Tracing VHF signals on a Tube Receiver

[killingtime]

Evening All,

I'm restoring my first valve receiver. A Dynatron TRV14.

It works on AM with the HF bands. Receives with audio output. FM is dead.

I have the schematic (attached) and I've listed the tubes, with what I think the roles are. I'd be grateful if those with greater knowledge than mine would check this. I have looked the tubes up, but many are multi role.

I plan on tracing the signal through from the input to the output to see where it stops.

1. ECC85 RF pre-amp. (Blue box)
2. ECC85 Local Oscillator (Signal-10.7Mhz) (Red Box)
3. ECH81 Mixer.

The ECC85 is in its own shielded box with assoc components.

The issue is, I'm not sure where to probe. These tubes have a high voltage on them, so figuring where the output of each stage is and sampling after the correct decoupling cap is crucial (to avoid blowing the Oscope input). The schematic is a bit of a rats nest. Stage outputs are not listed. You just have to know.

Lastly, I have access to an RF sign gen with FM mod. When connecting this to the VHF input, would it be a good idea to insert a high voltage non-polarized cap in series with the gen output? This set is old, and I would imagine many of the caps are leaky. This could put large voltages on circuit parts where there should be none, and I wouldn't want to blow the sig gen output. Better safe than sorry. What kind of cap & uF value? Max DC voltage in the circuit is 260V DC.

Are there any quick tests I could do before tracing signals? Like checking to see if all the valves get warm... That would be a quick win.

Any suggestions welcome.

Thanks.

[Cobaltblue]

Radiomuseum will ban you for publishing anything from their site.

Cheers

Mike T

[killingtime]

Originally Posted by Cobaltblue

Radiomuseum will ban you for publishing anything from their site.

Cheers

Mike T

OK. Can I post stuff that's not from RadioMuseum?

[Cobaltblue]

If it not available top right and its not from Radiomuseum then it can be published here providing it does not in contravention of the sources policies.

So if you have a physical copy that you have scanned and its out of copyright. or it has come from a source without such rules then that's OK.

Cheers

Mike T

[Nuvistor]

If you have little experience with repairing radios have a look at this web site, full of general repair information which should help answer your questions.
https://www.vintage-radio.com/repair...ion/index.html

The TRV14 uses a very conventional for the time circuit, slightly more complicated with the extra switching for the SW stations.

[VT FUSE]

Your last paragraph in your initial post may be a part solution to the VHF problem, you enquire about 1st principle checks- the ECC85 is notorious in multi band sets for going u/s due to cathode poisoning if the set was predominantly used on broadcast bands other than VHF, a good check is to substitute a known working or new valve. Also ensure the wave band switching gets exercised along with a little switch cleaner carefully applied.
Good luck in getting it going.

[unitelex]

Hi,
Firstly seems we are probably fairly close proximity, I am in Altrincham

The ECC85 has two sections, the first triode is the VHF RF amplifier and the second one is the mixer/oscillator, a standard configuration.
The schematic for this set has the 2nd triode unconventionally located below the first triode so it not so obvious that the signal passes through that stage on first inspection.
The pentode stage of the ECH81 is the first IF amplifier for VHF and frequency changer for AM with the triode stage of the ECH81 is operating as the local oscillator for AM.

If you have AM without FM then suspect the ECC85 stages or band selection switches particularly S1B which switches HT to the ECC85 stages when VHF band is selected.

Usually the VHF tuner's capacitors and resistors are OK, best left well alone because disturbance of these items can send it out of alignment.
The most common issue in this stage is a jaded ECC85 valve or band switches (as suggested by VT Fuse). The HT switch might be burnt out as it does switch DC HT and is prone to arcing.

It could also be possible that the IF stages are working on AM but out of alignment on VHF, or the VHF discriminator stage (V4) has a fault. The stage around V4 has an 8uF cap C72 which often fails, and one or both of the two OA79 crystal diodes D1, D2 could have failed.

Occasionally I get a VHF set in for repair and find a previous repairer has adjusted the front end to attempt extend the range above 100MHz with varied success. This is usually evident from disturbed VHF trimmer adjustments where the paint or wax seal has been broken. If it looks untouched I would leave it alone and focus on voltage measurements and signal injection and tracing.

Always best to start by checking voltages against those showing on the service sheet, have the set on VHF band since that is the one you have trouble with, but also a good idea to check on AM band also.

If voltages look about right, then to home-in on the causal area I would start checking signal at the ECH81, with VHF selected.

Does the set respond if you inject 10.7MHz on to G1 (pin7), a 100pF or less could be used to couple this signal in. If the set responds to this then the trouble is likely to be in the ECC85 stages. If it does not respond suspect Discriminator and the IF stages.

Paul has a more comprehensive guide to diagnosing these sections here :https://www.vintage-radio.com/repair..._vhf-sets.html (as suggested by Nuvistor).

Regards
Chris

[killingtime]

Evening All,

Thank you for the comprehensive replies.

Nuvistor: The guide is helpful.
Vt fuse: It may well be the tube, I've asked if I can borrow a tube tester from a friend.
Unitelex: Helpful and detailed reply. I've done some more testing...

With the VHF switch engaged (S1B - Orange Box Top), the ECC85 sees 225 VDC. Also tested that there is continuity with the IF output switch (S2A - Green Box Mid). Also tested the tube filament with an ohm meter - continuity, and I can see it glowing in service.

So that means I now have to start injecting signals to see if I get an IF out of the mixer. Could someone please confirm that the correct place to sample the IF out is at the red * in the green box on the schematic below? I should see 10.7MHz at this point right?
Between the rf transformer (7/8) and the decoupling cap (100p), meaning no bias voltage. The image has not come from RadioMuseum (no watermark), and is free from copyright.

Attached and in the link below.

i.ibb.co/qnsk9s7/vhf.jpg

Thanks.

[unitelex]

Yes you could attempt to measure a signal at the transformer side of the 100pF cap however this would be best done with a very low capacitance probe.

A better approach might be to inject 10.7MHz at the ECH81 side of the 100pF cap, ie the pentode G1 pin. Then look for a signal at the demodulator or monitor the audio output, with the waveband switch set to VHF.

Ideally your 10.7MHz signal injection would be Frequency Modulated but that is not essential, you should at least see the 10.7MHz at the demodulator.

Regards
Chris

[killingtime]

Well, after a bit more testing, it looks like it might well be the ECC85 tube as others suspected.

Pictures attached.

I first put a DVM on the ECC85 after the output transformer (at * on the schematic), to make sure there was no DC bias from a leaky cap between that and the ECH81 (there wasn't), then I probed it with an AC coupled Oscope to look for a 10.7Mhz signal. Nothing, other than Oscope noise.

So I injected a 10.7Mhz FM signal into the same point and got a clean audio tone out of the speaker. A few mV was all it took. 100KHz deviation. Had a play with the modulation frequency and could hear output all the way up 11 KHz which was nice.

The ECC85 is looking like the guilty party at this point. Will have to swap it for a working one.

[Sideband]

You'll be lucky to see the oscillator output on the scope unless you are using a low capacitance 100:1 probe. Just applying a standard probe is probably enough to stop the oscillator.

[killingtime]

Well, the plot thickens.

After just probing about the VHF section has sprung into life...but the volume is fading in and out, even on strong stations using the built in vhf dipole antenna or my roof mounted 3 beam. Only happens on VHF FM. Not replaced any tubes or components.

I've cleaned the open chassis switches (DC and VHF) with acetone which were black with dirt but that's made no difference. Even prodding the switches doesn't get rid of the fading but, switching out to AM and back to FM cures it every time for about 5 minutes. That sounds like a dirty switch to me, but they're clean, and the dc voltage to ECC85 is rock solid. It's not the volume pot either. Gave that 50 turns to liven it up but the fading persists

Originally Posted by Sideband

You'll be lucky to see the oscillator output on the scope unless you are using a low capacitance 100:1 probe. Just applying a standard probe is probably enough to stop the oscillator.

Wasn't an issue. See attached. that's a x10 probe (about 14pF at the tip) and weak stations did get weaker with the probe attached, but strong ones were unaffected. Didn't even pull the LO frequency. The FFT shows 10.7MHz center. Only a few mV though.

Worth mentioning the one school boy error I originally made was looking for a 10.7MHz IF with the radio un-tuned. It has to be on a strong station to see an IF, otherwise the IF drops to nothing.

[red-duck]

Simple but sometimes successful trick is rocking the valve, cleaning valve pins and checking they engage with the socket correctly. Many a loose valve has caused lots of trouble shooting headaches.