AR88 smoothing block - replacement with electrolytics?

[Martin G7MRV]

Along with my order for loads of polyesters to replace the micamolds and bathtub caps, I grabbed a set of three 4uF 600v electrolytics to replace the smoothing block...

But now i come to look at doing so, my understanding of the power supply fails me! Only one cap goes direct to ground, the other two go somewhere in the biasing circuitry. This leaves me a little confused as to whether I can actually use electrolytics to replace these two, and if so, which way around?

Im thinking that the connection to the bias circuit/T1 center tap, will be considerably lower voltage than the actual HT rail and so connecting electrolytics in the normal way here should be ok, but id very much appreciate the input of the groups experience!

Incidentally I have bridged the TRANS switch mode out, so the supply cannot be switched off load

[ms660]

Connect the replacements as per the schematic, the reason that the "-ve's" of C96 and C97 in the schematic are connected to the secondary center tap and not ground is because it prevents the main ripple currents (100Hz) from flowing through the bias chain and appearing on the bias voltages.

So the -ve's of the two replacement electrolytics for C96 and C97 connect to the secondary center tap and the -ve of the replacement electrolytic for C98 connects to ground/chassis.

Make sure that the metal cans of the first two are insulated from the chassis.

Lawrence.

[Station X]

I might have a NOS smoothing capacitor can. Here. If I have I'll test it for leakage and capacitance. Yours for the cost of the postage.

[Martin G7MRV]

Originally Posted by ms660

Connect the replacements as per the schematic, the reason that the "-ve's" of C96 and C97 in the schematic are connected to the secondary center tap and not ground is because it prevents the main ripple currents (100Hz) from flowing through the bias chain and appearing on the bias voltages.

So the -ve's of the two replacement electrolytics for C96 and C97 connect to the secondary center tap and the -ve of the replacement electrolytic for C98 connects to ground/chassis.

Make sure that the metal cans of the first two are insulated from the chassis.

Lawrence.

Ah, smashing. That means I can fit them close to the chokes and the mains transformer, where there is considerably more space available. The caps are insulated body but i'll be mounting them with an air gap anyway, and sleeving on the wires. Wouldnt trust the thin polymer film around them to withstand much.

Thanks for the offer Graham, but I have a feeling the postage cost on one of these blocks is far more than what I paid for these caps!

[G6Tanuki]

Yes it is essential to properly isolate the negatives of the capacitors from chassis or you can short out the bias circuit..

Particularly important if you are using metal can capacitors with no guarantee that the internal parts are isolated from the can.

In times past I have used 440VAC rated motor capacitors, Ducati, made in Italy, which have plastic cases so can be mounted using no extra insulation on the clamps.

[Radio Wrangler]

With the capacitance and voltage values needed and the volume available to package them, you could use plastic film non-electrolytic capacitors. I looked around for my AR88 but most I found were touted as audiophile types, but the prices weren't too bad. I'm looking forward to sonic improvements when I get my old receiver finished!

David

[Silicon]

The designers of military gear in the 1940s tried to avoid using electrolytic capacitors in the power supply.

The paper in oil cans were more reliable.

[turretslug]

Originally Posted by Radio Wrangler

With the capacitance and voltage values needed and the volume available to package them, you could use plastic film non-electrolytic capacitors.

When I renewed the innards of the smoothing filter block in my AR88D, necessary because of a split seam and consequent oil leakage, I used good-quality aluminium-shell ERO plastic film units (4.7uF 630V), they only occupied about a sixth of the outer can volume.

[thoyer454]

I have several '88 radios and I use a terminal strip (tag strip) to mount the new caps in the choke area. One end of caps connects to respective choke terminals, the other to the terminal strip wired to chassis or the -ve line.
Tom
W3TA