ECH81 triode anode voltage extremely high - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

indigo.girl · 5th Jun 2017, 9:31 pm

Looks to me like V5 (EB81) is only used by the FM signal

Radio Wrangler · 5th Jun 2017, 9:47 pm

Yes, it's only used in a ratio detector circuit (diodes point opposite directions and the big floating capacitor C46 is a give-away)

The diode parts of the EBF89 do AM demodulation and AGC detection.

David

indigo.girl · 5th Jun 2017, 10:46 pm

Have checked out the components and still no fault detected

R21, R49, R19 all matching spec
V5 glows, no shorts between any pins and the heater.
C46 replaced with a 4.7uF Electrolytic (spec says 2uF)
Running out of ideas now!

ms660 · 6th Jun 2017, 7:52 am

I would go back to the tuner, also make sure all switch contacts are good.

Lawrence.

Boater Sam · 6th Jun 2017, 10:29 am

There is a history of trimmer capacitors in VHF tuners going short circuit in Bush VHF heads, could this be similar?
Funny, never considered these as components likely to fail, but there is always a first time.

Radio Wrangler · 6th Jun 2017, 11:09 am

If you've been involved with Icom shortwave gear, you'll consider trimmer capacitors as very life-limited. They creamed an adhesive gunge all over the components in their frequency synthesisers to reduce microphony, and it rotted their trimmers several years later. Changing them is de-rigeur in amateur radio gear.

David

Boater Sam · 6th Jun 2017, 12:45 pm

There you go then, have a look at the trimmers.
Still getting to grips with Long Wave never mind Short!
The Bush ones are mica, I have never seen these fail anywhere else. A tiny tweak usually clears the short strangely.

indigo.girl · 6th Jun 2017, 10:19 pm

Lawrence - checked out the switch contacts - all OK.
Sam - twiddled the trimmers (returning to original position) and no change.

I did find that two bare wires that run into V4 pin1 and pin2 grids were in direct contact. I thought I'd found the fault but when I separated them there was still no change

The whole underneath is a mish-mash of bare wires - I can't believe they weren't insulated when assembled - surely one squash of the base and all sorts of erroneous contacts will be made.

indigo.girl · 10th Jun 2017, 8:49 pm

Quote:

Originally Posted by Boater Sam

There you go then, have a look at the trimmers.
Still getting to grips with Long Wave never mind Short!
The Bush ones are mica, I have never seen these fail anywhere else. A tiny tweak usually clears the short strangely.

Looking at the trimmers around the V1 circuit there are 4:

C8 and C10 run to earth so may have shorted
C14 and C16 are both in parallel across other circuits.

The pin3 cathode reading is much lower than 1.6V and eratic so I had changed the C8 1-15pF trimmer for 33pF and the C9 220pF cap above it for 330pF (the nearest I have available). The pin 3 cathode now reads 1.6V and matches spec. But still no reception on FM. Also the anode voltages (pin 1 and pin 6) have increased quite a bit (300V and 246V). Specs put them at 178V and 163V.

I have next changed the C10 (1-5pF) trimmer for 165pF cap. Anode voltages reduce back own to 212V and 184V but still no FM reception.

I think its time to give up

ms660 · 10th Jun 2017, 9:08 pm

A proper RF sig gen would probably help, I would say a must have for some radio signal faults.

I read earlier that your digital meter has been giving false readings, time for an analog meter as well if you haven't got one.

I would put all the original value trimmers etc back in the tuner and maybe come back to it at a later date with an analog meter and if you can manage it an RF sig gen that will cover up to at least 100MHz, the old Advance E2 used to be the trade's workhorse sig gen.

Lawrence.

5th Jun 2017, 9:31 pm	#41
indigo.girl Retired Dormant Member Join Date: Apr 2016 Location: Loughborough, Leicestershire, UK. Posts: 754	Re: ECH81 triode anode voltage extremely high Looks to me like V5 (EB81) is only used by the FM signal Attached Thumbnails

5th Jun 2017, 9:47 pm	#42
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: ECH81 triode anode voltage extremely high Yes, it's only used in a ratio detector circuit (diodes point opposite directions and the big floating capacitor C46 is a give-away) The diode parts of the EBF89 do AM demodulation and AGC detection. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

5th Jun 2017, 10:46 pm	#43
indigo.girl Retired Dormant Member Join Date: Apr 2016 Location: Loughborough, Leicestershire, UK. Posts: 754	Re: ECH81 triode anode voltage extremely high Have checked out the components and still no fault detected R21, R49, R19 all matching spec V5 glows, no shorts between any pins and the heater. C46 replaced with a 4.7uF Electrolytic (spec says 2uF) Running out of ideas now!

6th Jun 2017, 7:52 am	#44
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: ECH81 triode anode voltage extremely high I would go back to the tuner, also make sure all switch contacts are good. Lawrence.

6th Jun 2017, 10:29 am	#45
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: ECH81 triode anode voltage extremely high There is a history of trimmer capacitors in VHF tuners going short circuit in Bush VHF heads, could this be similar? Funny, never considered these as components likely to fail, but there is always a first time.

6th Jun 2017, 11:09 am	#46
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: ECH81 triode anode voltage extremely high If you've been involved with Icom shortwave gear, you'll consider trimmer capacitors as very life-limited. They creamed an adhesive gunge all over the components in their frequency synthesisers to reduce microphony, and it rotted their trimmers several years later. Changing them is de-rigeur in amateur radio gear. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

6th Jun 2017, 12:45 pm	#47
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: ECH81 triode anode voltage extremely high There you go then, have a look at the trimmers. Still getting to grips with Long Wave never mind Short! The Bush ones are mica, I have never seen these fail anywhere else. A tiny tweak usually clears the short strangely.

6th Jun 2017, 10:19 pm	#48
indigo.girl Retired Dormant Member Join Date: Apr 2016 Location: Loughborough, Leicestershire, UK. Posts: 754	Re: ECH81 triode anode voltage extremely high Lawrence - checked out the switch contacts - all OK. Sam - twiddled the trimmers (returning to original position) and no change. I did find that two bare wires that run into V4 pin1 and pin2 grids were in direct contact. I thought I'd found the fault but when I separated them there was still no change The whole underneath is a mish-mash of bare wires - I can't believe they weren't insulated when assembled - surely one squash of the base and all sorts of erroneous contacts will be made. Attached Thumbnails

10th Jun 2017, 9:08 pm	#50
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: ECH81 triode anode voltage extremely high A proper RF sig gen would probably help, I would say a must have for some radio signal faults. I read earlier that your digital meter has been giving false readings, time for an analog meter as well if you haven't got one. I would put all the original value trimmers etc back in the tuner and maybe come back to it at a later date with an analog meter and if you can manage it an RF sig gen that will cover up to at least 100MHz, the old Advance E2 used to be the trade's workhorse sig gen. Lawrence.