ARC SP-6A Tube Preamp

[chiily]

It was all going to well...

Having a listen tonight and the crackles came back...I tried tapping, and pressuring the main PCB and the volume/selector PCB with a chopstick, but couldn't invoke the noise. Nor wiggling the selector/left/right switches...

I did discover that the mute switch is on its way out. Sometimes switching it to the mute position generates hum in both channels. If I wiggle the switch the hum stops. My guess is a poor connection to gnd when switched to mute. From looking at the schematic the mute switch isn't in the signal path when in the Operate position. Tapping the switch when in the Operate position doesn't generate crackling.

I'll bypass the switch tomorrow...

Other than that, I need to check the PCB with the switches mounted on it.

Voltages are okay though..lol

 

[Atmasphere]

It was all going to well...

Having a listen tonight and the crackles came back...I tried tapping, and pressuring the main PCB and the volume/selector PCB with a chopstick, but couldn't invoke the noise. Nor wiggling the selector/left/right switches...

Is it in both channels?
Is in on all inputs or phono only?
Is it affected by the volume control?

If yes to the first then its likely a power supply or ground problem.

 

[chiily]

Two albums in and not a noise.

Remember I mentioned the mute switch and how it wasn't consistently grounding the output to gnd when in the mute position, sometimes on mute the speakers would hum and if I wiggled the switch the hum would die away? I also noticed that when in mid crackle and I switched to mute, then one speaker would continue to crackle at a reduced volume and then die away. It got me thinking, maybe it was just the switch? Looking at the circuit diagram if the mute switch has a poor/intermittent connection to gnd when the SP-8 is passing signal then this would cause the LDR to "flicker", cause the audio to drop in and out, not as on/off, but as an analogue/variable effect. Remember that we had managed to get all the voltages in the PSU close to the schematic, especially the voltage at the top of the Zener stack. What more could it be...

So, took the black front panel off by removing the dress nuts on the switches and the nuts from the pots. And for a time I considered swapping the operate switch with the mute switch, as the UK versions of the SP-8 don't have any switched outlets, so I would imagine that that switch hasn't seen much action. However, how ever much I tried to unsolder the switch legs it was having none of it, firmly fastened. I would think the only way would be to snip the legs...

I did look for a new switch and I found an eBay seller in the USA, but for some reason eBay won't let him ship them to the UK, though the seller is insistent that he has international shipping turned on. And would ship international if an order came through! In fact he looks to be the only place online that I can find selling these specific switches. https://www.ebay.co.uk/itm/19612817...nIfsU76RD2&var=&widget_ver=artemis&media=COPY

So, that avenue is not open...

Next best thing, try to clean the switch. I always thought that these switches are sealed, but not the ones on this SP-8 it seems. I managed to get some servisol super 10 to flow into the switch body by filling up the switch barrel and flicking the switch back and forth. All the servisol disappeared. I also took the opportunity to clean the volume pot with some Deoxit for carbon tracks, the stereo/mono switch and the input selection switch with the servisol...working them back and forth.

Clean switches and rotary knobs...

I went and tested the resistance from every gnd point I could find to the earth...no more than 0R4.

Can this really be the fix?

I would so love to get my hands on a set of those switches from eBay...I know that the switch clean is only a temporary measure really...

More music tomorrow and no crackles?

 

[Atmasphere]

Can this really be the fix?

It could... it would be a bit surprising that a preamp this old doesn't have a few noisy switches or controls.

 

[microstrip]

(...) I would so love to get my hands on a set of those switches from eBay...I know that the switch clean is only a temporary measure really...

I do not know if the handle has the proper size:
https://www.ebay.co.uk/itm/363808747149

 

[chiily]

I do not know if the handle has the proper size:
https://www.ebay.co.uk/itm/363808747149

Cool I shall check those out, thank you.

 

[chiily]

So occasional crackling now, can go a couple of hours without anything, then starts to come and go....does it with the volume turned all the way down...PSU still.

Time for freezer spray...?

 

[chiily]

Really stupid question....

Could the MM stage be affecting the Line stage? There's a bit of hiss and rumbling coming up from the MM stage when it is selected. I know the gain is more, but I would have expected a hiss, but not a on and off rumble sound.

I've fitted grid stoppers to the MM stage valves. I shall swap some valves around.

This pre-amp is testing my patience as it must everyone's, particular @Atmasphere . Again thank you for all your help, you kindness and knowledge is immeasurable, thank you. The pre-amp is much better than when this journey started. I'm just not a point that I can trust it in the main system.

 

[Atmasphere]

Could the MM stage be affecting the Line stage? There's a bit of hiss and rumbling coming up from the MM stage when it is selected. I know the gain is more, but I would have expected a hiss, but not a on and off rumble sound.

If you have a noisy tube it could easily rumble!

Just to be sure though put your 'scope on the B+ line that feeds it to make sure there's no noise. It should be a nice flat line.

So occasional crackling now, can go a couple of hours without anything, then starts to come and go....does it with the volume turned all the way down...

Both channels or just one?

If its still both and you have the freeze spray then yes. Alternatively you could use a solder iron to heat up a suspected component. Since it uses a TLO71 and its likely original, its not that far fetched to imagine one of the sections getting noisy. I've seen that often enough...

Happy to help!

 

[DonH50]

Be VERY careful using freeze spray around tubes. Easy to shatter a tube if you accidentally nail it with cryo spray, and even good components can suffer thermal shock when they are in a hot box. I used to use a big straw used for milk shakes and such and stick the little cryo can tube into the bigger straw to better control the overspray. This after taking out a tube by spraying a bias resistor and having the spray bounce off the resistor onto the tube. The results were undesirable.

I'd still be suspicious of switches and volume etc. controls. A good contact cleaner can be helpful.

 

[chiily]

Be VERY careful using freeze spray around tubes. Easy to shatter a tube if you accidentally nail it with cryo spray, and even good components can suffer thermal shock when they are in a hot box. I used to use a big straw used for milk shakes and such and stick the little cryo can tube into the bigger straw to better control the overspray. This after taking out a tube by spraying a bias resistor and having the spray bounce off the resistor onto the tube. The results were undesirable.

I'd still be suspicious of switches and volume etc. controls. A good contact cleaner can be helpful.

I was thinking about it this combination of freezer spray and hot valves...I remember as a child licking the end of my finger and touching the top of a hot incandescent lightbulb. Hmmm...crack, poof, and no light bulb. Ah, but we learn by our mistakes!

I've three new DTDP switches on their way from the USA. I'll replace all bar the on/off switch.

The selector switches I have cleaned with servisol super 10 and I have been able to get right to the contacts. If I really have to I could strip them down, but I don't fancy unsoldering them.

The volume control is not original, but it must be quite old as it is a 100k log blue ALPS pot, with indents!

The balance pot is original and I can't see a way to get any cleaner inside, well I can to one wafer, but not the other. If pushed I could unsolder and strip it down.

I have wiggled the pots and switches, reasonably vigorously but I can't invoke the crackle.

I did hook my 'scope up to the B+2, B+1, V4/V5 Va, and the top of the Zener stack (not all at the same time!) to monitor any voltage drops coinciding with the crackles...nope, no change to the voltages.

If you have a noisy tube it could easily rumble!

Just to be sure though put your 'scope on the B+ line that feeds it to make sure there's no noise. It should be a nice flat line.


Both channels or just one?

If its still both and you have the freeze spray then yes. Alternatively you could use a solder iron to heat up a suspected component. Since it uses a TLO71 and its likely original, its not that far fetched to imagine one of the sections getting noisy. I've seen that often enough...

Happy to help!

I need to confirm both channels because I've moved the speakers around.

I've replaced the original TLO71s with new ICs, socketed. In fact I've replaced a lot of components in the PSU. I'll have to update a PSU schematic.

 

[chiily]

I was considering removing the valves from the MM stage and adding a 33k 10W resistor across B+2 and not selected the phono input...just to focus onto the Line stage.

 

[Atmasphere]

I have wiggled the pots and switches, reasonably vigorously but I can't invoke the crackle.

Then they are fine.
Why and when did you replace the original opamps? Was that before or after the crackle was detected?
Are you sure the crackle is coming from the preamp? Could it be in your amps?

 

[chiily]

The crackling is with the new opamps in place. Why I replaced them, I've come across opamps in other ccts that have behaved that way before, half breaking down....

I did think about the Leak ST20 amp being part of the crackle so I swapped it out for a little T amp, D class, and the crackle is still there...

If the problem is still there after I install the 33k resistor in the MM stage and remove the valves, then it is time to reflow all the joints...I've done about 90% of them so far.

Just to give you an idea of what I have changed so far in the PSU section. Remembering that this amp came to me having been run with the zeners in the zener stack being all, bar one, O/C, so I was suspecting the silicon in the PSU section, the top part to be broken.



The green boxes are around the components changed so far.

Bridge diodes were replaced with UF4007. Everything else was like for like.

And in the MM and LIne stages these are the parts I have changed, from the original diagnosis that the caps in the RIAA had all shorted and smoked the cathode resistor R4 in one channel.

 

[microstrip]

IMO you need a noise sniffer - something that allows you to listen to noise in DC lines - it is basically an HV capacitor of around 100nF with some protection diodes and a small amplifier and speaker. Sometimes much more efficient than the scope for random noises.

But the fundamental check should be finding if the crackle happens in both or just one channel.

 

[Atmasphere]

IMO you need a noise sniffer - something that allows you to listen to noise in DC lines - it is basically an HV capacitor of around 100nF with some protection diodes and a small amplifier and speaker. Sometimes much more efficient than the scope for random noises.

But the fundamental check should be finding if the crackle happens in both or just one channel.

This is a great idea! in the old days in the US such a device was called a 'signal tracer'. Eico and Heathkit made examples for just this sort of thing. I have two of these on my bench for this and other purposes. Both are highly modified- I put RCA connections on them and rebuilt them to eliminate ground loops, improve fidelity and reliability.

Here's a tip: solder resin can cause noise and can occasionally arcing too since tiny bits of solder can be suspended in it. So I am careful to clean a circuit board on which I've worked to remove any resin. Quite often this results in lower noise.

 

[microstrip]

This is a great idea! in the old days in the US such a device was called a 'signal tracer'. Eico and Heathkit made examples for just this sort of thing. I have two of these on my bench for this and other purposes. Both are highly modified- I put RCA connections on them and rebuilt them to eliminate ground loops, improve fidelity and reliability.

Here's a tip: solder resin can cause noise and can occasionally arcing too since tiny bits of solder can be suspended in it. So I am careful to clean a circuit board on which I've worked to remove any resin. Quite often this results in lower noise.

Fortunately you haver better knowledge than me - yes the "signal tracer" , I could not remember the proper name anymore! Many decades ago I dreamed about the Heathkit one but it was not easily accessible in Europe.

No experience with it this particular device, but DIY people can get a cheap version in kit :

http://www.velleman.co.uk/contents/...J3V2tF26v5mEUYQTgoiRckoPVZ-6SE65bfw74Ezd9kZRi

 

[Atmasphere]

Fortunately you haver better knowledge than me - yes the "signal tracer" , I could not remember the proper name anymore! Many decades ago I dreamed about the Heathkit one but it was not easily accessible in Europe.

No experience with it this particular device, but DIY people can get a cheap version in kit :

http://www.velleman.co.uk/contents/...J3V2tF26v5mEUYQTgoiRckoPVZ-6SE65bfw74Ezd9kZRi

That one might have a problem if you want to connect to a point where there's a high Voltage.

 

[DasguteOhr]

You need this to find hum, for low noise you put headphones in the 3.5mm jack good luck.
I call it the superprobe follow the signal path where the hum is induced. switch on the preamp first and connect an oscilloscope to the output to see what frequency the hum is. e.g. 100hz~120hz are diodes, filtercaps and choke in power supply

Probe-PRO Tonortungssonde - VDV500-123 | Klein Tools

Die Probe-Pro Tonortungssonde ortet bei Verwendung mit dem Toner-Pro Tongenerator (VDV500-063, separat erhältlich) nicht aktive Verkabelungen. Sie enthält eine responsive, haltbare, austauschbare, nicht metallische leitende Spitze und ein Arbeitslicht für den Gebrauch in dunklen Räumen. Mit der...

www.kleintools.de

 

[chiily]

Had a "back to basic" day today. I was still hearing the odd disturbance, both channels. I couldn't pin is down to anything, heat applied, different valves, wiggling switches and knobs. So, I decided to start from the start, and check resistor values. I had done the pre-amp before and nothing jumped out, but I had tested all the resistors in cct, time to lift some legs.

I discovered that R9 in the RIAA section, on one channel was open cct, which is no big deal.


Also, R50, 15M, was far over spec in both channels. Both are carbon composite resistors.

And that R63 in the PSU was also open cct. R52 and R51 are within spec. R68 (12.1k) is a new part anyway.


Awaiting a delivery from Mouser now...