DR112 PSU Capacitors

[mikeydee]

Hi,

I have just started working on repair for a friends DR112 6 channel forum. The amp's symptoms are that it fades in and out of volume and I noticed that two of the capacitors show signs of rupture so first off I plan to replace them in the hope that the rusty output transformer is ok. However I am not certain whether to replace like for like or look at the schematics that I have seen in books, Mark Huss' website and other sites. I realise that the 79 diagram is a different era but wondered what others approach has been - 3 of the caps are as per the 79 schematic but the 16/32 one seems a bit different from the 220 MFD on the schematic.

Here are the values I have fitted (looking at amps front control panel, left to right). In particular capacitor number 2 is at odds with the diagram.

1 x 50 + 50 MFD @ 350v *
1 x 16 + 32 MFD @ 350v **
2 x 220MFD @ 350v *

It would be great if some could share their experience on this.

* appear to be date coded 78 and 80
** appears to be date coded 70

[Dr.HI-TONE]

that amp should have:
one - 16+32uf 500vdc
one 50+50uf 500vdc
three 200uf or 220uf cans minimum of 350vdc

can you post pics?

[OldSchoolDave]

I have just started working on repair for a friends DR112 6 channel

Welcome to the Vintage Hiwatt Forum!

At that age, all of the electrolytic capacitors are probably due for replacement. Nine times out of ten, that will cure the symptoms you report. Unless the transformer looks to have been submerged, I suspect it's o.k.. Surface rust is generally not a problem.

Keep us posted on your progress.

Dave

[mikeydee]

Hi glad to be here,

Thanks for confirmation of the cap values. I just double checked the schematic against the amp. I didn't notice that the 16/32 cap on the schematic is supplying the preamps so that makes sense - I was mixing it up with the 220MFD cap on HT3. So as far as the 220 MFD caps goes it looks like I am one missing.

Some pics below showing the four caps.

IMG_1121.JPG (468.34 KiB) Viewed 1688 times

DR112 chassis from above

IMG_1120.JPG (398.99 KiB) Viewed 1688 times

DR112 front panel

IMG_1119.JPG (359.31 KiB) Viewed 1688 times

[mikeydee]

I have just started working on repair for a friends DR112 6 channel

Welcome to the Vintage Hiwatt Forum!

At that age, all of the electrolytic capacitors are probably due for replacement. Nine times out of ten, that will cure the symptoms you report. Unless the transformer looks to have been submerged, I suspect it's o.k.. Surface rust is generally not a problem.

Keep us posted on your progress.

Dave

Absolutely agree I will be replacing all.

[Dr.HI-TONE]

can you post a pic of the underside?
what is the serial number of that amp?

[mikeydee]

Sure thing . . .

Serial number is 1112 (dates it at about 1970 from what I can see on mhuss.com).
Attached is the underside picture of the DR112 and I can get many others pics if they can help.

Mike

[Dr.HI-TONE]

I am surprised to see the older 1960's filtering in an amp that late.
You do only need two 220uf caps.

[mikeydee]

Thanks very much for your help. I will get this fix done over the next few weeks and report back on the difference

Cheers

[OldSchoolDave]

Some work has obviously been done to that amp previously. The diode leads are usually straight as arrows. These have been bent (and one looks to have been spliced/repaired?). Also, the two grey resistors(?) next to the diodes aren't original.

Dr.Hiwatt has seen far more PA heads than I. I'll let him speak to the capacitors on the preamp board (I'd expect to see more mustards).

Cool amp, though! I'm keeping an eye out for one of those myself.

Dave