advertisement


Leak TL/12 Plus / Point One Plus

Might just well be surface corrosion and all is fine, but I’d attack it by first trying Will’s suggestion, and seeing what happens. Then if need be desolder one end of that cap and get a reading of its value. As for fitting I think the leads are riveted to the cap.
 
Tony L said:
I’m confident ‘my’ end of the lead is solid, but I’d not put any money on the entry into the cap being good at all. I don’t understand the construction of this type of cap at all (I don’t even know what they are called), but it looks real manky to me!
Click to expand...

It has probably been dipped or covered in wax to prevent moisture ingress!

Burning the midnight oil here...

Nighty night.
 
John_73 said:
Might just well be surface corrosion and all is fine, but I’d attack it by first trying Will’s suggestion, and seeing what happens. Then if need be desolder one end of that cap and get a reading of its value. As for fitting I think the leads are riveted to the cap.
Click to expand...

I’ll do the Quad 34 test sometime later this morning as that will hopefully change the operating parameters of the input sufficiently to help test Will’s theory. To be honest I’m inclined to accept Will’s theory as-is regardless as it is the only credible one we have and simply replace C15. If I understand correctly, its role would appear to be to help prevent exactly this kind of instability. Makes sense to me.

As I mention above this is a capacitor I do not want to bother testing due to where it is. If I’m going to rework that area, which I’m perfectly happy to do, I only want to touch it once by actually replacing that capacitor with a brand new one. Those tags are likely the most fragile on the board as they look to have been drilled-out further to accept the two components. It will obviously involve desoldering the big chunky resistor too so I just don’t want to do it multiple times. The schematic says 15uuF, which I don’t understand (is that 150pF?). I also have no idea what this type/genre of cap is called, what voltage I need, whether it is polarised etc. If someone could let me know exactly what to buy and I’ll go grab some!
 
Tony L said:
C15? It does look more than a bit shabby. If it is suspect and anyone can point me towards a good replacement I’ll swap them out in both amps. It is awkward to get to as it is right under R5, so I don’t want to fart about trying to lift an end to test it, if it’s in the frame even slightly I’ll just replace it with new/known good. My meter isn’t great right down in the pF range anyway so I’m not confident I’d get a meaningful reading anyway. I have no idea what that type of resistor is even called, but if you point me to a vender I’ll go grab a couple.

PS You can spot some C15 crustyness in this pic:
Click to expand...

C15 is a Silver Mica capacitor. That one looks quite normal. (As the name suggests it is a mica structure with the flat leads riveted to the mica and dipped in wax to keep moisture out.)
There are millions in old radios that are much older than the TL12+ all working fine. But if you want to eliminate it, then swap it over with the one from the 'good' amp. (I would do C15 and R5 as a pair, easier than getting the cap out on its own.) The schematic says it is a 15pF (15uuF) so any axial 250 volt 15pF cap would do.
 
Tony, I was writing my response when your post popped up, so posted it straight away. But there are a couple of things to add.
First, if you measure the voltage across R5 with the amp on, that will give you the actual voltage C15 is subjected to. The manual suggests between 70 - 80 volts, so a polystyrene cap would do like: https://uk.farnell.com/lcr-components/fsc-160v-15pf-1pf/cap-15pf-160v-ps/dp/9519963

Second a question. On your passive pre, are the two channels totally independent of each other? Or is there a connection between the left and right RCA outer ground rings?
 
snowman_al said:
Second a question. On your passive pre, are the two channels totally independent of each other? Or is there a connection between the left and right RCA outer ground rings?
Click to expand...

It is totally independent, no continuity between the L/R RCA outers and they are fully isolated from the chassis. It is also worth mentioning this is a ‘shunt’ attenuator, i.e. the actual stepped aspect is on the ground/return, the signal is a single fixed resistor. I don’t really understand what if any relevance this has, but IIRC shunt attenuators present a varying load depending on the volume knob position. I also don’t know where it stands with regards to capacitance.

I’ll dig the Quad 34 out shortly as that is totally different being a modern (and I assume buffered) active preamp. I assume it will common the grounds and provide a very different output impedance.

PS I’ll measure the R5 voltage later. I really need to learn to read schematics properly as to my eyes it looks like it sees 230V which R5 then drops to 167V. There is a level of math/theory here that I’m totally missing as I was a total drop-out! I’m definitely at the ‘just enough knowledge to be really stupid/dangerous’ level!
 
Incredible amounts of patience being shown here, really hope you get a positive outcome. The need to disconnect & reconnect leads would drive me to distraction.
 
It’s a learning curve. I’ll clearly never know as much as Will, Alan etc, and I don’t expect to, but I already know more than when I started and my soldering skills are improving with practice. It’s all good! I’m in the lucky position of not having any budget or time limits, I’m certainly not planning to sell either these or the Stereo 20 when done, and it will be hugely satisfying to end up with a really nicely restored pair of amps and a bit more knowledge about electronics in the process. I’ll only be annoyed if I screw anything up!
 
I’ve done the Quad 34 test and amp #2 still comes up with a buzz, which as ever can be ‘cleared’ by unplugging the pre-power link on the rear of the 34 and replugging it. I guess this rules out any odd load, capacitance or lack of grounding being presented by the Audio Synthesis passive attenuator. Any instability is clearly within the amp, but may not exist if the input is actually shorted on start-up.

I’ll do the R5 voltage reading at the end once I’ve played a CD or two.

PS No standard ground-loop here at all. Once amp #2 was replugged everything is silent, and much to my surprise the volume knob ends up about where one would want it on the 34, i.e. these Leaks are perfectly usable, at least with fairly inefficient speakers such as my LS3/5As, with this kind of preamp. I’m playing an ECM CD at a fairly modest Sunday morning level and the volume knob is on ‘10’ out of a range of ‘21’ steps. The Audio Synthesis certainly sounds better, but in fairness the Quad has been sleeping in its box for over a year so will likely wake up a bit.
 
Really odd, and frustrating for you too! I'm wondering if somebody could talk you through a basic test with the oscilloscope to see if the amp is on the verge of oscillating when first powered up, before you 'clear' it with the plugging and unplugging?

Also, is that cap indeed 15pF or something different? Reason I ask is I did a little Googling and came up with the following hit from 'another place' and it's mentioned there that it should be 200pf?! Are there different schematic revisions for this amp like the Stereo 20? FWIW it is 300pF on my Stereo 20, but that's in line with a 4.7k resistor too.

https://www.vintage-radio.net/forum/showthread.php?t=54561
 
P.S. haven't been through the thread again, but have you tried changing over the EF86 input valve between amps?
 
John_73 said:
Also, is that cap indeed 15pF or something different?
Click to expand...

I am sure I saw a higher value written on it, 200 or 300 pF, something like that, but I didn’t think to take a photo of it which is really annoying. I’ll see if I can gently nudge it and read anything later once any high voltage has gone away.

The Vintage Radio thread absolutely reinforces what Will suggested so I’m getting confident we are getting to the end of this now. That cap has been served an eviction notice. I just need some help deciding what exactly to replace it with.

John_73 said:
P.S. haven't been through the thread again, but have you tried changing over the EF86 input valve between amps?
Click to expand...

Yes. The problem is unquestionably with the amp chassis, not the valves (I swapped the whole set. Neither is it the interconnect, mains lead, or CD/preamp channel. If I leave absolutely everything on the same sides and just swap the de-valved amp chassis over the problem goes with it. My money, based on what we have learned over the past 24 hours, is C15 is dead or wrong.
 
OK well that's good news that the valves are all OK. Yes must admit for the past few days I've been thinking C15 is very likely something to do with this issue, especially from reading that other thread over at The Vintage place and what Will has said. If it's 300pF then I have plenty of spares here of exactly the same type, but all NOS and not corroded. Used them for the same position in my Stereo 20. I have a feeling it might be 200pF in yours though. But either silver mica or polystyrene would do the trick. If it's a warm/hot area on the amp though I'd go for silver mica.

P.S. Might be an idea (even though it's a pain to do) to see what value it is in the good amp too, on the off chance either the 'bad' amp has the wrong value, or a dicky cap, or there are two different amp revisions that you own.
 
Further to what John is saying, 15pF would be too small a value, it's odd how that arrived on the diagrams, 200-300pF would make a lot more sense.
 
At this point I'd do what Alan suggested and swap over C15 from the good amp to the bad, just to see if that changes anything.
 
Barrymagrec said:
The circuit I`ve been looking at gives C15 as 0.00002 or 200 pf in more normal terms. Also in series with a 4k7.

I assume that some differences may be to cater for different versions of the output transformer.

http://44bx.com/leak/Leak/Circuits/TL12Plus3.gif
Click to expand...

Those values certainly make a lot more sense than a single 15pF capacitor - the latter would probably cause oscillation rather than cure it. The resistor is actually quite important here too - it terminates the indefinite rolloff and subsequent phase shift from just having the capacitor alone
 
Meter probes clipped across R5 gives 61V.

To the best of my eyesight ability and given how tightly I soldered R5 over the top I am pretty sure the existing value of C15 in both amps is 15pF. It is certainly two digits, and really the only alternative could be 25pF, but I think it is 15.

The TL12 Plus clearly went through some revisions and I have obviously implemented a few to make life easier on the EL84s (470k from 1m, plus the associated cap change to 0.1uF). There may well have been a corresponding change here, I just don’t know. I’m certainly happy to change the value of C15 if it could make the amps more stable.
 
Is that voltage too low? Looking at the diagram that Barry posted, it looks like 200V in that area, and divided over R5 and C15 should be around 100V rather than 61V?
 
Well a quick google confirms the early TL12+s had 15pF C15s. And a picture too.



A 'static' 60 volts is fine. A 160 - 200 volt replacement should be okay there.
I would still suggest you swap C15 and R5 to eliminate them.

@ John_73 the Resistor R5 and the EF86 form a potential divider. (HT voltage from R7 minus the EF86 anode voltage.) Alan
 
You must log in or register to reply here.


advertisement


Back
Top