Resurrecting a 1980s analogue preamp project

[Monkeyboi]

15 minutes ago, Leinster Lad said:

Alan, what are RV5 and RV6 ? ( and  ADLK1 and ADLK2 )

How are they connected ?

RV5 and RV6 are for the d.c. offset adjustments for the op amps in the monitor output stage.  See the attached schematic. 

 

To successfully adjust the d.c. offset to allow a capacitiveless coupling of the output you need to have access to a d.c. microvolt meter to affect the adjustment or a high sensitivity CRO.  It imperative that the d.c.voltage at pin 6 of each op amp be exactly 0 volts under no signal conditions after adjustment of RV5 and RV6 for each channel independently.  Any d.c present at the output will cause damage to the speakers connected to a fully d.c. coupled power amplifier, hence the adjustments are critical.  If any of the components in this part of the circuit are replaced or the power supply repaired of modified, the d.c. offset adjustment(s) must be checked prior to placing the pre-amp back into service.

 

The links LK1 and LK2 allow two possible modes of operation.  When the shorting links are in position A this keeps the output capacitor in each channel in circuit for a.c,.coupling.  In position D it facilitates d.c. coupling of the output by shorting out (bypassing) the output capacitor in each channel.

 

Values for RV5 and RV6 are 100k 25 turn cermet trimpots.  I used the Radytronic ^TM 2396W style vertically mounted units because of space constraints, ease of adjustment and its local availability.

R92 and R93 are 22k 0.25W metal film resistors.  I mounted them on the underside of the board.

 

If you have already commenced the build of the main board you will have to replace resistors R82 and R83 (6k8 025W metal film) as these need to the slightly physically located on the board to facilitate the modification.

 

Cheers,

Alan R.

 

DISCLAIMER:  This modification is still under test.  Your decision to implement this modification is at your own risk and I accept no liability for the suitability or long term performance of the pre-amplifier as a result.  I also have no control over the quality of the workmanship nor the testing ability of the person(s) undertaking this modification.  

 

 

[Leinster Lad]

Cheers Alan, another excellently explained modification

[Vik22]

Good work Alan. Have you got the second part of your upgrades?

 

I have two eti5000 preamps. So far I have rebuilt one but took different approach and below is the summary if anyone was interested.  I have removed MC phono and directly connected MM input to 47k socket reducing cabling. Main mods to MM stage included replacement of all electros with better ones, replacement of polyester with polypropylene, removal of the cap in the nfb, bypasing of input and output caps with polypropylene and change of opamps - the input one is OP37, the second is currently lme49710 but I think I'll replace it with opa627au or bp. I also increased the RIAA 1uF cap  to 2.2uF.

 

In the preamp I cut off the right side of the board with PS and output muting. In that place I inserted separate PS for the audio section (LM350, lt1085 - 3A), separate for the display and added relay based output muting. Also replaced pots with alps as Alan did: 10k half way balance and 10k volume. I kept the original muting for the tape section. Within the preamp I replaced all caps, reduced the overall gain to 5 or 6 (I do not remember exactly), replaced opamps with lme49710, got rid of electros in the second opamp nfb. Monitor output is now the preamp output as well as I needed one more output for the active sub,  display is connected to the output. 

 

The MC section mods include replacement of inputs with AS194 and ne5534 with lt1115. Caps and audio path resistors will be upgraded as in the MM phono and main part of the preamp. I use Dale resistors in the audio path everywhere. 

 

The second eti5000 will have green display diodes replaced with blue ones,  PS will be as in the first ETI but the main preamp will be replaced with mbl clone. I'll keep 400Hz oscillator and signal polarity reverse etc section from the original but on new separate boards. I haven't decided on op-amps yet. I may use the Russian version of OP37 (KR140ud26a) while the second and third op-amps will be LME49710 or opa627. I'll use Alps pots of course. 

 

cheers,

 

 

 

[Leinster Lad]

On 31/03/2019 at 7:43 PM, Vik22 said:

Good work Alan. Have you got the second part of your upgrades?

 

Hey Alan, just revisiting this.

My series 5000 pwr amp rebuild should be finished soon(ish) at which time I can get onto the pre-amp.

 

Just wondering if you ever managed to compile "part 2" of your refurb notes ?

[Monkeyboi]

Apologies guys.  I got side tracked with a few other projects and a number of repairs for some SNA members.

 

Nothing to really add since the d.c. offset mod, but expect if you are building from an original kit of parts to have tantalum capacitor failures.  The best solution is to simply buy new ones and heave those old ones out.  The old ones fail short-circuit but may trap a few players as some of these old tag tantalums will test okay out of circuit on your LCR meter but will fail the leakage test when a voltage approaching their working voltage is applied.

 

Happy to answer any specific questions you guys have.

 

Cheers,

Alan R.

[Leinster Lad]

Thanks Alan,

TBH I have been distracted by other projects/school holidays/xmas/life etc to even get near the series 5000 pre-amp, and not sure when I will get it back on the bench.

Once I have done the two series 5000 pwr amps, I should be able to get onto the pre.

 

Cheers

Dave

[Franksal]

Hi all

I just joined Stereonet.

I recently purchased a eti5000  preamp which I plan to refurbish - the only issue is that the main pcb was home made and of poor quality. I have being trying to find a copy of the original artwork on-line to make a new one with no luck. Due to the size of the artwork it was not included in the back of the magazine as usual - you had to write to eti for it. Just wandering if someone happens to have a copy.

 

Regards Frank

[surprisetech]

2 hours ago, Franksal said:

Hi all

I just joined Stereonet.

I recently purchased a eti5000  preamp which I plan to refurbish - the only issue is that the main pcb was home made and of poor quality. I have being trying to find a copy of the original artwork on-line to make a new one with no luck. Due to the size of the artwork it was not included in the back of the magazine as usual - you had to write to eti for it. Just wandering if someone happens to have a copy.

 

Regards Frank

 

Not sure if you've tried this avenue, but Silicon Chip owns the rights to ETI archives.

Might be worth enquiring if you haven't tried them already.

Silicon Chip Website - Contact - Silicon Chip Online

 

I was going to suggest RCS Radio in Sydney.  I got a few ETI PCBs from them over the years, but it looks like they shut up shop about 5 years ago.

 

Good luck.  It will be a worthwhile project if you can get that PCB sorted.

[audiofeline]

It might be easier to copy/re-draw the original pcb using software and get one made from that. 

[Franksal]

Thanks for the replies

I have tried Silicon Chip - they have the magazine articles but not what I am after.

 

The last resort, as suggested, may be to use the PCB I have to make a copy.

 

Thanks again

Regards Frank

[Leinster Lad]

Hi @Franksal and welcome. Unfortunately I do not have a copy of the main board, and as you mentioned, it was too big for the magazine and had to be sourced from ETI, which are no longer around. It is a shame that SC cannot help you.

Hopefully someone here will have a copy that you can scan (at least 600dpi) 

With a bit of stuffing around, that pdf can be made into a gerber file and then a board made.

You will need to use the old board as a drill template though.

 

Don't give up, this pre-amp is very well regarded and looks so cool with those LED VU's jumping around !  (and just quietly, its specs are right up there)

 

Alan  @Monkeyboi  shared a fantastic upgrade guide a couple of years ago. Should be easy to find here ( this whole thread actually )

 

I have two of these and are getting ready to refurb the one I got off @TrevorM  with some of the Mods that @Monkeyboi has posted about.

 

I have scans of the original SC article if you need them.

 

Lots of info in these DIY forums, good luck and keep us posted (with pics) !!

[Leinster Lad]

1 hour ago, audiofeline said:

It might be easier to copy/re-draw the original pcb using software and get one made from that. 

A lot of work, but a clever person would have no trouble.

Might even be able to bring it up to 2021 standards   ( AKA the @Monkeyboi mods )

 

The only person that I know that could do justice to a project like this would be @Suzyj, but that would be a massive ask 

[thatmetalman]

On 31/3/2019 at 10:43 PM, Vik22 said:

Good work Alan. Have you got the second part of your upgrades?

 

I have two eti5000 preamps. So far I have rebuilt one but took different approach and below is the summary if anyone was interested.  I have removed MC phono and directly connected MM input to 47k socket reducing cabling. Main mods to MM stage included replacement of all electros with better ones, replacement of polyester with polypropylene, removal of the cap in the nfb, bypasing of input and output caps with polypropylene and change of opamps - the input one is OP37, the second is currently lme49710 but I think I'll replace it with opa627au or bp. I also increased the RIAA 1uF cap  to 2.2uF.

 

In the preamp I cut off the right side of the board with PS and output muting. In that place I inserted separate PS for the audio section (LM350, lt1085 - 3A), separate for the display and added relay based output muting. Also replaced pots with alps as Alan did: 10k half way balance and 10k volume. I kept the original muting for the tape section. Within the preamp I replaced all caps, reduced the overall gain to 5 or 6 (I do not remember exactly), replaced opamps with lme49710, got rid of electros in the second opamp nfb. Monitor output is now the preamp output as well as I needed one more output for the active sub,  display is connected to the output. 

 

The MC section mods include replacement of inputs with AS194 and ne5534 with lt1115. Caps and audio path resistors will be upgraded as in the MM phono and main part of the preamp. I use Dale resistors in the audio path everywhere. 

 

The second eti5000 will have green display diodes replaced with blue ones,  PS will be as in the first ETI but the main preamp will be replaced with mbl clone. I'll keep 400Hz oscillator and signal polarity reverse etc section from the original but on new separate boards. I haven't decided on op-amps yet. I may use the Russian version of OP37 (KR140ud26a) while the second and third op-amps will be LME49710 or opa627. I'll use Alps pots of course. 

 

cheers,

 

 

 

G'day.

 

I have a 5000 preamp and I'm curious to see what you did with the reworked PSU and mute section you mentioned. I'd love to see any photos you have for some inspiration. I'm thinking of employing a separate PSU and mute circuitry (within a separate 1U case) but using the 'Denoizer' CCT posted by Mark Johnston over at diyAudio.

 

Cheers,

 

Tim

[Vik22]

3 hours ago, thatmetalman said:

G'day.

 

I have a 5000 preamp and I'm curious to see what you did with the reworked PSU and mute section you mentioned. I'd love to see any photos you have for some inspiration. I'm thinking of employing a separate PSU and mute circuitry (within a separate 1U case) but using the 'Denoizer' CCT posted by Mark Johnston over at diyAudio.

 

Cheers,

 

Tim

Hi Tim,

 

The second eti5000 is not yet finished. Simplified mbl6010 circuit diagram is attached.

 

The first eti5000 has its PS based on altronics kit K5501. It's been modified by replacing LM317/337 regulators with 3A ones mentioned above, input capacitors are now 4700uF/35V, regulating diodes are byw100-200 but you can use 3A FR30x instead. Protection diodes 1n4007 have been added across regulators and 1.1k resistors from ADJ to ground are replaced with 2k trimpots (20 turn) so output voltage can be regulated and set equal for positive and negative output voltages. The fixed output voltage from K5501 has been changed to suit display, that is to +15V / 1.5A. External transformer is 2x18V 50VA as my regulated voltages are +/-16.7V. 

 

I used muting circuit from ESP but first you have to remove muting circuits for monitor and line out on the main board. These are formed by transistors Q7, Q10, Q11, Q12 and their respective diodes and resistors. I disconnected monitor output from IC7 and IC8 and connected it with the line output so I can use one output to connect my active subwoofer and the other output is connected to two way active crossover. 

 

I hope you can read attached K5501 schematic. If you have any further questions let me know.

cheers,

 

 

 

[thatmetalman]

On 18/10/2023 at 6:16 PM, Vik22 said:

Hi Tim,

 

The second eti5000 is not yet finished. Simplified mbl6010 circuit diagram is attached.

 

The first eti5000 has its PS based on altronics kit K5501. It's been modified by replacing LM317/337 regulators with 3A ones mentioned above, input capacitors are now 4700uF/35V, regulating diodes are byw100-200 but you can use 3A FR30x instead. Protection diodes 1n4007 have been added across regulators and 1.1k resistors from ADJ to ground are replaced with 2k trimpots (20 turn) so output voltage can be regulated and set equal for positive and negative output voltages. The fixed output voltage from K5501 has been changed to suit display, that is to +15V / 1.5A. External transformer is 2x18V 50VA as my regulated voltages are +/-16.7V. 

 

I used muting circuit from ESP but first you have to remove muting circuits for monitor and line out on the main board. These are formed by transistors Q7, Q10, Q11, Q12 and their respective diodes and resistors. I disconnected monitor output from IC7 and IC8 and connected it with the line output so I can use one output to connect my active subwoofer and the other output is connected to two way active crossover. 

 

I hope you can read attached K5501 schematic. If you have any further questions let me know.

cheers,

 

 

 

 

Thanks, Janusz.

 

I appreciate the detailed response. I went and purchased the downloadable PDF of the Silicon Chip Oct 2005 article for that preamp to get the PSU info and other details. Good value for A$10.00. 

 

Re the muting ccts from the ETI 5000 preamp, I've certainly heard enough from others (including yourself of course) that convinces me to take the time.

 

I've commenced a complete tear down of the preamp which I purchased from a friend who built his in the mid 80s (I built mine in 1984 but sold it soon after - regretted that decision for decades, so how surprised was I when I was in my mate's shed one morning and saw his '5000 sitting within a large pile of retired gear!).

 

During the strip out of all components, some of the copper pads didn't survive. Mostly those belonging to the 5534ANs. Same issue with the MM board too. I also stripped down every rotary switch PCB (which feature genuine UK Lorlin switches), with the view of servicing the switches (which cost around $12 each to buy from, say, Mouser; I'll save the cost and attempt to service their contacts). All pots have been discarded of course and will be replaced with Alps or similar.

 

Here are some tear-down progress photos if anyone's interested

 

Cheers,

 

Tim

 

 

The main board prior to all pads having 40 year old solder removed (via a solder sucker for most excess, and Japanese braid to tidy up. Flux remover will be sprayed on over the coming days).

 

Main board with some of the discarded components. It's regrettable that this goes into land waste but I doubt there was a viable alternative given the age (and state) of the components.

 

The microphone DIN sockets were gladly removed! I have my eye on DC power sockets suitable for the task (from Altronics).

 

As I removed all the heavily corroded RCAs, I decided to recycle their insulators. At a push I could soak the RCAs in a suitable solution for a day in an attempt to clean them up but they're heavily oxidised and I wasn't convinced I'd bring them back. As with some tear downs, the amount of generated waste is often unavoidable.

 

The condition of both front and rear panels, is excellent. A quick clean is all that's needed.

 

The state of the boards isn't great after removing solder, and the pads didn't fare well in some instances. This is the MM board but it'll be okay once flux remover is applied and the pads re-tinned before loading. I am considering remaking the board in KiCad. I'll see how much energy I have... same consideration for the switch boards too, but the main board would be too great an undertaking IMO. As tempting as that it though...

 

The MM board which will also be stripped completely.

Edited November 5, 2023 by thatmetalman

[Leinster Lad]

3 minutes ago, thatmetalman said:

Re the muting ccts from the ETI 5000 preamp, I've certainly heard enough from others (including yourself of course) that convinces me to take the time.

 

What is this about please ?

[thatmetalman]

12 minutes ago, Leinster Lad said:

 

What is this about please ?

 

Alan mentioned (I think it was Alan?) some years back that the muting transistors could be removed. Also Janusz in his 2019 post discussed removing the mute circuitry from the PSU section of the main board. I'm trying to remember all the posts and comments around the mute stuff going back years!

[thatmetalman]

@Monkeyboi Hi, Alan. I've picked up errors in the original Tilbrook ETI Series 5000 schematic, one of which makes no sense if you assume the PCB overlay as being correct. The preamp I inherited appears to have the assumed error on the PCB, as does your board when I pour over your posted photos.

 

Referring to Tape 1 OUT of the PCB overlay:

• LEFT channel: Q5's base connects to a 4k7 resistor (R31)
• RIGHT channel: Q8's base connects to a 1N914 (D15) - as I believe it should

On the PCB, note that each transistor within the tape muting circuit (Q5 - Q10) has its base connected to a 1N914 signal diode with the exception of Q5 which connects to a 4k7 resistor. This doesn't make sense for only one channel. Working with the assumption that a 1N914 must connect to each (Q5 - Q10) transistor base, one would only notice the anomaly if a tape deck/source was hooked up to Tape 1?

 

So if we assume that a 1N914 should in fact connect to each transistor base, the schematic is also incorrect for Tape 2 Left Out (Q6) just to confuse matters even more.

 

Secondly, assuming that the PCB overlay is correct for the 400Hz oscillator circuit, the schematic has D18's and D19's polarity incorrectly shown; should be D18 cathode to pin 6, D19 cathode to pin 2. 

 

Project errata typically appeared in a magazine's following month/s, but without a library of the months that followed the preamp's articles, it's impossible to check this without asking Tilbrook himself (or anyone with a 1981 ETI binder!).

 

- Tim

[Leinster Lad]

on a technical level, what does the transposing of the diode and resistor do ?

[Wicked_XR]

Hi Alan, I'm impressed with the work you did. Mines been sitting on a shelf for a 20+ years, had to put it away with the power amp when the kids came along. I powers it up few years ago, the tag caps on the psu lit up and smoked, replaced all the tags and it worked again. I was looking at rewiring mine, maybe do a single PCB that holds the pots and switches. I layed out new vu Meters, using SMT, which made the PCB 1/4 of the size. Still need to get the boards made, maybe in a few weeks. I was thinking about putting in a motorised pot for the volume as thats the only downside of the preamp. I'll post when a get time to do stuff. I hope you still check your post.

 

- I currently have 3mm leds, the original rectangle leds never looked right as they weren't straight, might make a daughter board that uses 0603 leds...or solder the leds on the edge, maybe castellations...

Regards,

 

Bill

Edited May 12 by Wicked_XR
Led types

[Wicked_XR]

Hi All, handy site for the ETI mags

 

https://archive.org/details/ETIA1981/ETI 1981-01 January/

[Wicked_XR]

Hi all, i relayed the VU meters, started it back in 1990, shelved for a few years, got back onto it a few weeks ago... and now finally complete, waiting for PCBs to arrive this week...kept all the old ic footprints, as ill pull them out of the old boards, all the other stuff is smt...

 

The preamp is currently getting a refurb, the main board is getting redone......

 

 

[thatmetalman]

Hi everyone.

 

For about 12 months I've been reworking the preamp main board and all sub boards including the LED meters and RIAA boards. A labour of love. Not sure why, but I have a real affinity with the 5000 preamp (probably after working at Jaycar in the 80s for 20 years, and being super close to Tilbrook designs and so on).

 

The PSU has been swapped out with a low noise regulated supply based on LM317/LM337 regulators. The design was posted in various versions on another audio forum. Some of you may recognise it as the 'DeNoiser'. Yes it's overkill for the 5000 but I wanted to do something out of the ordinary. It is worth highlighting my approach, that cost was and is not a consideration because I felt that such an iconic Australian piece of vintage audio deserved the special "Blueprint" attention (as we said at Jaycar all those years ago - for those who remember Jaycar's 'Blueprint' approach to special audio projects).

 

The original Tilbrook muting circuit has been retained more out of a lack of electronics engineering/theory skills on my part. As a production manager in the field following the years these kits were published, I have 40+ years of know how, but not the engineering qualifications!

 

I included Alan's DC/AC selection and DC offset adjustment mod to see if it could fit onto the board. It was tight. I will provide due credit, or remove if Alan objects.

 

Decoupling mono caps are mounted under the board although I am torn about placing caps across each op amp supply rail pin given the illustrated objection by published engineer Douglas Self. Regardless, the pads are provided if caps are optioned in by the home builder.

 

In a recent email from Douglas some months back, he wrote in response to my question, "In your article you stated that a 100nF decoupling capacitor is only necessary across a NE5534's supply rails (i.e. pins 4 and 7), rather than one cap from each on the IC's supply rail pins, to ground (two caps per IC)". Douglas responded, "That is correct. In fact caps to ground just conduct rail noise into the ground if you're not careful". So, I guess that the builder can decide.

 

I have attempted to duplicate Tilbrook's PCB layouts as close to verbatim. This was a painstaking effort! However, there are differences but I have tried to be as close to the original especially for ground planes.

 

Below are some renders and screens for amusement. I have ordered most components including Alps pots, Lorlin make-before-break rotary switches and audio-grade electros where necessary (this was difficult as supply is poor and key manufacturers no longer tool up, however, there are alternatives). I also sourced CA3130 ICs for the LED meters and LM394CH equivalents.

 

I want to breadboard the PSU but otherwise I've checked over this for many, many months and I believe it to be ready to go into production.

 

You can see I've also included silkscreen component IDs as well some other embellishments such as test points, and so on. The CCT diagram of the PSU section of the preamp is included too for reference.

 

My plan is to go as far as re-tooling the chassis and front panel. I have chosen to keep the design verbatim (sans the PSU!) even though Tape and, say, moving coil functions are unlikely to be on most builder's requirements list. I think it'd be a shame to reimagine the 5000 preamp and remove such features.

 

I would have loved to include Alan's additional PCB to dial in turntable cartridge values, but as I stated at the top, I'm not an engineer and would have to research a lot to get it right.

 

Happy to take feedback, positive or critical.

 

I'm keen, too, to see Bill's efforts!

 

[thatmetalman]

On 16/6/2024 at 9:14 PM, Wicked_XR said:

Hi all, i relayed the VU meters, started it back in 1990, shelved for a few years, got back onto it a few weeks ago... and now finally complete, waiting for PCBs to arrive this week...kept all the old ic footprints, as ill pull them out of the old boards, all the other stuff is smt...

 

The preamp is currently getting a refurb, the main board is getting redone......

 

Hi, Bill. Are your LED boards a SMD version of the Tilbrook design or a revised design? It appears to be same as Tilbrook's 1981 design?

[Wicked_XR]

Hi metalman, yes the same cct, but changed to SMD, the old ones took up too much room. I've redone the preamp board as well, I'll post on here tomorrow. Your switch PCB look good, I'm doing those as well, I want to group input together, maybe come off at right Angles with daughter board as it's a lot better to solder.