Redmere Soloist in a bushel basket

[Sowhat]

I got a Redmere Soloist in for repair and restoration and it was in pieces... not all the pieces however. Some previous tech decided to pull all the higher power transistors for whatever reason and then lost some of them, didn't document what he did or anything resembling responsible work what so ever. This amp has some schematics up on the web but they are quite incorrect and incomplete, the +-15 volt regulator is completely missing as is the chassis wiring and circuitry.
I have reverse engineered a drawing detailing the missing circuitry and have done the same to the amplifier where I found that the schematics are pretty much worthless. Here's my tentative power supply schematic.



I have inserted values as good as I could read them from the board... I think I have it correct. Parts in question in this schematic have a question mark by them, it should be obvious what parts were missing, the two main transistors in the +-15 Volt regulator for one. Also there is a strange 15K ohm resistor where it appears a 1K resistor should be... there's been a lot of soldering, trace lifting and burning on this board so someone prior to me may have put that in by mistake.

Can anyone take a look and help me determine what parts should be where to make this thing work, my only alternative is to strip all that stuff out and put in a more modern dual regulator IC circuit, I'd really hate to do that however.

[Mick Bailey]

I would expect the unregulated +/- 50v to feed directly to the power amp. As drawn it connects to where the +/-15v output should be and wouldn't work.

Take a look here for a typical transistor-regulated PSU (although this shows just a positive supply);

Regulated DC power supply using transistors

[Sowhat]

The +-50V does feed the power amp it's just that it's also feeds to the +-15V regulators as shown as some sort of base reference for the zeners maybe. I have never seen this configuration before and it has me a bit puzzled. I carefully traced this circuit from the board and that's how it's hooked up.

Here's an x-ray image of the board with traces highlighted with red being +50VDC, green being -50VDC purple being 0VDC ground and light blue being other circuit nodes.



It sure looks to me as to having that circuit traced correctly... maybe I screwed up but I don't think so. The left side of the board is the +-15V regulators and the right side of the board is the power amplifier less the output transistors.

[Mick Bailey]

I just realized my observation was incorrect; as a series-regulator the +/- rails would be as they are marked (taken off the emitter of the series-pass transistor). I don't 'get' this circuit arrangement and the purpose of connecting the 50v rails as a reference appears unnecessarily complicated anyhow.

[Sowhat]

unnecessarily complicated... yeah!

Seems the engineers at PA:CE over there in jolly old England thought it was a problem child too... I figure this board is an early revision because I found a picture of the board on the web and there have been some serious changes, like scrapping that regulator and putting in an integrated solution like we all do these days.

Here's my board...



and here's the one I found on the web...



A bunch of changes there for sure. Rumor has it that PA:CE in Cambridge only produced about 10 of these Redmere Soloists back in 1978 or 1979 while others say it may be closer to 50. Needless to say they didn't catch on, maybe it was that whopping $2,600.00 pricetag for a solid state amp? Regardless, I wold like to keep things as original as possible because this thing is so rare, even if it's not as loved as other vintage amps.

[Sowhat]

I have finally confirmed a schematic by tracing the circuitboard. As I have said this is a really early version of the board but this one has at least been checked against the real thing:



Here's the one I started with that was published on www.backfromthesixties.co.uk/redmere_soloist:



I still don't have it completely working yet but it's become a vanity project at this point.

[tedmich]

Yikes Good luck! From an historical perspective this is a wacky and interesting amp that I (an perhaps some others) was unaware of, thanks for bringing it to our attentions.


Maybe PA:CE charged per knob?

Great site too! http://www.backfromthesixties.co.uk/

[Sowhat]

I believe I have figured out the problem with the amplifier... seems it was my stupidity and blind ignorance that was to blame, not to get into details but never assume something is good, check every suspect completely. Tomorrow will tell the tale when I wean it off the variac and plunk it straight on the mains.

I have documented this repair/restoration with great detail and made some one of a kind notes on how this version works, was constructed, it's pitfalls and dangers. I will publish what I have found and documented here when it's all signed sealed and delivered. I've tried to contact Chris Devine of backfromthesixties.co.uk to have him post my findings but it seems he's been abducted by aliens or something and is incommunicado.

[tedmich]

Chris is probably rebuilding an oddball 6-way switch from an old amp he found buried on the moors...

[Sowhat]

I have finally confirmed a schematic by tracing the circuitboard. As I have said this is a really early version of the board but this one has at least been checked against the real thing:

I try to publish stuff as soon as I get it made but that can be a double edge sword. After exhaustive checking and correcting I think the documents I've created for this amp are now correct... famous last words. I am publishing them again in this post, now, these are the official ones, yeah, right!.

Amplifier Schematic power_amp_sch.pdf
Regulator Schematic power_amp_sch_2.pdf
Board Layout place_ref.pdf
Mains wiring redmere_mains.pdf

[Sowhat]

Just an update of where things sit with this obscure amp...

I had to put it aside because it was eating up way too much bench time, not that I'm on some schedule or something, I'm retired kind of, but I had some other pressing jobs that needed to pass through.

At the last session I had reassembled the power amp, checked everything several times and started bringing it up slowly via variac and I didn't get very far with this. I could see DC building up on the output as I raised the mains voltage. I didn't let it get much beyond 5 volts before powering down. I got DC on my output just like someone getting chocolate in their peanut butter.

I have always had a love hate relationship with these old unprotected quasi complimentary designs, they always give me fits an provide a lot of excitement in my boring world when they let the smoke out. I remember building an old Tiger II back in the 60's that wound up being a reliable smoke generator. I never really got that thing running right.

I pretty much know how to track down the demons in these antiques but it takes a careful, deliberate methodology and with me, a lot of time. I am a bit wary of my reverse engineering, which I have checked several times but seeing that I did a lot of daydreaming during college, I never really fully trust my math.

Can someone give a look at the amplifier schematic I made up based on board tracing and dubious input information to see if what I made up is correct and would work.

Maybe someone can tell me what's going on with the ground/speaker output circuit with that FET switch shorting out that 10K resistor when the third preamp is engaged... what's that all about... some sort of mojo mechanism... basically I don't see that it would have much effect if any with that parallel 0.33 ohm resistor in there.

Any opinions or analysis would be very welcome.

[Dave H]

Maybe someone can tell me what's going on with the ground/speaker output circuit with that FET switch shorting out that 10K resistor when the third preamp is engaged... what's that all about... some sort of mojo mechanism... basically I don't see that it would have much effect if any with that parallel 0.33 ohm resistor in there.

I'm still trying to understand that VTX schematic you posted and here's another odd one.

It looks to have mixed mode feedback (voltage and current). I think the FET increases the current feedback when CH3 is engaged. The 0.33 ohm resistor senses the speaker current.

Shouldn't R52 and R53 on the +/-15V supply be the same value? (1k?)

[Sowhat]

Shouldn't R52 and R53 on the +/-15V supply be the same value? (1k?)

Good to see that someone notice that as well. I just documented what was there and when I came across that 15K resistor, the same thought came into my mind. They should be 1K on both sides but no, there's a 15K in there on one side. Maybe some previous tech read his color codes wrong and installed the wrong value in haste, or... maybe there's some more of that mojo magic going on here... David Gilmour used one of these for recording once, maybe this exact one, so it must be magic, right? This is a hand built amp and as I see, nothing more than an ever evolving prototype.

I have since pulled that entire low DC supply from the board and replaced it with some LM7815 and LM7915's, kind of a eliminating the unknown variables type of fix... the preamps, effects and control logic all run fine with the amp out of the loop using this modern style low voltage split supply.

[Sowhat]

Just thought some pictures would be nice...

Here's me testing out my new +-15VDC supply:


Here's beta matching the new output transistors... some MJ15003's: This little microcontroller based testing gizmo seems to work great, I'll need to get this put into a little box with a battery sometime. You can find these at: ESR Meter / Transistor Tester



Here's the amp all re-assembled and mounted back into the chassis:

[ragepta]

Hey,

I got myself one of these beasts a few years ago. I thought I would give a try repairing it in my spare time (first channel without effects works, the rest doesn't). But I realized quickly I don't know much about electronics and kids ate up all the leisure time I thought I had...

Let me know if some pictures of my unit's circuitry would help you.
Do you know anyone interested in the whole thing or in particular parts? I need space, so I will get rid of it anytime soon. Better to find someone giving it a second life than throwing it away...

How much is it worth today anyway?