Hello everyone,
First I want to thank all of you who have unknowingly contributed to my learning about electronics. I'm still a beginner, but this site is an invaluable resource for novices like me as we try to level up our skills. So...thanks! If I could buy you all a beverage I would.
I have a few EMT 928's in various states of disrepair. One motor doesn't spin, One spins but makes no sound, one has a broken counterweight....you get the idea. I've never had the pleasure of hearing let alone owning any EMT model so frankenstein-ing one together from the units I have was an exciting prospect. I passed that test, and now have a unit that makes most of the right noises, though all the normal age-averse pcb components are definitely in need of a refresher. For example, if the recording pushes anything above a mezzo forte the sound crackles with agony. This brings me to the preamp.
There are separate identical boards for left/right channels. What I'm hoping for is one of you kind experts to flex your knowledge on the attached pictures and help me identify exactly what a few of the components are that I need to replace. There are a few that seem obvious to me, but before I start slinging solder I'd like to make sure I'm not making big mistakes. I found the manual for the 928 in the library over at VinylEngine, and while it does have circuit layout, it tells nothing of values. I'm over at Digikey and Mouser being completely inundated and overwhelmed with options, and having a hard time knowing exactly which component types to get. I'm not agonizing over trying to go ultra high-end with the quality of the caps (at least not yet)--I just need to get fresh parts on so that I can move forward with at least some assurance that my sound problems aren't related to 40yr old electronics.
As examples:
C2- Orange on the left, what type is this?
C17- Green Top middle, Tantalum cap? (Its ESR was way out of spec)
C24- Green rectangles lower right and center
Hopefully my link out to an album will work here: the preamp pics are the first few.
EMT 928 - Google Photos
Thanks for reading.
First I want to thank all of you who have unknowingly contributed to my learning about electronics. I'm still a beginner, but this site is an invaluable resource for novices like me as we try to level up our skills. So...thanks! If I could buy you all a beverage I would.
I have a few EMT 928's in various states of disrepair. One motor doesn't spin, One spins but makes no sound, one has a broken counterweight....you get the idea. I've never had the pleasure of hearing let alone owning any EMT model so frankenstein-ing one together from the units I have was an exciting prospect. I passed that test, and now have a unit that makes most of the right noises, though all the normal age-averse pcb components are definitely in need of a refresher. For example, if the recording pushes anything above a mezzo forte the sound crackles with agony. This brings me to the preamp.
There are separate identical boards for left/right channels. What I'm hoping for is one of you kind experts to flex your knowledge on the attached pictures and help me identify exactly what a few of the components are that I need to replace. There are a few that seem obvious to me, but before I start slinging solder I'd like to make sure I'm not making big mistakes. I found the manual for the 928 in the library over at VinylEngine, and while it does have circuit layout, it tells nothing of values. I'm over at Digikey and Mouser being completely inundated and overwhelmed with options, and having a hard time knowing exactly which component types to get. I'm not agonizing over trying to go ultra high-end with the quality of the caps (at least not yet)--I just need to get fresh parts on so that I can move forward with at least some assurance that my sound problems aren't related to 40yr old electronics.
As examples:
C2- Orange on the left, what type is this?
C17- Green Top middle, Tantalum cap? (Its ESR was way out of spec)
C24- Green rectangles lower right and center
Hopefully my link out to an album will work here: the preamp pics are the first few.
EMT 928 - Google Photos
Thanks for reading.
BrassVinyl,
I envy you having 'a few' 928's in various states of disrepair - not that they are in various states of disrepair, but because you have several. C2 = aluminum electrolytic cap 10ufd/16volt, C17 = tantalum cap 100ufd/3volt, and C24 = 0.068ufd/250volt polyester. The 928 schematic shows the capacitor values, but not the voltages, which can generally be estimated from the power supply voltages. IME modern replacement caps are physically much smaller than vintage caps so it's safe to go to higher voltage rated replacements since they will fit in the same physical space as the originals. I've often had to go to higher rated voltage replacement caps in order to get into a size for which the lead spacing will match the originals on the PCB. A quick look on Digi-Key turned up many aluminum electrolytics that would fit in the space of the tantalums.
There are plug-in replacement upgrades for the 709 opamps - I'll let some solid-state experts make recommendations for those.
For me, the biggest dilema is whether to replace all the 40 year old electrolytics wholesale or just those that need it based on troubleshooting. EMT likely used premium quality long-life parts, and the photos reflect that. I think I would compromise by just replacing the 'orange' electrolytics and the 'green' tantalums (parts in the signal path) as routine and other caps only as required. Perhaps some EMT owners will chime in with their actual experience. I can only watch and drool.
Ray K
I envy you having 'a few' 928's in various states of disrepair - not that they are in various states of disrepair, but because you have several. C2 = aluminum electrolytic cap 10ufd/16volt, C17 = tantalum cap 100ufd/3volt, and C24 = 0.068ufd/250volt polyester. The 928 schematic shows the capacitor values, but not the voltages, which can generally be estimated from the power supply voltages. IME modern replacement caps are physically much smaller than vintage caps so it's safe to go to higher voltage rated replacements since they will fit in the same physical space as the originals. I've often had to go to higher rated voltage replacement caps in order to get into a size for which the lead spacing will match the originals on the PCB. A quick look on Digi-Key turned up many aluminum electrolytics that would fit in the space of the tantalums.
There are plug-in replacement upgrades for the 709 opamps - I'll let some solid-state experts make recommendations for those.
For me, the biggest dilema is whether to replace all the 40 year old electrolytics wholesale or just those that need it based on troubleshooting. EMT likely used premium quality long-life parts, and the photos reflect that. I think I would compromise by just replacing the 'orange' electrolytics and the 'green' tantalums (parts in the signal path) as routine and other caps only as required. Perhaps some EMT owners will chime in with their actual experience. I can only watch and drool.
Ray K
The oranges look to be Roderstein Bakelite caps.
Elvee mentions them here: Roederstein 'BAKELITE' series EK capacitors - "Living Legend" OR "Looney Legacy" ...?
The green box caps can probably be found on Google if the codes are clear enough to read (and you care) - that's how I found the Rodersteins - The values seem simple enough. Poly film.
Elvee mentions them here: Roederstein 'BAKELITE' series EK capacitors - "Living Legend" OR "Looney Legacy" ...?
The green box caps can probably be found on Google if the codes are clear enough to read (and you care) - that's how I found the Rodersteins - The values seem simple enough. Poly film.
Ray,
Thanks so much. This is exactly what I needed to get going.
It's good to hear you seemingly pretty easily saw some options that would fit physical dimensions, though I've seen some folks here work some leads into masterful bends to make newer components fit on old pcb designs. I don't mind getting creative in that regard if I have to.
For the opamps, I found a thread on the 928's somewhere where they were talking about these. There was a range of opinions as is usually the case with these topics, but if memory serves there was mention of using a 741 in place of the 709's. I recall a detail about the leg pinout being different but would still work as long as the orientation was correct. I'll have to go search for that. Either way this is something that's easy to change, and, if I can get clean sound after replacing some of the dried out electrolytics, then it will make the sound comparison for the 709/741 easier (or possible). This is assuming the op amps are good of course. My very limited experience tells me to start with the caps.
And generally I'm not a modder, and with regards to the components used being 'old school' as they may be, these things were designed by much more expert folks than I will ever be. I figure I owe it to the design to start fresh with good parts and see where I might need to go from there.
Thanks again for the reply.
Thanks so much. This is exactly what I needed to get going.
It's good to hear you seemingly pretty easily saw some options that would fit physical dimensions, though I've seen some folks here work some leads into masterful bends to make newer components fit on old pcb designs. I don't mind getting creative in that regard if I have to.
For the opamps, I found a thread on the 928's somewhere where they were talking about these. There was a range of opinions as is usually the case with these topics, but if memory serves there was mention of using a 741 in place of the 709's. I recall a detail about the leg pinout being different but would still work as long as the orientation was correct. I'll have to go search for that. Either way this is something that's easy to change, and, if I can get clean sound after replacing some of the dried out electrolytics, then it will make the sound comparison for the 709/741 easier (or possible). This is assuming the op amps are good of course. My very limited experience tells me to start with the caps.
And generally I'm not a modder, and with regards to the components used being 'old school' as they may be, these things were designed by much more expert folks than I will ever be. I figure I owe it to the design to start fresh with good parts and see where I might need to go from there.
Thanks again for the reply.
Thanks Hearinspace,
This is great. The composition of the cap is exactly what I'm missing. And you're right, I don't know why it didn't occur to me to try to put more of these codes into a search engine... I did a bit of that when I first started looking at the boards, I think I just got dazed and confused with all the options and decided I needed to look for a lifeline.
Thanks to you as well.
This is great. The composition of the cap is exactly what I'm missing. And you're right, I don't know why it didn't occur to me to try to put more of these codes into a search engine... I did a bit of that when I first started looking at the boards, I think I just got dazed and confused with all the options and decided I needed to look for a lifeline.
Thanks to you as well.
No.
The '741 is convenient and has significant problems with good audio.
The antique '709 is a very good amplifier, with reservations. The wise designer uses it as a can of parts, not a complete solution.
The 928 has a bunch of discrete transistors handling stuff the '709 is not so good at. T1 T2 is a low-hiss amplifier with gain of 6 to overwhelm the '709's hiss. Z1 (also Z4) '709 is decompensated for 8MHz gain-bandwidth... FAR more than any '741. The '709's output was good for the day but a short would kill it (be careful!!). But the outside line is not seen by the '709 but by a 4-transistor mini-power-amp. And there is an output transformer-- a non-ideal part but R30 R31 take a great whack of NFB around it to reduce all flaws.
2017年10月のブログ記事一覧-Decca Decolaがお嫁入り
Thanks PRR and Nigel. I was thinking of replacing all the caps across the board. But after sleeping on it (many nights now) I think on this first pass I'm looking to only refresh any known bad parts, keeping values the same, or near as I can get. The 709's will stay. Thanks for the tip on the danger of shorting them, I'll be taking extra care.
Also, thanks for that blog link. I thought I had found every electron related to the 928 all across the web, but that blog is new to me. It looks like a fantastic resource.
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