AR88 curious voltages

[cendoubleu]

Hi everyone, and a Happy New Year to you all.
Had a bit of time over the last couple of days to do some work on the AR88 (after 18 months!). I changed R43-R45 as they were looking dodgy, bathtub capacitors which house 0.05uF(C109,110,and C48 (i think)) and 0.22 uF(C99,C112,C133), plus caps C117,118 and 122. I have also put a output transformer from an old scrap piece of equipment in parallel with the O/P transformer as the primary was kaput. On power-up, I only have 24.5v at the juction of R43 and C97. Could this indicate that the large 4uf caps have gone leaky too. On inspection they appear to have moisture around their terminals. When I get a moment I shall lift R43 and check the voltage at C97, but any pointers would be much appreciated.

Charlie

[Station X]

If you're trying to measure HT, then this should be measured across C98. What you've been measuring is the voltage fed to the RF Gain Control, which is then fed to the grids of the variable mu valves. 24.5V seems reasonale for this.

[cendoubleu]

Thanks Graham for the clarification. I was looking at the resistor chain of R43,44, etc as a potential divider and not realising quite what was going on.

[Station X]

The resistors form a potential divider which is connected in series with the negative side of the HT supply. It's used to derive various bias voltages.

The diagram can be difficult to read, as it's drawn the American way, and to my way of thinking it's upside down. They probably feel the same way about our diagrams.

[cendoubleu]

Thanks for all the info Graham, I can see that now.
I think I may have problems also when I pop the o/p valve back in. I have just thought about where I sited the new o/p transformer - right under the mains transformer! If it doesn't hum like heck I shall be suprised. If so will have to think of a new site. It was just convenient as the bathtub cap I took out to replace with leaded caps, the holes lined up exactly with the replacement xformer. I will try and check it out today.
Hope the 'New Year head' is not too painful this morning!

[Station X]

I restuffed my bathtubs leaving the backs off. I think that forum member Sean Williams will be offering some AR88 Output Transformers for sale on here soon.

[cendoubleu]

Thanks the info about the transformers from Sean. There was one on ebay a few weeks ago but not at a price I could afford just before Xmas. I got the idea of using another xformer in parallel with the original from this site:http://www.btinternet.com/~ALLAN.ISAACS/ar88.html
and as I had one in the junk box, thought I would give it a try. If there are hum problems I shall try a different placement but if no good shall wait to see what Sean has to offer.

[Station X]

Colomor seem to be the only people selling these now. As you say though the price is a bit high:-

http://cgi.ebay.co.uk/ws/eBayISAPI.d...m=350023775469

[Sean Williams]

Yes, I will be offering some of these - will probably put an ad up on Saturday/Sunday....

Cheers
Sean

[dave walsh]

Thanks Sean. Still interested here. I'm confused about your link to Colomor [?] though Graham, as I understood that they stopped making what looked like an excellent replacement at least two years ago and only offer the occasional "new-ish" original one. The cendoubleu link suggests a relatively simple solution to this frequent problem anyway but it won't be the replacement for the original of course. There's quite a history of woe in previous O/P Trans threads.
Dave W

[Station X]

I may be wrong, but I think that Colomor trade on eBay under several names. The location "Billingshurst" gives it away. We shouldn't be discussing eBay sellers here though.

I vaguely recall a replacement item being sold, but I don't know who the supplier was.

The asking price for these transformers may be high but, having dismantled one with a view to rewinding, I found that the construction standard was very high. The only problem is that they're unreliable. Mine went open circuit when the speaker became disconnected for a few seconds due to a high resistance plug. I got a replacement from Colomor.

[cendoubleu]

further update on the AR88. Yes there was a loud hum (and nothing much else) with the replacement xformer parked under the mains xformer. Moved it a bit further along the side apron toward the front panel and voila! the thing sprung in to life. Sensitivity appears a bit down but there are some more caps/resistors to change and it will probably need some alignment (if I'm brave enough!). The only curiosity I have noticed so far is that V10 (af driver) plate is only 42V whereas it should be in the region of 80v so will have to investigate that. Another question I would pose is: why is the feedback for V10 taken from the o/p xformer, when presumably R54 could just be connected direct to chassis, rather than through the xformer.

[Station X]

will probably need some alignment (if I'm brave enough!).

I strongly advise you to leave the IF alignment well alone. I had great difficulty setting mine up and had to use an HP Sweep Generator for the job. A homebrew one wasn't good enough.

The only curiousity I have noticed so far is that V10 (af driver) plate is only 42V whereas it should be in the region of 80v so will have to investigate that.

Any number of components to suspect here. I'd start with R40, R41 and C113. If no joy try C117, C118 and C122. Goodness I sound like Steve P!

[cendoubleu]

thanks for the info Graham. Point taken over the alignment, will steer clear as I don't have a Wobbulator. I have changed C117,118 and 122. So will look to R40,41. I have measured them 'in-circuit' and they appear OK but will change em out anyway.

[dave walsh]

As I am in the idiot/armchair group bravely identified by Mark S on his thread
yesterday, I wonder if anyone has a response to the question about R54 posed here in post 12* by cendoubleu? Would be interested to learn. Dave W

[G4XWDJim]

Originally Posted by cendoubleu

Another question I would pose is: why is the feedback for V10 taken from the o/p xformer, when presumably R54 could just be connected direct to chassis, rather than through the xformer.

No-one seems to have commented on this:-
The series feedback resistor R54 determines the amount of voltage being fed back to the previous stage. The value of 2.7k must have been chosen by the designer to feedback the amount he felt was correct. It looks to be a fairly typical NFB circuit arrangement to suit a low impedance output. Could be interesting to replace R54 with a pot and note the change in sound output and quality when it's varied.

Best Wishes,

Jim G4XWD

[dave walsh]

Well somebody's commented now Jim-thanks a lot. I was expecting responses to cendoubleu's original comment [p12*] from the other very knowledgeable contributors to AR88 threads on here! Your suggestion seems to be quite interesting [I think ].
Dave W

[cendoubleu]

Yep thanks for that Jim. I still don't really understand why its taken from a tap on the o/p xformer rather than ground, as the voltage will be zero in both cases. According to the data, this provides f/b which results in holding the gain of V10 to 20. Without R54 it is something like 100. I guess this would overdrive V11 and produce distortion.
I have now changed R40 and acheived correct plate voltage on V10. It had only crept up to about 320K. I have also noticed that V11 plate and screen voltage are about 20v higher than spec, but this could be because I am using a 6V6GT rather than 6K6.
Now basic reception is fine but have some more faults to look at:

Tone Control is inop i.e. tone control does not make any difference
BFO inop
AFC doesn't appear to work very well.

I have some more bathtubs to replace, so this will probably make a difference. Slowly getting my way through it!

[G4XWDJim]

Originally Posted by cendoubleu

Yep thanks for that Jim. I still don't really understand why its taken from a tap on the o/p xformer rather than ground, as the voltage will be zero in both cases. According to the data, this provides f/b which results in holding the gain of V10 to 20. Without R54 it is something like 100. I guess this would overdrive V11 and produce distortion.

Although it's taken from what looks like a centre tap the end of the transformer is earthed so the tap will have the output signal on it. Try listening on a headphone connected by croc clips to verify it. My manual is temporarily buried so I can't look at it for complete verification.

Re the volt readings being too high:- The AR88 readings as given in the book were probably taken by a 1000 ohms per volt meter so if you're using a 20,000 ohms per volt meter as most of us do they could well read higher depending upon where in the circuit you're looking at.

Good luck,
Jim

[XTC]

As Jim says, one end of the tapped secondary is earthed, so both of the other taps are above earth.

The tap in the middle of the winding is for a 2.5R speaker. The whole of that winding is designed for 20K phones. The NFB is taken from the speaker tap. Note, it wasn't a specially wound tap for feedback, they just used the speaker tap in a conventional NFB arrangement.

There's a jack socket for phones which switches off the speaker and should switch in a 5 ohm load. It's a two position jack and is more complicated than that - see E772. As running AR88s with no speaker or load wrecks the O/P transformer primary, that part of the circuit is well worth checking out. A bad contact in that jack could cause a lot of grief. I recall there have been threads discussing it before.

See the REME document E772 paras 46 to 48 for a good description of the OP secondary circuit and "Getting The Most From Your AR88", both mentioned in the other current thread on AR88s.

Pete.