VHF90C, possible service data errors. - UK Vintage Radio Repair and Restoration Discussion Forum

GJR 11L · 11th Jan 2006, 5:20 pm

Well, the VHF90C is all but ready to go back together however, the discriminator stabilising cap and resistor that sits parallel to it are not of the values that the service sheet says they should be. C29 should, says the book, be a 5uF @50V, the resistor (R17) is meant to be 22K.
The cap, which looked original judging by the surrounding filth, had a value nearer to 47uF! A little further than the +100% allowed in the spec, no value readable on the thing itself. That resistor is actually a 310K or a 41K, but it's got no tolerance band visible and its markings are right in the middle of the body.

so I'm unsure which.

The metre tells a different tale, it's apparently failed to all intents and purposes as it's into the tens of MegOhms range.

Presumably, by replacing the cap with a new 5uF part and changing the resistor to the 22K specified, things will be ok. I only ask due to the resistor found there being so far from the book value.
The diagram I am using is for the VHF90A, but their chassis are identical and every other value matches.
Please advise, before I put the set away and ignore it for another dozen or so years.

ukcol · 11th Jan 2006, 6:00 pm

The manufacturers manual for the VHF90C gives the same values as you quoted, that is 22k and 5uF. The component numbers are different (R27 and C49).

I suspect the old electrolytic is a 4.7uF with the decimal point eligible, but the resistor value in your set is strange. R27 would not be a close tolerance component and neither of those values (41k and 310k) fit the E12 value scheme.

(For those who don't happen to know, the E12 range is [1] [1.2] [1.5] [1.8] [2.2] [2.7] [3.3] [3.9] [4.7] [5.6] [6.8] and [8.2]. Multiplying by powers of 10 gives the complete value range).

Sideband · 11th Jan 2006, 6:04 pm

I don't have the circuit handy but 5uF and 22k sound right. Use a 4.7uF at about 50v as a replacement and dont forget that the POSITIVE end goes to chassis

.

Rich.

GJR 11L · 11th Jan 2006, 6:31 pm

Quote:

Originally Posted by ukcol

The manufacturers manual for the VHF90C gives the same values as you quoted, that is 22k and 5uF. The component numbers are different (R27 and C49).

Cheers for that, but those numbers refer to the same components in the am/fm set that bears the same model number. Their functions are obviously the same, as is V5, but the correct manual for this VHF90C is the one for a VHF90A!

The more I examine the joints at these components, the more I suspect bodgery, in which case the question isn't "What are the values here and here?", but "How the

has this set been working at all?" At first it all looked original, but the solder melted away in two distinct layers when I sucked the posts clean prior to replacing these bits, which I shall replace with the values stated in the book and confirmed by your good self.
The morning will do for that though.
Rich, thanks also for your comments, I did notice the orientation of the cap, but such essential information as the correct fitment of electrolytics cannot be over-emphasised! I have some suitably small 5uF ones, ranging from 100 to 450 Volts working, so one of the former can go in there.
Thank goodness the UABC80 seems undamaged, it's the only one I have.

11th Jan 2006, 5:20 pm	#1
GJR 11L Heptode Join Date: May 2004 Location: Ellington, Northumberland, UK. Posts: 815	VHF90C, possible service data errors. Well, the VHF90C is all but ready to go back together however, the discriminator stabilising cap and resistor that sits parallel to it are not of the values that the service sheet says they should be. C29 should, says the book, be a 5uF @50V, the resistor (R17) is meant to be 22K. The cap, which looked original judging by the surrounding filth, had a value nearer to 47uF! A little further than the +100% allowed in the spec, no value readable on the thing itself. That resistor is actually a 310K or a 41K, but it's got no tolerance band visible and its markings are right in the middle of the body. so I'm unsure which. The metre tells a different tale, it's apparently failed to all intents and purposes as it's into the tens of MegOhms range. Presumably, by replacing the cap with a new 5uF part and changing the resistor to the 22K specified, things will be ok. I only ask due to the resistor found there being so far from the book value. The diagram I am using is for the VHF90A, but their chassis are identical and every other value matches. Please advise, before I put the set away and ignore it for another dozen or so years. __________________ John.

11th Jan 2006, 6:00 pm	#2
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: VHF90C, values of a couple of bits? The manufacturers manual for the VHF90C gives the same values as you quoted, that is 22k and 5uF. The component numbers are different (R27 and C49). I suspect the old electrolytic is a 4.7uF with the decimal point eligible, but the resistor value in your set is strange. R27 would not be a close tolerance component and neither of those values (41k and 310k) fit the E12 value scheme. (For those who don't happen to know, the E12 range is [1] [1.2] [1.5] [1.8] [2.2] [2.7] [3.3] [3.9] [4.7] [5.6] [6.8] and [8.2]. Multiplying by powers of 10 gives the complete value range).

11th Jan 2006, 6:04 pm	#3
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,566	Re: VHF90C, values of a couple of bits? I don't have the circuit handy but 5uF and 22k sound right. Use a 4.7uF at about 50v as a replacement and dont forget that the POSITIVE end goes to chassis . Rich. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....