The Workbench

Re: Penta signal path troubleshooting

PL212 — Wed, 06 Aug 2008 01:37:17 GMT

That did it!! C15 (2,2μ 50v) on the amplifier board was my Waterloo -- I broke the connection somewhere in the midst of replacing it. Now fixed via a patch wire, insulated with shrink tubing. Nearly all the capacitors are replaced in the crossovers (Dillen and I are tracking down the more obscure ones for this extremely early Penta I) and I'm taking a test listen with the speaker column lying on the floor. Sounds terrific!

Re: Penta signal path troubleshooting

PL212 — Wed, 06 Aug 2008 00:15:57 GMT

Double-checking my work, I already found one broken trace on the amplifier board (where a silver ring had pulled away from the board.) I guess you can't be too careful with these 1980s boards - very fragile! Will check everything again, make some repairs, and report back...

thanks for the tip!

Re: Penta signal path troubleshooting

Die_Bogener — Sun, 03 Aug 2008 02:58:07 GMT

Hi,

the WIMA caps have no polarity, no magnetism... that's why they sound so wonderfull. So this is not the problem.

Check the cable to the front clap. It's a common problem, that this cable breaks... especially after disassembling the amp.

Martin

Penta signal path troubleshooting

PL212 — Sun, 03 Aug 2008 00:47:54 GMT

I'm in the midst of refurbishing the BeoLab 150 amplifiers which are a part of a set of Penta I's. I've completed what I thought was a successful recapping of the amplifier board, together with an upgrade of the IC2 opamp, only to discover that I've unfortunately rendered the BeoLab (temporarily?) inoperable. Here's the situation right now:

Fuses are both intact.
Speaker itself works when signal is applied to internal connection in the bottom of the column, bypasssing the BeoLab 150.
The amp light glows a steady red, as normal, when connected to mains power.
When fed via either SpeakerLink or guillotine wires from an audio source, no sound is audible, even at high volumes.
The BeoLab 150 seems to turn on automatically and correctly (light turns green) when fed via 4-pin SpeakerLink and set to LINK, and the audio system is turned on. (However I believe this is the result of SpeakerLink signaling.)
The amp light does NOT turn green when set to AUTO instead of LINK, as it should when functioning normally
Moving the SENSITIVITY slider has no effect (not even a soft pop or click)
Moving the BASS switch does generate a soft pop in the speaker at each detent, but no other effect. (Does this tell us anything about what is or is not working correctly in the output signal path?)
Volume up or down does not produce the characteristic electronic interference sound as previously (although that could be something the servicing fixed.
The display works perfect in SpeakerLink mode, showing volume and source.

I have double-checked my soldering for bad connections, as well as checked the polarity of the new capacitors, but I suppose I might have done something wrong. (One question is whether the new small red WIMA capacitors are polarized -- I installed the under the assumption that the "-" next to the voltage rating was the negative side.) Another possibility is that my re-jiggering of the system has created the need to adjust a potentiometer somewhere? Finally I might have dislodged a wire in wrestling the heavy items in and out of the housing... I suppose my question is whether there's a logical troubleshooting process to follow, in terms of what to check first. I have a copy of the Service Manual and access to a voltmeter...

Thanks in advance!