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-   -   Ferrograph 4A: Restoration advice needed (https://www.vintage-radio.net/forum/showthread.php?t=151194)

ms660 18th Feb 2019 8:03 pm

Re: Ferrograph 4A: Restoration advice needed
 
I wouldn't test run of the mill usual suspect capacitors, by the time they've been unsoldered, tested and re-soldered might just as well replace them instead, having said that, each to their own.

I would replace C1 with a 1,000 VDC polypropylene type.

I've had suflex types give problems in radios, TV's, tape recorders and test equipment, just my observation.

Lawrence.

Uncle Bulgaria 4th Mar 2019 5:44 pm

Re: Ferrograph 4A: Restoration advice needed
 
Thanks again, Lawrence.

The power supply has been running all yesterday afternoon on a bulb limiter, with no ill effects and a steady 375V on the choke/reservoir capacitor junction. It is almost time to plug the amplifier section in...

I'm quite pleased with how well it's cleaned up! I've left the top plate in its original state to retain some of its history and I'm gradually reattaching parts to it. I've rebuilt the switches and re-soldered a number of broken connections in the plugs from over-flexed, work-hardened or corroded joints.

Uncle Bulgaria 8th Mar 2019 7:52 pm

Re: Ferrograph 4A: Restoration advice needed
 
4 Attachment(s)
I had to rather demolish the tape counter in order to get into it to clean it. There are two screws underneath the cardboard printed face - in heating it up to try to dislodge the press-fit body, as I thought, I melted the cover! Time to make a new acrylic dial glass. Out with the holesaw!

Then a nicely scrubbed-up switch, and a horribly dusty loudspeaker. There's been a lot of elbow-grease and hot water gone into this project so far! Don't worry, not on the speaker cone!

Friday night - time for the last motor lubrication, and :beer:

The other night I did start to pick up my jar of remarkably malt-looking ISO100 oil instead of my glass of whisky... I need to remember to put a lid on!

Uncle Bulgaria 12th Mar 2019 5:40 pm

Re: Ferrograph 4A: Restoration advice needed
 
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I am confused as to how the new set of pressure pads fit. The new ones have short legs and came with three springs that don't seem to have any place in my 4A. There is no detent in the collars they mount on for the springs to pull against. I also presume I don't need the third, as it would be for the stereo head that in mine is the 'dummy'.

Are these a later model that aren't compatible? You can see from the picture that the new ones (top row) can't hook onto the springs supplied on the head mounting plate, whereas one of the old ones has a bend in the wire which I assume is for the coil spring to engage with. This surmise is illustrated in the second photograph.

TIMTAPE 12th Mar 2019 8:58 pm

Re: Ferrograph 4A: Restoration advice needed
 
They may be from the Series Seven which IIRC was the last Ferro model to use pressure pads.

Tim

BillDWVA 12th Mar 2019 11:31 pm

Re: Ferrograph 4A: Restoration advice needed
 
Quote:

Originally Posted by Uncle Bulgaria (Post 1128307)
I am confused as to how the new set of pressure pads fit. The new ones have short legs and came with three springs that don't seem to have any place in my 4A. There is no detent in the collars they mount on for the springs to pull against. I also presume I don't need the third, as it would be for the stereo head that in mine is the 'dummy'.

Are these a later model that aren't compatible? You can see from the picture that the new ones (top row) can't hook onto the springs supplied on the head mounting plate, whereas one of the old ones has a bend in the wire which I assume is for the coil spring to engage with. This surmise is illustrated in the second photograph.

Hi Uncle B,

Those little coil springs are used on Series 5 and later decks; you donít need them as youíve got the tension springs under the deck as youíve shown.

Remember that you can simply glue new felt to your existing arms; I bought some piano tunerís felt and use it on all my decks when the old pads are compressed hard or packed solid with oxide dust.

Cheers,
Bill

Uncle Bulgaria 13th Mar 2019 5:50 pm

Re: Ferrograph 4A: Restoration advice needed
 
1 Attachment(s)
Thanks both.

Yes Bill, I've had in mind your recommendation - unfortunately I bought the arms before your initial post on the first page! However, these longer wire pieces look crucial for engaging the springs so I'll have to re-felt them rather than use the new ones.

New problem: R35 burning.

Attached is the PSU circuit section. On 'record' the manual suggests I should have 240V on the anode of V6, the EL84 oscillator. Each time I've engaged 'Record', R35 starts smoking. I've replaced it but it was a symptom not the problem.

Could this be a valve fault, in which case are there any measurements I can do with a multimeter or AVO to check? Or where else in the circuit should I be looking?

The heads are not connected, if that matters.

Radio Wrangler 13th Mar 2019 7:10 pm

Re: Ferrograph 4A: Restoration advice needed
 
Anode to g1 short in the valve? otherwise you have a lot of oscillator power going on because the only power reaching R35 should be erase frequency AC via the secondary of the oscillator coil... or has the thermistor gone low?

This circuit should survive without the heads, otherwise removing the erase/bias link will do damage.

David

Uncle Bulgaria 13th Mar 2019 7:28 pm

Re: Ferrograph 4A: Restoration advice needed
 
Thanks for the wrangling!

The EL84 has no continuity except h-h.

The thermistor out of circuit reads 110k Ohm at room temperature. Am I right in thinking that the thermistor and R35 are a divider to keep the screen current constant?

There is no indication I can see for the thermistor's values in the parts list: "Thermistor STC. A1 522/100" is all it says.

ms660 13th Mar 2019 7:55 pm

Re: Ferrograph 4A: Restoration advice needed
 
Just to note, the resistor is connected to the control grid not the screen grid, try powering up with the oscillator valve unplugged and see if the resistor still overheats when switched to record.

Lawrence.

Uncle Bulgaria 13th Mar 2019 11:29 pm

Re: Ferrograph 4A: Restoration advice needed
 
Yes, of course you're right Lawrence - I was confusing myself thinking about the pin numbers!

With the EL84 out, there's still smoke from R35.

Uncle Bulgaria 14th Mar 2019 2:10 am

Re: Ferrograph 4A: Restoration advice needed
 
Aha! The smoke is back in.

Realising it still did it with the EL84 out, I traced it back to the coil and found there was a wiring error. Either from when I replaced the old resistor, or the reason it failed in the first place. Often it's the impetus to check with fresh eyes that's all that's needed!

It's sitting on now with no smoke.

Next issue: the hold-in solenoid (though it appears to be more of an electromagnet) is not holding in. It has the volts it should have on input and output (perhaps more, but this is with a DMM rather than an AVO8 as the manual requires). Also the run switch is sparking, even with a new capacitor across it.

BillDWVA 14th Mar 2019 9:13 am

Re: Ferrograph 4A: Restoration advice needed
 
Hi,

The hold-in solenoid is in the HT circuit so needs to be carrying some current in order to work, I.e. to be magnetised. Have you got the set fully energised now? You mentioned in an earlier post that you had the power supply section running on its own.

Bill

Uncle Bulgaria 14th Mar 2019 2:00 pm

Re: Ferrograph 4A: Restoration advice needed
 
Yes Bill - I've got the top plate on and everything hooked up besides the heads. They're a little difficult to access so I didn't want to be dealing with that until I was sure the basic functions are working.

The motors all spin on their allotted settings (FF, RW, Play, Rec) and the speed change works. The 'run' slider is just not holding in. I notice it is shorted by the 'stop' button and the 'auto-stop' switch. The 'auto-stop' is out of circuit, so open. I believe this means it is not preventing the solenoid from energising. I can't see how mine works anyway as it's very different from the illustration in the book and doesn't seem to have any means of switching.

On 'Rec' the panel meter can be adjusted to zero. There is no output to the speaker, so I'm checking the valve voltages in the amplifier section and have already come across two further very high resistors.

The solenoid coil has a resistance of 355 Ohms.

ms660 14th Mar 2019 2:12 pm

Re: Ferrograph 4A: Restoration advice needed
 
Bypass switching aside, the current through the solenoid is dependant on the current through the sound output valve only (V4) According to the voltages in the manual the solenoid current should be approx. 43mA.

Lawrence.

Uncle Bulgaria 14th Mar 2019 6:25 pm

Re: Ferrograph 4A: Restoration advice needed
 
Thank you Lawrence - I can trace that now. It's off E & F on the deck into TR2 on the anode of V4 and the HT.

I checked the cathode bias resistor on V4. It was high so I put another in parallel until RS can send me the right one. With the deck in 'play', I got 8.9V across 218R so there was 40mA through the valve.

There was a slight sense of stickiness, as if the solenoid was pulling a little, but not enough to hold the slider in.

Uncle Bulgaria 17th Mar 2019 10:23 pm

Re: Ferrograph 4A: Restoration advice needed
 
3 Attachment(s)
Lawrence's comment led me to go through the voltages in the manual and discover some components that previously tested good, but changed in use. One cathode was grounded, for example.

However, some measurements are far enough out that I think they're beyond tolerance, so it would be interesting to see what others think.

V1a: 46V - 15% high.
V2a: 72V - 31% high.
V3a: 85V - 42% high.
V4a: 248V - 3% high

R28/R29: 238V - 11% high.
R27/R28: 206V - 23% high.
R7/R27: 154V - 28% high.

Attached is the relevant part of the circuit (full diagram download link earlier in the thread) and the pages from the manual for reference.

BillDWVA 18th Mar 2019 12:04 am

Re: Ferrograph 4A: Restoration advice needed
 
Hi Uncle B,

Other more experienced members here will probably correct me but there are two reasons that come to mind why you may be seeing higher voltages in this valve circuit than in the manufacturer’s service data. The first is that you are probably measuring the circuit using a modern DMM with a very high input resistance whereas there is a good chance the figures quoted were as measured by an AVO with a typical resistance of 20k/volt.
Secondly, if you have replaced the HT smoothing and reservoir capacitors with new, their lower leakage currents take some load off the power supply leading to a higher HT voltage measured at the rectifier cathode and consequently higher voltages throughout the set - I noticed this in my recently refurbished Vortexion. It has not lead to any noticeable operating problems though.

Bill

Uncle Bulgaria 19th Mar 2019 12:22 pm

Re: Ferrograph 4A: Restoration advice needed
 
Thanks Bill.

You're right, I was using a DMM as my AVO 4 is under piles of cleared things (I'm restoring this on the floor around the place as we build a house and soldering sometimes takes second fiddle to making walls), and the AVO8 I've got from my grandfather has a snapped binding post!

I noticed the 'measured with an AVO8 at X ohms/Volt', but thought that couldn't account for the small measured difference at the HT and the wildly varying readings on the anodes. Wouldn't it be a linear progression?

I haven't touched the filter/reservoir capacitors as they seemed to be running along just fine - none of the dual cans have been changed as yet.

I couldn't resist, so powered it all up yesterday and had guitar playing through both inputs with a working volume control! The solenoid still wasn't sticking, but I think it's entirely my fault - I'd put a 250mA fuse in the back panel as a fail-safe and it must have been on the edge with the motors running as it blew. Replacing it with the circuit diagram's 1A now that I thought there was limited danger of escaping smoke, the solenoid holds and the motors run!

ms660 19th Mar 2019 12:39 pm

Re: Ferrograph 4A: Restoration advice needed
 
Ohms per volt is range related, eg: if the meter is 20k per volt on DC and it's switched to the 250 volt range the resistance across the meter probes will be 5 meg ohms (20,000*250) on the 100 voltage range it would be 2 meg ohms (20,000*100) and so on etc. The loading effect on the voltage to be measured will vary according to the voltages source impedance, think potential divider.

Lawrence.


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