Capacitor foil deterioration?

[DMcMahon]

It says TFM on one of the caps pictured at Post 15, I did not realise that TFM went as high as 10uF

David

[WaveyDipole]

There is a 10uF film capacitor on this board and it is huge! Its about the size of a D cell and is not WIMA branded. Compared to the (much smaller) WIMA film capacitors it looks pristine! Must have cost a bit in the day. They are not that cheap to replace today either.

[DMcMahon]

That looks to be C6 for the output control voltage of the Automatic Record Level circuit, I wonder why they chose a MKT film capacitor as opposed to an electrolytic ?

David

[dazzlevision]

Originally Posted by WaveyDipole

There is a 10uF film capacitor on this board and it is huge! Its about the size of a D cell and is not WIMA branded.

SEL = Standard Elektrik Lorenz, a West German subsidiary of International Telephone and Telegraph corporation of the USA, until it was sold on in the 1980s when ITT pulled out of Europe and subsequently split itself up!.

[ms660]

Originally Posted by DMcMahon

That looks to be C6 for the output control voltage of the Automatic Record Level circuit, I wonder why they chose a MKT film capacitor as opposed to an electrolytic ?

I would say leakage current, same for C7.

Lawrence.

[WaveyDipole]

Yes, agree, I do believe this is C6 and the other 1uF mentioned earlier looks like C7.

Having changed that 0.1 with the split along its length and a couple of others (but not the 1uF yet) yesterday, I tried powering slowly up on the variac. It came up and I have some audio when tapping pins around the ECL86, but there is also a lot of hum and some crackling so further investigation required. There are at least two further WIMA caps that could do with replacing. One of them being C29 across the primary of the output transformer which is quite badly cracked. The other has a crack similar to the one pictured in the opening post.

The cause of the lack of audio was actually a popped fuse on the rectifier. The replacement it is holding for now but one of the rectifier tabs seems a bit wobbly in comparison to the others. The output seems a bit low at 233V when the circuit indicates 254V to 275V. The rail could be being dragged down by something so further investigation needed. In the meantime, a bit of Googling suggests it may be a Selenium (referred to as "silinium" in one particular post!) rectifier rated at 250V (presumably AC?). Not sure what the C 100 K4 stands for or any of the figures below. Can anyone confirm this please? I have not seen this type before so are they reliable, or best replaced?

[DMcMahon]

C 100 I believe is 100mA, B 250 is 250VAC.

The 254V figure is for Playback mode and 275V for Record mode.

Very common on these TKs for the HT voltage to be lower than shown on schematic, unless exceptionally low does not cause a problem, this is generally due to the ageing of the metal/Selenium bridge rectifier. Poor smoothing of the HT leading to high ripple voltage also can cause lower HT.

These bridge rectifiers are generally reliable apart from ageing effects causing lower voltage. You can replace with a modern silicon bridge rectifier or 4 discrete silicon rectifier diodes. These replacements can be more efficient than the old rectifier and you possibly could end up with too high a HT, then a series dropper resistor can be fitted.

David

[WaveyDipole]

Well I found the source of the crackling. It was a cold joint on the secondary side of the output transformer. That being re-flowed and the 1000pF across the secondary and also a 47nf being replaced due to cracking, the audio output now seems to work quite nicely. I was able to inject a 1kHz signal right at the head and hear the output on the speaker, but still remains to be heard what happens when a tape is played. I will hopefully have time to reconnect the motor and try that tomorrow.

For the time being, I have left the rectifier alone as it seems to be working just fine. I note your comment regarding the voltage on the low side being common, as well as the difference between playback and record voltages which I will confirm.

[DMcMahon]

Originally Posted by WaveyDipole

I will hopefully have time to reconnect the motor and try that tomorrow.

Is there a motor problem ? don't remember it being referenced.

David

[WaveyDipole]

No, I disconnected one of the power leads to stop it continuously running. It was making a right racket when the tape deck was upside down and I was probing the pcb! It seems to run quietly enough when the tape deck is the right way around although it will probably need lubricating. I would welcome advice on what lubricant to use.

[DMcMahon]

The motor itself generally runs smooth and pretty quiet, I have never lubricated one.

It is normal for the terrible racket if upside down, also to a lesser extent if upended onto one of its sides, this is the flywheel knocking.

If there is any loud sound like rumbling when the unit is in its normal position it most likely is coming from the rotating flywheel, sometimes indicating a lack of grease in the lower bearing assembly as referenced in Post 6 of this linked Thread -

https://www.vintage-radio.net/forum/...d.php?t=205452

Would not recommend removing the flywheel unless it is noisey or to replace the belts, it is a little difficult more so on the TK 18 series compared to the TK 20 in above link.

David

[knobtwiddler]

Originally Posted by Maarten

I think Wima Durolit are paper capacitors.

A 10u 100V capacitor is likely to be plastic foil or electrolytic, not paper.

For what it's worth, I took the 2 x 10u and 1 x 2u2 TFM caps out of the scope, as well as the smaller ones with red writing. Absolutely no problems at all - tolerance and ESR are perfect on all 5 caps. Annoyingly, because the leads were wrapped around the PCB, it was impossible to remove them without damaging the leads, so I had to replace them with modern caps (about £30 worth of caps). No difference to scope problem... The scope's impossible to probe live due to construction, so I am now faced with making up test fixtures in order to take readings with it powered...

[WaveyDipole]

Originally Posted by DMcMahon

The motor itself generally runs smooth and pretty quiet, I have never lubricated one.

It is normal for the terrible racket if upside down, also to a lesser extent if upended onto one of its sides, this is the flywheel knocking.

If there is any loud sound like rumbling when the unit is in its normal position it most likely is coming from the rotating flywheel, sometimes indicating a lack of grease in the lower bearing assembly as referenced in Post 6 of this linked Thread -

https://www.vintage-radio.net/forum/...d.php?t=205452

Would not recommend removing the flywheel unless it is noisey or to replace the belts, it is a little difficult more so on the TK 18 series compared to the TK 20 in above link.

David

Noted. What you describe is consistent with what you are describing in para 2 as normal operation so I will leave it alone.