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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 9th Oct 2017, 9:10 am   #1
jascha
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Smile Ferrograph Series 4 issue! Help pls

Hi everyone,

My Ferrograph series 4 has developed a fault that is only noticeable when the volume is cranked up all the way, resulting in a motorboating type of effect! I have been advised to replace all the signal caps however aren't these looking like mustard capacitors inside these Ferrograph? I understand that mustard capacitors are very reliable and don't wish to replace them blindly if the culprit happens to be just one cap! I recently worked on a Revox G36 and has these mustard caps which I left alone as they're all fine. I am rather confused if the caps in the Ferrograph are indeed the same type or whether they should be all replaced with more modern polypropylene ones. I mainly refer to the 0.1uf, 0.5uf and I believe 0.22uf caps? I have already replaced just the 0.5uf and the nightmare to get to 0.1uf right behind the eq board! I appreciate any feedback please.
Also there are 2 can caps that are axial (don't believe they are electrolytic). Any ideas what these caps are please and why they are in a can form?
Cheers
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Old 9th Oct 2017, 11:17 am   #2
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Default Re: Ferrograph Series 4 issue! Help pls

Motorboating usually implicates decoupling capacitors in the power supplies rather than signal capacitors.

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Old 9th Oct 2017, 1:04 pm   #3
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Default Re: Ferrograph Series 4 issue! Help pls

This is a Ferrograph stock fault. As David says it's the decoupling capacitors.

A curiosity is that it's often the pre-amp decoupling electrolytics, rather than those nearer the rectifier. It seems electrolytics like to see a bit of ripple current and those 'hidden' behind series resistors fail first. I once had a PA amp where the main electrolytics re-formed nicely whereas I ended up changing the pre-amp decouplers.
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Old 9th Oct 2017, 7:43 pm   #4
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Default Re: Ferrograph Series 4 issue! Help pls

Perhaps I didn't explain myself well enough. The issue I have is only surfacing when the volume is full on (machine in stop mode and mode selector on play), and the bass control is not on minimum! I get this thumping sound like a rumble but it disappears when I turn either the bass control all the way down or lower the volume level.
Today I restuffed and replaced the 100uf electrolytic cathode bypass capacitor can but so far it hasn't made a difference. I have also replaced the other cathode bypass electrolytic cap that I believe is a 50uf/12v cap. I have not replaced the power supply electrolytics yet but have measured them and both value and esr seem ok. Sorry to ask but which caps are the decoupling capacitors that you refer to? Are they the 3 can capacitors that house the high voltage electrolytics (C9/C13, C11/C12, C29/C30)? Their esr & value seem to measure ok.
Thanks again guys for all your help.

Last edited by jascha; 9th Oct 2017 at 8:11 pm.
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Old 9th Oct 2017, 9:40 pm   #5
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Default Re: Ferrograph Series 4 issue! Help pls

You are describing "motorboating" it's caused by amplifier stages coupling together unintentionally at low frequencies via the power supply rails. The decoupling capacitors would normally prevent it by providing a local reservoir of power, bypassing any unwanted AC signal to ground.

Have you also checked the main smoothing caps C24 & C26?
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Old 9th Oct 2017, 9:50 pm   #6
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Default Re: Ferrograph Series 4 issue! Help pls

I have checked C24 & C26 for esr and capacitance measurements only. I have not done any fault findings with the deck on. To be quite honest it's rather difficult to work on these machines while they're powered up as the cables are rather short and prevent the deck to be opened for one to comfortably work on, say to check for any dc on grids etc.
I attach a picture of the amplifier section showing that my deck has already mustard capacitors which I believe don't need replacing.
A question: can the can caps be restuffed with 400v/450v rated caps, although the originals seem to be rated atv500v? Not easy to find radial caps of 500v rating to restuff the original cans, to keep the authentic looks.
Cheers
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Old 10th Oct 2017, 11:33 pm   #7
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Default Re: Ferrograph Series 4 issue! Help pls

450v should be fine. Running HT at rectifier is likely to be around 300v.

It's some time since I opened my series 4 so I can't remember what the access is like.

If you want to narrow down which capacitor is causing the problem you could tack a modern cap in parallel with the decouplers and move it down the circuit. Do not do this to C26 however as it will strain the rectifier.
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Old 10th Oct 2017, 11:38 pm   #8
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Default Re: Ferrograph Series 4 issue! Help pls

Many thanks. I took a chance today and ordered both 22uf and 10uf caps rated at 450v so I can start recapping the can capacitors when they arrive. Thanks for the tip of attaching capacitors in parallel one at a time temporarily to try and narrow the problem down. I just hope the problem isn't on more than one cap however. It's impossible to find capacitors rated at 8uf and 16uf so I opted for the closest values further up the line. I trust this should be ok?

Yesterday I successfuly restuffed the EL84 bypass capacitor with a modern cap and the operation was very successful at least. Didn't cure the problem of course but it was on the to do list ! I attached a couple of photos

Cheers
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Old 11th Oct 2017, 6:09 am   #9
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Default Re: Ferrograph Series 4 issue! Help pls

" It's impossible to find capacitors rated at 8uf and 16uf so I opted for the closest values further up the line. I trust this should be ok? " You can, see here - https://www.watfordvalves.com/products.asp?ID=3&man=260 but your right they'll be fine as long as any rectifier valve "max capacitance " value isn't exceeded.

Nice coloured filling.: ) Playdoh or wax?

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Old 11th Oct 2017, 6:37 am   #10
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Default Re: Ferrograph Series 4 issue! Help pls

Apart from bad capacitors, the feedback that causes motorboating can be caused by bad chassis connections. The root cause could turn out to be a dirty tag connection or a dirty clamp on a canned capacitor. Take care in restuffing to make sure your -ve connections are very solid. other metals riveted or clamped onto aluminium can be dodgy.

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Old 11th Oct 2017, 8:01 am   #11
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Default Re: Ferrograph Series 4 issue! Help pls

I drilled through the other end to pass the negative terminal of the axial capacitor through and soldered it to the original tag! Nothing was changed in that respect. Quite hard to drill I must admit. I hope that the motorboating is caused by one of these caps and nothing else as it will be a pain to solve otherwise!

As for restuffing the cap I first inserted the axial capacitor through and surrounded it with bits of polystyrene to keep it steady. I cut a disc out of a plastic flimsy material but still quite strong out of a container that came with some turning tools I had purchased online. I drilled it and inserted it for a tight fit! The positive terminal was soldered to a crimping terminal that I had so it's very solid. I intend to to similar jobs for thecHT & decoupling caps but using radial caps. I have to work out a way how to secure the negative terminal to the cans, mainly how to feed it through the top safely and securely away from the other terminals. Will update if it turns out nicely
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Old 11th Oct 2017, 5:30 pm   #12
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Default Re: Ferrograph Series 4 issue! Help pls

its worth ensuring that the silver foil which runs under the chassis, attaches to the front of the cabinet has a good earth. If not, this has be known to cause stability problems at high gain

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Old 14th Oct 2017, 2:25 pm   #13
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Default Re: Ferrograph Series 4 issue! Help pls

I am not quite sure what you refer to as a silver foil attached to the front of the cabinet. I see a black earth cable running from one of the speaker mounting screws leading to the front of the cabinet, underneath a white cardboard like material. If this is what you refer to then it looks like it's connected well to earth. (?)

I have tried so far connecting 4 parallel caps (one at a time) to both the dual can capacitors that live on the amplifier chassis and although did notice that the motorboating was reduced considerably, the exercise did lower down the noise level (perhaps the gain as a result??). I still have to try to do the same exercise with the two caps that are next to the rectifier caps on the oscillator chassis. I only did the experiment with the machine in play position put without any tape and checking the gain level on maximum while increasing the bass control. Am I on the right track here?

Cheers again
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Old 14th Oct 2017, 10:30 pm   #14
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Default Re: Ferrograph Series 4 issue! Help pls

The 'silver foil' is a sheet of tin plate covered in the white insulating laminate you can see pinned to the inside of the cabinet front. It is earthed by a wire soldered to the back of the bottom left corner that with a similar wire from the speaker chassis bolts to the power supply chassis.

To work on the amp 'live' the easiest way is to also remove the power supply and plug both together on a workbench. You'll need to short pins 6 and 7 of the octal socket on the amp to get power to the output valve (the lead from the deck does this by routing the HT through the deck solenoid/auto stop switch). Note that the amp will be unstable and probably oscillate if the gain is turned up without the tape head plugged in.
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Old 16th Oct 2017, 5:06 pm   #15
jascha
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Smile Re: Ferrograph Series 4 issue! Help pls

Thank you all so much for all your help.
I have sorted out the motorboating problem which was due to the decoupling capacitors C11/C12 that reside on the amplifier chassis. I recapped all 4 can electrolytic capacitors just to be safe, all 4 seemed to be desperate!
All motorboating noise has totally gone
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