HP 200CD voltage issue - Page 1
Forum section: electronics / repair

author: Crambone
author_rank: Frequent Contributor
timestamp: June 19, 2025, 12:22:42 am
content: I am seeing excessive high voltage on this unit, it’s wired 120VAC.When I received it, it was popping fuses after a few seconds. I tested all the tubes and they tested ok but not strong so I replaced them with known good ones. I found C12 wired backwards + to - & - to + replaced all the electrolytic filter capacitors and that stopped the fuse from popping. I was waiting for it to warm up and it fried R17 & R18.Now I am still seeing high voltage across the entire unit with the exception of the 6.3VAc. I also removed and tested RT1 & 2 Lamps. I also tested the Air Variable Capacitor for shorts thinking maybe the person working on it prior to me didn’t use caution to not bend them fins but no short found.What could cause this issue?I’ve attached a pic of the schematic with voltages I’m seeing and location of parts mentioned highlighted.Thanks Andrew

author: TimFox
author_rank: Super Contributor
timestamp: June 19, 2025, 12:58:54 am
content: Is the filter choke L5 shorted?I don't see anything besides that choke that would reduce the Vbb to 215 V.Also, were the original rectifiers old-style selenium that were replaced later with modern silicon?

author: floobydust
author_rank: Super Contributor
timestamp: June 19, 2025, 02:23:34 am
content: It's tricky because HP xfmer CT is not grounded. They make a -ve rail with R40 I think.I would expect higher B+ as we are no longer in the 115VAC days, and if the tubes are off, not drawing much current. But you measure 440V instead of 215V?It looks like the output tubes should draw 60mA each (R27 and R28) on the plate+screen.The 6AU6 with 440V on the screen should be roasted dead or they've gone open?I would check some resistors R17, R17, R27, R28, R30, R31 and the lamps. Schematic CR1, CR2 shown backwards?

author: Crambone
author_rank: Frequent Contributor
timestamp: June 19, 2025, 02:32:35 am
content: I don’t see any reduction in voltage from one side to the other on the filter choke.

author: TimFox
author_rank: Super Contributor
timestamp: June 19, 2025, 02:57:55 am
content: My bad: I didn't notice that the transformer CT produced a negative voltage instead of being grounded.I would look for a short from that node to ground, which would place the full 400-odd volts at the Vbb.

author: floobydust
author_rank: Super Contributor
timestamp: June 19, 2025, 03:56:00 pm
content: It's not easy to troubleshoot especially when someone else dim has been in there.I would quickly measure the DC voltages at each tube pin and write it down. Voltage across R40 too.I think 440V would damage the 6AU6 due to there being no -ve grid bias. Or temporarily wire it for 220V mains or use a variac to tone it done while measurements are being done.

author: factory
author_rank: Super Contributor
timestamp: June 19, 2025, 06:31:57 pm
content: Quote from: Crambone on June 19, 2025, 12:22:42 amC12 is supposed to be a non-polarised capacitor, original part is HP #0180-0013 (or 18-25HP) & marked "VNP", the other clue is the lack of polarity marking on the circuit diagram. This part is used in a number of different models of HP 200 series audio frequency signal generators.The later transistor based 200 series also use a non-polar capacitor too.Wonder how many people notice when deciding to replace capacitors?The fins of the variable capacitors have sections that can be bent, this is for alignment at the factory.David

author: factory
author_rank: Super Contributor
timestamp: June 19, 2025, 06:35:58 pm
content: Quote from: TimFox on June 19, 2025, 12:58:54 amEarly 200CD used a tube rectifier, 1965 manual gives silicon diodes for the rectifier.David

author: elecdonia
author_rank: Frequent Contributor
timestamp: June 29, 2025, 02:17:27 am
content: R40, 10k ohms, will have more than 100V dropped across it when everything is working correctly. Note that this 100V is negative with respect to circuit ground. This would lower the measured 440V by more than 100V.Another reason for the excessive B+ voltage is if little or no plate current is flowing through the 2 output tubes. Perhaps these tubes are old and weak? Or maybe there is a fault in their grid bias circuitry?