Kia Ora (Covid-Free greetings from the other side of the Tasman) I’m on the scrounge for knowledge again – if you’d be so kind... A lovely ruby-red Type 61 Little Nipper followed me home a while back.

Beautiful on the outside, troubled on the inside. Possible fire (lotsa smoke damage) under the chassis, output transformer primary open, 6M5 no-go. I seem to remember reading the pitch-cased transformers can go open. So I decided on a complete rebuild. (Under.jpg)

Upon powering it up & got a very harsh whistle & a motorboat. I swapped the capacitor from the secondary to the 6M5 cathode to the other speaker terminal & the screech stopped. Unfortunately the motorboating didn’t. & I can also hear it in the new o/p transformer! I’m using the workshop speaker, the hum is quite low, the HT figures look normal but it’s very unstable.

The new O/P transformer is designated OP3 (Front.jpg) & should be suitable for a 6M5.
I’ve not heard stations yet but I’m loathed to run it too long. Any measurements/checks result in a loud splat from the speaker – I can also hear these in the O/P transformer.
I’ve swapped out all the valves apart from the 6X4 – the HT is good - & there’s no difference. When I pull out the 6AV6 the motorboating stops. I can stop the motorboating in certain positions of the tuning dial but putting my hand near the 6BE6, 6BA6 & 6AV6 starts it off again. It probably something stupid I’ve done & would welcome any & all criticism

On the tuning side, one winding of the aerial transformer (Aerial.jpg) was open so I’ve used a donor one. Is the original a special type? Also, according to the circuit diagram the gang is tuning the smaller coil. Is this usual?

I hope you are all well in your bubbles, mines filled with wine!
Best,
John

Sounds like feedback in the IF stage. Have you replaced all the paper caps? (no photos yet)

Yep, all small components are new. I've sent the photos to Brad. Thank you for your interest
John.

61-51 uses feedback from the secondary. Phase that the wrong way around and it should motor boat. Reverse the primary wires.

It was probably me with the comment with tarred & nor feathered OP transformers I note on that one R22 & R23 are two separate 22K resistors in parallel. On some of that type its one 10K & it cooks as the current is too great for a one watt resistor.

61-51 has a shielded transformer: Fit an earthed cable. 10Meg resistor on 6AV6 has an attrition rate do check. Putting new caps around it can cause issues as the modern ones are not shielded. If the issue is in the RF try probing with a chopstick & see if that changes the frequency of the oscillation. That will be the problem area. There may be a scrap one of that type floating around here? They are popular on my bench, fixed a dozen or more. I won a pile of sets from a deceased estate awhile back, most are parts only, although the Kriesler may fix.

Marc

Photos uploaded to Post 3.

Yes Marc has the most likely explanation. Reversing the speaker transformer primary OR secondary, which ever is more convenient, should fix it.

6A8G, You are giving the poor old 6X4 a hard time with 100 mfd and 47 mfd filter caps.
I would prefer low mfd’s and higher working voltage.
JJ

If it is a 61-51 they should be 16μF (22 uF) Those others will strip the cathode.

6X4 does not produce a surge like 5Y3 / #80 albeit some of those 450V RB'S are not good quality & I won't use them.

Marc

Thank you all for the replies, swapping the feedback from the secondary - this stopped the scream but not the motorboat.
Thanx for the pointer for the power supply caps - I will replace.
When I pull out the 6BA6 the motorboat continues - it stops when I pull out the 6AV6 however.
All caps except the ones around the oscillator coil are new or NOS.
Is the aerial transformer a special type? The one which is open looks pregnant - there's a donor from a Bell Colt in service at the moment.
& yes there's a three-core plug & yes, the earth is connected.
The motorboat is very low frequency.
I have yet to prod the IF channel

Motor boating is a low frequency feedback through the HT circuit.
The audio amplifier forms a oscillator action, the output valve pulls the HT line down a bit by drawing current.
That pulse is normally damped by isolating resistors and bypass caps and nothing happens.
If that pulse gets back into the grid of a preamp valve that then feeds forward the pulse amplified and pushes the output valve harder.
That then pulses the HT line a bit harder and the system will settle going putt putt putt putt etc.
Normally if you scope a HT line you will see a bit of sawtooth ripple, but on a motorboating set you will see a big pulse in time with the motor boating. Even watching a needle voltmeter on the HT line bob up and down will display the pulsing.
There is a component not doing its job!
Usually a bypass capacitor missing/faulty, or, some modification deleting a crucial resistor/capacitor.
As a test, on a set with simple filtering if you deliberately open circuit the second filter you can make it motor boat nicely.
Can also be a cathode/screen bypass faulty.
Fred.

As for the first filter cap and the rectifier tube, you should use the 16μF. subsequent filter caps, if separated from the first by either a choke coil or resistor, may be very big in uF's.

6AV6 can be very cranky and I have seen them mounted on rubber & it may pay to try shieling it. The 10M or so grid resistor is often around 13+. There is a batch of resistors in some of them where most of them were duds.

The first IF tube is susceptible to oscillation, if the lead dress is bad & flying trough the air & not run close to the chassis and around the 6AV6 and most second detectors induction & radiation are common issues especially with new caps.

A chopstick is very useful for probing to see if that changes the oscillation frequency.

OK, re the motorboating:

The circuit is only marginally stable with that negative feedback method and would have issues if some of the caps or the speaker transformer in particular was replaced - which has happened.

I'd suggest removing C19 (.001uF) and replacing it with 100pF fitted directly onto pin 7 of the 6AV6 to ground. Use short leads.

Also, C16 is now .015μF, it should be .01μF or even .0047μF. There might be a ground loop or common impedance from the speaker transformer ground somewhere near the volume control. This might be original. Making C16 .0047μF should move the lower pole up a bit and avoid the problem.

If that doesn't help, try simply removing C22.

New old stock capacitors? How old are they. The only NOS caps I'm happy to use are the philips mustard ceramic caps. Can we elaborate on this. What sort of caps are they?

NOS also rings alarm bells here. Waxed paper and Oil filled types are seriously unreliable. Polyester & Styrene not so, albeit the latter can be damaged soldering them. seen examples of that. Plastic ones like those yellow ones can be killed by a careless hit by a soldering iron.

NOS NP caps at minimum should be tested for leakage, at their working volts & for HV ones an insulation tester with different voltage ranges is ideal. There are other nasty things you can do to and with Insulation testers: Handy