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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 20th Jun 2022, 7:53 pm   #41
Lucien Nunes
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Default Re: Dansette HiFi Mk2 smoking amplifier

Those are the two triode anode loads, so being open-circuit would stop the amp working completely. You have previously had sound so they must have been working then, even if out of tolerance. Due to their value and position it's more or less impossible for them to have been damaged by a fault elsewhere, and they can't both be from a bad batch of parts as they are different values. So that is distinctly odd and while not impossible if I came across that scenario I would suspect my own measurements to be in error e.g. from locking the range on the meter too low to measure them.

Some notes:

There is what looks like a dry joint on C8 at the V2A end. This will not completely stop the unit working if open, but if the dry joint includes the adjacent end of V2A's anode load (the 180k that's not numbered in the circuit diagram) then that would be as much of a showstopper as an O/C resistor.

The diagram appears to have V1B and V2B labelled the wrong way round. Looking at the PCB, V2A's anode drives its own pentode not V1's, and vice versa for the cathode.

The feedback resistor R11 has been replaced with the wrong value. The pic in post #1 shows it to have been 15k which matches the circuit diagram, but a 33k part has since been fitted. This won't stop the unit working.

What is that resistor on the underside of the board? It was there in the first pic but doesn't seem to have a purpose as all the necessary parts are present topside. It looks like 1M, and while I can't see its connections my best guess is that it is in parallel with R8. Is it?

The speaker lead connections to the tags topside, currently have the black lead to signal and the blue to ground. That might not be a problem if the black lead isn't grounded anywhere else, and indeed it might be correct. But if the black lead is grounded, the output will be shorted and no sound will be the result.

Next post will be about taking voltage measurements. It's time to bite the bullet.
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Old 20th Jun 2022, 11:20 pm   #42
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Default Re: Dansette HiFi Mk2 smoking amplifier

I'm wary of ECL86's. An auction buy of a Blaupunkt "Sultan" sounded pretty good after a bulk replacement of the large C's and 'that' capacitor along with a largish resistor on the output transformer tags, a 1kohm 3 watt job. After 15 minutes or so it started to sound very 'bassy' which got worse and a hot smell - the 1kohm looked a bit toasty. All of the current through the 1k goes into the pentode side of the output ECL86. I attached a voltmeter across the 180ohm cathode resistor to monitor the current. Immediately after warm-up it was passing about 15mA and it stayed there for about ten minutes but then started to climb - when it got to 45mA panic set in. I asked a couple of friends at work and they both said 'that capacitor' which I had already changed but another one made no difference but then someone said 'grid emission' which I had to look up. Apparently ECL86's are renowned for it - I sorted though the spares and found four of them - two Mullards, one unreadable and a Telefunken - only one passed the test and stayed stable at 15 to 18mA - the Telefunken. I wonder if it has a slightly different construction but I couldn't see as it was well 'gettered'. So beware the ECL86! I'm sure that not the same for all circuits, perhaps it's just running near its limits in this one. The data says Va 250v and Ia 36mA so it's running about half the current.
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Old 21st Jun 2022, 12:50 am   #43
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Default Re: Dansette HiFi Mk2 smoking amplifier

I've just magnified that picture and several solder joints look to need re-doing before going any further.

David
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Old 21st Jun 2022, 9:13 am   #44
Herald1360
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Default Re: Dansette HiFi Mk2 smoking amplifier

I wonder if unleaded solder is getting in on the act?
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Old 21st Jun 2022, 10:20 am   #45
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Default Re: Dansette HiFi Mk2 smoking amplifier

Okay, thanks for the pictures.
Evidently there is a need to improve some of the soldering, I guess you knew that already.

I'm still not sure what you understand as 'dead' is. Open circuit, but why / how? Anyway you have replaced them, so to move on...

There is very limited further testing you can do without measuring power on voltages. You are limited to resistance checks, so let's do a table and see if it throws anything up. I've marked up a picture with the valve pin numbers on it and re-named the valves as Vx and Vy as there is a bit of confusion re. the circuit, but the table will sort that out.

So try this,
-ve Meter lead on the Black wire capacitor terminal and measure round Vx.
Pin 1 =
Pin 2 =
Pin 7 Should be 180Ω =
Pin 8 Should be 560kΩ =

Now do Vy negative to the Black wire still.
Pin 1 =
Pin 2 =
Pin 7 Should be 180Ω =
Pin 8 Should be 560kΩ =

Now put the negative lead on the the point Ht2. Measure to
Ht1 Should be 33kΩ =
Vx Pin 9 Should be either 470kΩ or 180kΩ =
Vy Pin 9 Should be either 470kΩ or 180kΩ =

Please fill in the values you get, it will show an obvious open or short or incorrect value. Also might give you a familiarity with the test points if you do want to try with power on at another date.
Alan
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Old 21st Jun 2022, 11:56 am   #46
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Default Re: Dansette HiFi Mk2 smoking amplifier

Quote:
Originally Posted by Herald1360 View Post
I wonder if unleaded solder is getting in on the act?

That's a very good point - the OP may be unfamiliar with using the wretched stuff...


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Old 21st Jun 2022, 3:15 pm   #47
hammom02
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Default Re: Dansette HiFi Mk2 smoking amplifier

Many thanks, I'll get on to it as soon as I can.
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Old 21st Jun 2022, 6:14 pm   #48
Sideband
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Default Re: Dansette HiFi Mk2 smoking amplifier

Quote:
Originally Posted by Boulevardier View Post
Quote:
Originally Posted by Herald1360 View Post
I wonder if unleaded solder is getting in on the act?

That's a very good point - the OP may be unfamiliar with using the wretched stuff...


Mike
It's like quoting 'Scotty' from Star Trek when he said 'Ye canna mix matter wi' antimatter'! Similarly ye canna mix leaded wi' unleaded'!
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Old 21st Jun 2022, 6:41 pm   #49
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Default Re: Dansette HiFi Mk2 smoking amplifier

Quote:
Originally Posted by hammom02 View Post
I'm very wary of testing anything with the power on though as I'm not sure what I'm doing in this area.
The safest way is to attach the meter to the relevant points in the circuit with crocodile clip leads while the device is turned off. Set the meter to the relevant setting and turn on the unit. Note the reading and switch the unit off before moving the leads to the next test point. From the rest of this thread it looks as though you've got a few people who can tell you exactly where to test.

Where high voltage is involved, or I'm particularly wary of a piece of equipment, I have been known to perform the switch on with a broom handle while wearing goggles. I've been close to an exploding capacitor before and it was not pleasant.

There really is no substitute for DC voltage readings at known points. The service manuals usually help you by showing what these readings should be, allowing for speedy diagnosis.
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