69 am/fm stereo with multiplex

3/10/2011 7:33:53 PMScott

Hello,
I am trying to solve a problem with my radio. I have replaced all the electro caps in the radio (91bfm2) and the multiplex (91famp-11). All things work except that the volume out of the right channel speakers(multiplex) is very low when output on ds-501 set at 1.5v. I can get good volume if I over adjust output to 2+ volts but any increase in speaker volume produces very scratchy sound. all other transistor voltages seem normal. Anybody else had this problem? Thanks

3/11/2011 7:55:02 AMDaniel

The symptoms you describe remind me of Germanium output transistors and how easy it was to damage them. So I'm hoping you have Silicon Transistors and not the touchy unstable germanium variety.

What I would do is get a "pencil" style signal injector, start at the output transistors and work your way back to the preamp stage until you can be certain which right channel audio stage is the real problem. And put a magnifying glass to the circuit board and look for hairline cracks and suspect solder joints. Don't forget to check all of the "ground" connections. Depending on your loudspeaker arrangement while much less likely, there might even be a problem with a crossover network.

:Hello,
: I am trying to solve a problem with my radio. I have replaced all the electro caps in the radio (91bfm2) and the multiplex (91famp-11). All things work except that the volume out of the right channel speakers(multiplex) is very low when output on ds-501 set at 1.5v. I can get good volume if I over adjust output to 2+ volts but any increase in speaker volume produces very scratchy sound. all other transistor voltages seem normal. Anybody else had this problem? Thanks
:

3/12/2011 7:28:39 PMScott

:The symptoms you describe remind me of Germanium output transistors and how easy it was to damage them. So I'm hoping you have Silicon Transistors and not the touchy unstable germanium variety.
:
:What I would do is get a "pencil" style signal injector, start at the output transistors and work your way back to the preamp stage until you can be certain which right channel audio stage is the real problem. And put a magnifying glass to the circuit board and look for hairline cracks and suspect solder joints. Don't forget to check all of the "ground" connections. Depending on your loudspeaker arrangement while much less likely, there might even be a problem with a crossover network.
:
:
::Hello,
:: I am trying to solve a problem with my radio. I have replaced all the electro caps in the radio (91bfm2) and the multiplex (91famp-11). All things work except that the volume out of the right channel speakers(multiplex) is very low when output on ds-501 set at 1.5v. I can get good volume if I over adjust output to 2+ volts but any increase in speaker volume produces very scratchy sound. all other transistor voltages seem normal. Anybody else had this problem? Thanks
::
:
:
Daniel,
Thanks for the tips....From what I have found out the Ds-501 is a silicon transistor. I have scoured the board looking for any breaks and have found none. I have access to a signal generator but it is not the pencil type. I have to replace the .47 fuse resistor before I can continue any troubleshooting. Thanks

3/12/2011 10:16:38 PMMAG

::The symptoms you describe remind me of Germanium output transistors and how easy it was to damage them. So I'm hoping you have Silicon Transistors and not the touchy unstable germanium variety.
::
::What I would do is get a "pencil" style signal injector, start at the output transistors and work your way back to the preamp stage until you can be certain which right channel audio stage is the real problem. And put a magnifying glass to the circuit board and look for hairline cracks and suspect solder joints. Don't forget to check all of the "ground" connections. Depending on your loudspeaker arrangement while much less likely, there might even be a problem with a crossover network.
::
::
:::Hello,
::: I am trying to solve a problem with my radio. I have replaced all the electro caps in the radio (91bfm2) and the multiplex (91famp-11). All things work except that the volume out of the right channel speakers(multiplex) is very low when output on ds-501 set at 1.5v. I can get good volume if I over adjust output to 2+ volts but any increase in speaker volume produces very scratchy sound. all other transistor voltages seem normal. Anybody else had this problem? Thanks
:::
::
::
:Daniel,
:Thanks for the tips....From what I have found out the Ds-501 is a silicon transistor. I have scoured the board looking for any breaks and have found none. I have access to a signal generator but it is not the pencil type. I have to replace the .47 fuse resistor before I can continue any troubleshooting. Thanks
:

Scott,
DS-501 is a PNP Germanium transistor. When you replaced “ALL” of the electrolytic capacitors; did you check or replace the small 10mfd coupling cap on the base of the DS-66 Pre-driver transistor in the multiplex unit? Double check your testing hook up on the bench. With all the back and forth cabling and faders on this set-up, it’s very easy to get something hooked up wrong on the bench.
Are you working from the Sams schematic or the Delco service literature?

Meade

3/12/2011 10:37:59 PMDS 501

http://semiconductormuseum.com/Transistors/LectureHall/JoeKnight/JoeKnight_EarlyPowerTransistorHistory_Delco_Page2.htm

3/13/2011 7:57:26 AMMAG

:http://semiconductormuseum.com/Transistors/LectureHall/JoeKnight/JoeKnight_EarlyPowerTransistorHistory_Delco_Page2.htm
:

Regardless of the statement in the link that Delco came out with a silicon DS-501, I maintain that the DS-501 transistor as used in Delco auto radios is PNP germanium. Delco service literature and the NTE cross reference guide both agree on germanium. NTE recommended replacement for DS-501 is NTE 105 (PNP-Ge, Audio Power Amp, case style: TO-36)
Meade

3/13/2011 2:13:20 PMDS 501

http://www.radiomuseum.org/tubes/tube_ds-501.html

3/13/2011 4:58:25 PMScott

::http://semiconductormuseum.com/Transistors/LectureHall/JoeKnight/JoeKnight_EarlyPowerTransistorHistory_Delco_Page2.htm
::
:
:Regardless of the statement in the link that Delco came out with a silicon DS-501, I maintain that the DS-501 transistor as used in Delco auto radios is PNP germanium. Delco service literature and the NTE cross reference guide both agree on germanium. NTE recommended replacement for DS-501 is NTE 105 (PNP-Ge, Audio Power Amp, case style: TO-36)
:Meade
:
I think my DS-501 is ok but I have thought about replacing it with an NTE 105 anyway. I replaced the 4 volt 100mf with a new 50 volt 100mf. I am using the delco service manual but I do have the photofact for the 91bfm-2. I have checked and double checked the speaker wiring and grounds. I have spent hours troubleshooting with no luck. I am going to replace the fused resistor which was ok but blew during the last attempt. I am getting some good response from this site and the help is greatly appreciated !!!

3/14/2011 2:04:28 PMClifton

Another source of similar transistors may be on the surplus market. GE Land Mobile used a similar transistor in the transistorized power supply of some of their mobile hybrid radios back in the 60s. The GE number was/is 5490810P1.

Clifton