Dansette conquest output transformer

[simonbridge]

Hi All
I have an issue with a dansette which uses a single ecl 82 with
I refurbished it an sold it on ebay. Unfortunately the delivery driver went through a ford and filled his van with water and the dansette.
Then the recipient decided to turn it on(i know!!) anyway it now doesn't work and is back with me at my expense(long story)

Circuit diagram attached..



Took the electronics out and got it on my bench through a light limiter and the light limiter is showing that something is a miss. Anyway traced it to the output transformer.
Looks like its had some damage to the windings and was measuring just 200ohms across the primary windings. Taking this out of the circuit and its pulling little or no current now.

Anyway my questions are.
1. Is 200 ohm normal
2. Where can i get a replacement.

Not had a lot of experience with output transformers so sorry for my ignorance,

simon

[Station X]

The DC Resistance of the windings of an output transformer doesn't matter too much. What matters is the impedance ratio, which is the square of the turns ratio. The transformer's purpose is to match the high impedance of the valve's anode circuit to the low impedance of the speaker.

Removing the transformer will result in the player drawing less current, because no current will flow in the anode circuit. Don't power the player in this state for long, as the current which should flow in the anode circuit will flow in the screen grid and destroy the valve.

I suggest trying to dry out the transformer in the airing cupboard or by passing a small current through the primary winding.

Have you done simple things like checking the DC HT voltage on the reservoir and smoothing capacitors?

Do the resistors measure OK?

[simonbridge]

Its been well over a week since it got soaked.
The HT is fine not checked the resistors.

The transformer(see the image) looks like its been burned out the insulation on the windings looks like its been pulling some current so I suspect this defo needs to be replaced.

[Station X]

If you're convinced it's the output transformer that's faulty, this will do the job:-

https://uk.rs-online.com/web/p/audio...rmers/2106475/

Just make sure that the anode current doesn't exceed 40mA.

[simonbridge]

Thanks mate.
Does it just connect up the same?

[Station X]

My valve data book says the recommended anode load impedance for an ECL82 is 9k, so use the 9k taps on the primary and taps on the secondary to suit your speaker. Tap details are given in the technical data on the RS Components website.

[electrogram]

does the valve see 1.5 ohms reflected as there are two 3 ohm speakers in parallel ? this would require a ratio of 77:1. if the transformer you have measures 200 ohms, the valve should draw almost normal current even f it has shorted turns. 200 ohms sounds like it might be ok, it doesnt have too bad an appearance either

[crackle]

I am not convinced you have found the problem. If the output transformer has suffered water damage then personally I would also be worried about the mains transformer, pickup cartridge and motor.
I would do some more checks before buying a new transformer.
What are the symptoms when you try to use the player normally via the lamp limiter? Working? no sound? humming?
What exactly is the lamp limiter doing?
What size bulb have you in the lamp limiter?
What is the voltage on pin 6 of the ECL82.
What is the voltage across the primary of the OP transformer?

Mike

[Station X]

HT is said to be fine, so I assume the mains transformer, rectifier and smoothing caps are OK.

[snowman_al]

You do not say what the 'fault' is, but the paper cone(s) on the loudspeaker will have suffered badly if it has been in water. Have you tested it / them?
As was said above, 200 ohms is a reasonable figure for the OP transformer primary.
Measure the 100 ohm cathode resistor next, a low value indicates a faulty 25uF capacitor. If normal put everything back and measure the voltage on pin 2, from that you can work out the current the valve is drawing.

[simonbridge]

crackle.. I am using a light limiter as its been recommended in order to limit the damage.
When I had the unit before this occured the light(60w) was nowhere near as bright as it is now.
With the transformer removed the voltage on the heater is around 7vac and it glows nicely/

With the transformer connected the voltage drops. Which i assume is down to the voltage drop across the light limiter.

The speakers are fine and did hum a bit with a sort of odd crackle when the power first applied.

[Diabolical Artificer]

Have you got a signal generator? If so test the OPT by putting a 1v RMS sine into the primary, measure what comes out. Do a continuity test from the pri to OPT metal clamp, same for secondary. It could really do with a HV leakage test on the windings If all these test ok...

Measure resistance from the reservoir smoothing cap to ground, make sure there is no DC stored in the cap first. Next if that shows no short (lower than 100 ohms ish) connect the OPT pri and take voltage readings on all pins, this with the LL in circuit will give lower readings but should give some idea of what's, what.

Andy.

[TrevorG3VLF]

A rough check of the output transformer can be made by disconnecting the primary and then feeding it with 6.3V from the heater circuit.
Take the valve out to protect the screen.

[simonbridge]

Thanks
Will try these things.

[crackle]

Originally Posted by simonbridge

crackle.. I am using a light limiter as its been recommended in order to limit the damage.
When I had the unit before this occured the light(60w) was nowhere near as bright as it is now.
With the transformer removed the voltage on the heater is around 7vac and it glows nicely/

With the transformer connected the voltage drops. Which i assume is down to the voltage drop across the light limiter.

The speakers are fine and did hum a bit with a sort of odd crackle when the power first applied.

These tests you have done do not in themselves indicate a fault with the output transformer. It is to be expected that the voltage will drop with the OP transformer connected up. This is because the valve is now conducting and drawing its full current.

Is the motor running when you do these tests?

Do you have a DC ammeter capable of measuring 1 amp and know how to use it?

Has the .01uF capacitor from pin 9 to the 10k resistor on pin 3 been changed?

Has the 3 in 1 electrolytic been changed or checked for leakage?

What you need to do is measure the voltages on the valve pins in relation to the chassis.
The important ones to check are cathode pin2, grid1 pin 3, anode pin 6.
Check the voltage across the 100 ohm cathode resistor and also with the power turned off check the value of the 100 ohm resistor.

Please report back with your findings.

Mike

[snowman_al]

Simon, ^^^ this (as requested before).
It would help a lot if you explained what the fault or symptoms are that you are trying to fix and a brief description of what work you did previously?
Alan

[Robert Gribnau]

The value of the cathode resistor of the pentode section (100 Ohm) looks very low to me. A mistake in the schematic, or?

[John10b]

The cathode resistor values are shown between 100 to 330 ohms depending on which Model Dansette you look at.
Cheers
John

[simonbridge]

Right chaps.
Im sorry for any lack of knowledge with valves. I have a lot of experience in digital electronics but never touched a valve amp before so I apologise for my ignorance.

I have the electronics on my bench plugged in to the light limiter. I have so far deduced this.
1. With the valve in circuit there is no HT on the primary of the output transformer measure from the half wave rectifier on the 32uf Cap.
2. With the valve removed the situation is the same no HT and the bulb(light limiter) seems to be drawing the same current.
3. Disconnecting the HT supply from the top of the primary winding at the junction with the 18k resistor I get 300VDC again no valve installed

I started checking the resistors 100ohm and 18ohm that lead from the secondary winding(at the bottom of the schematic)
Lifting the leads from the 18ohm caused disintegrated it as i pulled out the lead. The 100ohm reads 78ohm

I have attached images of the disintegrated 18ohm
I am not sure about the old style resistors but seems to have the bands brown, grey, black and silver.
which i would have thought was 180ohm with as 10% tolerance.(image attached)


Anyway have I got a knackered output transformer and an obvious 18ohm resistor or is it 180ohm?

[John10b]

Have you carry out the checks on the output transformer as suggested by Diabolical Articifer #12, check between primary and ground? It’s possible you have a leak causing the ht to drop.
Cheers
John