AR88D no audio

[HamishBoxer]

I am confused here as I thought there was HT coming from the rectifier valve?

What voltage across C96 and C97? Are R43 R44 R45 ok?

[Keith]

Alan only said there were volts "to" the rectifier. As Lawrence says, if the rectifier filament is glowing, most likely fault is an o/c resistor in the bias chain (R43/44/45), These are prone to failure due to high dissipation (especially if decouplers have been leaking).

[AlanG4MOM]

It looks like the rectifier has failed.
It's not lit. There is no continuity between pins 2 and 8 when pulled out.
I now need to change out the bathtub C79, C84, C92 and R22.
Then look for a spare 5Y3GT rectifier.

[CambridgeWorks]

You could try re-soldering the pins see if that works.
Rob

[HamishBoxer]

Just a point,try re soldering the heater pins on the rect before condemning.

Clashed post!

[AlanG4MOM]

I tried re soldering the connections but no joy.
The original rectifier is marked 5Y3WGTA.
I do not have a direct replacement but I do have an RCA 5AR4/GZ34.
It looks like this could be used.
Comments please ?

[Keith]

GZ34 should be fine.

[Radio Wrangler]

The GZ34 has the advantage that as an indirectly heated valve, it will warm up slower than the 5Y3, allowing the other valves to heat more before the HT comes up. This should reduce the surge of off-load HT voltage due to the mismatch in heating times.

On the other hand, the GZ34 is a significantly more efficient rectifier with lower voltage drop, so the running HT voltage will be a bit on the high side.

My AR88's had a 5R4 in the rectifier socket since the 1960's

David

[turretslug]

Look carefully at the 5Y3's tape filament- if it has actually failed (as opposed to there simply being a poor joint in one of the hollow base pins- by no means unknown), then you may see that it has parted at a fold-back. They seem to get to an age where the emissve tape simply weakens and parts, sometimes from being slightly jarred or just of its own accord. Indirectly-heated substitutes, such as the 5Z4 family or 5AR4 (GZ34) you have should be long-lived with the provisos highlighted by Radio Wrangler.

Mine's had a broken-spigot Russian "GZ32" (as labelled- actually, it's more like a 5Z4G with its 2A, rather than 2.3A, heater current) in it for yonks.

[HamishBoxer]

Is it now up and running again I wonder with fresh rectifier?

[AlanG4MOM]

I fired it up with the GZ34 in circuit.
It lit up fine and I waited in anticipation.
After a minute or so there were a few flashes within the GZ34 envelope which gradually worsened so I powered off. I do not know the status of the GZ34 and have no way of testing it. Continuity is still ok between pins 2 and 8.
I have re flowed the solder on the hollow pins on the original 5Y3GT but still can't read
any continuity between pins 2 and 8.
Time for a cuppa !

[AlanG4MOM]

Quick update.
I have just noticed what I believe to be bits of filament tape floating around loose
in the 5Y3GT envelope so I think we can safely say it is not recoverable.

[Radio Wrangler]

It sounds like there may be something wrong which destroys rectifiers. Is there something going short circuit downstream of it? This can induce flashovers.

Possibilities are the block capacitors, the tone correction capacitor, the 6V6, the grid blocker cap, crumbly wiring?

David

[AlanG4MOM]

David,
Thanks for the feedback.
When you say block capacitors due you mean the 3 x 4uF ?
I have already replaced the tone correction capacitor C119 and wired it across the primary of
the output transformer.
I may have spare V11 I could substitute to test.
Alan

[HamishBoxer]

Might be worth monitoring the HT current with the next rectifier.A meter in series with the cathode of same.

[AlanG4MOM]

I tried the GZ34 once again. No ask me why.
After a few sparkles and a purplish glow (similar to the VR150) it settled down to just
the red glow of the filaments. Looking at the front panel the S meter is now responding
to the rf gain control and I have audio out once again.
So where to go now.
I think I'll work through all the 3 section capacitors first.

[Keith]

Sounds as if there was a short which has now cleared. You need to eliminate it as it will invariably come back at some point. The AR88 is very poor in the fusing department. On my LF I added a 500mA fuse in the lead to the centre tap of the HT winding. This was very easy as there was a spare fuseholder on the rear apron (used only to hold a spare mains fuse!). At least this will protect the transformer and rectifier if/when the problem comes back.

[AlanG4MOM]

Keith,
I did stumble across your post regarding the fusing of the centre tap on the HT winding few days ago. There are a few holes in the rear apron which look like being from the original steel stamping. I'm sure I can encourage a fuse holder to fit. Can you remind me of the fuse size.
The receiver seems to have a fault with the noise limiter. Whenever I select N/L in Manual or AVC the audio becomes distorted. Go back to MAN or AVC without the noise limiter and the audio is back to normal. Maybe I should start a new thread with this one.
Thanks to all for your ideas and suggestions which have guided me along the way.
Alan

[Keith]

I'm fairly sure I used 500mA - difficult to check without moving the heavy beast! Noise limiters always introduces some distortion as the top of the wanted signal is clipped. The AR88 NL is variable so this helps. I only use it to clip lightning crashes, the rest of the time it stays switched off.