Danson D540 mono amp "can we make it work" project..

[Bomber county]

Ok so I'm a bit green when it comes to valve stuff I will admit and maybe I shouldn't really have put power to it without checking this and that first.Anyway with a 60 watt bulb in the mains feed I thought I'd see what happens.... Not much happened really, no smoke or explosions and just a faint mains hum from the loudspeaker. All the valves get warm and the heaters glow but no output from either a mic or an MP3 player attached to the input. Input bulb was reasonably bright (but not the full output) for a few seconds then settled to "quite dim".

I see a few aluminium can type capacitors with "Hunts" on them on the underside. I understand these have a reputation for being less than reliable so if I start by changing them is it likely to cure it? what sort should I replace them with? All three pots for vol, tone etc where stiff and sticky but after a blast with Servisol cleaner are at least moving freely and the "hum" increases as I turn them.

Valves are EZ40, EL41 x2, ECC40 and an EF40. Am I about right with the test bulb wattage? no rating plate on the chassis.

I have a scope and a decent Fluke 87 meter that apparently tests capacitors but no means of re-forming or a dedicated cap tester.

['LIVEWIRE?']

The hunts capacitors that I, and most other forum members regard as 'change on sight' have/had! brown plastic cases which will invariably have split apart. Typical values ore 10nf (0.01uf) 100nf (0.1uf0 and similar. I don't know the amplifier in question, but one of the Hunts caps may be 'that cap' as it's referred to here. This must be replaced to prevent damage to the output & mains transformers and output valve, since, if it is electrically leaky, excess current flow will result.

[GrimJosef]

Just to explain that 'that cap' (one of those coded phrases, the meaning of which is only revealed once you've been admitted to 'the club' - welcome, by the way ) is known to everyone else as the interstage capacitor which couples the anode of one valve in the chain to the control grid of the next valve.

As Livewire says, if any of these is a Hunts Micamould (they can be black as well as brown) then you'd be well advised to replace them. If, on the other hand, they are Mullard 'mustards' (Google will show you lots of pictures of those) then they'll be fine and replacing them will do you no good at all. The same is very probably true if they're red rectangular Wimas, although amps with EL41s in tend to pre-date these.

If you get the chance, a photo of the component side of the amp might allow us to focus our advice a bit more tightly. If you're confident that it'll be safe to make measurements on the running amp then it would be interesting to know what the valves' grid voltages and cathode voltages relative to signal ground are.

Cheers,

GJ

[PJL]

A circuit has been found in this thread https://www.vintage-radio.net/forum/....php?p=1100818.

I would check the continuity of the output transformer primary and mains transformer primary and HT windings first.

To prevent their catastrophic failure, the electrolytic capacitors should be reformed next. There are various threads on this in the forum.

Then replace the output valve grid coupling capacitors (0.022uF) before powering up again.

[Station X]

Copy of circuit from thread linked to by PJL attached.

[AC/HL]

Originally Posted by Bomber county

I see a few aluminium can type capacitors with "Hunts" on them on the underside.

If they are electrolytics, and there are quite a few in this amplifier, they are no worse than any other make. It was the crumbling encapsulation that made the paper Hunts capacitors notorious.

[Bomber county]

I'll upload a couple of pics when I get a minute. The caps all appear to be electrolytics though. Whats involved in making a capacitor re-former? or are they something I might find at a rally for "not much"?

[Station X]

The construction of a capacitor reformer is shown here:-

https://www.vintage-radio.com/projec...-reformer.html

Given that you've already applied power it's unlikely the smoothers will explode and they should reform via the lamp limiter. Just feel them from time to time and switch off if they get hot. POWER OFF of course and remember that the caps will retain a charge.

Change the coupling caps first though.

[Bomber county]

Photos. The "aerial" socket went to a GS60 photo cell and the big empty "valve socket" was power out to the two lamps and the motor so I assume don't affect anything by being left disconnected but if anyone knows different please tell...

[AC/HL]

The blue sleeved ones are electrolytics. The others are all paper dielectric, so quite probably leaky, even though they are metal cased and not plastic. The two .022 (22nF) capacitors from the triode anodes to the output pentode grids are the twin "That capacitors" and will need replacing to avoid expensive and possibly terminal damage.

[Bomber county]

Ok, I'll check the transformer tomorrow and order some capacitors for it if it checks out ok. Transformer has what looks like a bit of melted fat/wax oozing out of it which has solidified, should I be concerned? Transformer also ran the projector lamp as well which I would imagine drew a fairish current..

[martin.m]

It can be difficult to repair a piece of equipment if you don't understand how it works. Fortunately valve theory is not complicated. Have a read about how they work and what sort of voltages are needed on the various electrodes. If you don't have a circuit diagram then valve data from a book or the Internet will tell you which pins are connected to the various electrodes. Bear in mind that valve radios and amplifiers contain dangerous high voltages. As others have stated, some types of capacitor are best changed on sight. Other common faults are carbon resistors which have changed value or gone open circuit. With some knowledge of basic valve theory it should be possible to find the fault logically using a multimeter. Good luck with this project.

Kind regards
Martin

[Bomber county]

I was an 70's/80's child (unfortunately) so I'm more 555's, 741's, BC108's and the like and didn't follow that particular career path in the end (which I regret...). Now trying to get my head round the whole valve thing whilst there are still a few people left to learn from

[martin.m]

As suggested in post #10, I would change the two capacitors that connect to the anodes of the ECC40 (pins 2 and 5) before going any further. Next measure the DC voltages on these two pins which according to the circuit drawing in post #5 should be around 100v and 150v respectively. Lack of voltage could indicate failure of the 100k resistors that feed HT to the anodes. Turning to the EF40 check the voltages on the screen grid (pin6) and anode (pin2). Expect around 70v and 50v respectively. If voltages are low or missing suspect the 1.8M and 470K resistors which connect pins 6 and 2 to the HT supply. Hope this helps you.

Regards
Martin

[Diabolical Artificer]

" the heaters glow but no output from either a mic or an MP3 player attached to the input" It's a bit hard to trace a signal without a scope and sig gen, but I guess you could do it with a small speaker one lead with a croc clip, clipped to ground, tother lead with a 630v cap and series resistor soldered onto an old screwdriver.

Basically you check you have a signal on the grid, and a signal out of the anode or cathode, then follow along till you get the the other end.

Your welcome over anytime if you want to bung the amp on my bench and use my scope etc, am happy to share what little I know about valve amps.

Andy.

[Bomber county]

I have an old Scopex scope which works (old 15Mhz one though) and a big old Marconi signal generator which someone gave me (havn't got round to trying that yet but it is alleged to work..). Might sound like a silly question but if I want to check for the various voltages at different points on this amp where do I connect the negative lead of the meter? to the earthed chassis or is there a negative line from the transformer. I have ordered some caps from eBay so can't do a lot now for a few days anyway.

[GrimJosef]

Find the 200 ohm resistor which has one end connected to the output valve cathodes and connect your negative lead to the bottom (i.e. not the cathodes end) of that. In all likelihood it will also be connected to the chassis (the circuit suggests that it is) but there is a small possibility that it won't be. The centre tap of the mains transformer's HT winding will also go to that point.

Cheers,

GJ

[Boater Sam]

Its unlikely to do much if you still have the lamp limiter in series...........

[Bomber county]

That is a good point, I did wonder if that might be the case. I was watching a bit on YouTube earlier about running old radios up through a lamp and the guy on there mentioned it as well. Hopefully the capacitors shouldn't be too long coming but I have shelved it now whilst I await their arrival...

[crackle]

Originally Posted by Bomber county

Anyway with a 60 watt bulb in the mains feed I thought I'd see what happens.... Not much happened really, no smoke or explosions and just a faint mains hum from the loudspeaker. All the valves get warm and the heaters glow but no output from either a mic or an MP3 player attached to the input. Input bulb was reasonably bright (but not the full output) for a few seconds then settled to "quite dim".

Your initial test would seem to indicate that you probably have an amp capable of working.
Once the 2 audio coupling caps to the grids of the output valves have been changed further tests can be carried out. You should get some output from the MP3 player into a speaker connected to the amp, even with the your 60w lamp limiter connected in series, so it looks like there is a fault somewhere stopping that from happening.
Leave the lamp limiter in circuit (that will give the smoothing electrolytics an easier time and chance to reform. Monitor the HT voltage as it comes up and check the smoothing caps do not get warm.
Its difficult to tell from the small images this site creates but it looks like your 9 pin valve holders are the type which have brass forks for the pin connectors, these go brittle with age and can crack and break. Check the valve sockets for dirty or broken pin connectors.

Mike