives the woofers in my home system, was a Dynaco accommodation sale of a factory wired unit; it is from the very first months of Stereo 400 production (early enough that the Whisper Fan® kit was not yet available) and it still has its original ceramic fuses, despite being worked on twice for bad transistors in the Class A amplifier section, stem-to-stern capacitor upgrades in the audio path, and a complete bypass of the PC-29 board.
GP49
I have a question for you, GP49. Exactly how did you bypass the PC29 board, anyway?
NOTE that I only bypassed the AUDIO section of PC-29: the filters and the two-transistor stage that serves to raise the input impedance of the Stereo 400. If you do this, the Stereo 400 will no longer be usable with the Dynaco PAS preamps; its input impedance will be too low.I disconnected the twisted-pair wiring at eyelets 19, 20 and 22 of PC-29 to eyelets 1 and 2 of each PC-28; and ran an extra twisted pair, one signal and one ground, from the input jacks way down on the heatsink, to eyelets 1 (signal) and 2 (ground) of each PC-28.
You could also run that extra twisted pair from eyelets 21, 23 (signal) and 22 (ground) of PC-29 to the corresponding eyelets on each PC-28. Much shorter wiring and easier to do.
This change duplicates the operation of the bypass switch on the Stereo 416, in its bypass position. It leaves Dynaguard in operation; since Dynaguard is out of circuit until actually triggered, disconnecting Dynaguard is not worth the effort if it's working properly. Leaving the inputs connected at PC-29 will provide Dynaguard with a signal to work with.
You still need to fix the protection and relay driver circuits on PC-29, so this is not an easy way around diagnosing and fixing the Q105, Q106 problem.. . unless you want to bypass the relay and depend on fuses to protect your speakers (Dynaco did that on the Stereo 410). I don't recommend it because fuses depend on excess current to blow, and by then your speakers could be fried.
GP49
Edits: 02/23/12 02/23/12
Just thought I'd mention, GP49, that awhile back, I bought semiconductor repair kits from an e-bay vendor for both PC-28 and PC-29 boards. Of course, all of the original transistors have been replaced, presumably with better parts. With the one I built in 1975, aside from the original power switch having developed a short in it, one of my bigger concerns was the 10-watt resistors on PC-30 getting too hot. That question was addressed on this forum earlier, so I might have a good solution for that problem. Back to original point, I have no intention of bypassing anything in any of the ST-400's that I have, although on one I picked up on e-bay last year, I noticed that input volume controls had been bypassed, and also noticed some film caps on the back of the PC-28 boards, I just haven't gotten into any of them yet.
Mark
That's good that you've replaced transistors, particularly the input devices on PC-28. Dynaco acknowledged that a whole batch of them were bad!
GP49
Once again, GP49, I have yet to replace anything on any of these amps at this time, but I thought I'd share with you some of the parts replacements that came with the kits. On PC-28 boards, Q1,2 and 3, originally 2N4889's, are now 2N5401's, Q4,5 and 6, originally 2N3440's, remain the same, as does Q7, a 2N5415, also Q8 and 11, BC308B's, I got BC307B's, Q9 and 10, originally SE6020A's, now TN3019A's, Q12 and 13, still TIP41c's, Q14 and 15, still TIP42C's, and diodes appear to be the same, D5 listed in kit as 1N5244B, other diodes as 1N4148, which is what I find in Dynaco parts list. I'm also considering redrilling the heat sink to double the number of output transistors, as the 416 (and some 400's were done, even though I know this could be a problem.
Mark
THANKS! For future reference I'm going to keep your message filed away.
I know of one Stereo 400 that was drilled in our shop for the additional TO-3 devices. I think it was from before the Stereo 416...but after Frank Van Alstine announced his "Double 400." A steel jig was made with the pattern for drilling
the holes, and I remember the placement of the jig had to be exact; there wasn't
any room for error.
Frank could be a curmudgeon; he even said that Dynaco stole his Double 400 design for the Stereo 416...right down to an error he made. But he has never, to my knowledge, ever said what that error was.
GP49
GP49, as the saying goes, wait, there's more. For the PC-29 boards, the parts list is: Q1, originally an EN3962, replaced with PN4249, Q2, originally an SE4010, replaced with 2N5961 (came with the kit) or BC550B, older version, Q3, not listed in my manual, replaced with BC237A or BC550A (came with the kit), Q4, originally 2N5061, same, Q5, originally FT4356, replaced with TN4033A, Q6, originally 2N4889, replaced with 2N5401 (came with the kit) or MJE172, IC1, originally NE5558, replaced with MC1458N (came with the kit) or MC1558N, IC2 and 3use the same LM301AH. Original D3, a BZ102, no longer available, now use 2 1N4148's. Older version replaces D8 and D10 with 1N5231B, newer version lists replacing D7,D8, D9, D10,D11,D13 and D14 with 1N4148's. More? OK, how about replacing the DB301 with a GBPC 35 A 800V? By the way, the BC550B's are cheap at Mouser.
Mark
All appear to be good suggestions, and thanks again.
One problem with working on Dynaco equipment is the transistors that are marked only with Dynaco part numbers. They can make it tricky for a technician in some cases. An early example: in the Dynaco Stereo 120, the output devices were originally identified in the early-production construction manuals as 2N3055, a very widely available TO-3 power transistor. But what Dynaco really put in there, and sent out if you bought replacements from them by the Dynaco part number, were transistors in the same case but of higher rating (perhaps they were "selected" 2N3055). Many Stereo 120 were "fixed" with locally-purchased or mail-ordered 2N3055s, because they were very inexpensive; even back then, about a dollar apiece. However, a transistor could meet the minimum specifications for a 2N3055 and yet be inadequately rated for operation in the Stereo 120. I don't know how many of those Stereo 120s are still functioning and still capable of being operated today, but I still have two that were used for years as loaners for customers, because they cost me next to nothing and I wouldn't cry if one blew up. Probably many of the ones "fixed" with cheap or surplus-store 2N3055s blew up again after being repaired, and their owners gave up on them, selling them for next to nothing. Later, Dynaco finally updated the construction manual with 2N transistor numbers for devices that did meet the requirements for safe, stable operation in the Stereo 120.
GP49
Yes, GP49, I went to www.avahifi.com and read Frank's comments on both the ST-120 and ST-400. What surprised me was his comments about the PC-28 driver boards occasionally overheating (which I can't comment on, as I never remember feeling any heat rising off of them). What he did say about the ST-120's 2N3055's was that these parts were run almost constantly just under their maximum voltage rating, which was why these parts blew so frequently. You did mention that you had access to a jig for redrilling the heat sink on an ST-400. Any chance you could make a paper pattern of it and send it to me?
Mark
I don't have access to the jig any more. It stayed with the shop when I moved away. That was when my hair was 0% grey instead of 90%.
GP49
Yes, I hear you on that one. I'm almost 57 years old, and hair has gone from almost entirely black to largely white.
Mark
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