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Old 18th May 2022, 4:12 pm   #21
John10b
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Default Re: Dansette HiFi Mk2 smoking amplifier

Apart from the hum are you getting any sound from LS if you touch the stylus?
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Old 18th May 2022, 4:31 pm   #22
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Default Re: Dansette HiFi Mk2 smoking amplifier

We are assuming you have replaced the faulty ECL86, or a buzz is all you will get.

The soldering on the new cap needs a tidy up as Lucien noted.

Do a resistance check on the output transformer first, power off of course.
Measure from the Red wire from the rectifier to each pin 3. Then again from the Red wire to each pin 6. The readings should be similar for each pin 3 (100/150 ohms) and again similar for each pin 6. (150/200 ohms ish) I do not have service data so am guessing. Any doubt tell us what they are.
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Old 18th May 2022, 5:08 pm   #23
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Default Re: Dansette HiFi Mk2 smoking amplifier

Yes, I'm getting sound through the pickup. I'll tidy the soldering on the 50+50 can and do some readings. And I have replaced the faulty ECL86 and all the other caps.
Thanks.
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Old 18th May 2022, 8:11 pm   #24
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Default Re: Dansette HiFi Mk2 smoking amplifier

I'm getting a reading of about 130 for pin 3 and around 450 for pin 6. This is about the same for both valves. I don't know what pin 6 is supposed to be but that seems quite high.
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Old 18th May 2022, 8:23 pm   #25
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Default Re: Dansette HiFi Mk2 smoking amplifier

Pin 6 is the anode of the pentode section:-

http://www.r-type.org/exhib/aaa0034.htm

450 (Volts I assume) is way too high! Is your meter set to read DC Volts with the lead inserted in the correct sockets?
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Old 18th May 2022, 8:34 pm   #26
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Default Re: Dansette HiFi Mk2 smoking amplifier

Google "ECL86 r-type" and in the results you'll find an entry for The Valve Museum. It'll take you to their pages for that type number.

You'll see a table giving what each pin number is connected to.

This recipe will tell you what every pin of just about every valve (if you use its type number) does.

The ECL86 has two valve structures in its single glass bottle. The small one is a high gain audio triode (THREE electrodes - a cathode spewing electrons shaken off by heat, A grid to control their flow, and an anode to catch those allowed through by the grid) The large valve in the package is an audio power pentode (5 electrodes, a cathode and an anode as before, but THREE grids to control electron flow and to act as screens to kill of any capacitive coupling from the anode to the first (control) grid.

Pin 6 is the anode of the power pentode.

THat site will give you some explanation on what the valve is for and what it was derived from. You can click through to full data sheets with graphs of characteristics.

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Old 19th May 2022, 7:44 am   #27
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Default Re: Dansette HiFi Mk2 smoking amplifier

Quote:
Originally Posted by hammom02 View Post
I'm getting a reading of about 130 for pin 3 and around 450 for pin 6. This is about the same for both valves. I don't know what pin 6 is supposed to be but that seems quite high.
@ StationX, fortunately they are resistance readings, not voltage readings. (I asked for a continuity test of the output transformer and used the valve pin numbers as I did not know how it is wired.)
@ hammom02, at least they are they same both sides which is good news, a large difference each side would have been a certain fail. Without better data assume the transformer is okay.

Does swapping the 2 ECL86's round make any difference to the buzz?
Then we will need some voltage readings.

There is a copy of the schematic previously posted here: https://www.vintage-radio.net/forum/...78&postcount=1
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Old 19th May 2022, 9:21 am   #28
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Default Re: Dansette HiFi Mk2 smoking amplifier

Quote:
Originally Posted by snowman_al View Post
Quote:
Originally Posted by hammom02 View Post
I'm getting a reading of about 130 for pin 3 and around 450 for pin 6. This is about the same for both valves. I don't know what pin 6 is supposed to be but that seems quite high.
@ StationX, fortunately they are resistance readings, not voltage readings. (I asked for a continuity test of the output transformer and used the valve pin numbers as I did not know how it is wired.)
Hence the importance of showing units. I did query whether it was volts.

A picture of how the new rectifier is connected could be useful, just in case.......
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Old 19th May 2022, 3:32 pm   #29
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Default Re: Dansette HiFi Mk2 smoking amplifier

Strangely enough, I swapped the valves round as suggested and the buzz is significantly lower. Down to an acceptable level. Why that should be I've no idea, maybe some of the pins were bent. I'll live with that for a while and see how it goes. Thanks for all the suggestions and useful tips.
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Old 19th May 2022, 3:51 pm   #30
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Default Re: Dansette HiFi Mk2 smoking amplifier

That suggests one of the ECL86s is not working as well as its pair or one of them is developing some internal leakage. Unfortunately the pentode sections share a cathode resistor, so it will be difficult to measure in the set.
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Old 20th May 2022, 2:23 pm   #31
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Default Re: Dansette HiFi Mk2 smoking amplifier

I'm getting some distortion in the sound now, so I'll replace the other ECL86 so hopefully they will be a better match.
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Old 20th May 2022, 3:40 pm   #32
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Default Re: Dansette HiFi Mk2 smoking amplifier

Noting you location, it might be worth contacting a local Forum Member who might offer to 'Scope the Waveforms for you. There appears to be some instabilty occuring.
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Old 19th Jun 2022, 6:13 pm   #33
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Default Re: Dansette HiFi Mk2 smoking amplifier

Hello again! I abandoned this for while and worked on a Dansette Conquest Auto which was a lot more straight forward. Coming back to it, I plugged it in and I got a buzz and some smoke again.

I decided to check the components again and found there were 2 dead resistors and a couple that were out of tolerance. I replaced 5 resistors in total.

So, switching on again, I get a nice quiet mains hum (no smoking) but nothing coming from the pickup through the amp (the cartridge is ok). If I remove the valves, I get the same. Could the valves be blown even though they are lighting up?

So I've replaced all the capacitors, 5 resistors and the two valves. I'm at a bit of a loss as to what to do next. Should I test or replace the output transformer? How do I do that? Any suggestions?
Thanks.
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Old 19th Jun 2022, 9:46 pm   #34
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Default Re: Dansette HiFi Mk2 smoking amplifier

As a couple of people have mentioned, including myself in post 20, one needs DC voltage readings. They are the single most important diagnostic for most valve circuits and take precedence over most other tests as they will locate most problems in minutes or seconds. In particular, they will locate problems that are likely to cause further damage, such as a valve or transformer that is being stressed by overcurrent due to some other cause.

Once anything that is upsetting the DC conditions has been found and dealt with, many circuits will already be in working order, without smoking parts, hum or distortion. If you are unlucky and there is still a fault that is breaking or affecting the audio signal path, then at least the fact that the DC conditions are OK lets you know that it is probably safe to keep the thing powered on as long as you like, while you trace the signal fault.

To put this into a running-order for the repair I would have gone along something like this:
1) Replace the valve that was visibly down to air
2) Test / reform the main electrolytics and replace if suspect
3) Replace coupling capacitors unless of a known dependable type such as Mullard mustards.
4) Test the rectifier for major, obvious faults (minor loss of performance can be inferred from the voltage readings)
5) Power up, take DC voltage readings starting with main HT and output valve cathodes as these are the most important for stable operation
6) If all within reasonable bounds, it will probably produce sound. If voltages indicate faults, deal with those.
7) Once voltages all good, if still no sound, trace signal path.

Note that with the exception of known trouble-makers and those that should be replaced on sight before power-up, I would not tend to test or replace components or valves until circuit conditions indicate them to be at least suspect. Testing individual resistors etc. is not normally necessary.

So from where you are now, I would power it up and, taking the necessary precautions against shock, measure with respect to the DC negative rail, the voltages on all the valve electrodes except the heaters E.g. V1 triode a=100V, g1=0V, k=2.5V. Also the voltages at the reservoir and smoother and any other HT decouplers. Post your findings.
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Old 20th Jun 2022, 7:45 am   #35
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Default Re: Dansette HiFi Mk2 smoking amplifier

Thanks for the reply. I've done 1-4 including replacing the rectifier with a new bridge rectifier as suggested.
I'm very wary of testing anything with the power on though as I'm not sure what I'm doing in this area.
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Old 20th Jun 2022, 8:13 am   #36
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Default Re: Dansette HiFi Mk2 smoking amplifier

In which case all you can do is check the output transformer primary winding for continuity. The fact that you can hear mains hum suggests it's OK though.
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Old 20th Jun 2022, 8:29 am   #37
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Default Re: Dansette HiFi Mk2 smoking amplifier

You checked the output transformer in post #22 and #24.
Check you get the same results now.

Sorry lots of questions...
Which resistors were 'Dead'? What did dead mean?
Did you replace them with the correct values?
You get hum from the speaker without the valves installed?

Please post some more pictures of the top and bottom of the amp so we can have a look too.

The circuit diagram and resistor values are repeated here. https://www.vintage-radio.net/forum/...78&postcount=1
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Old 20th Jun 2022, 3:36 pm   #38
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Default Re: Dansette HiFi Mk2 smoking amplifier

I checked the readings as suggested in post#20 and they are the same. By 'dead', I meant I was getting no readings from them, so they have been replaced with the same values. I'll upload some images.
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Old 20th Jun 2022, 4:12 pm   #39
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Default Re: Dansette HiFi Mk2 smoking amplifier

Quote:
By 'dead', I meant I was getting no readings from them
Strictly speaking, when you make a test you always get a reading, even if it is off the scale of the meter range you are testing with. What I infer here is that the resistors you tested were open-circuit, and while certain types can go open-circuit by themselves it is unusal. Resistors that have been overheated or grossly stressed and gone open-circuit usually point at faults elsewhere and can offer important diagnostic evidence. E.g. a cooked cathode resistor in the output stage points at a faulty coupling capacitor from the preceding stage or possibly valve. Which resistors, specifically, were open-circuit?

Take care with that soldering. The joints on the rectifier look a bit flimsy and whilst the fact that you seem to have HT more or less shows that those are making contact, it only takes one dry joint on the PCB where a component has been replaced to stop the thing working. FWIW it still looks like there is at least one dry joint at the smoothing can negative pin, although it's hard to see in that pic. That's also one reason for not changing components without good reason, as each change can create new faults - dry joints, broken tracks etc - even with the most careful work.

If you are not comfortable with doing diagnostics with the power on, now might be the time for someone to have a look at it with you. OTOH we can give guidance if you want to develop that skill. It is a good example to learn with as it is simple, transformer isolated and PCB based so things are pretty accessible and easy to probe.
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Old 20th Jun 2022, 5:43 pm   #40
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Default Re: Dansette HiFi Mk2 smoking amplifier

The two that were showing no readings are marked on the attached image (180k and 470k). I'll check the soldering again.
Thanks.
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