Ferrogaph 2A amplifier hum.

[stevie23]

Hello people. I am doing a little more work on one of my Ferrographs. This one is the 2nd of 2As I have. Transport seems to be working. These wearite decks seem to be able to stand the test of time. I've take the amplifier and power units out of the machine and I have found a date stamp for 1958 so only 63 years old. . I've removed the double capacitor (C24/26). The 16uf side (red) is measuring 9.6uF with a vloss of 18% and an ESR of 47 ohm. The 8uF side (yellow) is measuring 7.065uF with a vloss of 17% and an ESR of 47 ohm. I'm using one of these new fangled chinese toy things (TC1). I'm not sure I trust it fully. I've been advised to change this cap in particular. I can order them from Cricklewood. Awful humming with oscillation upon turning the gain control. (I couldnt upload the video - whats the criteria?). The other caps are mainly TCC metalmite/metalpack although there is also a paper cap at C10 (50mfd). The smaller value non electrolytic I believe are mostly harmless (or am I wrong )?

[Uncle Bulgaria]

I'd say it's best to measure DC voltages in situ first, before testing at low voltage. The capacitor testers can be useful, but there's no substitute for seeing how the circuit is behaving when in use, and the fault (the hum) is present.

A particular capacitor may measure fine, but if tested at its working voltage of a few hundred volts, it may have great leakage making it unusable.

If you have an oscilloscope, you can probe the PSU capacitor connections and see if 50Hz ripple is coming through, indicating the the capacitor is not doing its job properly. I recently had motorboating on my 4A, and was able to see the failure on my oscilloscope at the junction of the choke and C24. Replacing this dual can (from Cricklewood) cured the fault.

If you have 'The Manual of the Ferrograph' you can use it to diagnose the problems by reference to the DC voltages referenced in the text, then go from there.

The danger with taking things out and testing them individually is that you can introduce a raft of new faults with replacement components without knowing if the originals were part of the existing fault.

[Uncle Bulgaria]

Originally Posted by stevie23

(I couldnt upload the video - whats the criteria?).

The forum doesn't support video uploads. If you 'Go Advanced' at the bottom of your post, and choose 'Manage Attachments' you can see the list of maximum file sizes for different file types. A jpeg for instance will be compressed to 800 x 600 with a maximum file size of 200kB.

I have uploaded videos to Dropbox and then posted a link in my forum post in the past.

Originally Posted by stevie23

The other caps are mainly TCC metalmite/metalpack although there is also a paper cap at C10 (50mfd). The smaller value non electrolytic I believe are mostly harmless (or am I wrong )?

When restoring my 4A, I injected a signal at different points after sorting the DC conditions, and discovered quite a few resistors out of tolerance, as well as failed capacitors. The forum's general consensus appears to be not to fix what isn't broken, rather than the American trend to blanket-replace capacitors. I found most of my TCC capacitors to be failed through leakage at working voltage (I have built a Practical Electronics Magazine leakage tester, which measures leakage current at up to 630V), but my machine was a Freecycle find and in poor condition.

I use this to remind me: "Steps to first powering on old equipment", but take a more test-first approach when it comes to electrolytics. The canned ones in my Ferrograph are a similar age to yours, and I've only had one failure so I don't agree with replacing without testing.

[ms660]

Re: Post#2...It should be 100Hz ripple, if it was 50Hz ripple that would suggest that there's a fault with one of the rectifiers anode circuits, eg: open circuit.

Under load there will always be a noticeable ripple voltage across the reservoir capacitor even if it's doing it's job properly.

Lawrence.

[Uncle Bulgaria]

Thank you, Lawrence. Listen to him stevie23, as he was instrumental in getting my 4A working, and knows an awful lot more than me!

My haste to be helpful makes the advice less useful.

"Festina lente"...!

[stevie23]

Thanks guys. Invaluable information from all of you. I've ordered 2 of the double capacitors from Cricklewood to replace the canned caps on the power supply block . I'm guessing the grounding connection can be wired between the nut and bolt that tightens up the metal band securing the capacitor? Or should it be soldered?

[stevie23]

I have the manual. I won one a while back for all of £2.95. As for changing more of the capacitors, would I be right to swap out ALL of the Canned doubles and Metalmite/Metalpack type silver varieties? I have spotted one of the notorious "Hunts" varieties as C2 strapped underneath the flywheel. ....

Another question: Is there a safe way of taking out the amp and power modules and powering them on so I can then more easily get to the components / voltages I want to test?

[Uncle Bulgaria]

Originally Posted by Uncle Bulgaria

The forum's general consensus appears to be not to fix what isn't broken, rather than the American trend to blanket-replace capacitors.

I would recommend taking it a step at a time, checking the DC conditions first, as I mentioned above. Replacing components that aren't broken isn't good practice. It can introduce other faults, and it's inefficient.

The metal cans are usually expected to be the most reliable, as it seems they survive most readily in many types of equipment even long after their design life.

My Ferrograph's metal cans were no exception, with only one being found leaky and unresponsive to reforming on initial repair. I then changed one of the PSU ones when it was the motorboating culprit, a symptom that arose after I'd been using it daily for a few months.

Having said that, I had to replace a lot of my resistors and capacitors in the amplifier stage as they were largely out of tolerance. If you unsolder one leg you can measure them reasonably accurately.

Originally Posted by stevie23

Another question: Is there a safe way of taking out the amp and power modules and powering them on so I can then more easily get to the components / voltages I want to test?

Safe if you are careful, but I haven't found a way that's not very awkward! See the attached picture. The amplifier section is hard to access, but with the PSU working it's possible to clip test leads onto most parts if you prop the top open, and mount the amplifier section upside down.

The PSU contacts that are useful to measure can mostly be accessed from the top with it remaining in the box - at least for the 4A, the output is present on some joints above the chassis because of the L4 choke.

Originally Posted by stevie23

I have spotted one of the notorious "Hunts" varieties as C2 strapped underneath the flywheel. ....

C2 is probably just fine - Hunt's have a poor reputation because we're finding some of their components have failed, decades after their designers would have expected them to be obsolete. On mine, C2 is a motor capacitor, 0.75µF 450V. I haven't changed it - it's probably a paper in oil type, and reliable, without the failure modes of waxed paper or wet electrolytic. If you don't notice any motor problems it's likely fine.

[flywheel]

Originally Posted by Uncle Bulgaria

I would recommend taking it a step at a time, checking the DC conditions first, as I mentioned above. Replacing components that aren't broken isn't good practice. It can introduce other faults, and it's inefficient.

The metal cans are usually expected to be the most reliable, as it seems they survive most readily in many types of equipment even long after their design life.

My Ferrograph's metal cans were no exception, with only one being found leaky and unresponsive to reforming on initial repair. I then changed one of the PSU ones when it was the motorboating culprit, a symptom that arose after I'd been using it daily for a few months.

Having said that, I had to replace a lot of my resistors and capacitors in the amplifier stage as they were largely out of tolerance. If you unsolder one leg you can measure them reasonably accurately.

C2 is probably just fine - Hunt's have a poor reputation because we're finding some of their components have failed, decades after their designers would have expected them to be obsolete. On mine, C2 is a motor capacitor, 0.75µF 450V. I haven't changed it - it's probably a paper in oil type, and reliable, without the failure modes of waxed paper or wet electrolytic. If you don't notice any motor problems it's likely fine.

The motor capacitor is filled with a grease-like* compound that's almost certainly the now-banned polychlorinated biphenyl. The Visconol branded caps also contain a more oil-like substance that's likely to be a version of the same. So, if it's still working, it's best to leave where it is!

*I once opened one up out of curiosity, but now wish I hadn't.

[stevie23]

OK, thanks guys. I've swapped out the reservoir caps. The humming has now stopped but it now goes into oscillation/motorboating when the gain is up past the halfway point and playing stops.

[DMcMahon]

Motor boating can be caused by several things including bad/leaky decoupling capacitors.

See Post # 7 of this link :-

https://www.google.com/url?client=in...PymiV0wgQsMSiv

I have never done this fault finding technique of adding a capacitor in parallel, if attempted be very careful as some of the decoupling capacitors will have appreciable DC voltage.

I cannot lay my hands on my Ferrograph 2A schematic but wonder if your paper C10 could be a decoupler/bypass, if so then definitely worth checking/changing.

David

[DMcMahon]

If C10 is actually 50mfd/50uF then it is unlikely to be paper.

David

[ms660]

Originally Posted by DMcMahon

Motor boating can be caused by several things including bad/leaky decoupling capacitors.

See Post # 7 of this link :-

https://www.google.com/url?client=in...PymiV0wgQsMSiv

I have never done this fault finding technique of adding a capacitor in parallel, if attempted be very careful as some of the decoupling capacitors will have appreciable DC voltage.

It was normal procedure when I was in the servicing trade, if bridging the reservoir capacitor charge the bridging capacitor across the filter capacitor first....don't forget to discharge it when finished.

Just to note that one of the HT decoupling capacitors relies for its connection to circuit by PL5-SK5 (pin 3)

Lawrence.

[stevie23]

David, great advice. I've done piggybacking of caps before just not on Valve equipment. So I thought i'd try. I seem to have found the caps that cause motorboating. Its another canned jobbie (C9/C13) decoupler. Motorboating seems to have stopped when I do this and the amplifier sounds clean if not a bit crackly upon moving the bass/treble knobs... I'll order another 16/16 from Cricklewood and replace it. Then we'll see how the land lies. ... ooh, on a side note, I'm picking up a Vortexion WVB tonight. I'm ****** excited!!

[DMcMahon]

That sounds like really good news. Sounds like the tone controls could do with a squirt of electrical contact cleaner.

Have fun with the Vortexion.

David

[DMcMahon]

Just to confirm (as Uncle Bulgaria suggested) capacitor C2 is the 0.75uF 450V AC Paper Capstan motor capacitor and unless any problems with the motor or capacitor showing signs of leakage, is unlikley to be requiring replacement.

C10 50uF/12V electrolytic (not paper) is the cathode decoupler/bypass for valve V1

David

[stevie23]

C9/13 now replaced and I hear sound from one of my tapes. Huzzah! Treble control works but the bass doesn't do much except produce an audible *Thump* when turned to zero left. So back to it. ��